Wireless MAC Protocol

1
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

2
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

3
Design Goals for Wireless MAC

• Requirements for a wireless MAC protocol
• robustness
• efficiency
• fairness
• support for priority and QoS
• support for multicast

4
Hidden Station Problem

• Hidden Stations within the range of the intended
  receiver, but out of range of the transmitter
• hidden sender C

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

• hidden receiver C

A
B
D
C
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
5
Exposed Station Problem

• Exposed Stations within the range of the
  intended sender, but out of range of the receiver
• exposed sender B

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

• exposed receiver B

A
B
D
C
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?)
6
Summary of hidden and exposed station problem

• Receivers 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

7
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

8
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

9
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 (m1) slots to let the sender
  receives ACK
• hidden sender defers (m1) slots when sees CTS
• solves hidden/exposed sender problem

10
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

11
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.

12
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.

13
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

14
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