Wireless Medium Access ControlRomit Roy ChoudhuryWireless Networking LecturesDuke University
Wired Vs Wireless Media AccessBoth are on shared media.Then, what’s really the problem ?
The Channel Access Problem • Multiple nodes share a channel • Pairwise communication desired • Simultaneous communication not possible • MAC Protocols • Suggests a scheme to schedule communication • Maximize number of communications • Ensure fairness among all transmitters A B C
The Trivial Solution A B C • Transmit and pray • Plenty of collisions --> poor throughput at high load collision
The Simple Fix Don’t transmit A B C • Transmit and pray • Plenty of collisions --> poor throughput at high load • Listen before you talk • Carrier sense multiple access (CSMA) • Defer transmission when signal on channel Can collisions still occur?
CSMA collisions spatial layout of nodes Collisions can still occur: Propagation delay non-zero between transmitters When collision: Entire packet transmission time wasted note: Role of distance & propagation delay in determining collision probability
CSMA/CD (Collision Detection) • Keep listening to channel • While transmitting • If (Transmitted_Signal != Sensed_Signal) Sender knows it’s a Collision ABORT
2 Observations on CSMA/CD • Transmitter can send/listen concurrently • If (Transmitted - Sensed = null)? Then success • The signal is identical at Tx and Rx • Non-dispersive The TRANSMITTER can detect if and when collision occurs
Unfortunately … Both observations do not hold for wireless Because …
Wireless Medium Access Control C D A B Signal power Distance
Wireless Media Disperse Energy A cannot send and listen in parallel C D A B Signal power Signal not same at different locations Distance
Collision Detection Difficult B • Signal reception based on SINR • Transmitter can only hear itself • Cannot determine signal quality at receiver A C D
Calculating SINR B A C D
Red < Blue = collision Red signal >> Blue signal C D X A B Signal power Distance
Important: C has not heard A, but can interfere at receiver B C is the hidden terminal to A C D X A B Signal power Distance
Important: X has heard A, but should not defer transmission to Y Y X is the exposed terminal to A C D X A B Signal power Distance
Any Questions at this point?
So, how do we cope with Hidden/Exposed Terminals?
How to prevent C from trasmitting? C D X A B Signal power Distance
An Idea! • A node decides to intelligently choose a Carrier sensing threshold (T) • The node senses channel • If signal > T, then node does not transmit • If signal < T, then transmit • Possible to guarantee no collisions? C D A B
An Idea! C D X A B Signal power Distance
A Project Idea! Do not transmit in this region Will this solve the wireless MAC problem? C D X A B Signal power T Distance
Whatever the answer … This is an example of a good class project If you came up with the idea, Showed that it’s a new idea, And evaluated it to demo how it performs
The Emergence of MACA, MACAW, & 802.11 • Wireless MAC proved to be non-trivial • 1992 - research by Karn (MACA) • 1994 - research by Bhargavan (MACAW) • Led to IEEE 802.11 committee • The standard was ratified in 1999
RTS = Request To Send CTS = Clear To Send IEEE 802.11 M Y S RTS D CTS X K
IEEE 802.11 silenced M Y silenced S Data D ACK silenced X K silenced
802.11 Steps • All backlogged nodes choose a random number • R = rand (0, CW_min) • Each node counts down R • Continue carrier sensing while counting down • Once carrier busy, freeze countdown • Whoever reaches ZERO transmits RTS • Neighbors freeze countdown, decode RTS • RTS contains (CTS + DATA + ACK) duration = T_comm • Neighbors set NAV = T_comm • Remains silent for NAV time
802.11 Steps • Receiver replies with CTS • Also contains (DATA + ACK) duration. • Neighbors update NAV again • Tx sends DATA, Rx acknowledges with ACK • After ACK, everyone initiates remaining countdown • Tx chooses new R = rand (0, CW_min) • If RTS or DATA collides (i.e., no CTS/ACK returns) • Indicates collision • RTS chooses new random no. R1 = rand (0, 2*CW_min) • Note Exponential Backoff Ri = rand (0, 2^i * CW_min) • Once successful transmission, reset to rand(0, CW_min)
RTS CTS RTS/CTS • Does it solve hidden terminals ? • Assuming carrier sensing zone = communication zone E F CTS A B C D E does not receive CTS successfully Can later initiate transmission to D. Hidden terminal problem remains.
Hidden Terminal Problem • How about increasing carrier sense range ?? • E will defer on sensing carrier no collision !!! RTS E F CTS A B C D Data
Hidden Terminal Problem • But what if barriers/obstructions ?? • E doesn’t hear C Carrier sensing does not help RTS E F CTS A B C D Data
Exposed Terminal • B should be able to transmit to A • RTS prevents this E RTS CTS A B C D
Exposed Terminal • B should be able to transmit to A • Carrier sensing makes the situation worse E RTS CTS A B C D
Thoughts ! • 802.11 does not solve HT/ET completely • Only alleviates the problem through RTS/CTS and recommends larger CS zone • Large CS zone aggravates exposed terminals • Spatial reuse reduces A tradeoff • RTS/CTS packets also consume bandwidth • Moreover, backing off mechanism is also wasteful The search for the best MAC protocol is still on. However, 802.11 is being optimized too. Thus, wireless MAC research still alive
Takes on 802.11 • Role of RTS/CTS • Useful? No? • Is it a one-fit-all? Where does it not fit? • Is ACK necessary? • MACA said no ACKs. Let TCP recover from losses • Should Carrier Sensing replace RTS/CTS? • New opportunities may not need RTS/CTS • Infratructured wireless networks (EWLAN)
MACA-BI [GerlaUCLA] • RTS/CTS/ACK are control overhead • Needed to reduce it • Rx predicts trasmission from the Tx • Traffic estimation (???) • If Rx thinks Tx has pending packets for Rx • Rx transmits RTR to Tx • Tx replies with Data • Improves MACA with no RTS/ACK • improvement but not too much
Tx Busy tone Tx Busy tone Rx Busy tone RTS CTS CTS RTS Signal X Signal X A B Y X DBTMA [HaasCornell98] A B Y X
Implicit MACKnowledgment • APs typically backlogged with traffic • Persistent traffic possibility of optimzation • We propose an implicit ACK optimization • Piggyback the CTS with ACK for previous dialog 802.11 Gain Implicit ACK
Hybrid Channel Access • The optimization timeline 802.11 Hybrid Channel Access Implicit ACK T R T R T R RTS RTS RTS CTS CTS CTS Data Data Data Backoff ACK Backoff Backoff RTS Poll +ACK CTS +ACK RTS Data Data CTS Backoff Data Backoff Poll +ACK RTS ACK Data CTS +ACK Backoff
Seedex [KumarUIUC03] • Forget channel reservation and backoff • Instead, let nodes pick sequence of time slots • Decides to probably transmit in some, else listen • Transmit slots chosen using a random seed • Publishes the seed to 2-hop neighbors • When PT slots arrive, nodes transmit with • Probability “p” • “p” chosen as a function of overlapping neighbors
Hot Research Topics • Power control increases spatial reuse • Whisper in the room so that many people can talk • Rate control based on channel quality • Expolit channel diversity • Utilize multiple channels to parallelize dialogs • Exploit spatial diversity • Use directional antennas to interfere over smaller region (next class) … and many more topics
Announcements • Reviews: • You are forgetting to appreciate the paper • There is a reason why the paper was accepted • Please organize your papers/reviews • Would be valuable later in career • You never know what you will do after 5 years • Meet me with slides before you present • Email for an appointment • Don’t have to review if you are presenter
Announcements • Review Template: • Problem definition? Why is it important? • Validity of models and assumptions • Solution • Evaluation • Email review to TA (CC to me) • Bring print out to class • Name-date-subject in email subject • Will post example reviews on webpage • Some of you still doing summaries.
Backup slides on IEEE 802.11 Read for more details
Today’s Discussions • IEEE 802.11 overview - some raw data • Architecture • PHY specifications – Spread Spectrum radios: FH & DS • MAC specifications – DCF and PCF • Synchronization, Power management, Roaming, Scanning • Security • Deliberations on 802.11 (DCF) MAC • Hidden terminal & Exposed terminal issues • Carrier sensing • Some other ideas & open challenges • Could be interesting for the project
IEEE 802.11 in OSI Model Wireless
802.11 Scope & Modules To develop a MAC and PHY spec for wireless connectivity for fixed, portable and moving stations in a local area LLC MAC Sublayer MAC Layer Management MAC PLCP Sublayer PHY Layer Management PHY PMD Sublayer