Question

Describe how frame transmission takes place in a LAN with a physical ring topology.

Answer 1

A physical ring topology consists of all devices, computers connected in a loop. device connection creates a circular data path. Each device on the network is connected to two other devices. In the ring topology or ring network, packets transfer from each device in way untill they reach their destination. Ring topology permits data in one direction only (Unidirectional).

Ring topology uses a communication protocol called the Token ring. It uses a frame called token A token is a special frame of 3 bytes that travel across all ring of stations.

When there is no data or frames transfer from any channel, the Token frame is repeated from station to station. The token frame is 3 bytes in size.

Token frame

Start Delimiter (SD)	Access control(AC)	End Delimeter(ED)
8 bits	8 bits	8 bits

Data frame

SD	AC	FC	DA	SA	Data	CRC	ED	FS
1	1	1	6	6	>=0	1	1	1

Data frames carry information for upper-layer protocols.

SD - It alerts stations for the arrival of the token/ data frame.

AC- It consists of in form of most significant to least significant. It has priority and reservation bits in order P, P, P, T, M, R, R, R. P for priority, R for reservation, and T for specifying token frame.

FC - It has 2 bits which indicate if the frame contains data or control information.

DA - SA - it contains a source and destination address. (MAC Address)

Data - It can have a length field of 0 or maximum bytes.

CRC-Its 32 bits, It checks for errors in frames and corrupted frames.

ED - Used to mark the end of the frame.

FS - terminates the frame.

There are 5 processes to be followed for token insertion.

1. Phase 0, Lobe check: A station checks whether frames sent are received.
2. Phase 1, Physical insertion: station sends a 5-volt signal to the MSAU(multistation access unit) to open the relay.
3. Phase 2, address verification: station transmits MAC frames with its source address and destination address. It returns AR(address recognized) and FC(frame copied) if bits are ) then there arent any other station transmitting.
4. Phase 3, Ring poll participation: A station creates a ring map by learning the address of neighbors and showing its address to the neighbor. It waits for AR and FC to be 0, and when they became 0, it sets them to 1 and starts transmission.
5. Phase 4, Request initialization: stations ends a special request to a parameter server to get configurational info. The frame is sent to the Token ring bridge, which holds the timer and ring number so that other stations can know.