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

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Presentation on theme: "Medium Access Control Nick Feamster CS 4251 Computer Networking II Spring 2008."— Presentation transcript:

1 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: doesnt 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 AB

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 A B C RTSCTS

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 A B C RTSCTS Physical Carrier Sense


21 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

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