MORE ABOUT DATA LINK LAYER


Carrier Sense Multiple Access Protocols:

CSMA- Carrier Sense Multiple Access

§  Carrier sense means that a transmitter attempts to determine whether another transmission is in progress before initiating a transmission.

§  That is, it tries to detect the presence of a carrier signal from another node before attempting to transmit.

§  If a carrier is sensed, the node waits for the transmission in progress to end before initiating its own transmission.

§  In other words, CSMA is based on the principle “sense before transmit” or “listen before talk”.

§  Multiple accesses mean that multiple nodes may send and receive on the medium.

§  Transmissions by one node are generally received by all other nodes connected to the medium.

CSMA/CD- CSMA with Collision Detection:

§  In this protocol, while transmitting the data, the sender simultaneously tries to receive it. So, as soon as it detects a collision (it doesn’t receive its own data) it stops transmitting.

§  Thereafter, the node waits for some time interval before attempting to transmit again. Simply put, “listen while you talk”.

§  But, how long should one wait for the carrier to be freed? There are three schemes to handle this.

·         1-Persistent: In this scheme, transmission proceeds immediately if the carrier is idle. However, if the carrier is busy, then sender continues to sense the carrier until it becomes idle. The main problem here is that, if more than one transmitter is ready to send, a collision is GUARANTEED!!

·         Non-Persistent: In this scheme, the broadcast channel is not monitored continuously. The sender polls it at random time intervals and transmits whenever the carrier is idle. This decreases the probability of collisions. But, it is not efficient in a low load situation, where numbers of collisions are anyway small. The problems it entails are:

o    If back-off time is too long, the idle time of carrier is wasted in some sense.

o    It may result in long access delays.

·         P-Persistent: Even if a sender finds the carrier to be idle, it uses a probabilistic distribution to determine whether to transmit or not. Put simply, “toss a coin to decide”. If the carrier is idle, then transmission takes place with a probability p and the sender waits with a probability 1-p. This scheme is a good trade off between the Non-persistent and 1-persistent schemes.

·         O-persistent: Each node is assigned a transmission order by a supervisory node. When the transmission medium goes idle, nodes wait for their time slot in accordance with their assigned transmission order. The node assigned to transmit first transmits immediately. The node assigned to transmit second waits one time slot. Nodes monitor the medium for transmissions from other nodes and update their assigned order with each detected transmission.

CSMA/CA- CSMA with Collision Avoidance:

§  In CSMA/CA collision avoidance is used to improve the performance of CSMA.

§  If the transmission medium is sensed busy before transmission, then the transmission is deferred for a random interval.

§  This random interval reduces the likelihood that two or more nodes waiting to transmit will simultaneously begin transmission upon termination of the detected transmission, thus reducing the incidence of collision.

Virtual time CSMA:

§  VTCSMA is designed to avoid collision generated by nodes transmitting signals simultaneously, used mostly in hard real-time systems.

§  The VTCSMA uses two clocks at every node, a virtual clock (vc) and a real clock (rc) which tells “real time”.

§  When the transmission medium is sensed to be busy, the vc freezes, when the transmission medium is free, it is reset. Hence, calculating vcruns faster than rc when channel is free, and vc is not initiated when the transmission medium is busy.

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