1. Nick Feamster CS 4251 Computer Networking II Spring 2008
Medium Access Control
Nick Feamster CS 4251 Computer Networking II Spring 2008

2. Some Multiple Access Protocols
CSMA/CD
Token passing
Wireless LAN Protocols

3. 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.

4. 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

5. 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

6. Questions
How does sender detect collision?
How long does it take?

7. 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

8. Ethernet MAC Protocol

9. 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

10. Wireless Interference
Two transmitting stations interfere with each other at the receiver
Receiver gets garbage A B C

11. Carrier Sense Multiple Access with 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

12. 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

13. 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.

14. Carrier Sense Multiple Access with 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

15. Carrier Sense Multiple Access with 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

16. 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

17. Virtual Carrier Sense Provided by RTS & CTS
Prevents hidden terminal collisions
Typically unnecessary
RTS
CTS A B C

18. 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 NS simulations
Long carrier sense range helps protect from interference
Receive Range
Carrier Sense Range

19. Hidden Terminal Revisited
Virtual carrier sense no longer needed in this situation
RTS
CTS A B C
Physical Carrier Sense

21. 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 transmissions)
High BER disrupts transmission but not detection