Token ring | Introduction | History | Working | Token ring vs. Ethernet | Remove data packet
What is Token ring?
Token ring is a communication protocol based on LAN technology. It uses token, a 3-byte frame, which travel around a logical ring of workstations. It is based on channel-access method that provide all time access for all workstations without collisions of contention access methods.
In this technology all nodes are connected either in a ring or in a star topology and pass logical tokens from host to host. Only one host that hold the token can send data and tokens are released when the data receiving confirmation is received from the recipient.
Token ring network prevent data packets to collide on a network segment because the data can only be sent by a token holder and the number of tokens available are well controlled.
How Token ring work?
The token represents the authority to control the transmission line. It allows the sending station in the network to transmit data when token arrives to their location.
Whenever a station wants to send a frame, it inverts a single bit of the 3-byte token which instantaneously changes it into a normal data packet. Because there is only one token, there can be only one transmission at a time.
As the token rotates in the ring, every node in the network gets the token in an interval. So there is a maximum limit on the time of waiting to grab the token so that starvation situation can be avoided. The maximum limit for the number of nodes in the network is 250.
To distinguish the normal data packets from token (control packet) a special sequence is assigned to the token packet. When any node gets the token it first sends the data it wants to send, then recirculates the token.
If any station transmits the token and nobody wants to send the data, the token comes back to the sender.
The token-holding time for a station in the network is 10 unless the installation sets a different value. If there is enough time left after transmission of first frame, the station can continue to send more frames. If token-holding time exceeds in between, the station regenerates the 3-byte token frame and puts it back on the ring.
Token ring History
IBM developed token ring as an alternative to Ethernet in 1984. It was standardized by IEEE 802.5 and became very successful and popular in corporate industries but significantly decreased in popularity because of later versions of Ethernet.
The IEEE standard versions provides data transfer rates 4Mbps, 16Mbps or 100Mbps.
How to remove data packet from ring?
After transmission done data must be removed from the ring. The following 3 ways are there to remove data packet from the ring:
- The sender itself removes the packet after one full round in the ring.
- The receiver removes packet after accepting it. This has two potential problems. First, the solution won’t work for broadcast or multicast, and second, there would be no way to acknowledge the sender that the packet has received.
- To have a specialized node only to discard packets. This is a bad solution as the specialized node would know that the packet has been received by the destination only when it receives the packet the second time and by that time the packet may have actually made about one and half (or almost two in the worst case) rounds in the ring.
Thus, the first option is best. Broadcasting and multicasting can be handled as well as the destination can acknowledge the source about the receipt of the packet (or can tell the source about some error) by adopting this option.
Token ring vs. Ethernet
- Token ring require additional hardware and software to setup with direct cable connection while Ethernet supports direct cable connection between interfaces.
- Access of token is deterministic in token ring compared to Ethernet.
- Token ring technology use single token in the network to avoid collision of data packets in the network while Ethernet use carrier sense multiple access and intelligent switch.
- Token ring support multiple identical MAC addresses while Ethernet doesn’t support duplicate MAC address.
- Token ring was more complex than Ethernet.
- In Token ring network cards and router ports were very expensive compared to Ethernet.
- In Token ring network certain stations could have priority over token by ‘access priority’ while Ethernet provide equal priority to every connected nodes in its network.