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A Brief Overview on IEEE 802.11s

Young-Bae Ko, Ph.D., Associate Professor

Dept of Info & Computer Engineering, Ajou Univ. Korea (Visiting Professor, CSL, UIUC)

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Contents

Introduction and Backgrounds

Architectural and Usage Models in the 802.11s Draft

Framework of the 802.11s Mesh Network Topology creation MAC layer forwarding MAC functionality enhancements Potential Issues of the current 802.11s

References

33

Introduction & Backgrounds

44

Introduction (1/2)

Wireless Mesh Networks

Wireless multi-hop infra networks, where a few nodes provide a connection to the external world (e.g., Internet) through a cable

Alternative wireless access technology, which can replace the traditional sets of IEEE 802.11 wireless LANs

Commercialized and managed ad hoc networks, which Introduce a hierarchy in the network architecture with fixed, special routers and mobile, general clients

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Introduction (2/2)

Many vendors have developed their own WMN solutions and put them on the market, because it is flexible and more cost effective than the typical wired APs.

Motorola Tropos Belair PacketHop

However, though most of them are based on the common 802.11 MAC, these products are not interoperable.

Need for defining a standard architecture for WMNs!

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WMN Standardization Efforts in IEEE 802

IEEE has been playing a key role in the development of wireless mesh standards [1].

IEEE 802.11s – WLAN Mesh IEEE 802.15.5 – WPAN Mesh IEEE 802.16a/d/j – WMAN Mesh

Motivation of WLAN Mesh standards Current 802.11 ad hoc mode is not sufficient for multi-hop mesh.

Recent efforts for the advance of 802.11 standards, such as 11e for QoS support[2] or 11n for high data rates (>100 Mbps), are still limited due to their inherent dependency upon the wired infrastructure backbones and the last, single-hop wireless communication.

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WLAN (Layer 2) Mesh Networks

The 802.11 Task Group “s” (TGs) Formed in May 2004 to design mesh networks consisting of

different WLAN devices performing routing at link layer (layer 2) To be based on extensions to the current IEEE 802.11 architecture

and protocols:

Internet

STA

STA

STASTA

STA

APAP

BSS

BSS

IBSS

ESS

DS

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IEEE 802.11s: Meshed WLAN Networks

It will provide an IEEE 802.11 Wireless DS that supports both broadcast/multicast and unicast delivery at the MAC layer using radio-aware metrics over self-configuring multihop topologies.

The objectives: Increased range/coverage & flexibility in use Possibility of increased throughput Reliable performance Seamless security Power efficient operation Multimedia transport between devices Backward compatibility and interoperability for interworking

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Current Status of IEEE 802.11s

802.11 TGs has defined the following: Scale: up to 50 mesh nodes Architectural model Usage models: 4 usage scenarios Functional requirements

Several standard drafts released since early 2006: The initial proposal was released at 2006 March meeting [3]. Draft 1.0 (released in Nov. 2006) was failed to be approved in the

first trial of a letter ballot. Draft 2.0 (May 2008) again failed in their 2nd letter ballot because

many issues still remain. Draft 3.0 (March 2009) is the most recent one [4]! The target releasing date of an official 802.11s standard is 2010.

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Architectural & Usage Models in 802.11s

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802.11s WLAN Mesh - Network Architecture

Mesh Portal: Acting as a gateway/bridge to external networks Mesh STA (station): Relay frames in a router-like hop-by-hop fashion Mesh AP (Access Point): Mesh relaying functions + AP service for clients

MeshSTA

STA

STASTA

STA

External Internet

Mesh Portal

Mesh AP

Mesh

AP

Mesh

AP

Portal

Mesh

Mesh Links

Non-mesh STAs

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Mesh Basic Service Set (MBSS)

Internal L2 behavior of WLAN Mesh is transparent to higher layers An MBSS (Mesh Basic Service Set) appears as a single access domain.

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Usage Models

Residential Inside home or a residential building High bandwidth application, such as multimedia content

distribution

Office Small to medium sized enterprise buildings

Campus/Community/Public access Out-door deployment environment Seamless connectivity

Public Safety Emergency sites

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Functional Framework in 802.11s

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Functional Requirements

The set of services provided by the WLAN Mesh that support the control, management, and other operation, including the transport of MSDUs between MPs within the WLAN Mesh.

PHYs

Mesh Topology Learning, Routing &

Forwarding

Medium Access Coordination

Discovery & Association

802.11 service integration

Mesh Configuration & Management

Mesh Measurement

Mesh Security

Mesh Interworking with other 802 networks

LAN metaphor, 802.1 bridging support

802.11i link security based

MAC enhancements

Unmanaged, autonomic

management

Legacy 802.11 a/b/g/n

Single-hop/multi-hop neighbor

discovery, Extensible path

selection & forwarding

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Key Functionality of 802.11s Networks

Mesh Topology Creation Self-configuring neighbor discovery , named as “Mesh Peering” Channel selection

L2 Routing MAC address based mesh path selection and forwarding Radio-aware metrics for routing

MAC Enhancement For supporting QoS, and increasing the network throughput Power management, Multi-channel operation, and so on

Security IEEE 802.11i as basis

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Mesh Peering Mechanism

To discover peer Mesh STA devices and their properties:

MSTA performs passive scanning (via periodic beacons) or active scanning (via probe messages)

The received beacon or probe response frame contains mesh related information

Mesh ID: name of the mesh (SSID like string) Mesh configuration element (including version and support functions)

A discovered MSTA will become a peer MSTA after peering processes by 4-way handshaking.

2-way handshaking with peering-open-frame/peering-confirm-frame exchange in each direction

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Channel Selection

Support for single & multiple channels/interfaces Each logical interface on one RF channel, belongs to one “Unified

Channel Graph (UCG)”

MP specifies one of the two channel selection modes for each interface:

Simple Unification mode – enables the formation of a fully connected UCG

Advanced mode – not fully defined in the proposal (opened to the venders)

Example Unified Channel Graphs

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Mesh Path Selection and Forwarding

To select single/multi-hop path(s) and to forward data frames across these paths between MPs at the link layer.

Extensible path selection framework A WLAN Mesh may include multiple path selection metrics and

protocols for flexibility.

A mandatory protocol and metric for all implementations are specified.

Hybrid Wireless Mesh Protocol (HWMP) Airtime link metric function

Only one protocol/metric will be active on a particular link at a time.

A particular mesh will have only one active protocol at a time.

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Airtime Link Metric Function

A default link metric to be used by a path selection protocol to select the best paths.

Other metrics can also be used.

Its cost function is based on airtime cost (Ca), which reflects the amount of channel resources consumed by transmitting the frame over a particular link.

f

ta er

BOc

1

1

Parameter Description

O Channel access overhead including frame headers, training sequences, access protocol frames, etc (depending on PHY)

Bt Test frame length in bits (Constant)

r Transmission data rate in Mb/s for the test frame size Bt

ef Test frame error/loss rate for Bt

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Example

48Mb/s, 10% PER

54Mb/s, 8% PER

12Mb/s, 10% PER

54Mb/s, 2% PER

54Mb/s, 2% PER

48Mb/s, 10% PER

This path having the minimum airtime cost is the Best!

Unicast Cost Function based on Airtime Link Metrics

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Hybrid Wireless Mesh Protocol (HWMP)

A default path selection protocol for interoperability.

To combine the flexibility of on-demand route discovery with extensions to enable efficient proactive routing to mesh portals.

On-demand mode Used in intra-mesh routing for the route optimization When a root portal is not configured or it can provide a better path

even if root is configured.

Proactive, Tree based mode If a root portal is present, a distance vector routing tree is built. Tree based routing avoids unnecessary discovery flooding during

discovery and recovery

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HWMP: On-demand Path Selection Mode

1. Source broadcasts PREQ (path request) with the destination and metric initialized.

2. Upon receiving PREQ, MPs update the path to source if sequence number is greater and offers a better metric

3. If a new path is created or the existing one is modified, PREQ is forwarded further.

4. PREQ provides “Target only” (TO) and “Reply and Forward” (RF) flags.• If TO=1: Only destination sends PREP (path reply) after selecting best path.• If TO=0 and RF =0: Intermediate node with path sends a unicast PREP to

the source MP and does not forward PREQ• If TO=0 and RF =1: The first intermediate node with the path to the

destination sends a PREP and forwards PREQ setting TO =1 to avoid other intermediate nodes to send back PREP.

5. When source receives the PREP, it creates a path to the destination.

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HWMP – Proactive tree building mode

Proactive PREQ mechanism

Root MP periodically broadcast PREQ

To create paths between the root mesh and all mesh nodes in the network proactively (2-way handshaking)

Proactive RANN mechanism

Root MP periodically broadcast RANN

Distribute path information for reaching the root mesh but the actual paths to the root mesh can be built on-demand (3-way handshaking)

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Example – Proactive PREQ mechanism

1) The root MP periodically propagates a PREQ into the network

- Destination Address set to all ones

- The TO flag set to 1 and the RF flag set to 1

2) Upon reception of a PREQ, each MP has to create or refresh a path to the root MP

3) The recipient MP’s action

- If “Proactive PREP” bit set to 0, MP may send a proactive PREP if required.

- If “Proactive PREP” bit set to 1, MP shall send a proactive PREP.

4) Tree path construction is completed

RPREP

PREQ

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Example – Proactive RANN mechanism

R

1) The root MP periodically propagates a RANN into the network.

2) Upon reception of a RANN, each MP has to create or refresh a path to the root through sending a unicast PREQ to the root MP.

3) The root MP sends a PREP in response to each PREQ.

4) Tree path construction is completed

PREP

PREQ

RANN

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The existing 802.11 MAC layer is being enhanced for

Supporting QoS: EDCA(Enhanced Distributed Channel Access) specified in 802.11e, as

the 802.11s’ basic operation mechanism Other features of 802.11e, like HCCA, are not considered.

Improving the network capacity: The usage of multiple channels and multiple radios Efficient handling of the two different kinds of traffic (BSS traffic &

Forwarding mesh traffic) Intra-mesh congestion control Mesh coordinated channel access (optional)

802.11s MAC Enhancements

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Handling BSS and mesh traffic by Mesh AP Giving priority to mesh traffic may starve STAs Giving priority to STAs might waste resource utilized by mesh traffic Advanced solutions: separate radio for mesh and BSS traffic

Intra-mesh congestion control A simple hop-by-hop congestion control implemented at each MP Local congestion monitoring Congestion control signaling Local rate

control

Mesh Coordinated Channel Access (MCCA) Optional scheme based on the reservation of contention free time slots Lower contention (more deterministic) mechanism for improved QoS for

periodic flows

MAC Enhancements – More Details

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Mobility is of little concern (do not support seamless handover).

No mechanism for multi-channel operation One proposal called “CCF (Common Channel Framework) was adopted in

the early version of the draft (before draft 1.0), but removed from the draft.

Limitations caused by the EDCA Performance limitations in multi-hop environments End-to-end QoS limitations

And many more More reliable and stable metric for link quality measurement and routing? Better solutions for power management? More robust approaches than its current security solution inherited from

802.11i, in terms of routing security or end-to-end security?

IEEE 802.11s MAC – Remaining Issues [5,6]

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References

[1] “Emerging standards for wireless mesh technology”, by M.-J. Lee, J. Zheng, Y.-B. Ko, and D.M. Shrestha, IEEE Wireless Comm., Apr. 2006.

[2] “Wireless LAN MAC and PHY specifications, Amendment 8: Medium Access Control (MAC) Quality of service Enhancement”, IEEE std 802.11e-2005., Nov. 2005.

[3] Joint SEE-Mesh/Wi-Mesh Proposal to IEEE 802.11 TGs, Feb. 2006.

[4] “Draft Amendment to Standard for Information Technology - Telecommunications and Information Exchange Between Systems - LAN/MAN Specific Requirements - Part 11: Wireless Medium Access Control (MAC) and physical layer (PHY) specifications: Amendment: ESS Mesh Networking”, IEEE P802.11s/D3.0, March 2009

[5] “IEEE 802.11s wireless mesh networks: Framework and challenges”, by X. Wang and A.O. Lim, Ad Hoc Networks, vol. 6, 2008.

[6] “IEEE 802.11s: WLAN Mesh Standardization and High Performance Extensions”, by G.R. Hiertz, et. al., IEEE Network, May/June 2008.

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Thanks !Thanks !

Q&A – youngko@illinois.eduQ&A – youngko@illinois.edu

http://uns.ajou.ac.kr