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 C D 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 C D 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 C D 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 C D 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 • 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

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 (m+1) slots to let the sender receives ACK • hidden sender: defers (m+1) 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