Token Ring IEEE 802.5

Token Ring IEEE 802.5

IEEE 802.5 Token Ring  

A token is passed from station to station in sequence. If any station wants to send data, first it will grab the token then transmit the data. Data passes through all stations; only the intended receiver gets a copy of data. Finally the original data is received at the sender and then the sender release the token.Here there is no collision as it using token method.If multiple stations attempts to get the token simultaneously the one which high priority will get the token.

Token Reinsertion Strategy in Token Ring

1. Delayed Token Reinsertion

     Token is released only after getting the entire data packet back at the sender

     Efficiency is Low

     Reliability is high

    Useful under low load conditions.

2. Early  Token Reinsertion

     Token is released immediately after the transmission of data packet

     Efficiency is High

     Reliability is Low

     Useful under high load conditions.

Monitor Stations in Token Ring:

Any station can become a monitor station.If multiple stations are ready to become monitor station then station with high priority will become the monitor station.The monitor station will check the availability of token in the ring .If the token is missed or vanished the monitor station will issue a new token.

The monitor station makes identification on data packet as it passes through the monitor station.So it will not allow the same packet to pass through the monitor station again.This solves the orphan packet problem.

The monitor station checks the data packet for all the fields.If it finds any problem the data packet, then the monitor station will not allow the packet to continue in the ring.
Monitor Crash
The monitor station has to send message to all other stations in regular intervals indicating that the monitor is active.
 If this message is not received a new monitor is selected.

Malfunction of Monitor Station:
Monitor Station is alive, but is unable to perform its functions.

Issues in Token Ring:

1. Token Problems
         Vanished Token
         Corrupted or unhealthy token

2. Source Problem
         Orphan Packet
         Stray Packet

3. Monopoly of High priority stations over the low priority ones.

4. Destination Problem : If the destination is busy or crashed the data will not be get delivered.

    Destination Busy: If the destination is busy then sender retransmits the packet.
    Destination Crashed: If the destination is crashed then sender does not retransmit the packet.

5. Ring Problem: Any problems in the ring affect the physical connection.


Solutions to issues in Token ring.

1. Token Holding Time (THT):
    No station can hold the token beyond the maximum Token Holding Time.This avoids the monopoly of high priority stations over low priority ones.

2. Monitor Stations:
    Monitor station  controls the activities in the ring - Token Management, Packet Delivery,Error Detection