08-02-2013, 11:21 AM
Direct Link Networks (Continued)
1Direct Link .ppt (Size: 71.5 KB / Downloads: 73)
Token Ring
Set of nodes are connected in a ring.
Data always flows in one direction
Node receiving frames from upstream neighbor passes it to downstream neighbor.
Features
Distributed algorithm dictates when each node can transmit.
All nodes see all frames : destination saves a copy of the frame when it flows past.
Token used to control who transmits.
The Token
The token allows a host to transmit -- contains a special sequence of bits.
When a node that wishes to send sees the token, it
picks up the token
inserts its own frame instead on the ring.
When frame traverses ring and returns, the sender takes frame off and reinserts token.
Some details
Remember -- nodes use network adaptor.
Contains a receiver, transmitter and storage.
802.5 token is 24 bits in size.
Total storage in the network must be enough to hold token.
Medium Access Control
Token circulates around the ring.
Every station that has data to send may seize the token off the ring
Modify 1 bit in the first two bytes
these now become the preamble for the subsequent packet
Transmitted packet contains dest addr.
Each node looks at the packet -- if packet meant for the node, copy packet onto buffer as it flows through the adaptor.
Sending node responsible for removal of frame from ring.
Token Holding Time
How long can a node hold onto the token ? Dictated by the token holding time or THT.
If lightly loaded (as in Ethernet) you may allow a node to hold onto it as long as it wants -- very high utilization.
But it is unfair ... multiple nodes might have data to send and if we do this, heavily loaded nodes are at an advantage.
Protocol for setting priorities
Say, X has a priority “n”.
If it sees d data frame going past and the bits haven’t already been set to a higher value, it can set the bits.
When the station that sent the frame gets back the frame, it sets the priority bits in the token to reflect the value in the reservation bits.
Any node with that priority can seize the token.
Strict Priority: No lower packets may be sent when higher priority packets await.