Wireless Medium Access Control Protocols

A Survery by Ajay Chandra V. Gummalla and John O. Limb

Introduction

Survey Distributed vs. Centralized Networks Wireless MAC Issues

Low Power Sensor Nodes Random Access Guaranteed Access Hybrid Access

Introduction Cont’d.

Distributed MAC Protocols Distributed Foundation Wirelesss MAC

(DFWMAC) Eliminate Yield – Non-Preemptive

Priority Multiple Access (EY-NPMA)

Introduction Cont’d.

Centralized MAC Protocols Random Access

Idle Sense Multiple Acces (ISMA) Randomly Addressed Polling (RAP) Resource Auction Multiple Access (RAMA)

Guaranteed Access Zhang’s and Acampora’s Proposals Disposable Token MAC Protocol (DTMP)

Introduction Cont’d. Hybrid Access

Random Reservation Protocols (RRA) Packet Reservation Multiple Access (PRMA) Random Reservation Access – Independent

Stations Algorithm (RRA-ISA) Distributed Queuing Request Updated

Multiple Access (DQRUMA) Moble Access Scheme based on Contention

and Reservation for ATM (MASCARA)

Introduction Cont’d. Dynamic Slot Assignment ++ (DSA++)

Distributed Wireless Network

ad hoc network No central administration Multi-hop wireless networks Wireless Sensor Nets

Centralized Wireless Network

Last Hop Network Very common

Corporate, Academic, and Cellular uses.

Has a controlling Base Station, with variable intelligence Wireless Access Point Cellular Tower

Wireless MAC Issues Half-Duplex

No Collision Detection Uplink and Downlink must be multiplexed

Time Varying Channel Reflection, Diffraction, and Scattering Different version of signal are superimposed

on each other Multipath Propagation Coherence Time = time signal strength

changes by 3dB

Wireless MAC Issues Cont’d.

Burst Channel Errors Higher BER Errors occur in long bursts Link Layer retransmission based on

immediate ACKs

Wireless MAC Issues Cont’d. Location Dependent Carrier Sensing

Hidden Nodes: Node A doesn’t know Node B is also talking to BS

Exposed Nodes: Node A knows node B is talking, but doesn’t know that it will not affect Node A’s conversation with BS

Capture: Node A and B are both transmitting to BS, but since Node A’s signal strength is stronger Node A’s transmission is used no collision is detected.

Random Access

Random Access is based on a “Talk whenever you want” way of thinking

Collisions are resolved by a contention resolution algorithm

Distributed Networks

Guaranteed Access

Access to medium is scheduled Round Robin Master/Slave (Polling) Tokens TDMA, FDMA

Hybrid Access

Melds best qualities of Random and Guaranteed Access

Request-Grant mechanisms Requests are Random Access, and

once reserved transmission is guaranteed

Random Reservation Access Demand Assignment

Distributed Foundation Wireless MAC (DFWMAC)

802.11 Standard 4-way exchange: RTS-CTS-DATA-ACK No ACK causes sender to retransmit No CTS causes exponential backoff RTS and CTS contain a NAV which

details how much data is to be sent

Elimination Yield – Non-Preemptive Priority Multiple Access (EY-NPMA)

HIPERLAN Sense channel for time to send

(TTS) 1700 bits, if clear, then send If busy, N slots; When done listen

again If still busy, abort; Else listen again,

and if not busy then transmit until finished

Idle Sense Multiple Access (ISMA) Carrier Sensing and Collision detection

are performed by the BS When medium is idle BS broadcasts idle

signal (IS) Nodes with data send If collision BS cannot decode signal, does

not send ACK and broadcasts IS again Otherwise BS sends ACK/ISA (ISA) Efficiency is improved by using small

Reservation packets

Randomly Address Polling (RAP)

Nodes with data broadcast orthogonal “codes” simultaneously

BS receives all codes, using a CDMA receiver

BS then polls each code All nodes with that code transmit If only one node the BS sends ACK More than one node with code causes BS to

send NACK Reservation RAP supports nodes with

streaming traffic

Resource Auction Multiple Access (RAMA)

Each node has and N-bit ID and transmits it, in contention phase

BS then echos back ID it heard bit-by-bit Once a node receives a bit it did not

transmit, it drops out Since BS does an OR operation on

received IDs then node with highest ID always wins

Zhang’s Proposal

BS polls each node for data, round robin

Node responds with data request, or a keep alive if queue is empty

BS then polls each node that responded with a data request

Disposable Token MAC Protocol (DTMP)

Improves on Zhang’s proposal When polling nodes BS indicates if it

has data to send to nodes If no data, then remain silent Otherwise send short message Transmit any data to send Channel is assumed to be reciprocal

Acampora’s Proposal Poll, request, data phases BS polls each node, if the node has

data to sends it responds The BS the broadcasts this nodes

ID so that all nodes know the order in which to send

BS then polls nodes each node in turn for its data

Random Reservation Protocols (RRA)

Uplink is time slotted Each slot large enough to carry one

voice packet Downlink is broadcast channel Nodes use random access to request

reservations for data to send BS enforces a policy of reservations

Stream Reservation Complete BS scheduling

Packet Reservation Multiple Access (PRMA)

A node with a back-logged voice packet transmits with probability p

If successful, reserves that slot for following packets

Data is similar, though no reservations are made Different access probabilities are used for voice

and data Introduction of data packets into voice only

network decreases efficiency Improvements include limited data reservations,

separating voice and data channels (FRMA), separating request and data channels (PRMA++)

Centralized PRMA uses scheduling to achieve QOS

Random Reservation Access – Independent Stations Algorithm

(RRA-ISA)

BS polls a subset of all nodes Subset is defined by the probability

of a single transmission in a slot is maximized

BS uses channel history to compute subsets

Distributed-Queuing Request Update Multiple Access

(DQRUMA)

Uplink and Downlink are duplexed Uplink has request channel and packet

channel Request channel is for contention requests Packet channel is for data (and piggyback

new contention requests) Downlink has 3 messages: ACK for

current slot, transmission permission for node to use next uplink slot, and data to the nodes

Better than RAMA and PRMA

Mobile Access Scheme based on Contention and Reservation for ATM

(MASCARA)

Frame consists of three periods: broadcast, reserved, and contention

Broadcast informs nodes of structure of current frame and scheduled uplink transmissions

Reserved period consists of downlink data, and uplink data as defined in broadcast period

Contention is random access and used to send new requests to BS

Dynamic Slot Assignment++ (DSA++)

MAC on uplink is TDMA Both uplink and downlink are slotted Each downlink slot contains some data

and a MAC message MAC message contains ACK for

transmission on previous uplink slot and a reservation for next uplink slot

BS collects all requests and schedules uplink transmissions

Comparison Summary

Comparisons QoS guarantees are not suited to

Random Access protocols because delay cannot be bounded

Demand Assignment protocols are best suited to multimedia applications

Random Access lends itself to large networks

Polling protocols are efficient only for smaller networks

TDD protocols perform poorly at high data rates due to increase in switching