1 K. Salah Module 4.2: Media Access Control The Media Access Control (MAC) sublayer –Random Access (CSMA), IEEE –Token Passing, IEEE Ch

2 K. Salah MAC Concepts MAC sublayer provides the protocols that define the manner in which nodes share the singly physical transmission media. The protocols define the who, the how, the when, and the for how long of channel allocation. These protocols are sometimes called contention protocols. Random Access – Stations access medium randomly Contention –Stations contend for time on medium Random Access Protocols –Pure ALOHA –Slotted ALOHA –1-persistent CSMA –Nonpersistent CSMA –P-persistent CSMA –CSMA/CD –CSMA/CA

3 K. Salah ALOHA Pure ALOHA –Packet Radio –When station has frame, it sends –Station listens (for max round trip time)plus small increment –If ACK, fine. If not, retransmit –If no ACK after repeated transmissions, give up –Frame check sequence –If frame OK and address matches receiver, send ACK –Frame may be damaged by noise or by another station transmitting at the same time (collision) –Any overlap of frames causes collision –Max useful throughput (frames/s) is 18% Slotted ALOHA –Time in uniform slots equal to frame transmission time –Need central clock (or other sync mechanism) –Transmission begins at slot boundary –Frames either miss or overlap totally –Max useful throughput (frames/s) is 37%

4 K. Salah Carrier Sense Multiple Access Propagation time is much less than transmission time All stations know that a transmission has started almost immediately First listen for clear medium (carrier sense) If medium idle, transmit If two stations start at the same instant, collision Max utilization depends on propagation time (medium length) and frame length –Longer frame and shorter propagation gives better utilization

5 K. Salah CSMA 1-persistent CSMA –Similar to telephone with persistent redial feature. –P=1 means the probability of transmitting when sensing idle channel. Nonpersistent CSMA –If channel is busy, wait a random period of time and rechecks the channel. P-persistent CSMA –Uses slotted channel –If channel is idle, transmit with a probability of p. Wait one time slot and sense again idle channel with probability of 1-p. Next time slot, transmit with p probability.

6 K. Salah CSMA/CD Step 1. “ Listen before talking”. Step 2. If channel is quiet for a certain time, called interframe gap (IFG), then transmit. “Talk if quiet”. Step 3. If channel is busy, monitor the channel until it is quiet for IFG period before transmitting. “Wait for quiet before talking.” Step 4. Monitor the channel continuously during transmission to detect collisions. “Listen while talking”. Step 5. If collision occurs, the first node recognizes it will send a jam signal to ensure that all other stations detect the collision. Transmitters should stop immediately, receivers should reject data, and others wishing to transmit should recalculate their backoff period. “A buzzer sounds off indicating we have more than one talker at a time.” Step 6. All nodes wish to transmit must now wait a random period called “backoff” and attempt again to spread out collisions. “Backoff” is based on BEB algorithm (Binary Exponential Backoff). After 16 collisions, drop the frame to be transmitted.

7 K. Salah Notes On baseband bus, collision produces much higher signal voltage than signal Collision detected if cable signal greater than single station signal Signal attenuated over distance Limit distance to 500m (10Base5) or 200m (10Base2) For twisted pair (star-topology) activity on more than one port is collision Why largest network diameter for Ethernet LAN 10Base5 is 2500 meters? –5-4-3 Repeater Placement Rule.

8 K. Salah Token Ring (802.5) MAC protocol –Small frame (token) circulates when idle –Station waits for token –Changes one bit in token to make it SOF (Start Of Frame) for data frame –Append rest of data frame –Frame makes round trip and is absorbed by transmitting station –Station then inserts new token when transmission has finished and leading edge of returning frame arrives –Under light loads, some inefficiency –Under heavy loads, round robin