CHANNEL ALLOCATION TECHNIQUES 

Random Access Protocols

ALOHA: ALOHA is a system for coordinating and arbitrating access to a shared communication Networks channel. It was developed in the 1970s by Norman Abramson and his colleagues at the University of Hawaii. The original system used for ground based radio broadcasting, but the system has been implemented in satellite communication systems.

A shared communication system like ALOHA requires a method of handling collisions that occur when two or more systems attempt to transmit on the channel at the same time. In the ALOHA system, a node transmits whenever data is available to send. If another node transmits at the same time, a collision occurs, and the frames that were transmitted are lost. However, a node can listen to broadcasts on the medium, even its own, and determine whether the frames were transmitted.

There are two different types of ALOHA:

(i)Pure ALOHA
(ii) Slotted ALOHA       

(i) Pure ALOHA 

• In pure ALOHA, the stations transmit frames whenever they have data to send.

• When two or more stations transmit simultaneously, there is collision and the frames are destroyed.

• In pure ALOHA, whenever any station transmits a frame, it expects the acknowledgement from the receiver.

• If acknowledgement is not received within specified time, the station assumes that the frame (or acknowledgement) has been destroyed.

• If the frame is destroyed because of collision the station waits for a random amount of time and sends it again. This waiting time must be random otherwise same frames will collide again and again.

• Therefore pure ALOHA dictates that when time-out period passes, each station must wait for a random amount of time before resending its frame. This randomness will help avoid more collisions.

• Figure shows an example of frame collisions in pure ALOHA.

• In fig there are four stations that .contended with one another for access to shared channel. All these stations are transmitting frames. Some of these frames collide because multiple frames are in contention for the shared channel. Only two frames, frame 1.1 and frame 2.2 survive. All other frames are destroyed.

• Whenever two frames try to occupy the channel at the same time, there will be a collision and both will be damaged. If first bit of a new frame overlaps with just the last bit of a frame almost finished, both frames will be totally destroyed and both will have to be retransmitted.

(ii) Slotted ALOHA

• Slotted ALOHA was invented to improve the efficiency of pure ALOHA as chances of collision in pure ALOHA are very high.

• In slotted ALOHA, the time of the shared channel is divided into discrete intervals called slots.

• The stations can send a frame only at the beginning of the slot and only one frame is sent in each slot.

• In slotted ALOHA, if any station is not able to place the frame onto the channel at the beginning of the slot i.e. it misses the time slot then the station has to wait until the beginning of the next time slot.

• In slotted ALOHA, there is still a possibility of collision if two stations try to send at the beginning of the same time slot as shown in fig.

• Slotted ALOHA still has an edge over pure ALOHA as chances of collision are reduced to one-half.

Key Differences Between Pure ALOHA and Slotted ALOHA

1. Pure ALOHA was introduced by Norman and his associates at the university of Hawaii in 1970. On the other hand, Slotted ALOHA was introduced by Roberts in 1972.
2. In pure ALOHA, whenever a station has data to send it transmits it without waiting whereas, in slotted ALOHA a user wait till the next time slot beings to transmit the data.
3. In pure ALOHA the time is continuous whereas, in Slotted ALOHA the time is discrete and divided into slots.
4. In pure ALOHA the probability of successful transmission is S=G*e^-2G. On the other hand, in slotted ALOHA the probability of successful transmission is S=G*e^-G.
5. The time of sender and receiver in pure ALOHA is not globally synchronized whereas, the time of sender and receiver in slotted ALOHA is globally synchronized.
6. The maximum throughput occurs at G=1/2 which is 18 % whereas, the maximum throughput occurs at G=1 which is 37%.

CSMA, or listen with random access carrier

Technical CSMA (Carrier Sense Multiple Access) is to listen to the channel before transmitting. This significantly reduces the risk of collision, but does not eliminate them completely. If during the propagation time between the couple of the more remote stations (vulnerability period), a coupler does not detect the transmission of a frame, and there may be signal superposition. Therefore, it is necessary to subsequently retransmit lost frames.

Numerous variations of this technique have been proposed, which differ by three Features:

• The strategy followed by the module after detecting the channel status.

• The way collisions are detected.

• The message retransmission after collision policy.

Its main variants are:

• Non-persistent CSMA. The coupler the listening channel when a frame is ready to be sent. If the channel is free, the module emits. Otherwise, it starts the same process after a random delay.

• Persistent CSMA - A loan coupler to transmit the channel and previously listening forwards if it is free. If it detects the occupation of carrier, it continues to listen until the channel is clear and transmits at that time. This technique allows lose less time than in the previous case, but it has the disadvantage increase the likelihood of collision, since the frames that accumulate during the busy time are all transmitted simultaneously.

• P-persistent CSMA - The algorithm is the same as before, but when the

Channel becomes free; the module transmits with probability p. In other words, the coupler differs his show with probability 1 - p. This algorithm reduces the likelihood of collision. Assuming both terminals simply making the collision is inevitable in the standard case. With the new algorithm, there is a probability 1 - p that each terminal does not transmit, thereby avoiding the collision. However, it increases the time before transmission, since a terminal may choose not to transmit, with a probability 1 - p, while the channel is free.

• CSMA / CD (Carrier Sense Multiple Access / Collision Detection) - This technique normalized random access by the IEEE 802.3 working group is currently the longer used. At a preliminary listening to the network is added listening during transmission. Coupler to issue a loan that detected free channel transmits and continues to listen the channel. The coupler continues to listen, which is sometimes indicated by the CSMA / CD persistent acronym. If there is a collision, it interrupts its transmission as soon as possible and sends special signals, called padding bits so that all couplers are notified of the collision. He tries again his show later using an algorithm that we present later.

Figure shows the CSMA/CD. In this example, the couplers 2 and 3 attempt broadcasting for the coupler 1 transmits its own frame. The couplers 2 and 3 begin to listen and transmit at the same time, the propagation delay around, from the end of the Ethernet frame transmitted by the coupler 1. A collision ensues. Like the couplers 2 and 3 continue to listen to the physical media, they realize the collision, stop their transmission and draw a random time to start the retransmission process.

The CSMA/CD creates an efficiency gain compared to other techniques of random access because there are immediate collision detection and interruption of current transmission. Issuer couplers recognize a collision by comparing the transmitted signal with the passing on the line. The collisions are no longer recognized by absence of acknowledgment but by detecting interference. This conflict detection method is relatively simple, but it requires sufficient performance coding techniques to easily recognize a superposition signal. It is generally used for this differential coding technique, such as differential Manchester code.

• CSMA / CA - Less known than the CSMA / CD access CSMA / CA (Carrier Sense Multiple Access / Collision Avoidance) starts to be heavily used in Wi-Fi networks, that is to say, the wireless Ethernet IEEE 802.11. This is a variation of the CSMA / CD, which allows the CSMA method to run when collision detection is not possible, as in the radio. Its operating principle is to resolve contention before the data are transmitted using acknowledgments and timers.

The couplers are testing wishing to transmit the channel several times to ensure that no activity is detected. Every message received shall be immediately paid by the receiver. Sending new messages takes place only after a certain period, so as to ensure a transport without loss of information. The non-return of an acknowledgment, after a predetermined time interval, to detect if there was a collision. This strategy not only makes it possible to implement an acknowledgment mechanism in frame but has the advantage of being simple and economic, since it does not require collision detection circuit, unlike the CSMA/ CD.

There are various techniques of CSMA with collision resolution, including the CSMA / CR (Carrier Sense Multiple Access / Collision Resolution). Some variants use the CSMA also priority mechanisms that may come under this term, that avoid collisions by separate priority levels associated with different stations connected to the network.