Module 4'2: Media Access Control

1
Module 4.2 Media Access Control

• The Media Access Control (MAC) sublayer
• Random Access (CSMA), IEEE 802.3
• Token Passing, IEEE 802.5

Ch 13- 14
2
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
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
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
CSMA

• 1-persistent CSMA
• Similar to telephone with persistent redial
  feature.
• P1 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
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
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
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