1. Lab 7 – CSMA/CD (Data Link Layer Layer)
MPC8360 Computer Networks I
Lab 7 – CSMA/CD (Data Link Layer Layer)
Semester B, 2011

2. The Open System Interconnection Reference Model (OSI)
Application 1 2 3 4 5 6 7
Presentation
Session
Transport
Network
We are Here
(Data Link Layer)
Data Link
Physical

3. Data Link Layer
The Data Link Layer is the second OSI Layer. It usually divided into two sub layers, MAC Layer and LLC Layer,
MAC- Media Access Control sub layer It provides addressing and channel access control mechanisms that make it possible for several terminals or network nodes to communicate within a multi-point network
LLC – Logic Link Control sub layer This sub layer multiplexes protocols running on top the Data Link Layer, and optionally provides flow control, acknowledgment, and error notification

4. Shared Medium
Multiple access protocol – a protocol which handle access of several stations on a shared medium and detect or avoid data packet collisions.
Station A
Station B
Collision
Shared Bus
Station C

5. CSMA - Carrier Sense Multiple Access
CSMA – (Carrier Sense Multiple Access) When the sender (station) is ready to transmit data, it checks if the physical medium is busy, this actions is called carrier sense. If so, it senses the medium continually until it becomes idle, and then it transmits a piece of data (a frame).
Station A
Station B
Carrier Sense
Shared Bus
Carrier Sense
Station C

6. Collision Detection
A transmitting data station that detects another signal while transmitting a frame, stops transmitting that frame, transmits a jam signal, and then waits for a random time interval before trying to send that frame again.
Station A
Station B
Carrier Sense
Shared Bus
Collision
Carrier Sense
Station C

7. Carrier Sense Multiple Access with Collision Detection (CSMA/CD
Frame Transmission procedure
Is medium idle? If not, wait until it becomes ready.
Start transmitting.
Did a collision occur? If so, go to Collision Detected procedure.
Reset retransmission counters and end frame transmission.
Collision Detected procedure
Continue transmission until minimum packet time is reached to ensure that all receivers detect the collision (64B).
Send jam signal
Increment retransmission counter.
Was the maximum number of transmission attempts reached? If so, abort transmission.
Calculate and wait random backoff period based on number of collisions.
Re-enter Frame Transmission procedure.
After c collisions, a random number of slot times between 0 and 2c - 1 is chosen

8. Quick Overview