Explain the terms topology and access methods used in LANs. Discuss the CSMA/CD and CSMA/CA protocols.

Explain the terms topology and access methods used in LANs. Discuss the CSMA/CD and CSMA/CA protocols.
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Answer :

(i) LAN topologies: Network topology is a physical schematic which shows interconnection of the many users. There are four basic topologies as under:

 (a) Direct Connection or one to all topology

 (b) Star topology

 (c) Bus Topology

 (d) Ring topology 

(ii) Access methods used in LAN:

i. Switched access: It is used in LANs that are assigned around CBXs.

Electronic switching is techniques are used to provide access.

ii. Multiple access: In multiple access schemes such as CSMA, whenever

a station gets ready for transmission, it first listens to the bus to see if there is any ongoing transmission. If there is one, the new transmission is not initiated until the bus becomes free. This ensures that an ongoing transmission is not corrupted by a new transmission.

iii. Token passing access: In this scheme, a token packet is introduced to the network. This packet continues to circulate through the network as long as no user accepts it. When a user works to transmit he waits for the token packet to reach him and accepts the token, this acceptance removes the token packet from the network.

(iii) CSMA/CD: It is an access method used primarily with LANs configured in a bus topology. With CSMA/CD, any station (node) can send a message to any other station (or stations) as long as the transmission medium is free of transmissions from other stations. Stations monitor (listen to) the line to determine if the line is busy. If the station has a message to transmit but the line is busy, it waits for an idle condition before transmitting its message. If two stations transmit at the same time, a collision occurs. When this happens, the station first sensing the collision sends a special jamming signal to all other stations on the network. All stations then cease transmitting (back off) and wait a random period of time before attempting a retransmission. The random delay time for each station is different, and therefore, allows for prioritizing the stations on the network. If successive collisions occur, the back off period for each station is doubled. With CSMA/CD stations must contend for the network. A station is not guaranteed access to the network. To detect the occurrence of a collision, a station must be capable of transmitting and receiving simultaneously. CSMA/CD is used by most LANs configured in a bus topology. Ethernet is an example of a LAN that uses CSMA/CD.

CSMA/CA: It belongs to a class of protocols called multiple access methods. CSMA/CA stands for: Carrier Sense Multiple Access with Collision Avoidance. In CSMA, a station wishing to transmit has to first listen to the channel for a predetermined amount of time so as to check for any activity on the channel. If the channel is sensed "idle" then the station is permitted to transmit. If the channel is sensed as "busy" the station has to defer its transmission. This is the essence of both CSMA/CA and CSMA/CD. In CSMA/CA (Local Talk), once the channel is clear, a station sends a signal telling all other stations not to transmit, and then sends its packet. In Ethernet 802.3, the station continues to wait for a time, and checks to see if the channel is still free. If it is free, the station transmits, and waits for an acknowledgment signal that the packet was received
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