1
Comprehensive survey of handoff management
challenges in wireless mesh networks and the evaluation
of MPLS-based solution for providing an efficient
transport
Anand Patil
1. Research Topic
A wireless mesh network is a self-organized backbone wireless network that can
dynamically maintain connectivity between its wireless nodes. The mesh here refers to an
interconnected group of mesh routers that help maintain connectivity throughout the mesh
network. A mobile terminal which is connected to a mesh network can maintain the network
connectivity as it moves from one Access Point to another within the mesh network. The
traditional wireless access methods such as Wi-Fi networks, WLAN, Bluetooth, and cellular
networks use single hop communication; for this reason they are connected to an Access Point
which connects directly to a wired backbone network. Expanding the coverage of traditional
wireless access methods is expensive because the wired backbone infrastructure which provides
the backhaul service requires new cables to be laid. In many situations laying new cables is not
feasible for practical and economical reasons. In contrast to single hop communication, the
wireless mesh networks (WMN) use multi-hop communication. In the wireless mesh networks
each Access Point (AP) is wirelessly connected to mesh routers. The mesh routers form the
wireless backbone for WMN networks by routing packets. In wireless mesh networks extending
the wireless coverage to larger areas is simple and cost-effective. This is the reason that service
providers such as Aruba Networks and Cisco are deploying next generation wireless mesh
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networks because they are becoming the preferred way to deliver Video, Data and Voice in
outdoor environments. “A wireless mesh can deliver the same network capacity, reliability and
security that were once reserved for wired networks – but with the flexibility of wireless. With
today‟s state-of-the-art solutions, municipalities, public safety agencies, port authorities, and
industrial organizations can rely on mesh networks to provide essential connectivity to their
workers and constituents” (Aruba Networks, 2011). When the city of Austin, TX decided to offer
cheaper way of providing wireless access to its public on-the-go, the Austin city officials decided
to build a wireless mesh network in the city that would link the public places. The city of Austin
successfully installed wireless mesh solution with the help of Cisco‟s mesh infrastructure (Cisco,
1996-2006).
Handoff management which is part of the mobility solution for mesh networks is a
determining factor for successful deployments of wireless mesh network. WMN design poses
challenges in terms of the mobile device changing its attachment to the network across subnets
or IP domains. Wireless service providers such as Cisco and Aruba Networks are faced with the
problem of providing a consistent mobility management solution in a wireless mesh network.
Aruba Networks has adopted „High-speed outdoor roaming‟ which is a modified version of the
Mobile IP mobility solution. Aruba network‟s proprietary „MobileMatrix‟ technology for fast
roaming of mobile terminals across IP subnet is an example of a mobility solution (Aruba,
20011, pp-9). This paper will look at few of these vendor implementation of mobility
management solution. Two solutions for mobility management namely mobile IP and MPLS-
based modification of Micro-mobile IP are described later in the paper. This paper
analyzes handoff and routing needs of the WMN networks, examines the MPLS application
micro mobility solutions, thus evaluating the suitability of MPLS as an efficient transport
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solution. The paper also looks at the current status of IETF and IEEE standards in terminal
mobility area.
2. Research Question
What are the mobility management challenges unique to the wireless mesh networks and
how have the standardization efforts in this area addressed the WMN deployment issues? Can
the MPLS routing features help achieve efficient handoffs for wireless mesh networks?
3. Background
A wireless mesh network is basically a collection of fixed wireless nodes, most of the
time consisting of regular wireless routers running adapted software. Its main goal is to provide
an inexpensive and easily deployable wireless backhaul that will connect distant LANs or
WLANs (Carrano, et al., 2011, p. 54). As opposed to wireless mesh network, the wired backhaul
refers to a wired backbone network capable of routing between nodes. Typically wired
backbones are an internet network composed of access routers, edge routers, and gateways
interconnected with each other using backbone wired network. The Mobile Terminals (MT)
accesses the network using end devices. These end devices are connected to part of networks
which are usually lower capacity network referred to as access networks. Access networks are
the network which MT are attached to. WLAN, for high-speed wireless data network, cellular
network, for voice and data connectivity, satellite communications for wireless access to
commercial applications, and Wi-Fi hotspot networks in public places for wireless network
connectivity are all examples of access networks (Akyildiz et al., 2004). A WMN has to be able
to integrate with these heterogeneous access networks with acceptable handoff latency.
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The term mobility can refer to either nomadic(terminal) mobility, vehicular mobility,
service mobility or session mobility. Service mobility refers to the ability to maintain sessions
and obtain services without any interruption while user changes terminals and session mobility
allows the user to maintain a session while he/she changes terminals such as a laptop to a PDA
(Mohammad et al, 2009). Only the nomadic/terminal mobility is in the scope of this paper. The
nomadic mobility refers to maintaining the identity of the terminal as it moves from one location
to another. So as the mobile terminal moves from one location to another location, its peer
devices need to be able to continue communication with it using the terminal‟s known identity
irrespective of its location (Schiller & Voisard, 2004, p. 213). Nomadic mobility is also referred
to as terminal mobility. Mohammad et al. (2009) define terminal mobility as “The ability of a
terminal, while in motion, to access telecommunication services from different locations, and the
capability of the network to identify and locate that terminal either in the same or a different
administrative domain.”.
IP protocol is suitable for static networks. In wireless networks, the nodes are in constant
movement. In such environment IP protocol does not result in efficient operation and instead
cause lot of overhead traffic resulting in noticeable latency and unacceptable user experience.
Mobile IP (MIP) refers to protocol enhancements to standard IP protocol that allows transparent
routing of IP datagrams to the mobile nodes on the internet (Perkins, 2002). Some authors have
defined mobile IP as a routing protocol in itself. Authors Raab et al (2005) define mobile IP as
dynamic routing protocol where end devices signal their own routing updates and dynamic
tunnels eliminate the need for host route propagation. This means that instead of routing tables
updating in response to the terminal movement, the tunneling feature hides the address change in
the routing table. A handoff in the wireless mesh network occurs when a mobile terminal
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changes it point of attachment to the wireless backbone network. A change of Access Point (AP)
while maintaining the connectivity is typically called a Handoff (Valko, 1999). Handoff latency
measures the delay between the point in time from when MT was connected to wireless mesh
backbone from a point of attachment and when it moves and is able to connect to the backbone
from another point of attachment. The delay is especially significant in multi-hop transmission
characteristics of WMNs due to multiple route discoveries, signaling message propagation when
multiple hops are involved (Xie & Wang, 2008). Handoff Management is the process by which a
mobile terminal keeps it connection active when it moves from one access point to another
(Akyildiz, Xie, & Mohanty, 2004). The handoff process in the WMN can happen between two
mobile nodes belonging to the same network domain, in which case the handoff is referred to as
micro-mobility, or between WMN nodes belonging to different network domains where it is
called Macro-mobility. Mobility Management enables telecommunication networks to locate
roaming terminals for call delivery and to maintain communication as the terminal is moving to a
new service area (Akyildiz et al., 1999). Mobility Management encompasses a set of tasks for
supervising the mobile user terminal (or mobile Station, MS), in wireless network. The tasks are
divided into registration and paging, admission control, power control and handoff (also called
handover) (Giannattasio et al., 2009). Recently the Multi-Protocol Label Switching has been
explored as an efficient tunneling protocol to be used in MIP. MPLS is a packet forwarding
solution where labels are assigned to packets. Routing is enabled by looking at labels. Label
lookup forwarding enables fast end-to-end routing without need for protocol specific lookups.
What this means is that packet header does not need to be parsed at each hop. If IP routing were
used, IP header has to be parsed to determine the next hop. MPLS label does away with parsing
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for layer-3 or layer-2 specific protocols since it adds a label to the data before the layer-3 header
and after layer-2 frame.
4. Hypothesis
The wireless mesh networks are typically characterized by mobile users traversing across
homogeneous and heterogeneous access networks. The access network could be organized as
one whole layer-2 access network or sub divided as subnets. The layer-2 access mechanisms
could be different in heterogeneous access networks. Within the wireless networks, the access
methods used by mobile device to attach to the access network are different (CDMA, Wi-Fi,
802.11, etc.). As the user moves from one subnet to another or even across the IP domain,
maintaining the mobile user location information becomes an overhead. The handoff
management process needs to deal with location and addressability challenges of mobile nodes.
Since the protocol parameters can be different across each wireless access network, it is difficult
to achieve a smooth transition at each network boundary. The overhead caused by need to
maintain the location and addressability information for each node, results in higher latency in
wireless mesh networks compared to static wired network. MPLS inherently supports QoS
features and this coupled with its lower overhead in network stack lookups should result in
lowering handoff latency and reducing network resource allocation. Due to the lack of adequate
IEEE standards for addressing WMN mobility issues, the vendors such as Cisco and Aruba
Networks have adopted proprietary mobility solutions.
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5. Scope and Assumptions
The scope of this paper is limited to a survey of handoff management issues inherent in the mesh
networks and application of MPLS to address those concerns. Current deployments of wireless
mesh networks by vendors such as Aruba Networks and Cisco is evaluated. The intended
audience is service providers who are planning to provide WMN solutions in enterprise,
municipalities, and emergency services. The IEEE standard 802.11s which is relevant to mesh
networks is covered. The IEEE 802.16 standard is out of the scope as well as the 802.21 standard
which deals with media independent handover between the cellular networks and the WLAN
network (De La Oliva et al. 2008). This paper is concerned with mobile terminal‟s terminal
mobility solution also referred to as nomadic mobility. The scope of the paper mainly focuses on
the intra-domain mobility issues. Thus cross-protocol handoffs which span across an
administrative domain are out of scope. Wireless Metropolitan Mesh Networks (WMAN) is the
deployment of mesh network over metropolitan area and not in the scope of this paper.
6. Importance
The past decade has seen tremendous growth in wireless LAN deployments such as in
enterprises, universities, and public wireless hotspots such as airports, restaurants, etc. However
providing mobility support for mobile devices which move from one wireless access structure to
another often necessitates fixed wired infrastructure as a backbone network to carry and route
traffic to the Internet (Nandiraju et al., 2007). This can be expensive for service providers. WMN
deployment can save CAPEX (capital expenses) for service providers because mesh nodes can
self-configure and acts like a router of backbone traffic without need for expensive T1 and T3
network pipes to be laid.
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Signaling and traffic management are crucial aspects of any communications network.
Mobility Management plays an important role in signaling and traffic analysis as efficient traffic
management load analysis studies involve studying the mobility management (Markoulidakis,
Lyberopoulos, Tsirkas, & Sykas, 1997).
7. Contestable
Even though there are several research papers which focus on the individual aspects of
the handoff management solutions, such as solutions at layer-2 MAC, network layer, micro-
mobility and macro-mobility solutions, a comprehensive survey paper on the handoff and
mobility challenges of WMN, which evaluates MPLS technology for solving mobility issues is
lacking. This paper brings together and provides a comprehensive survey of the handoff
management challenges enabling a better definition of the problem areas in WMN deployment. It
also explores the applicability of MPLS solution to micro mobility solution. The paper
contributes to the WMN deployment scenarios by analyzing the proprietary mobility solution
adapted by the service providers, especially Cisco and Aruba Networks.
Because the IEEE standardization attempts for WMN mobility are still in draft stage (De
La Olivia et al. 2008), this study has importance for service providers (such as Cisco and Aruba
Networks) who in absence of standards have implemented their own proprietary solution to
mobility problem in WMN. The IETF and IEEE work on mobility management solution is
discussed in the „Standards‟ section later in this paper. Even though the handoff process in
wireless networks has been studied, but real-world solution that is acceptable to service providers
has been difficult to achieve (Kretschmer & Ghinea, 2010).
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8. Methodology
This paper identifies the micro-mobility requirements for mobility management in WMN
networks, identifies the current micro-mobility solutions, discusses their inherent issues, and
finally evaluates whether use of MPLS technology in micro-mobility solution can offer
significant advantages. In order to accomplish these goals the paper answers the following sub-
problems.
8.1 Sub Problem 1
The first sub-problem is to evaluate how the wireless mesh network architecture differs
from the traditional Wi-Fi wireless access architecture.
8.1.1 Data collection and research
Identify the issues specific to WMN mobility and the requirements of mobility
management for WMN deployments.
8.2 Sub Problem 2
Second sub-problem is to identify the handoff challenges for wireless mesh deployment
and to examine how the efforts by the standards committee have helped in standardizing the
WMN deployment issues.
8.2.1 Data collection and research
Identify the handoff mechanisms in the WMN networks. Explain the OSI layered
solutions to mobility management to understand the handoff challenges at link and network
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layer. Identify and evaluate the standards committee efforts at standardizing the network and data
link layer mobility solutions.
8.3 Sub Problem 3
Does MPLS offer a better solution towards solving the mobility issues apparent in the
wireless mesh networks in term of reduced complexity and efficient route path selection?
8.3.1 Data collection and analysis of results
Evaluate how MPLS architecture can help towards efficient handoff in WMN. Can
MPLS tunnels provide reduced complexity compared to the IP-to-IP tunnels of the traditional
micro-mobility solutions in the WMN network?
9. Wireless mesh network Architecture
9.1 Traditional wireless infrastructure mode:
The traditional wireless access methods such as Wi-Fi networks, WLAN, Bluetooth, and
cellular networks use single hop communication; for this reason they have to be connected to an
access point which connects directly to a wired backbone network. In a traditional wireless
network each access point connects to a wired backbone network, which could be Ethernet LAN
or any other wired access methods that in turn is connected to the Internet backbone. The AP
serves to provide the mobile terminals with network access. When the mobile terminal moves
from one access network to other, the user has to re-establish the connection. The major
disadvantage of this configuration is that coverage can be increased only by adding new APs.
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Since APs require a backhaul network, providing large wireless coverage areas becomes
expensive.
9.2 Wireless mesh networks:
In contrast to single hop communication the wireless mesh networks use multi-hop
communication. In the WMN networks each AP is wirelessly connected to mesh routers. The
mesh routers form the wireless backbone for WMN networks by routing packets between mesh
routers. A WMN network is composed of Mobile MT connected to AP, from which they get the
network access. The APs wirelessly connect up to the mesh routers. Two or more mesh routers
are managed by mobile gateway routers (MGRs) (Garroppo, Giordano, & Tavanti, 2009). The
mobile gateways routers (MGRs) are the WMN‟s connection point to the Internet backbone. The
service providers typically use a single IP domain in which all their equipment is located. This
single IP domain is divided in to subnets, with each MGR being part of a distinct subnet. Instead
of being connected to a wired backbone, the MRs wirelessly connects to mesh gateway routers.
Each of the MGR is an internet gateway (IG) and connects to the wired infrastructure. The entry
point in the wired infrastructure is via Access routers (AR). The mesh routers are interconnected
to each other and are capable of multi-hop communication. The mesh routers are the first point
of network access to the Access Points because they work at layer-3 and are equipped with
complete IP stack (Garroppo et al. 2009). Just like routers and gateways form the backbone for
the Internet, the MRs and MRGs form the backbone for WMN. This architecture enables mobile
nodes to send multi-hop messages to distance the Foreign Nodes (FN) or the Corresponding
Nodes (CNs).
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Figure 1: Wireless mesh network diagram
Internet Backbone
Domain 1
MTMT
AP
APAP
MTMT
AP
APAPSubnet A
Subnet B
Domain 2
MR
Subnet C
MR
MR
Wireless Mesh BackboneAP
MR
MGR
MGR
MGR
ARARAR Access Routers
9.2.1 Basic Service Set and Enhanced Service Set:
As shown in the below figure, a simplest configuration for WMN is one which consists of Basic
Service Set. A BSS consists of an Access Point several MTs. A MT is confined to a single AP.
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Figure 2: BSS and ESS movement of a MT
Access PointAccess Point
BSSBSS
ESS
BSS: Basic Service Set
ESS: Extended Service Set
MT 2MT 1 MT 3MT 4
AssociationAssociation
Wireless Mesh Client Network
SUBNET A SUBNET B
9.2.2 Homogeneous and Heterogeneous deployments
Because the WMN deployment is expected to cover a large area compared to traditional
WLAN deployment, the expectation is that there will be different access systems, service
providers, backbone networks which could become part of one single mesh network. The WMN
architectures can be divided in two categories, namely Heterogeneous WMN deployment and
Homogeneous WMN deployment. Homogeneous deployments are limited to a single service
provider‟s domain. The entire network is managed by a single provider as a single IP domain.
Single IP domain deployments contain homogeneous access network such as Wi-Fi, cellular
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network, WLAN, and Bluetooth across the mesh network with similar protocols and interfaces.
As shown in the figure above, the domain 1 network has its access network divided into subnets,
the MT movement is within single domain but across subnets. In contrast a heterogeneous
deployment will typically span across IP domains. Each IP domain may be using different access
networks, assuming that the backbone is all IP; the signaling overhead and mobility management
can introduce delay and latency whenever mobile terminal moves from one IP domain to
another.
10. Mobility Management:
A Mobility management solution for a WMN is concerned with ensuring that as the MT moves
in the mesh network, the mesh router can keep track of MT‟s location information as well as
maintaining the user‟s active connections to the backhaul network. From MT‟s perspective the
network adjusts to accommodate MT‟s changing location, reachability, and active TCP
connections. Mobility can refer to either terminal mobility, service mobility, session mobility, or
vehicular mobility. These various types of mobility solutions were briefly discussed in the
section 3 titled „Background‟. In this paper the term Mobility refers to Terminal Mobility.
Mobility management is a broad term referring to various aspects of mobility of a mobile
terminal in the wireless mesh network. Depending on the architecture of the WMN, the entire
WMN may consist of one single IP domain consisting of separate subnets or may be composed
of different IP domains, each domain consisting of several subnets. The former architecture
consisting of a single IP domain is usually a homogeneous wireless access environment because
service provider uses a single access method to access all of its MTs. An architecture consisting
of multiple IP domains is a heterogeneous environment as different IP domain may implement
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different access systems in them. One IP domain may use WLAN connection where as other
domain may use a cellular network to communicate between MTs. The mobility management of
MTs is needed in both the homogeneous and heterogeneous environments, but in heterogeneous
environments a layer-3 handoff solutions are needed, where as for homogeneous environments
both layer-2 and layer-3 are candidate solutions. Depending on the movement of the MT within
or out of its IP domain, the mobility solutions can be classified as intra-domain also called
Micro-Mobility and Inter-domain also called Macro-Mobility.
10.1 Macro and Micro Mobility of mobile Terminal:
When the mobile terminal changes it Point of Attachment (PoA) within an IP domain
from one subnet to another this kind of mobility is referred to as micro-mobility, also called
intra-system mobility. This kind of mobility is the result of network and access systems having
similar protocols and interfaces. In the above shown figure 2, the MT movement from Subnet A
to Subnet B is referred to as Micro-mobility (intra-system). If the MT were to move from one
domain to another, the movement is called macro-mobility (inter-system mobility) (Akyildiz et
al. 2004, p. 18). Macro-mobility is a result of the network spanning service providers, and the IP
domains each having a different protocol stacks and heterogeneous access networks.
10.1.1 Mobility Solution:
IETF introduced mobile IP as a solution for mobile terminals which needed to remain connected
to the internet when changing their point of attachment. The Mobile IP uses concept of Home
Agent, Foreign Agent and registration between the agents as a way to keep track of the changes
in the mobile terminal location and address. The Home agent and Foreign Agents are mesh
gateway routers (MGR) as shown in the figure 1. Because MGR (HA and FA in mobile IP) are
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required to keep the location data for the mobile terminals in database, whenever there is a
handoff initiated by MT movement, the location change information needs to be registered and
propagated to mesh gateway router. This requirement for a MT to register every time it changes
the PoA even within the same MR attachment causes lot of overhead registration messages.
Under mobile IP solution, even when the MT moves from one subnet to other within the same
home domain, the MT is required to send the registration update to the Home Agent for that
domain (Akyildiz et al. 2004, p. 19). Due to inherently frequent handoffs in the wireless mesh
networks, such registrations introduce latency. Due to the latency introduced by the registration
and handoffs in mobile IP, the research community has adopted several variant of mobility
management solution. To reduce the latency due to mobile IP registration and overhead traffic,
the WMN architecture has divided the mobility architecture in two separate problem domains.
The mobility of MT when it moves within the IP domain is referred to as Micro-mobility and
when the mobility spans across IP domains, the mobility is referred to as Macro-mobility.
10.1.1.1 Micro-Mobility:
Due to registration and tunneling overheads of mobile IP solution, the mobile IP has not
been used in micro-mobility domain instead the Micro-Mobility solutions have adopted modified
versions of the mobile-IP. When mobile terminal moves, mobile IP requires a new tunnel to be
setup. The tunnel between Home Agent and Foreign Agent is needed so that mobile terminal can
get the packets destined to it when it associates with the Foreign Agent‟s Access Point. The
delay is inherent in the round trip incurred by the mobile IP as registration request is sent to the
HA and the response sent back to the FA (Campbell et al. 2002). Registration and routing of the
packets via tunnel between HA and FA becomes more significant when the handoffs frequency
increases. Micro-mobility is characterized by frequent handoffs due to the mobile terminals
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switching frequently between APs. Various micro-mobility solutions are available such as
Cellular IP, Hawaii, and Hierarchical mobile IP (Campbell et al. 2002). A common approach in
the micro-mobility solutions to reduce the signaling overhead associated with MT registration is
to localize the signaling overhead generated by the MT movement. For this purpose typically a
mobile Agent gateway is designated closer to the MT and signaling and registration need not
propagate all the way up to the Home Agent gateway (Xie & Wang, 2008, p. 37). Signaling
(location info) is derived implicitly or via ways transparent to a mobile terminal. The idea being
that MT should be freed from signaling overhead, after it has initiated a handoff process (Caceres
& Padmanabhan, 1996, p. 59). Instead of all the handoff signaling flowing to root node which is
the mesh gateway router in MIP, the micro-mobility solutions implement a sort of Hierarchical
router distribution. The Hierarchical router distribution helps with localizing the registration and
signaling traffic because registration updates as MT moves from one PoA to another is kept with
the nearest mobile router. Micro-mobility solutions such as Cellular IP, HAWAII, and HMIP
tend to keep the mobility changes visible in smaller and local area. This avoids the registration
messages between HA and FA from long trips, for example when HA and FA are far apart and
have to cross several network devices to reach one another. Cellular IP, HAWAII, HMIP are
essentially effect proprietary control messages for location management and routing within a
regional area of network (Yokota et al. 2002, p. 132).
10.1.1.2 Routing based and Tunnel based Micro-mobility:
The Micro-mobility solutions are either tunnel based or routing based (Chiussi et al. 2002). The
Cellular IP and Hawaii protocols fall under routing based solution where as Hierarchical MIP
falls under tunnel based approach. Routing based approaches leverage on the IP forwarding and
lookups to send the packets destined for mobile terminal to mobile terminal even when the
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mobile terminal has changed its Point of Attachment (PoA). The tunnel based micro-mobile
solution use the tunnels similar to mobile IP, but the mesh routers are divided into hierarchical
router domain. The division of hierarchical routers helps to localize the tunnels and registration
overheads (Chiussi et al. 2002) and thus avoid the mesh gateway router updates every time a
handoff occurs. Whether the handoff is tunnel based or routing based, the Micro-mobility
solutions need location and handoff management techniques to enable efficient handoff. Every
mobility management solution has two parts, namely location management and handoff
management (Akyildiz, Xie, & Mohanty, 2004). The location management and handoff
management will be discussed in later sections.
10.1.1.3 Routing Based Handoffs
Cellular IP: Cellular IP overcomes the mobile IP limitation of having to propagate the
MT handoff registrations all the way to mesh gateway router. Instead of designating mesh
gateway router as root router (which is always part of every routing decision), the mesh router
closet to the Access Point assumes that functionality. This way when handoff occurs within the
MR domain, mesh gateway router is not even involved. Cellular IP uses the packets transmitted
by MT to mesh gateway router to determine the path information, thus reducing on the signaling
required to keep the location database updated. Instead of a Home Agent maintaining the
location database in a centralized manner, the Cellular IP requires that each mesh router keep a
node to IP address of the MT. Thus each MR knows which port to forward the packet to. This
hop-by-hop routing means that no single point of failure exists (Valko, 1999, p. 55). However to
keep the node-to-MT ipaddress mapping updated, a periodic beacon transmission is required to
be send by the MT. Since the MT may move between the APs, the mapping can become
outdated. For this reason cellular IP utilizes timers to reduce packet loss due to packets delivered
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to old AP. Search and lookup times are improved by using caches. Cellular IP uses IP addresses
to identify the mobile terminals (Campbell, & Gomez 2000, p. 48).
Figure 3: Cellular IP architecture
Internet
AR AR
MR1
MR2
MGR
MR3
MT1 MT2
MT1 -- MR2MT2 – MR3
Host Specific Routing
Figure : Cellular IP Architecture
Mesh Gateway/Domain root Router
Mesh Router
Access Router
Host Specific routing Table
Home Domain
HAWAII: Like Cellular IP, Handoff Aware Wireless Access Internet Infrastructure
(HAWAII) allows the mobile terminal to retain its IP address as it moves in the access domain
from one mesh router to other. Thus the Home Agent (usually the mesh gateway router) is
unaware of the MT movement, avoiding expensive handoffs between HA and FA for each
address change of the MT (Ramjee et al. 2002). Instead of extracting signaling information such
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as node reachability and port to next-hop address mapping from MT‟s normal data traffic the
Hawaii protocol requires the mobile terminal to send explicit signaling messages so that HA can
use these signaling messages to determine the IP routing path to MT. Location information (i.e.
mobile-specific routing entries) is created, updated, and modified by explicit messages sent by
mobile Host (Campbell et al. 2002, p. 73). HAWAII works on hierarchical network segregation
by diving access network in domains. Each MT has a home domain and foreign domain. Inside a
Home Domain, a domain root router is designated. Each MT send periodic infrequent signaling
message to root domain router. This establishes a „path setup‟ between Domain root router and
MT (Ramjee et al. 2002). By selecting only a few designated routers which participate in the
„path setup‟ updated messages, the signaling traffic overhead is minimized. Further HAWAII is
similar to Cellular IP in respect that the MT retains its IP address as it moves within the home
domain or within its foreign domain. But rather than using hop-by-hop tables like in Cellular IP
where each node maintains next-hop table with Host IP address and port mapping, the HAWAII
location management uses IP address forwarding technique, so as far as routing is concerned
HAWAII uses IP routing mechanism to reach mobile host.
10.1.1.4 Tunnel Based Handoffs
Hierarchical MIP: Hierarchical micro-mobility solutions do not use IP addresses or
hop-to-node mapping to reach from gateway to mobile terminal, instead they employ tree like
structure of Agents, and each Agent maintains destination MT‟s address to next Agent address
mapping. Thus the location database is distributed across several Agents. Once a MT registers
with it‟s HA, after that it maintains location database update only with its immediate next Agent
(Campbell, Gomez 2000). This way location database updates when MT moves is visible only to
the lowest Agent in the hierarchy of the Agent tree.
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10.1.2 Location Management:
Reaching the roaming MT for delivering the packets destined to is a challenge that
location management tries to solve. In traditional wired infrastructure when an IP device moves
from one subnet to other the TCP connections are broken. TCP connections work on static
location addressing; IP addresses are assigned in a hierarchical manner. So when an IP device in
a wired network moves from one subnet to other, its new subnet will assign it new an IP address.
With the new IP address, the TCP connections need to be re-established. Whereas when a MT
moves from one subnet to another, the expectation is that the MT will still be reachable with the
same connection end points as if it has not moved from its original location from a network
perspective. Location Management involves keeping track of the MT while it moves from one
AP to another. This is necessary because packets destined for MT need to be routed to its current
location and not its home location. To make it possible to route MT A‟s packets to a location that
is MT A‟s current location, there is a need for maintaining a database with MT‟s current foreign
and home location information. The database needs to maintain the mapping and every routing
decision will necessitate a lookup for current location before packets destined to a MT can be
routed. Each of the three Micro-mobility solutions maintains the location database at different
mobile nodes in the mesh network. Also the composition of the location database varies
depending on whether mobility solution. In case of Cellular IP the location database contains the
mapping of destination MT IP address and interface port use to forward the packet. In case of
HAWAII, the location database consists of IP address to next mobile node address. In case of
Hierarchical MIP, specific set of agent nodes maintain database of destination mobile node
address and next Agents address for forwarding the packet. Location management involves
keeping the location data refreshed and current. This necessitates paging and beacon messages
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between mobile gateway nodes and MT (Mohammad et al. 2009, p. 679). The location
management can be classified into (Xie & Wang, 2008):
10.1.2.1 Address Management:
Address management involves keeping track of the identity of the MT while it is
roaming.
10.1.2.2 Movement detection:
A MT has to know that it has entered a new location or area. This can involve either an
active periodic probe request message by the MT to the mesh routers or it can be a passive router
advertisement beacon message from mesh routers to the MTs. When this happens the MT knows
that it has entered a new area and thus registration has to begun. MTs can look at the subnet
mask to identify if it has moved under new AP and new MR or just done a layer-2 handoff.
10.1.2.3 Paging:
Paging involves determining the MR of the MT in question. This is needed because the
MT gets its network connection from its associated MR.
10.1.3 Handoff Management:
Handoff Management deals with keeping the connections active as the MT changes its
Point of Attachment. Handoff of the MT involves update of the location database because MT‟s
new location needs to be communicated to the mesh routers and mesh routers contain the
mapping data to reach the MT. The various solutions to handoff depend on which OSI stack
layer the handoff is being done. The handoff solutions fall in three categories and they are link
layer handoff, network layer handoff and cross layer handoff. There are tradeoffs in each
solution whether it is link layer, network layer or cross-layer, these handoffs which will be
23
discussed in subsequent sections. There are two types of MT movement in the wireless mesh
network. In first case the MT may move from attachment to a MR to another MR. In this case
the MGR attachment still remains the same. If the MT changes its MR attachment then it
performs Link-layer handoff, where as when the MT changes its MGR attachment it performs a
Network layer handoff (Xie & Wang, 2008, p. 39).
10.1.3.1 Design Issues for Handoff Management:
10.1.3.1.1 Handoff detection:
A MT needs to detect a handoff is necessary before it can initiate a handoff. Forced
Handoff occurs when the point of attachment of the MT or other mesh Hosts such as routers
changes in the mesh network. The MT initiates procedure to find new MR attachment. Unforced
Handoff occurs when MT finds a better path to the MGR and thus initiates a handoff to connect
to new MR which may or may not lead to new MGR attachment (Xie & Wang, 2008, p. 39).
10.1.3.1.2 Mesh gateway router selection:
If a handoff occurs and MT finds a new MGR point of attachment then a network-layer
handoff needs to be initiated by the MT. This leads to the third handoff issue namely, the QoS
maintenance.
10.1.3.1.3 QoS Maintenance:
If during the handoff a MT finds several MGRs to choose from then the MT needs to
consider the QoS maintenance issues. The MT needs to make sure that the minimum QoS that it
had with previous MGR is what it can get with the new MGR. The QoS guarantee involves
maintaining resource reservation along the path that the MT will reach the new MGR.
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The MT‟s change in attachment to the mesh Network can result in link-layer or network-
layer handoff. The handoff can be broadly classified as Link-Layer handoff where the MT
changes its association or PoA from one mesh router to another and the Network-Layer handoff
where the MT changes its PoA with respect to the mesh gateway router, also called the Access
router. The MT gets its network layer address from the MGR/AR.
10.2 Link Layer Handoff and Network Layer Handoff Issues:
10.2.1 Link Layer Handoff:
The link layer lives in second layer of the OSI protocol stack. This layer is responsible
for node to node movement and the addressing is based on MAC addresses instead of IP
addresses. Link layer handoff happens when a MT moves away from the radio range of one AP
to another. The MT and its attached AP form a Basic Service Set (BSS). At this point it leaves
one BSS and enters other BSS. During this link layer handoff the MT exchanges management
frames with the new AP to form a new BSS. Forming new association, exchanging credentials
leads to latency in handoff. When MT moves in the subnet such that its connection to the serving
MGR remains the same, the handoff is still link layer handoff. This is also known as access
handoff or intra-system handoff because the devices attached to MGR, such as MRs, are link
layer devices connecting to the MT by 802.11 link interface. The MT may move from on one AP
to other AP which maps to a different MR. This is still a link layer handoff, as long as the
serving MGR remain the same (Cisco, 2009). The roaming in the layer-2 network can be of two
types:
25
10.2.1.1 Layer-2 roaming in layer-2 network:
When a mobile terminal moves from one AP to another and both the APs are still
attached to the same MGR and in the same subnet, the roaming is called layer-2 roaming in
layer-2 network. This type of deployment allows the mobile client to roam from one AP to
another without needing a change in client IP address (Cisco, 2009).
Figure 4: Layer-2 roaming at Layer-2 OSI layer
Access PointAccess Point
BSSBSS
MT 2MT 1 MT 2MT 3
AssociationAssociation
Wireless Mesh Network
MGR
MGR
Mesh Routers
S U B N E T A
MGR
MR MR
Mesh Gwy Routers
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10.2.1.2 Layer 3 roaming in Layer-2 network:
When the mobile client moves from being attached to one AP to another AP which is controlled
by a different MGR and in a different subnet, then this kind of mobility is called layer 3 roaming
in layer-2 network (Cisco, 2009). This type of mobility supports mobility from one type of
access network to a different type. For example Subnet A could be a WLAN 802.11 radio
network and Subnet B a cellular network. Thus heterogeneous access networks are supported by
layer 3 roaming. Figure below illustrates the layer-3 mobility:
Figure 5: Layer-3 roaming at Layer-2 OSI layer
Access Point
MT 2MT 1 MT 2MT 3
AssociationAssociation
Wireless Mesh Network
MGR MGR
MGRMGR
Mesh RoutersMR MR
Mesh Gway Routers
Subnet A Subnet B
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10.2.2 Link layer handoff delays:
The link layer handoff follows the 802.11 handoff procedure. This procedure has several
steps (CodeAlias, n.d.)
10.2.2.1 Scanning delay:
The MT scans for suitable AP to connect to. This is needed when current AP‟s SNR is
low or other conditions where continued attachment will result in signal loss.
10.2.2.2 Association delay:
After selecting suitable AP, the MT has to associate with the new AP. The association,
also, allows the new AP to inform the link layer devices (bridges, switches) to update their L2
table so that packets in destination to the STA get forwarded to the new location.
10.2.2.3 Authentication delay:
The new association between MT and AP need to be authenticated. 802.1X enabled APs
only accept 802.1X frames from non-authenticated STAs. The 802.1X frames contain EAP messages that
are forwarded by the AP to a back-end RADIUS server over the RADIUS protocol. More detailed
description of the delays due to authentication can be found in CodeAlias (n.d.).
A link layer assisted handoff has been proposed by Yokata et al. (2002). This architecture
requires a layer-2 bridge to filter the MAC addresses before forwarding to the home domain.
When a MT finds a neighboring AP with which it associates, the neighboring AP broadcasts the
MT‟s MAC address in its local segment. This causes the MT‟s MAC address to be registered in
the MAC bridge (the MAC bridge filters the packets before handing over to the Home/Foreign
domain root router). With a 2 port bridge, the subsequent packets will now flow from HA to
MAC bridge and to Foreign Agent to new AP and thus to the MT. After the MAC bridge port
28
mapping timer expires, the flow is tunneled from HA to FA and to the new AP and the MT. With
the link layer approach using the MAC bridge, the need for HA registration does not need to
happen before the MT in new domain starts receiving the packets. The registration delays are
thus not a requirement before MT can start communicating after handoff. This avoids the
registration related delays evident in the mobile IP before communication can resume between
the MT and the Internet.
10.2.3 Network Layer Handoff:
10.2.3.1 Mobile IP
Mobile IP (RFC 3220) which is IETF‟s specification for network level mobility, incurs
lot of registration overheads, suffers from triangular routing problem, packet loss during handoff,
and does not support paging (used to locate the terminal‟s mesh router). In contrast to the link
layer handoff, the network layer handoff allows handoffs to occur between two domains or
subnets. When a MT moves from one subnet in a domain to another, often the Access router
needs to update its routing entries. This scope of the handoff is now not within a single domain
but across domains. To maintain the MT‟s IP continuity in such cases needs MT to execute a
network layer handoff. PMIPv6 (Proxy mobile IPv6) is one network layer handoff protocol
which a proxy agent helps the MT execute network layer handoff.
10.2.3.2 Proxy MIPv6
Proxy MIPv6 (RFC 5213) (Gundavelli, et al, 2008), the IETF proposed standard, is a
network-based local mobility protocol. It defines Mobile Access Gateway (MAG) and Local
Mobility Anchor (LMA). MAG is the access router which provides access to the Internet, where
as LMA is local home agent router (Lee & Min, 2009). The LMA provides the MT with its
29
anchor point connection to the network layer of the mesh network. The MAG assists the MT in
the handoff by detecting the movement of the MT from one subnet to another. When this
happens the MAG searches for the LMA in the new subnet and finds if the new LMA has the
binding Cache entry for the MT. The binding cache entry is mapping between the MT‟s Home
address and its Care of Address in the new subnet (RFC 5213, 2008). If not then MAG sends the
binding update to the new LMA in the moved to subnet. A tunnel is established between the
MAG in the home domain and the LMA in the visited domain. This enables the MT to be
addressed with its Home Address even though it has moved to a different subnet and has separate
Care of Address, called the Proxy CoA (RFC 5213, 2008), from the visited subnet.
The predecessor to the PMIPv6 namely the MIP required stack modification in the MT
because it needed the MT to be part of the handoffs. However the proxy MIPv6 which is network
based handoff takes that responsibility away from the MT and network layer instead handles the
handoff for the MT (Garroppo et al. 2009). There are many mobile devices which are not
necessarily mobile aware, so a implementation was needed which could free up the mobile
terminal from handoff mechanism. Thus the proxy MIP was born. In the PMIPv6
implementation, the entire handoff is performed for the MT by the network layer and initiated by
the MAG. Network layer involvement has known to cause handover latency and packet loss
before the MT in the new subnet can start receiving the packets destined to it (Lee & Min, 2009
p. 1085), for this reason modified version of PMIPv6 namely PFMIPv6 (Proxy Fast mobile IPv6)
was developed. In PFMIPv6, the handover procedure is done before the MT executes a handoff.
This saves on latency and packet loss is avoided. In the PMIPv6, the MAG detects the MT
movement and then initiates the network handoff by establishing tunnel with foreign LMA. In
PFMIPv6, the MAG depends upon the link layer trigger to initiate the tunnel before handover is
30
started. For example the MT link layer can detect the presence of a nearby AP which has
stronger signal and trigger signal to its home domain MAG that a handoff is necessary. The
home domain MAG sends MT identifiers such as MT id, MT-LMA id etc to the foreign LMA in
Handshake Initiate message. Thus the Foreign LMA has handover parameters that will be needed
for binding before the handover is executed (bi-directional tunnel between MAG and foreign
LMA establishment) (Lee & Min, 2009).
10.3 Mobility requirements for handoff:
When mobile clients move across the network, the following need to be incorporated for
a successful handoff:
10.3.1.1 Location Database:
Location database is information about the mobile terminals association with AP and
MAC address. The database is located in the MGR‟s client database. The entries stored for each
mobile terminal are client MAC and IP address, security context and associations, QoS, WLAN
and associated AP (Cisco, 2009, pp2-17). As the mobile terminal association with AP changes,
the MGR‟s client database is updated. This update is either the responsibility of MGR and
happens at the network level by communication between the MGR‟s or can be the responsibility
of the mobile terminal to update the client database.
10.3.1.2 Move Discovery:
There are two ways for move discovery. The mobile client‟s layer-2 service detects
disconnect at layer-2 level and issues a notification called Media Sense to the Windows OS
(Cisco, 2009, pp 12-4). This enables the mobile terminal to recognize a move and request new IP
from DHCP server. The other way is for mobile Client to receive a Foreign Agent advertisement.
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Within a subnet the mesh router periodically sends a beacon message to all the devices
advertising its existence. This way any visiting mobile client can detect a change in association
with AP in the visiting subnet, request new IP and initiate a handoff.
10.3.1.3 Location discovery:
Once the move discovery is done and if it is a move to a different subnet, then either a
tunneling or routing update method is used for routing packets to the mobile client in the new
subnet. For tunneling method the mobile Client has to update the Host MGR about the MTs new
AP association and new Foreign MGR‟s address. The MAC being a flat assignment remains
same.
10.4 Challenges of Link Layer Handoff:
As we note above during the link layer handoff involves scanning, management frame
exchange, and authentication delays. There could be several APs served by a single mesh router,
all the network layer-2 elements and when the AP is involved in a handoff, the signaling
messages have to pass through all the layer-2 devices generating lot of network traffic and
overhead. Thus pure layer-2 handoff is not an efficient solution. Combination of layer-2 triggers
with layer-3 handoff establishment is a good working solution for handover.
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11. Standardization efforts in wireless mesh network:
11.1 Current mobility standards and their status
11.1.1 IETF mobility standards
The Mobile IP (MIP), which is the RFC 3344 (Perkins, 2002), is the current IETF
standard for supporting mobility on the Internet. IETF introduced mobile IP as a solution for
mobile terminals which needed to remain connected to the internet when changing their point of
attachment. But the IETF Mobile IPv4 suffers from longer handover delays mainly due to AAA
(authentication, authorization and Accounting) signaling, IP address configuration, and packet
loss during handoff (Adibi, Naserian, & Erfani, 2005). To alleviate the Mobile IP related issues,
a hierarchical Mobile IP has been proposed by IETF as proposed standard in October 2008 called
the Hierarchical Mobile IPv6 Mobility Management (HMIPv6) in RFC 5380. Successor to the
Mobile IP was the Proxy MIPv6, which is the IETF proposed standard (RFC 5213). The PMIPv6
handles all the handoff related signaling in the network layer nodes, this way mobile terminal
does not have to get involved in the signaling and thus overhead is reduced. There are many
mobile devices which are not necessarily mobile aware, so a implementation was needed which
could free up the mobile terminal from handoff mechanism. Thus the proxy MIP was born. The
Proxy MIPv6 is discussed further in the section 10.2.3.2. Mobile IP with Regional Registration
(MIP-RR) was proposed in RFC 4857 to reduce the registration delays. MIP-RR uses
hierarchical levels for home and foreign agent, so that registration related signaling can be
handled locally and does not need to travel across to one single common home/foreign domain
agent for the entire domain. This reduces the signaling traffic at the upper level home agent.
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11.1.2 IEEE Standards for mobility management
The IEEE standards committee established the 802.11 Task Group named 802.11s TG for
the „s‟ amendment to the existing 802.11 WLAN standards to address the Wireless Local Area
Mesh Networking. The 802.11s is presently in the draft stage. The latest activity in the 802.11s
task group has been to forward the draft 802.11s document to the RevCom (Review Committee)
(IEEE, 2011).
The 802.16 working group is IEEE‟s effort to standardize the mesh architecture in Wide
Metropolitan area Networks (WMAN). The 3G cellular network‟s 3GPP program (Third
Generation Partnership Program) uses IETF defined Mobile IP for its mobility solution in
cellular network). The 3GPP networks handle the issue of mobility in two ways; the 3GPP2 uses
Mobile IP for IP mobility (i.e., terminal mobility) management and SIP for session mobility
management (Munasinghe & Jamalipour, 2008).
11.2 IEEE 802.11s standards Overview
A device which confirms to the 802.11 MAC and PHY specifications is identified as a
Station. A Station which can acts as a central device for other wireless LAN stations is called an
Access Point (AP) and this topology is called a Basic Service Set (BSS) (Hiertz et al. 2007). A
BSS forms a single hop network and all the devices on the basic service set depend on the central
AP to relay frames to each other. When several APs interconnect with each other, they form an
Extended Service Set (ESS). Stations can roam between APs in an ESS area. The IEEE 802.11s
takes this topology further and defines a mesh network. It defines a mesh Point (MP) as a mesh
network element which can relay frames between MPs using multi-hop communication. The
802.11s link management protocol is used to discover peer MPs which can then establish the link
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layer connection with each others. The peer MP discover can involve passive scanning, which is
based on listening to the beacon frames from nearby MPs or active scanning by sending probe
request to nearby MPs. Once the neighborhood MPs are identified, they can form a mesh
network. The IEEE‟s 802.11 amendment „s‟ describes the necessary functions to form a wireless
mesh network. The 802.11s is an MAC layer approach to multi-hop communication between the
mesh Points, versus the earlier approaches to using network layer for multi-hop frame exchanges
(Carrano et al. 2011, p. 53). Typically 802.11s defines a wireless mesh network with point of
view of mesh routers being fixed in their location relative to each other. This contrasts with
mobile Ad hoc Networks (MANETS) in which there are no fixed routers. In a MANET the
routers are mobile and ad hoc without the need for infrastructure of mesh routers for routing
frames. The mesh routers in the WMN are responsible for exchanging the frames. Since the
mesh routers form the backbone wireless network, they need to constantly update their routing
tables. There are two route discovery methods for updating the routing table (Carrano et al. 2011,
p. 54):
11.2.1 Proactive Routing Updates:
In proactive approach of the route discovery, the mesh routers are constantly exchanging
the routing information regardless whether there is a need for data transmission between the
mobile terminals. The proactive approach tries to keep the routing tables updated. As evident this
approach results in the excessive traffic, and since the effort is to keep the routing tables updated
at all the time and since the nodes in wireless network are mobile, the tables can quickly get
outdated and thus greater need to exchanges routing entries frequently resulting in excessive
traffic overhead. Two examples of proactive routing are Optimized Link State Routing (OLSR)
and Destination-Sequenced Distant-Vector Routing (DSDV).
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11.2.2 Reactive Routing Updates:
Reactive approach to route discovery takes an on-demand approach. When a mesh router
finds that there is a need for the routing tables to be refreshed, it initiates the path discovery
mechanism. Examples of reactive routing protocols are Dynamic Source Routing (DSR) and Ad
hoc On-Demand Distance Vector (AODV).
Recently however the focus of the routing protocols for WMN has shifted to layer-2
multi-hop routing. Layer-2 allows easy embedding in the network cards for the mobile devices
(Carrano et al. 2011). The 802.11s defines three entities that make up a wireless mesh network,
the mobile Station (MT), the mesh Station/router (MR), the mesh Access Point (AP), and the
mesh gateway router (MGR). The mesh gateway router is the portal for the mesh network to
connect to other networks. Each element in the mesh network needs an identity and this identity
is called the mesh ID. Mesh ID along with the path selection protocol in use in the mesh network
and the path selection metric constitutes the unique mesh Profile of each mesh element. All the
elements in the mesh cloud (formed by interconnected mesh routers) share the same mesh
Profile. mesh beacon frames are used by the MR to discover peer MRs. MRs form mesh peer
links with each other, each link identified by the MAC addresses of participating MRs and link
identifier.
11.2.3 Adaptive wireless routing
Recently there have been number of Adaptive routing protocols that have been suggested
fir wireless mesh networks. Traditional wireless network protocols such as OLSR and AODV
use static parameters which are preset and may not be suitable for all network conditions,
resulting in network degradation performance when used in environments that they were not
36
designed for (Azzuhri et al. 2010, p.145). The adaptive WMN routing protocols will dynamically
change certain parameters that are initially pre-set in the network. Adaptive AODV defines three
mobility levels low, normal and high. By checking the number of 1-hop neighbors it will use
appropriate mobility level (Azzuhri et al. 2010, p.145). So if it finds that there are a lot of 1 -hop
neighbors with which it has association it will reduce the HELLO message interval. Similarly in
adaptive routing version of the OLSR routing (OLSR is link state routing). Based on the link
condition the periodic link messages are either reduced or increased.
11.3 WMN Challenges for the Standards and Research bodies:
With the „s‟ amendment in the 802.11 specifications, IEEE redefined the MAC layer so
that multi-hop communication is enabled in the MAC layer for the mesh routers participating in
the mesh networks (Carrano et al. 2011). The challenges for WMN are:
11.3.1 Routing protocol:
Due to inherent stability issues of the wireless links, the wired routing protocols are not
suited for wireless mesh networks. For this reason ad hoc wireless routing protocols are used in
the WMN. These protocols broadcast frequent routing messages to gather routing metrics in
order to provide better QoS on the mesh (Hiertz et al. 2008). In wired links the layer-3 routing
protocols can access the neighboring links and determine the routing metrics, enabling them to
determine the routing paths and update the routing tables, but in the wireless mesh environments,
due to presence of radio links which IP layer cannot directly access, using IP protocols in
wireless mesh networks in a challenge. For this reason and since the MAC layer is closest to the
radio access physical layer, the standards organization foresee the integration of the routing and
the frame forwarding in to the MAC layer (Heirtz et al. 2008). Thus L2 routing which supports
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MAC address based mesh path selection and forwarding using radio aware routing metrics is
what is proposed as part of the 802.11s standard. An essential component of the routing solution
is the use of metrics to determine the preferred route between source and destination (Faccin et
al. 2006). The airtime metrics used for radio links in the wireless mesh networks should ideally
be multi-dimensional, taking into account the link condition, bandwidth, QoS considerations,
power requirements etc… Thus the WMN networks call for routing protocols operating at layer-
2 MAC layer.
11.3.2 Link Management
Mesh points use passive scanning or active beacon transmission for finding the candidate
peer mesh point. Once the candidate mesh point is identified and mesh link established, the
airtime metrics is calculated (Heirtz et al. 2007).
11.3.3 Path selection
The airtime metrics of the peer mesh links calculated above is used during the path
selection. Path selection protocols at layer-2 are mentioned in the section___. The path selection
helps mesh point determine the path to the root or portal mesh point.
11.3.4 QoS:
In case of mesh Networks two main QoS issues are identified, namely for access network
traffic and backbone traffic (Faccin et al. 2006). For both of these traffic networks, since the
routing solutions for the WMN operate at layer-2 and layer-2 MAC layer is responsible for
forwarding frames from hop to hop, the challenge is to provide consistent QoS over end to end
link which covers multi-hop route path. Cross layer routing solutions have been proposed to help
layer-3 routing take advantage of the layer-2 proximity to the radio medium for providing QoS
38
over multi-hop links and at the same time improve up on the route optimization that comes with
layer-2 triggers.
12. MPLS for Mobility Management:
12.1 MPLS for routing:
In the conventional IP based routing, the packets are assigned to the stream based on the
longest prefix match of the destination IP address. This longest prefix match is done at every hop
at intermediate routers. At every router the network header is required to be parsed to compute
the longest prefix match. In the inter-domain routing where the routing tables are much bigger
than the inter-domain routing, this lookup and computation for longest match can add substantial
overhead to routing process. Multi-protocol label switching (MPLS) uses labels attached to the
packets for routing from hop to hop, the packets are assigned to stream based on the packet label,
unlike the IP address based routing where at each hop the packet may be assigned to a different
stream based on the longest prefix match computation. MPLS allows use of traffic engineering
and policy based routing. Because the routing is based on labels and not the destination address,
policy routing defines the stream to which the packets can be attached. The routers which assign
the labels to the packets are called the Label Edge routers (LER) and the routers which use the
labels for forwarding the packets are called the Label Switching routers (LSR). The path between
two LSR is called Label Switched Path (LSP). Packet with labels mapped to a common Forward
Equivalence class (FEC), will all flow through the same route.
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12.2 Mobile nodes’ name and location identity with IP routing:
In the IP world, a node is identified and named by the IP addressed assigned to it. The IP
address identifies not only the identity but also the location binding of a network node. This
works fine in static wired networks where the routers are stationary. In wireless networks where
the mobile nodes are mobile, this kind of routing which is based only on IP address does not
work because as the mobile nodes move for example from a point of attachment in subnet A to
point to attachment in subnet B, the change in its location in IP domain necessitates an IP address
changes. The tying of the IP address with location information is needed in IP world because IP
addressing is hierarchical. Without the IP address change the other hosts on IP network would
not be able to communicate with mobile node. The IP address change of the mobile node causes
all the routing tables in the upstream network to change and this result in delays when handoff
needs to take place.
The mobile IP tries to solve this problem by keeping the moving mobile node‟s address constant
as it moves from one domain to other. This is achieved by using mapping between its home and
foreign addresses at the network anchor point. However bi-directional tunnels are necessary to be
setup between the mesh gateway router and the home anchor point. This tunneling introduces
delays because frequent mobile node movement causes registration messages to be sent to mesh
router gateway.
12.3 How MPLS fits in the mobility solution:
12.3.1 MPLS as routing layer binding the layer3 and layer2:
Mobility solutions have been explored which try to separate mobile hosts‟ IP identity
from the mobile nodes‟ location identity. One such solution by Sethom, et al. (2004) implements
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a virtual layer between the layer-2 and layer-3 to separate the mobile nodes‟ physical internet
identity from its location identity. This virtual layer uses MPLS routing mechanism for
independence for layer-2 and layer-3 routing. The MPLS helps achieve independence between
the layer-2 forwarding mechanism from the application identity (IP address) (Sethom et al.
2004). As noted earlier in the paper the layer-3 network handoff introduces latency because the
routing updates need to be made in the routing table for each handoff made by the mobile
terminal. The layer-2 handoff can reduce this handoff latency, but suffers from broadcast
messages and probe messages in entire layer-2 domain. The MPLS routing mechanism lies in
between the layer-3 and layer-2 routing solutions, on one hand it provides routing by using fast
label look-ups at each hop without needing label decoding function (unlike network headers that
are decoded at each Layer-3 devices), thus also providing layer-2 like fast MAC lookup
functionality.
12.3.2 Static binding between the mobile nodes identity and physical
location:
MPLS breaks the static binding between the mobile nodes identity and its physical
location in the internet addressing scheme. When a node physically moves from one location in
the internet addressing scheme to another, its IP address also has to change. MPLS breaks this
static link between the node identity and node location by using labels to do the routing function
while maintaining fixed IP address as its identity. The dynamic binding between labels and
terminal‟s physical points of attachment breaks the static association between the location and
identification in the current internet architecture, and enables transparent and efficient mobility.
Routing is then accomplished by labels instead of IP (Sethom et al. 2004, p. 66). The static
binding between the nodes identity and at the same time its use for routing packets destined for
41
the node creates limited scope for using the routing entry for anything other than routing
function. By decoupling the routing entity from the nodes identity, the MPLS labels can be used
for multiple functions at the same time. For example labels can identify the type of service that
packets should receive, the resources that the packet stream in the FEC should be allowed to use,
and the routing policies based on labels.
12.3.3 MPLS as hierarchical isolation between mobility anchor point
and the mesh routers:
As noted in the section 10.1.1 „micro-mobility‟, the micro-mobility solutions such as
Hierarchical MIP, Cellular IP, and HAWAII help in reducing the handoff delays by partitioning
the mobility domain into local mobility regions. This way the mobile terminal movement is
localized and home agent router is not involved for every handoff thus the registration delays
caused by the registration between the mobile terminal and the home agent router is avoided.
MPLS solutions for micro-mobility take similar approach in localizing the mobile agent
movement updates. The gateway home agent which is the anchor point for mobile terminals and
is the designated labeled switched router, the mesh router serving as an acting base station and
mobile terminal are the only nodes involved in the registration and handoff process.
12.3.4 MPLS with traffic engineering support in wireless mesh
networks:
MPLS brings with it the traffic engineering concepts which are critical to ensuring a user
acceptable QoS experience when moving from mobile terminal‟s home domain to foreign
domain. With WMN networks requiring carrying voice, data, and video there is a need for
reliable QoS mechanism which will not add signaling overheads to what can be sometimes be
42
unstable and bandwidth starved radio links between the mobile nodes in the mesh network. The
MPLS RSVP-TE can be used to implement an IntServ model. In the case of RSVP-TE the end
host-to-host connections are replaced by reservations between network elements. This reduces
the signaling overhead. By employing signaling protocols such as RSVP-TE a LSP can be setup
and managed dynamically by using dynamic or static routes. A LSP can be configured to provide
QoS guarantees and to follow automatic reconfiguration when a failure appears of network state
changes (Boringer et al. 2005).
12.3.5 MPLS tunnels in the Micro-Mobility solution:
As discussed in the section 10.2.3 under „Network layer handoff‟, when the mobile
terminal moves from one subnet to another, the first step that MT does it register with its Home
agent. The registration involves making the Home Agent aware of the MT‟s care of address
which could be the Co-located COA or Foreign Agent‟s CoA (RFC 3344). From that point on
when HA receives packets destined for the MT, it establishes tunnel between itself and the
visited Foreign agent. Even though the packets destined for MT‟s home IP address is
encapsulated in the IP header with CoA, the routers that are part of the tunnel have to re-examine
the network header for determining the next hop using longest prefix match. If this tunnel is
replaced with the Label Switch Path, then the label lookup will need to be done only at HA and
at FA and not at the routers in between these two edge routers. The HA and FA act as Label
Edge routers (LER) attaching and stripping the labels respectively. Once the label is stripped the
conventional IP address routing is used to reach the MT in the visited network. This saves 16
bytes per packet transmitted (Boringer et al. 2005). Thus the MPLS is an efficient light-weight
tunneling technology, using Label switched paths between the home and foreign agents an
overlay network is efficiently created and managed (Chiussi, Khotimsky, & Krishnan, 2002).
43
MPLS by providing LSP paths create an overlay network, this means that existing network nodes
by incorporating label switching capability turns the internet network into a more efficient label
based switching network riding on top of the existing IP network.
12.4 MPLS based handoff:
The handoff mechanism in MPLS based micro-mobility solution uses established LSP
paths between the Home agent and the foreign agent. When the mobile terminal moves from one
subnet to another, it sends the registration message to the new mesh router, in response to the
MR‟s agent advertisement message. The new mesh router registers with the gateway home agent
using registration request message. The LSP path is established between the gateway agent in the
domain and the foreign agent in the moved to subnet. The gateway updates its label database and
sends registration reply message to the new MR. The new MR sends the registration message to
old MR as well, so that old MR can send the mobile terminals packets to new MR.
12.5 MPLS compared to traditional micro-mobility solution for
registration and handoff delays:
Micro-mobility solution using MPLS defines a new mobility agent called Label Edge
Mobility Agent (LEMA). LEMA functions as any other Label Switched router (LER) in that it
maps the IP address of the mobile host/agent to a FEC. The FEC itself consists of next-hop and
the MPLS label. Thus FEC defines the LSP. In addition to a regular LER functionality, the
LEMA also accepts registration message and on receiving one, it maps the IP address in the
registration message to a FEC class. Thus the LEMA can on fly define new LSP when it receives
registration request message from the mobile host. LEMA is a mobility enhanced version of LSR
(Chiussi, Khotimsky, & Krishnan, 2002). The LEMA is any LSR which has the mobility
44
function added to it. When a mobile node moves from one AP to another in the LEMA domain,
on receiving the registration message from mobile node, the LEMA enabled node maps the new
CoA of the mobile node in the new subnet to a new FEC, thus defining new LSP path to the
mobile node dynamically. The overhead to create new LSP is much less than overhead of
creating routing table entry in the traditional micro-mobility solutions like HAWAII, and
Cellular IP.
12.5.1 LEMA network and attachments
As the mobile node in the MPLD network moves from one AP to another, the LSP paths
are setup dynamically based on which advertised LEMA path the mobile node chooses. In
contrast with other traditional micro-mobility solutions (such as Cellular IP, HAWAII) the
MPLS based solution allows the mobile terminal to chose a predetermined LSP path based on
which LEMA it chooses to register with. In the figure below the registration and path selection is
described (Chiussi, Khotimsky, & Krishnan, 2002).
45
Figure 6: LEMA registration and path selection
AR 1
LEMA 4
LEMA 7
LEMA 8
AR 2 AR 3
LEMA 6
LEMA 5
MT MT
LSP path advertisement LSP path advertisementLSP Path: (2, 4, 7, 8)
LSP path advertisementLSP Path: (3, 5, 7, 8); (3, 6, 8)LSP Path: (1, 4, 7, 8)
LEMA 1 1 2 3
4
5
6
7
8
Access Network 1 Access Network 2
When MT is attached to AR1, its LSP path is (1, 4, 7, and 8). When the MT moves and attaches
to AR2, based on the advertisement from AR2 about the available LSP paths, the MT can chose
path (2, 4, 7, 8). As can be seen the change is only at the first node of attachment from LEMA
node 1 to 2. When the MT completes its link-layer attachment to AR2 and parses the
advertisement (2, 4, 7, 8), it recognizes that the LEMA node match is at level 2 at node 4. Thus
in this case it needs to only change attachment at node 4. Thus the MT sends a registration
message to LEMA node 4, with change request to send the packets destined for MT to LSP path
46
(2, 4) instead of (1, 4), the rest of the LSP path (4, 7, 8) remains same for the MT whether it is
attached to AR1 or AR2. When the MT moves from coverage of AR2 to AR3, the AR3
advertises the LSP path (3, 5, 7, 8) and (3, 6, 8). When MT compares these new paths to the
current LSP path, it sees that change needs to happen at level 3 node. The MT has the option of
choosing any one of these LSP paths. The MT sends registration messages to the nodes which
changed in the handoff so that they insert the mapping of the MT‟s IP address to the new FEC
along the LSP path (3, 6, and 8).
12.6 Key Points in MPLS label routing compared to traditional
micro-mobility routing:
12.6.1 Simplified Registration:
The registration process is simplified, because when MT moves from one access point to
other, it just need to change the LSP path that it can chose based on path selection criteria and
this involves simple label swap and new FEC generation. The traditional route updates in
traditional micro-mobility solution is more time consuming, thus increased latency during
handoffs.
12.6.2 Flexible Path Selection:
The mobile terminal based on the advertisement received from the LEMA need to make a
path selection and then swap the Label for generating new FEC at the root LEMA for Path
change. In traditional approach the MT would have to propagate the mapping change all the way
to Home Agent. Though the Hierarchical approaches help in localizing the registration
propagation, the overhead is increased number of agents to keep track of local mobility. In
47
MPLS, tunnel redirection, which is a crucial ingredient of any mobility scheme, happens quickly,
at a change of a node in a single node in the network (Chiussi, Khotimsky, & Krishnan, 2002).
12.6.3 QoS Guarantee and Link Reliability:
LSP paths can setup RVSP-TE QoS guarantee mechanism which is lacking in the
traditional routing. Link reliability is much improved in MPLS networks. This is because when a
node experiences failure in IP routing network, since in the traditional IP addressing network the
packets travel nodes are all the link
12.6.4 Packet loss during handoff:
Traditional micro-mobility solutions experience packet loss when MT moves from one
AP to other. When the MT is in movement the packets that are received by the old AP may be
lost because it is not able to contact the MT. Some implementations of traditional micro-mobility
solutions use buffering at all of the nodes which avoids this problem. MPLS based nodes avoid
this problem due to implementation of the redirect message to the old node to send the packets to
new LEMA registered node to which the MT has moved to.
12.6.5 IP-to-IP Tunnel overhead is avoided:
The IP-to-IP tunnels are completely avoided with MPLS implementation. Instead of
tunnel establishment during handoff, the nodes setup LSP path and MT has ability to chose
which LSP path, instead of hierarchical IP address path setup in traditional micro-mobility
solutions based on network hierarchy.
48
12.7 MPLS throughput compared to other micro-mobility during
handoff process
Xie et al. (2003) have studied the throughput that the MPLS based routing can achieve
when compared to the other micro-mobility solutions such as HAWAII and Cellular IP. In their
measurements, the MPLS based micro-mobility can support throughputs of 1.375 mbps
compared to throughput of 1.22 mbps when the mobile nodes speed is 15 meters per sec during
the handoff process. At higher speeds such as 40 meters per sec. the throughput for MPLS based
WMN falls gradually to 1.335 mbps where as for HAWAII based WMN the throughput falls
significantly to 0.8 mbps.
Figure 8: Throughput in mbps vs. the mobility handoff speed using MPLS and non-MPLS
micro-mobility solutions.
MPLS vs. Traditional Micro-mobility Handoff Performance
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
10 mps 20 mps 30 mps 40 mps
HAWAII (mbps)
MPLS (mbps)
49
13. Vendor implementation for mobility Solution
There have been several wireless mesh implementations in the commercial industry.
Aruba, Cisco, and other have implemented their own proprietary solutions for mesh networks.
13.1 Aruba Network’s wireless mesh network solution
Aruba Network‟s proprietary wireless mesh network implementation is called the
Airmesh multi-service mesh solution. It uses Adaptive wireless Routing (AWR) and
MobileMatrix in its mesh network deployment.
13.1.1 Adaptive Wireless Routing (AWR):
AWR is Aruba‟s proprietary routing solution for wireless networks. It works with layer-2
triggers for RF awareness and layer-3 for network routing intelligence to provide routing
solution that is optimized for wireless network. A routing protocol which is entirely layer-3
based can overwhelm the network, because it needs routing updates to be sent to all the layer -3
modes. In fast moving mesh network where nodes are highly mobile such routing update
overheads introduces significant latency. Aruba‟s AWR routing uses cross layer approach. The
layer-3 intelligence is combined with layer-2 information such as radio link strength and airtime
matrices discussed earlier in the paper to calculate. Such layer-2 parameters provide AWR
routing with intelligence to use wireless links based on conditions suitable for radio links. Using
this feature allows AWR routing to balance the overall traffic in case of radio interference
(Aruba Networks, 2011). For optimal video transmission, Aruba uses deep packet inspection,
MAC layer frame prioritization, and buffering techniques to achieve optimal video transmission.
Using the link layer metrics allows the network to use Air quality metrics which is specific to
50
wireless communications. Indicators for assessing current link quality include the link‟s data
rate, received signal-strength indicator (RSSI) and external interference (Aruba, 2010, p. 5). The
result is that AWR makes the routing decisions not just on network layer intelligence but adds
layer-2 metrics for better routing decisions in wireless networks.
13.1.2 Aruba Networks Handoff process enhancement:
Aruba Network uses MobileMatrix for mobile terminals moving from one subnet to
another. The MobileMatrix takes approach similar to mobile IP, but uses the AWR fast routing
convergence which helps alleviate the latency inherent in the mobile IP based handoff (Aruba,
2010, p. 10). When a handoff occurs the MobileMatrix roaming allows fast convergence for
routing table updates. The location data is maintained in the routing table with each node. For
mobile node position change, the routing tables are updated. The convergence happens faster
than traditional layer-3 convergence, due to proprietary AWR routing.
13.2 Cisco’s mesh network deployments:
Cisco uses three components for deploying the wireless mesh network:
1. Cisco 1500 Series mesh AP: This is the access point for the wireless clients.
2. Cisco Wireless LAN Controller (WLC): WLC is the central point for controlling APs.
3. Cisco Wireless Control System (WCS)
The Client mobile terminals connect to Cisco WMN with help of the Series 1500 mesh APs.
The mesh APs are of two types, the Roof-top APs RAP) and mesh APs (MAP). Both the APs use
dual radio channel for better bandwidth distribution between the client access channel (2.4 GHz)
and backhaul channel (5.8 GHz). The WLC manage the mesh APs in that it dictates the security
51
policy, the QoS on the links and mobility functions.WCS in-turn centralizes the management of
network management functions and dictates the RF prediction, network optimization functions
(Cisco, 2009). MAPs use Adaptive Wireless Path Protocol (AWPP) to determine best path
through other MAPs to WLC. There are two kinds of traffic on MAPs
Bridge traffic: This is the traffic between the devices connected MAP Ethernet ports.
Traffic between MAPs and WLC: This traffic carries WLAN traffic through the LWAPP
tunnels.
13.2.1 Cisco’s Light Weight Access Point Protocol (LWAPP):
LWAPP is Cisco‟s underlying protocol used in communication between the MAPs and
the Wireless LAN Controller (WLC) (Cisco, 2009, p.2-1). A MAP node establishes two types of
tunnels between itself and the WLC. Both of these tunnels are LWAPP tunnels. The first tunnel
is between MAP and WLC and carries the mobile client‟s layer-2 frame data, and the second
tunnel carries the LWAPP control signaling traffic. LWAPP supports both layer-2 mode of
operation where it transports layer-2 frame traffic between the MAP and WLC and layer-3 UDP
traffic. The Layer-3 USP traffic mode of operation is more preferred as it allows network layer
intelligence to be used in traffic distribution.
52
Figure 9: LWAPP tunnel setup
Wireless LAN Controller (WLC)
MAP/
RAP
MAP/
RAP
MAP/
RAP
MAP/
RAP
MTMT MT
LWAPP Tunnel Traffic
control messages
LWAPP Tunnel Traffic
W. Client Layer-2 traffic
13.2.2 Mobility Management in Cisco wireless mesh networks
Cisco defines a „mobility group‟ to group WLCs such that wireless clients can freely
roam between WLCs in the same „mobility group‟. If the client moves from attachment to a
WLC „A‟ to another AP which maps to WLC „B‟ and both WLC „A‟ and „B‟ both belong to the
same mobility group, then a layer-2 handoff is performed because the handoff involves same
subnet. The movement of wireless clients across mobility group, which is across subnets, is
defined as inter mobility group roaming.
53
Figure 10: Cisco mesh network mobility solution
WLC
WLC
WLC
WLC
Mobility Group ‘A’
WLC
WLC
WLC
WLC
Mobility Group ‘B’
MAP
13.2.2.1 Intra ‘Mobility Group’ roaming (layer-2 handoff):
To enable groups of WLCs to communicate with each other, Cisco defines a „mobility
group‟. A mobility group is a group of WLCs that together, act as a single virtual WLC by
sharing essential information such as Radio resources, and VLAN parameters (Cisco, 2009, p.2-
13). Any WLC in the mobility group can directly contact other WLAN in the same group using
pre-authenticated tunnels. This enables the wireless mesh clients to freely roam between the
54
WLC which are part of the same mobility group without wireless clients having to re-
authenticate. Cisco deployments use the mobility groups to facilitate seamless wireless mesh
client roaming between APs that are joined to different WLCs which are part of same mobility
group of WLCs. This way virtual WLAN domain is formed. The roaming within the „mobility
group‟ is movement within a subnet. Thus this involves layer-2 roaming. WLCs which
implement Cisco‟s LWAPP protocol use frame forwarding in layer-2 to exchange frames.
13.2.2.2 Inter Mobility Group roaming (layer-3 handoff):
If the wireless client is moving from one subnet to another a layer-3 handoff is initiated.
In this case an anchor/home WLC is defined and the anchor WLC becomes the gateway for the
mobile client. The layer-3 handoff can be asymmetric or symmetric. In asymmetric handoff the
path to wireless client is always via the anchor/home WLC and the path from the wireless client
is directly to the foreign WLC. In symmetric layer-3 handoff both the path to and from the
wireless client is via home/anchor WLC (Cisco, 2009, p.2-19).
55
Figure 11: Asymmetric layer-3 roaming
Anchor WLCA-WLC
Foreign WLCF-WLC
S U B N E T A S U B N E T B
Ethernet IP Tunnel
Who is MT’s WLC?
MTMT
MT’s details, home IP..
Mobility Group
MAP A MAP B
MAP C
56
14. Conclusion
The paper discussed the short coming of the micro-mobility management solutions as
well as the requirements of a mobility management system for the wireless mesh networks. The
IEFT and IEEE standards in the area of mobility management and their current status were
discussed. The paper found that the MPLS solution has several advantages in reducing the
complexity, satisfying the mobility management requirements, and simplifying the mobility
architecture. MPLS advantages were evaluated in terms of suitability for mobility management
solution. In conclusion, it is evident that the MPLS brings with it many features which help
reduce complexity of the handoff management and location management for wireless mesh
networks. This results in reduction in the handoff latency thus solving one of the critical issues
inherent in traditional wireless mesh network deployment. MPLS is particularly suited in the
mobile wireless architecture due to its dynamic path selection and ability to define end to end
QoS. Due to MPLS‟s inherent tunneling support, many of the disadvantages of IP-to-IP tunnels
in the micro-mobility solution are avoided. Since MPLS is a layer-2.5 solution it is much closer
to the layer-2 generated link metrics such as airtime metrics in deciding which nodes provide the
best backhaul path. Thus the conclusion of this paper is that MPLS with its inherent tunneling
technology and QoS support is best suited as routing technology for wireless mesh network
deployment.
57
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