Prerequisite – Token Ring frame format, Efficiency Of Token Ring
Token Ring protocol is a communication protocol used in Local Area Network (LAN). In a token ring protocol, the topology of the network is used to define the order in which stations send. The stations are connected to one another in a single ring. It uses a special three-byte frame called a “token” that travels around a ring.
- Topology Used: Ring
- Access Control: Token Parsing
- Data Flow: Unidirectional
- Data Rates: 4Mbps, 6Mbps
- Acknowledgement: Piggy Backing Acknowledgement
- Encoding: Differential Manchester Encoding
In this article we are going to discuss about some problems with Token Ring. There are four types of problems: Source Problems, Destination Problems, Token Problems and Monitor Problems. These are explained as following below.
1. Source Problems: It can be of two types:
- (i) Orphan Packet Problem:
In case, if a station shuts down then the data transmitted by the source will remain circulating in the ring for a long time. In order to solve this problem a special station is introduced in the Ring known as Monitor Station.
There is a monitor bit in the Data packet, whenever the Monitor encounter the packet, it changes the monitor bit to 1 which was initially 0 and after that if the Monitor encounter the packet whose monitor bit is 1, it will discard them.
This is how we remove the Orphan Packets.
- (ii) Stray Packet Problem:
Data get corrupted in such a manner that sender can’t recognize it. Hence it will circulate in the ring for infinite time. To solve this problem, the Monitor compute the CRC every time and when the CRC is changed, Monitor is going to remove it.
2. Destination Problems: Generally three are three types of Destination Problems:
- (i) If the Destination is Busy:
If the destination is busy or its all the buffers are full then in this case the sender will re-transmit the data again & again. This as a wastage of CPU and Time.
- (ii) If the Destination is Down:
If the Destination is Down the sender will also re transmit the Data again & again, assuming that destination is Busy.
- (iii) If there is some error in the Data:
In case of corrupted Data, Destination is not going to accept it. It will discard the packet.
In order to solve these problems at Destination level, three bits are added into the frame, to let the sender know about if the Destination is Down or if there is an error in the Data.
These Three bits are A (Available), C (Copied) and E (Error).
Initially all the bits will be 0. If the Data is copied or accepted by the Destination, A & C bit will become 1.
In case the sender is available and the data is not copied due to some error in Data or all the Buffers are full, then Bit A will become 1 and C will remain 0. To detect if there is an error in the packet or the Destination is busy, we use the Error bit. Error bit will become 1 if there is any error in the Data, else it will remain 0. means Destination is Busy.
|A (Available)||C (Copied)||E (Error)|
On the acknowledgement of these bits sender will work efficiently.
3. Token Problems: There can be three types of problems with Token.
- (i) Captured Token Problem:
One station holds the Token and send lot of Data without giving the chance to others. It will lead to Monopolisation.
To avoid this problem we have the maximum Token Holding Time. A station can hold the Token maximum for that time limit. Hence, there will be no Monopolisation.
- (ii) If Token is lost:
It may be possible hat after taking the Token, Station Switched Down. In this case the token will be lost.
To deal with such situations, it is the responsibility of Monitor to generate new Token. Monitor will wait for Max Token Return Time and if the token is not received then it will generate a new Token.
- (iii) If Token got Corrupted:
Monitor will assume that this is a disturbance in Ring so, it’ll remove it. and after the Maximum Token Return Time it will generate a new Token.
Note: Token is very small (3 Bytes) and CRC is of 4 Bytes so, to prevent the Token we can’t use CRC. It will increase the size of the Token.
4. Monitor Problems : There are two types of problems.
- (i) Monitor not working :
If Monitor goes down then entire communication goes down . To overcome this the Monitor gives heartbeat information to all systems on the ring.
This heartbeat packet is called AMP(Alive Monitor Packet).
If the Monitor goes down, polling is done and any one of the systems will act like monitor.
- (ii) Monitor Malfunctioning :
If Monitor gets hacked then it is called monitor malfunctioning . The only solution for this problem is human intervention i.e we have to bring it back to normal state manually .
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- Token Ring frame format
- Token Bus (IEEE 802.4)
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