CSMA vs. Token Passing

Difference between CSMA And Token PassingĀ 

CSMA

Carrier Sense Multiple Access is a listen-before-talking system. Each member of the network may transmit only if it does not detect another member using the network. This method works like a meeting in which each person waits for the others to finish talking before speaking.

But what if the network is “quiet,” and two stations begin to transmit at once? If two (or more) computers transmit messages or data at the same time, the information will become distorted and unusable. One way of taking care of this situation is CSMA/CA (collision avoidance). In this scheme, when data get garbled, the sending station does not get an acknowledgment from the receiving station. Then the transmitting computer retransmits. The transmissions take place in fractions of a second, so the likelihood of a second collision in succession is very low.

CSMA/CD (collision detection) does not wait for an acknowledgment. Instead, it has the stations listen before transmission to see whether the system is free, and listen during transmission to see whether two or more stations are transmitting at once. If a collision does take place, each transmitting station stops and waits for a fraction of a second before retransmitting. The amount of waiting time is chosen by each computer at random. Thus, it is unlikely that two stations will wait precisely the same amount of time before trying again. Remember that each transmission takes only a fraction of a second to complete.

Token-passing.

Token-passing is a protocol used in ring networks. It avoids collisions altogether because two or more stations cannot transmit at the same time.

Token-passing is much like a children’s game. The token is a pattern of bits, or a message that goes around the ring to each computer in turn. A computer can transmit only when it is “in possession” of the token. The computer then changes the bit pattern from “free” to “busy.” It transmits a block of data or information called a frame immediately after the “busy” token. Part of the transmission also carries the “address” of the receiving computer. The token and frame pass through each computer on the network until they reach the one that is supposed to receive the frame. The receiver copies the data or message in the frame. The token and frame continue around the ring back to the transmitting computer. The transmitting computer then clears the frame and generates a new “free” token.

Today most LANs are developed for PCs so that individual workers can communicate with each other. The connections in a PC network are made with various types of cables. Depending on the type of network hardware used, the cabling may be ordinary telephone wire, coaxial cable, fiber-optic cable, or twisted-pair cable (copper wires twisted to reduce electromagnetic induction). The most popular type of LAN today is Ethernet carried over twisted-pair cable.

 

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