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CS 635 Lecture 4, Part 1


The IEEE 802.3 standard is based on the Ethernet specification. It defines both a medium access control layer and a physical layer.


CSMA/CD and its precursors can be termed random access, or contention, techniques. Random - there is no predictable or scheduled time for any station to transmit; contention - stations contend for time on the medium

ALOHA - developed for radio packet networks, applicable to any shared transmission medium. Two types:
Pure ALOHA (true free for all):

  1. Station that has a frame to send does so at will.
  2. Station listens for the maximum round-trip propagation delay plus a small fixed time increment.
  3. If the station does not receive an acknowledgment during that time, it retransmits the frame.
  4. The receiving station verifies the FCS, if OK sends an acknowledgment; if not, drops the frame
Problems: Slotted ALOHA - channel is organized into slots whose size equals the frame transmission time. Needs a central clock or some other synchronization technique. Transmission is allowed only at a slot boundary. The frames that do overlap will do so totally. Maximum utilization is increased to 37%.

Conclusion: both algorithms ignored the fact that propagation delay between stations is very small compared to the frame transmission time. Short delay time provides stations with better feedback about the state of the network; this information can used to increase efficiency.

CSMA/CD - If the medium is in use, the station must wait. If the medium is idle, the station may transmit. If two stations transmit simultaneously, a collision occurs, the data is garbled and not received. To account for this a station waits for an ack a reasonable amount of time (taking into account the round-trip delay and processing time) and then transmits the frame again. If a station begins to transmit a frame and there are no collisions during the time it takes for the leading edge of the packet to propagate to the farthest station, there will be no collision.

The maximum utilization achievable using CSMA can far exceed that of ALOHA and Slotted ALOHA. The max. utilization depends on the length of the frame and on the propagation time; the longer the frames or the shorter the propagation time, the higher utilization.

Three approaches in determining what to do when the medium is found busy:

Non persistent CSMA

  1. If the medium is idle, transmit; otherwise, go to step 2.
  2. If the medium is busy, wait an amount of time drawn from a probability distribution (the retransmission delay) and repeat step 1.
A problem with this approach is that capacity is wasted because the medium will generally remain idle following the end of a transmission even if there are one or more stations waiting to transmit.

To avoid idle channel time, the 1-persistent CSMA can be used.

  1. If the medium is idle, transmit; otherwise, go to step 2
  2. If the medium is busy, continue to listen until the channel is sensed idle; then transmit immediately
If two or more stations are waiting to transmit, a collision is guaranteed.

The p-persistent CSMA rules

  1. If the medium is idle, transmit with probability p and delay one time unit with probability (1-p). The time unit is typically equal to the maximum propagation delay
  2. If the medium is busy, continue to listen until the channel is idle and repeat step 1
  3. If the transmission is delayed on time unit, repeat step 1
The question is what is an effective value of p, especially under heavy load. Consider n stations having frames to transmit while a transmission is taking place. At the end of the transmission, the expected number of stations that will attempt to transmit is equal to the number of stations ready to transmit time the probability of transmitting, or np.  Top  Next