Flow control strategies are developed to allow to smooth network traffic without causing congestion or deadlock situations.
PACKET COLLISION RESOLUTION
To move a flit between adjacent nodes , 3 elements must be present:
- Source buffer holding the flit
- Receiver buffer accepting the file
- the channel being allocated
when two packets reach the same node, they may request the same outgoing channel.
There are four methods for resolving the conflict between 2 packets competing for the use of the same outgoing channel.
They are:
- VIRTUAL CUT-THROUGH ROUTING
- BLOCKING FLOW CONTROL
- DISCARD AND RE-TRANSMISSION
- DETOUR
VIRTUAL CUT-THROUGH ROUTING
Packet 2 is temporarily stored in a packet buffer. When the channel becomes available later,it will be transmitted then. One advantage of it is that it does not waste resources, but it needs a large amount of buffer storage.
Virtual cut-through routing combines both store and forward and wormhole schemes.When collision occurs wormhole routing is used otherwise store and forward routing is used.
BLOCKING FLOW CONTROL
Pure wormhole routing use a blocking policy in case of packet collision. The second packet is being blocked from advancing ; however it is not being abandoned.
DISCARD AND RE-TRANSMISSION
It simply drops the packet being blocked from being transmitted. It may result in a serve waste of resource, and it demands packet retransmission and acknowledgement. This policy is rarely used now.
DETOUR
The blocked packet is routed to a detour channel. It may waste more channel resources.
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