9. WIRELESS ATM

• Anywhere, Anytime Access to ATM Networks.• Voice, Data, Video, and Images in Any Combination, Anywhere,

Anytime with Convenience and Economy.• Fixed Wireless & Mobile Users Wireless Equipment.• Problems

– Noisy Wireless Channels High BER.– Wireless Channel

Very bandwidth limited.ATM designed for bandwidth-rich environment.

– OverheadEvery ATM cell has overhead of 10%.For wireless channel, we need more control information which can far exceed the overhead limit.

Wireless ATM Network Architecture

Base Station

VLR

MSC

HLR

ATM Network

VLR: Visitor Location RegisterHLR: Home Location RegisterMSC: Mobile Switching Center (also ATM switch)

Wireless ATM in Digital Battlefield

Fixed ATM Network

FSC: Fixed Switch Center (ATM Switch)MBS: Mobile Base Station

Wireless LAN

Satellite

Military and CommercialWireless ATM Networks

Commercial MilitaryStatic network topology Highly dynamic topologyTypically single hop Multi hopStatic allocation n-band userand data channels

Dynamic bandwidthallocations; Priorities

Maximum number ofusers/hertz/area

Maximum transfer rate inhostile environment

Privacy High securityFixed infrastructure for openaccess

Constrained access

Quality of Service (QoS) Parameters

1. Throughput2. Delay3. Jitter4. Loss Probabilities5. Probability of Dropping the Call6. Expected BER; Packet Error Rate7. Expected Disruption Time During Handoffs8. Minimum or Maximum Level of Mobility9. QoS Renegotiation

Also in wired ATM network

Personal Mobility vs. Terminal Mobility

Network

Network

Wired

Wireless

User Terminal

Terminal Mobility

Personal Mobility

Protocol Stack For Wireless ATMQuality Critical

ApplicationsTime Critical Applications

TCP UDP

IP

AAL Layer

ATM Layer

Error Control

Medium Access Control

Physical Layer (Wireless Channel)

Link

Layer

ATM Layer

IP Layer

Specific Requirements for PHY Layer

Low Speed Wireless PHY HIGH Speed Wireless PHY

Frequency Band

Cell Radius

Frequency Reuse Factor

Transmit Power

Channel Bandwidth

Data Rate

Modulation

MAC Interface

Fixed Packet Length

5.15-5.35 GHz, 5.725-5.875 GHz 59 GHz - 64 GHz

80 m 10 - 15 m

100 mW 10 – 20 mW

up to 12 7

30 MHz 150 / 700 MHz

25 Mbit/s 155 / 622 Mbit/s

16 tone DQPSK 32 tone DQPSK

par., transf. speed 3.127 Mbyte/s par., transf. speed 87.5 Mbyte/s

PHY header + MAC header + 4*ATM cells

System Architecture and Protocol Model

ATMBackbone Network

Host

Host

Wireless Workstation

Wireless Workstation

User Applications(Quality-Critical Traffic)

TCP/IP

AAL Subsystem

ATM

Sonet DL Subsystem

Wired Line Wireless Link

Error Control

FEC

Hybrid ARQ

Time CriticalApplications

Quality Critical

Applications

Why FEC?

• ATM HEC performance is too low for

wireless ATM.

• High CLR and payload errors

• Cell delineation problem

FEC (for Time-Critical Applications)

• To correct channel errors at the expense of bandwidth by adding redundancy

Concatenated FEC Scheme

RS OuterEncoder

Symbol Interleaver

Conv. InnerEncoder

Bit LevelInterleaver

WirelessChannel

Bit LevelDeinterleaver

ViterbiDecoder

SymbolDeinterleaver

RS OuterDecoder

Cells

Cells

Transmitter FEC

Receiver FEC

• Why Hybrid ARQ? (for Quality Critical Traffic)– ARQ provides high reliability at good and moderate

channel qualities.– The throughput drops rapidly, if the channel error

rate is high as in wireless channels.

• Hybrid ARQ– FEC first tries to correct the frequent error patterns.

If it fails, then ARQ takes over.

• Hybrid ARQ Types– Type I Hybrid ARQ scheme– Type II Hybrid ARQ scheme: only additional parity

bits are retransmitted to combine with the previous packet (incremental redundancy).

Medium Access Control for Wireless ATM Networks

MT

MT

MT

Categorization of MAC Protocols

• Based on Channel Organization– TDMA-Based MAC Protocols– CDMA-Based MAC Protocols– Random MAC Protocols– Hybrid MAC Protocols

• Based on Duplex Mode of Uplink and Downlink– Time Division Duplex (TDD) (One Carrier

Frequency)– Frequency Division Duplex (FDD) (Two Carrier

Frequencies)

* Frequency Division Duplex (FDD) (Two Carrier Frequencies)

– Uplink frequency carries traffic from terminal to BS while downlink frequency carries traffic from BS to terminal.

– FDD allows almost immediate feedback from the BS enabling terminal to find out quickly if its contending reservation request was unsuccessful and a retransmission is necessary.

– Thus, FDD impacts the delay encountered by user traffic as well as the resource availability of the wireless channel.

TDMA Based MAC Methods– Dynamic Packet Reservation Multiple Access (DPRMA), by

Dyson and Haas in 1999. FDD– Mobile Access Scheme Based on Contention and

Reservation for ATM (MASCARA), by Bauchot et al. in 1996, and Passas et al. in 1997. TDD

– PRMA with Dynamic Allocation (PRMA/DA), by Kim and Widjaja in 1996. FDD

– PRMA with Adaptive TDD (PRMA/ATDD), by Priscoli in 1996. TDD

– Dynamic TDMA with Piggyback Reservation (DTDMA/PR), by Qiu et al. in 1996. FDD

– Distributed Queuing Request Update Multiple Access (DQRUMA), by Karol et al. in 1995. FDD

– Dynamic TDMA with TDD (DTDMA/TDD), by Xie et al. in 1995. TDD

Packet Reservation Multiple Access (PRMA) Protocol

(Goodman’91)

• Time is divided into slots of equal duration, and slots are grouped into frames.

• Each slot in a frame is either “reserved” or “available”. • BS controls the upstream traffic and broadcasts a

continuous stream of packetized information through the downstream channel

• The status of a slot is provided in feedback information supplied by BS.

• Terminals can send two types of information: “Periodic” information such as speech or “Random” information such as data.

• Frame rate is identical to the arrival rate of the speech packets.

• Uses S-ALOHA for time slot reservation and TDMA for data transmission.

Packet Reservation Multiple Access (PRMA) Protocol

(Goodman’91)

• A station contends for an available slot using S-ALOHA.

• If transmission is successful, BS responds with an ACK message and the slot is reserved in subsequent frames until the terminal relinquishes it by leaving the slot empty.

• A terminal with “random packets” contends for slots in the same way, but cannot reserve the same slot in a subsequent frame even after a successful transmission.

• Thus, terminal must contend again for another available time slot.• For unsuccessful transmission, a terminal with “periodic” packets

retransmits the packet with certain probability in subsequent unreserved slots until it receives an ACK signal from BS.

• Similarly, a terminal with “random” packets retransmits a packet in unreserved slots with certain probability.

Packet Reservation Multiple Access (PRMA) Protocol

(Goodman’91)

Advantages:• Simple

Disadvantages: * Upon congestion, the speech packet dropping rate

and data packet delay both increase. * Feedback information may cause waste of

bandwidth.

PRMA/DA — Services and the Frame Structure• Supports Multimedia Traffic

– Constant Bit Rate (CBR), Variable Bit Rate (VBR), Available Bit Rate (ABR).

• Frame Structure– It is organized according to traffic types.– Downlink transmission is not considered. FDD

… … … …1 2 Na 1 2 Nc 1 2 Nv Nd1 2

Available slots CBR reservation slots VBR reservation slots ABR reservation slots

ATM cellPRMA/DA

headerPRMA/DA

trailer

Wireless Packet

ATM cellPRMA/DA

headerPRMA/DA

trailer

Request packet

Variable Variable Variable

Operation Procedures of PRMA/DA

• Send Requests in Available Slots– Contention-based transmission.– Slotted ALOHA is used.

• Reserve Time Slots for each Successful Request– Dynamic allocation algorithm is used to allocate time

slots for CBR, VBR, and ABR connections.– The allocated time slots are reserved for the lifetime

of a connection.– Dynamic allocation algorithm is also used for

updating available time slots for the transmission of requests.

• Transmit Packets in Reserved Time Slots– Since time slots are reserved, contention is free in

this phase.

Contributions and Shortcomings of PRMA/DA

• Contributions– Dynamic allocation of slots for each sub-frame.

• Variable boundary can be easily implemented.• Bandwidth can be utilized efficiently.• Collisions can be resolved quickly

– No mini-slots; Easy for synchronization.– Multiple traffic classes supported.

• Shortcomings– A request packet has the same length as a data packet.

• If traffic rate high, this would cause inefficiency.– No mechanism is used to dynamically update VBR

resources.• VBR bandwidth is allocated according to the average

rate. The bursty requirement has to rely on the leftover bandwidth. QoS of VBR cannot be guaranteed.

MASCARA(Mobile Access Scheme based on Contention and Reservation

for ATM)• Supports CBR, real-time VBR (rt-VBR), non-real-time VBR (nrt-VBR), ABR, UBR traffic.

• Demand assignment scheme with contention based reservations.• Uplink subframe is divided into a contention period to transmit reservation

requests or some control information, and uplink period for uplink data traffic.• Each period within a frame has a variable length depending on the instantanous

traffic to be carried.

FH Period Uplink Period Downlink Period ContentionPeriod

MPDU 1 MPDU n… …

1 time slot n time slots

MPDU 2

PHYHdr

MPDUHdr MPDU payload: Cell train (many ATM cells)

Operation Procedures of MASCARA• If a terminal has cells to transmit, it sends a reservation request

either piggybacked in the MPDUs uplink period or in special control MPDUs sent in the contention period.

• Base station schedules transmissions of the next frame according to reservation requests, arriving cells for each downlink connection, traffic characteristics and QoS requirements of all connections.

• In the Frame Header of the downlink, BS broadcasts information which contains a descriptor of the current time frame (including the lengths of each period), the results of the contention procedures from the previous frame and the position of the slot allocated to each downlink and uplink connection.

• To minimize PHY layer overhead, MASCARA uses the concept of a CELL TRAIN (a sequence of (1-n) ATM cells belonging to a terminal and having a common header).

• Length of overhead plus that of the MPDU header is equal to one time slot, which is defined as the length of an ATM cell.

Priority Regulated Allocation Delay-Oriented Scheduling (PRADOS)

* Assigns priorities for each connection according to its service class.

* PRADOS combines priorities with a leaky bucket traffic regulator.• Regulator uses a token pool introduced for each connection.• Tokens are generated at a fixed rate equal to the mean ATM cell rate of

each VC.• Size of the pool is equal to the maximum number of ATM cells that can

be transmitted with a rate greater than the declared mean.• Starting at priority 5 and ending with priority 2, scheduler satisfies

requests for connections as long as tokens and slots are available. • For every slot allocated to a connection, a token is removed from the

corresponding pool.

Traffic Priority Token Pool

CBR 5 Yes

rt-VBR 4 Yes

nrt-VBR 3 Yes

ABR 2 Yes

UBR 1 No

Contributions and Shortcomings of MASCARA

• Contributions– Cell train concept is used.– A novel scheduling scheme - PRADOS.– Dynamic TDD is implicitly implemented.– Multiple traffic classes are supported.

• Shortcomings– With each request corresponding to a time slot, too

many requests are transmitted in the protocol. This results in wasting bandwidth.

– Large size of request packet results in reduction of good throughput.

– Connection admission control (CAC) is separate from the MAC protocol. The overall performance of the integrated system is unpredictable.

Comparisons of TDMA MAC Protocols

Protocols PRMA/DA MASCARA DPRMA

Duplex Mode FDD TDD FDD

Frame Type Fixed Variable Fixed

Random Access Slotted ALOHA

Slotted ALOHA

Reservation ALOHA

Mini-slot No No No

CAC In MAC Separate Separate

Traffic Classes CBR, VBR, ABR

CBR, nt-VBR, nrt-VBR, ABR, UBR

Voice, video, data

Network Layer ATM ATM ATM

Control Overhead Medium High Medium

Mobility Management in W-ATM Networks

• Location Management Handoff Handoff ManagementManagement

Base StationBase Station

MT A is receiving a call !MT A is receiving a call !How will the networkHow will the networkdeliver the call to A ?deliver the call to A ?

A

Types of Mobility

• TERMINAL MOBILITY

(network should route calls to the MT

regardless of its point of attachment)• PERSONAL MOBILITY

(users should access the network wherever they are; UPT (Universal Pers. Tel #))

• SERVICE PROVIDER MOBILITY

(allow user to roam beyond regional networks).

Location Management

Call Delivery(Paging)

Location Update(Registration)

Cost Tradeoff

Too Many Location Updates

Too Few Location Updates

Low Paging CostsHigh Update Costs

High Paging CostsLow Update Costs

Solution

• Location Areas (GSM) = Registration Areas (IS-41)

Registration Area BoundaryRegistration Area Boundary

Center CellCenter Cell

Handoff Types

Intra-CellIntra-Cell Inter-CellInter-Cell

Soft HandoffSoft Handoff Hard HandoffHard Handoff

W-ATM Architecture

ATMSwitch

ATMSwitch

ATM Backbone Network

ATMSwitch

ATMSwitch

CellBS

MT

Wireline connections to ATM switch

Wireless connections to BS

LOCATION MANAGEMENT

LOCATIONSERVICE

TERMINALPAGING

TWO-TIERDATABASES

LOCATIONREGISTERS

LOCATIONADVERTISEMENT

VIRTUALCONNECTION

TREE

INTEGRATEDLOCATION

RESOLUTION

MOBILE PNNI

LOCATION MANAGEMENT TECHNIQUES FOR W-ATM

• LOCATION SERVICE * Use of DATABASES to maintain records of MTs. * When location information is obtained from

DATABASE, TERMINAL PAGING is used to deliver calls to MTs.

* Requires signaling, querying and paging.

• LOCATION ADVERTISEMENT * No databases but location information is

broadcast throughout the network.

Location Service: Method 1: Two Tier Database (Akyol/Cox’96)

PREVIOUSZONE

HomeTier

VisitorTier

HomeTier

VisitorTier

CURRENTZONE

ZoneManager

HomeTier

VisitorTier

HOMEZONE

(1)(2)

(5)

(3)

(4)

Explanation: * Bi-level databases are distributed to ZONES throughout the network.

* Each zone is maintained by a ZONE MANAGER controlling the zone’s location update procedures.

* Each MT has a home zone where it is permanently registered. 1. MT transmits a location registration request message to the new zone. Message

contains User ID Number, authentication data and ID of the previous zone.2. Current zone manager determines the home zone of the MT from the previous

zone ID.3. Current and home zone managers authenticate the user and update home user

profile with the new location information.4. Home zone sends a copy of the profile to the current zone manager which stores

the profile in the visitor tier of its database.5. Current zone manager sends a purge message to the previous zone manager so

that user’s profile is deleted from the visitor tier before.

Location Advertisement: Method 1:

Virtual Connection Tree (Veeraraghavan et.al.’97)

Portable Base Station (PBS) Cell

Boundary

De-registrationmessage

MT’s Formerposition

Registrationmessage

• VCT advertises location information via provisioned virtual paths.• A collection of PBSs connected via provisioned VPs forms a connection tree.• PBSs are equipped with switching capabilities and limited buffering capabilities.• Trees are based on the mobility indications of the MT.• Each PBS maintains a running list of resident MTs in its coverage area.• Location registration occurs when MT is on/off or it moves to a new service

area.• On/Off case, MT sends a message to its local (current) PBS which then

adds/deletes the MT to/from the service list.• When MT moves to a new service area of a PBS, the PBS sends a de-registration

message to the old PBS on behalf of the MT and enters the MT’s ID into its current list.

Comparison of LocationManagement Techniques

Advantages Disadvantages

No Paging No Scalability

No Database Wasted

Admin Bandwidth

Advantages Disadvantages

Flexibility Database Admin

Scalability Signaling Load

Location AdvertisementLocation Service

Handoff Management

Full Connection Re-Routing

RouteAugmentation

Partial ConnectionRe-Routing

Multicast Connection Re-Routing

Virtual Connection

Tree Re-Routing

Nearest Connection

Node Re-Routing

InterWorking Devices

Connection Extension

InterWorking Devices

Connection Re-Routing

HybridConnection Re-Routing

Homing BaseStation

Re-Routing

Full Connection Re-Routing: Maintains the connection by establishing a completely new route for each

handoff as if it were brand new call.

Route Augmentation:Extends the original connection with a hop to the MTs next location.

Partial Connection Re-Routing: Re-establishes certain segments of the original connection, while preserving the

remainder.

Multicast Connection Re-Routing:Combines the 3 techniques but includes the maintenance of potential handoff

connection routes to support the original connection, reducing the time spent in finding a new route for handoff.

Comparison of Handoff ManagementApproaches

Full Extension Partial MulticastAdvantages

Optimalroute;

existingmethodology

Fast;maintains

cellsequence

Maintains cell sequence;reduced

resource utilization

Fast;maintains

cellsequence

Slow;inefficient resource

re-assignment

Disadvantages

Wastes bandwidth;inefficient

connection route

Complex;added switch

processing reqs

Added bufferingrequirements;

bandwidthpre-allocation

References:

1. J. McNair, “Mobility Management Protocols for Wireless

ATM Networks”, BWN Lab Technical Report, 1997. (Available on the WEB).

2. I.F. Akyildiz, J. McNair, J. Ho, H. Uzunalioglu, W. Wang,

“Mobility Management in Next Generation Wireless Systems”,

Proceedings of the IEEE Journal,

Vol, 87, No.8, pp.1347-1384, August 1999.