104/19/23

Wireless MAC Protocol

Outline:• design challenges for wireless

MAC– hidden/exposed stations– flexible control for QoS support

• two design paradigms– multiple access based– token based

• rationale for design choices

204/19/23

Wireless Networking Environment• A simple model:

– A single shared physical channel among users– Omni-directional antenna, limited transmission range– Same transmission rate for all users

• Channel characteristics(illustrated with examples)– wireless transmission is spatial and local– sender & receiver: different views of the world– relevant contention is at the receiver side– contention may induce collisions– contention/collision/congestion is location dependent– channel access is a collective behavior from the fairness

perspective: the notion of “local” is misnomer

• Wireless MAC: how to address channel access in a wireless environment

304/19/23

Design Goals for Wireless MAC• Requirements for a wireless MAC

protocol:– robustness– efficiency– fairness– support for priority and QoS– support for multicast

404/19/23

Hidden Station Problem• Hidden Stations: within the range of the

intended receiver, but out of range of the transmitter– hidden sender C

A B DC

A B DC

Problem: A transmits to B, if C transmits (to D), collision at BSolution: hidden sender C needs to defer (Question: who tells C, A or B?)

Problem: A transmits to B, if D xmits to C, C cannot reply. D confuses (4 cases)Solution: D needs to be notified that its receiver C is hidden

– hidden receiver C

504/19/23

Exposed Station Problem• Exposed Stations: within the range of the

intended sender, but out of range of the receiver– exposed sender B

A B DC

A B DC

Problem: C transmits to D, if B transmits (to A), B cannot hear from ASolution: exposed sender B needs to defer

Problem: C transmits to D, if A xmits to B, B cannot reply. A confuses (4 cases)Solution: A needs to be notified that its receiver B is exposed (how can B hears A?)

– exposed receiver B

604/19/23

Summary of hidden and exposed station problem

• Receiver’s perception of a clean/collided packet is critical

• Hidden/exposed senders need to defer their transmissions

• Hidden/exposed receivers need to notify their senders about their status

704/19/23

MAC Protocol

Resolve channel contention & access:• Channel access arbitration

– know who are there– allocate the channel among multiple senders &

receivers who share the channel

• Collision avoidance– multiple access based– token based

• Collision resolution– backoff based

804/19/23

Solution Space for channel contention

• Multiple access approach– with carrier sensing

• carrier sensing: provides collision information at the sender, NOT the receiver

• FAMA, 802.11

– without carrier sensing• MACA, MACAW

– cons and pros: robust, solves hidden/exposed station problem, hard to provide QoS

• Token based approach– TDMA, DQRUMA– cons and pros: easy to provide QoS, less robust,

hard to handle hidden/exposed stations

904/19/23

Collision Avoidance• Basic approach: when a station needs to send,

– listens to the channel– if it overhears an ongoing transmission, waits until it completes before

re-executing the channel access – otherwise, it initiates a control packet handshake– after successful handshake, starts data transmission

• RTS-CTS-DS-Data-ACK sequence– draw the basic handshake sequence– explain why they are necessary– deferral:

• exposed sender: defers 2 slots to hear DS when sees RTS– not hearing DS, cease to defer– hearing DS, defers (m+1) slots to let the sender receives ACK

• hidden sender: defers (m+1) slots when sees CTS

– solves hidden/exposed sender problem

1004/19/23

Collision Avoidance (contd)• How to solve hidden/exposed receiver

problem ?– Hidden receiver: needs to send an out-of-

band signal– exposed receiver: needs to receive the

initial control packet in the presence of ongoing data traffic

– one solution: dual (data and control) channel + NCTS packet

1104/19/23

Collision Resolution• Backoff algorithms: BEB and MILD

– BEB: unfair in the sense that it favors the last transmitter to aggressively contend for the channel again

– MILD: still favors a successful transmitter, better than BEB

• What is the definition of fairness ?– per station versus per flow– (spatial congestion) independent versus dependent

• techniques for collision resolution– collision measurement for spatial congestion

• most collisions are contention-related if CA is effective

– backoff advertisement• since contention is spatial, advertising backoff values helps neighbors to

share information & make collective decisions.

1204/19/23

Multiple Tokens Approach• Someone controls the distribution of tokens, only those with tokens are allowed to send

• Effective in cellular environment• Two major components

– distribution of tokens• provides an instrument for QoS support• interact with higher layer scheduling

– identification of transmitters• adding new comers: periodically initiating an identification

phase• deleting leaving/idle/sleeping transmitters: indicating

whether you have more to send when transmitting.

1304/19/23

Integrating multiple access with multiple tokens

• How to put these two together– remember only the stations with tokens

can transmit– define several token types: unicast,

broadcast– unicast token: pure token-based

allocation– broadcast token: use multiple access

1404/19/23

Further Issues

• Two channels ?• Multicast ?

– How do multiple receivers ack ?– How to solve hidden/exposed stations ?

• Fairness ?– AIMD in congestion control to MAC contention

• Energy efficiency issue– RTS-CTS-DATA-ACK keeps the interface on all the

time

• Performance evaluation