Medium Access Control Nick FeamsterCS 4251 Computer Networking IISpring 2008
Some Multiple Access Protocols • CSMA/CD • Token passing • Wireless LAN Protocols
Random Access MAC Protocols • Non-Carrier-Sense protocols: doesn’t “listen” to the channel before transmitting • ALOHA • Carrier-Sense protocols: senses the channel before transmitting • CSMA (Carrier Sense Multiple Access): does not detect collision. • CSMA/CD (Ethernet): A node “listens” before/while transmitting to determine whether a collision happens.
ALOHA • Radio-based communication network • Developed in 1970s at the Univ of Hawaii • Basic idea: transmit when a node has data to be sent. • Receiver sends ACK for data • Detect collisions by timing out for ACK • Recover from collision by trying after random delay • Too short: large number of collisions • Too long: underutilization
Ethernet MAC • If line is idle (no carrier sensed) send packet immediately • If line is busy (carrier sensed) wait until idle and transmit packet immediately • If collision detected • Stop sending and jam signal • Jam signal: make sure all other transmitters are aware of collision • Wait a random time (Exponential backoff), and try again
Questions • How does sender detect collision? • How long does it take?
Ethernet Performance • Ethernets work best under light loads • Utilization over 30% is considered heavy • Peak throughput worse with • More hosts • More collisions needed to identify single sender • Smaller packet sizes • More frequent arbitration • Longer links • Collisions take longer to observe, more wasted bandwidth
Carrier Sense Multiple Access (CSMA) • Listen to medium and wait until it is free(no one else is talking) • Wait a random backoff time • Advantage: Simple to implement • Disadvantage: Cannot recover from a collision
Wireless Interference • Two transmitting stations interfere with each other at the receiver • Receiver gets garbage A B C
Carrier Sense Multiple Accesswith Collision Detection (CSMA-CD) • Procedure • Listen to medium and wait until it is free • Start talking, but listen to see if someone else starts talking too • If collision, stop; start talking after a random backoff time • Used for hub-based Ethernet • Advantage: More efficient than basic CSMA • Disadvantage: Requires ability to detect collisions • More difficult in wireless scenario
Collision Detection in Wireless • No “fate sharing” of the link • High loss rates • Variable channel conditions • Radios are not full duplex • Cannot simultaneously transmit and receive • Transmit signal is stronger than received signal
Solution: Link-Layer Acknowledgments • Absence of ACK from receiver signals packet loss to sender • Sender interprets packet loss as being caused by collision Problem: Does not handle hidden terminal cases.
Carrier Sense Multiple Accesswith Collision Avoidance (CSMA-CA) • Similar to CSMA but control frames are exchanged instead of data packets • RTS: request to send • CTS: clear to send • DATA: actual packet • ACK: acknowledgement
Carrier Sense Multiple Accesswith Collision Avoidance (CSMA-CA) • Small control frames lessen the cost of collisions (when data is large) • RTS + CTS provide “virtual carrier sense” • protects against hidden terminal A B
Random Contention Access • Slotted contention period • Used by all carrier sense variants • Provides random access to the channel • Operation • Each node selects a random backoff number • Waits that number of slots monitoring the channel • If channel stays idle and reaches zero then transmit • If channel becomes active wait until transmission is over then start counting again
Virtual Carrier Sense • Provided by RTS & CTS • Prevents hidden terminal collisions • Typically unnecessary RTS CTS B A C
Physical Carrier Sense Range • Carrier can be sensed at lower levels than packets can be received • Results in larger carrier sense range than transmission range • More than double the range in NS2 802.11 simulations • Long carrier sense range helps protect from interference Receive Range Carrier Sense Range
Hidden Terminal Revisited • Virtual carrier sense no longer needed in this situation RTS CTS B A C Physical Carrier Sense
Physical Carrier Sense • Energy detection threshold • Monitors channel during “idle” times between packets to measure the noise floor • Energy levels above the this noise floor by a threshold trigger carrier sense • DSSS correlation threshold • Monitors the channel for Direct Sequence Spread Spectrum (DSSS) coded signal • Triggers carrier sense if the correlation peak is above a threshold • More sensitive than energy detection (but only works for 802.11 transmissions) • High BER disrupts transmission but not detection