1
Link Layer: Multiple
Access, Ethernet,
Components
CS 242: Computer Networks
Week 12: Monday, 11/25/07
Daniel Bilar
Wellesley College
Fall 2007
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MAC Protocols: Frequency Division Multiple Access (FDMA) and Random Access (CSMA/CD), Study notes of Computer Systems Networking and Telecommunications
An overview of mac protocols, focusing on frequency division multiple access (fdma) and random access (csma/cd). Fdma is a channel access method where the channel spectrum is divided into frequency bands, and each station is assigned a fixed frequency band. Csma/cd is a random access method where nodes transmit at full channel data rate without coordination, and collisions are detected and recovered from. Key ideas of random access include carrier sensing, collision detection, and randomness.
Typology: Study notes
Pre 2010
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Link Layer: MultipleAccess, Ethernet,Components
CS 242: Computer NetworksWeek 12: Monday, 11/25/07Daniel BilarWellesley CollegeFall 2007
Goals today
Understand, in layer 2 and below, a broadcast channelcan be shared on a LAN (multiple access)
Partitioning
Taking Turns
Random Access
Multiple Access Protocols
Aloha, CSMA/CD
Physical components for link layer traffic
Older: Repeaters, Bridges / Switches
Modern: Virtual LANs Some slides adapted and pictures gratefully acknowledged
from Schul, Rexford, Kurose
Responsibilities: Link Layer
Responsibilities:
Transferring frame
(layer 2 datagram) from one node toadjacent node over a link
Before break, we had a look at encoding
Signal propagation over physical links
… and framing
Break sequence of bits into a frame
Today we look at the link access: How toshare a channel
Multiple Access Protocol
Single shared broadcast channel
Avoid having multiple nodes speaking at once
Otherwise, collisions lead to garbled data
Multiple access protocol
Distributed algorithm for sharing the channel
Algorithm determines which node cantransmit
Channel Partitioning: TDMA
TDMA: time division multiple access
Access to channel in "rounds"
Each station gets fixed length slot (length =packet transmission time) in each round
Unused slots go idle
Example: 6-station LAN
1,3,4 have packet, slots 2,5,6 idle
1
3
4
1
3
4
6-slotframe
Channel Partit. MAC protocols: FDMA
FDMA: frequency division multiple access ^
Channel spectrum divided into frequency bands ^
Each station assigned fixed frequency band ^
unused transmission time in frequency bands go idle ^
example: 6-station LAN, 1,3,4 have packet, frequencybands 2,5,6 idle
frequency bands
time
FDM cable
“Taking Turns” MAC protocols^ Token passing:^
Control
token
passed from onenode to nextsequentially.
Token message
Concerns:
Token overhead
Latency
Single point offailure (token)
T data
(nothingto send)
T
Random Access Protocols
Idea
“Happy go lucky!”
When node has packet to send
Transmit at full channel data rate R
No a priori coordination among nodes
Two or more transmitting nodes
➜
collision
Random access MAC protocol specifies:
How to detect collisions
How to recover from collisions
Examples
ALOHA and Slotted ALOHA
CSMA, CSMA/CD, CSMA/CA
Random Access: Slotted ALOHA Assumptions
All frames same size
Time divided into equalslots (time to transmit aframe)
Nodes start to transmitframes only at start ofslots
Nodes are synchronized
If two or more nodestransmit, all nodes detectcollision
Operation
When node obtains freshframe, transmits in nextslot
Case
No collision
Node can send new framein next slot
Case
Collision
Node retransmits frame ineach subsequent slot withprobability
p
until success
Slotted ALOHA (cont) Pros
Single active node cancontinuously transmitat full rate of channel
Highly decentralized:only slots in nodesneed to be in sync
Simple
Cons
Collisions, wasting slots
Idle slots
Nodes may be able todetect collision in lessthan time to transmitpacket
Clock synchronization
Pure (unslotted) ALOHA^
Unslotted Aloha: simpler, no synchronization
When frame first arrives, transmit immediately
Collision probability increases:
frame sent at t
0
collides with other frames sent in [t
1,t
+1] 0
Worse than slottedALOHA:
channel
used for usefultransmissions 18%of time!
Random Access: CSMA
We see that collisions hurt the efficiency ofALOHA protocol
At best, channel is useful 37% of the time
Idea:
Do not interrupt
CSMA (Carrier Sense Multiple Access) is atechnique that listens before transmission
If channel sensed idle: Transmit entire frame
If channel sensed busy, defer transmission
CSMA/CD (Collision Detection)
Idea:
Be a polite conversationalist
CSMA/CD: carrier sensing, deferral as inCSMA
Collisions detected within short time
Colliding transmissions aborted, reducingwastage
Collision detection
Easy in wired LANs: measure signal strengths,compare transmitted, received signals
Difficult in wireless LANs: receiver shut offwhile transmitting