Today we will start our lessons on Media Access Control, called MAC. Broadcast networks are of high benefits and a low cost. All information will be sent to all users, so no routing is really necessary in a broadcast network. The primary function of Media Access Control is to minimize or eliminate the instance of the collisions to achieve a reasonable utilization of the medium. There are two broader categories of schemes for sharing a broadcast transmission medium. The first category involves a static and a collision free sharing. We refer to these as channelization schemes because they involve the partitioning of the medium into separate channels that are then dedicated to particular users. The second category involves a dynamic access sharing of the medium on a peripheral basis that is better matched to situations in which user traffic is bursty. In this category, two basic approaches are random access and scheduling. Satellite networks use channelization. Each station is assigned a channel in an uplink frequency band that is used to transmit user satellite. Satellite uses it to broadcast back on channels that occupy a different downlink frequency band. Cellular network is another example using channelization. Channelization techniques are suitable when stations generate a steady stream of information that makes efficient use of the dedicated channel. But, channelization is inflexible in allocation of language to users with different requirements and inefficient for bursty traffic. Dynamic Media Access Control schemes are much better at handling bursty traffic. There are different channelization approaches, frequency division multiple access FDMA, time division multiple access TDMA and coded division multiple access CDMA. Ring networks often use scheduling approaches. It involves the use of a token for Media Access Control. A station holds a token transmits into the ring and token is passed to the next station. Scheduling techniques are also used in multi job lines that were used in early data networks to connect a number of terminals to a central computer. In a random access approach, a station can transmit whenever ready. The problem is, simultaneous transmissions can occur and therefore collision may occur on a signal must be gobbled, it requires re-transmission strategy. The shared medium is the only means available for station communicating between each other. That implies some transmission resources will be utilized implicitly or explicitly to transfer coordination information. We say that the delayed bandwidth product plays a key role in the performance of ARQ protocols. It is also a key parameter in media access control protocol performance. Previously call that delayed bandwidth product is often referred to the wrong chip propagation delay times bandwidth. Let us use a simple two station examples to show the impact of delayed bandwidth products. In this two station case, a station with the frame to send listens to the communication channel and transmits if channel found idle. Stations monitor the channel to detect collisions. If collision occurs, stations that begin transmitting earlier, re-transmit. The propagation time between the two stations is fixed, unknown between those two stations. Let us develop an Access Protocol for this position. Say station A transmits at a time 0 when it found that the channel is idle, station A continues to observe the signal in the channel to make sure that the single is not collided by a signal from the other station. Please note, the signal from station A does not reach station B until one propagation time. In case one, station B has not begun a transmission before one propagation time and therefore A is assured as station B will refrain from transmitting there after. Unless station has captured the channel, an entire message will go slow. In case two, station B transmits right before one propagation time because at that time a signal has not reached yet, so there is collision. B detects collisions soon thereafter, while A detects collision almost at two propagation times. That is their long ship delay. At this point, both stations are aware that if they are competing for the channel, they need to figure out a way to resolve their contention. If they know the value of propagation delay, they can measure the time from when they began transmitting to when a collision occurs. The stations that have began transmitting earlier will re-transmit as soon as the channel becomes quiet. We observe, that time approximately equal to two propagation delay is required to coordinate the access for each frame transmitted. The maximum efficiency on a throughput are therefore derived. Normalized delayed bandwidth product, the ratio of propagation delay over the frame transmission time plays a key role in performance measurement or Media Access Control protocol.
