4. Internal Switching Fabric > Functional Behavior
87
Tsi578 User Manual
June 6, 2016
Integrated Device Technology
www.idt.com
4.2.1
Transfer Modes
The ISF supports both cut-through and store-and-forward transfer modes. These modes are selectable
on a per-port basis. By default, all ports are configured for store-and-forward mode. To change the
configuration, write the TRANS_MODE field in the
“RapidIO Port x Control Independent Register”
when traffic is not flowing through the port.
4.2.1.1
Store-and-Forward Mode
When a port is configured for store-and-forward mode, the port must receive the entire packet before
the ISF delivers the packet to an egress port. This increases the latency of all packets received on the
port. The increase in latency is directly proportional to the packet size and bit rate of the port.
4.2.1.2
Cut-through Mode
When a port is configured for cut-through mode, the port is permitted to start sending the packet before
the packet has fully arrived at the Tsi578. This is possible because the RapidIO destination ID (routing
information) appears near the front of a RapidIO packet.
Congestion
Configuring a port for cut-through mode does not guarantee that the packet is sent to the ISF
immediately after the destination ID arrives for the packet. Congestion in the ISF can mean that some
or all of the packet is received before the switching operation begins.
Cut-through mode, generally, provides better system performance. However, in cases where there is a
mix of high-speed and low-speed ports, a packet sent from a low-speed port to a high-speed port in
cut-through mode prevents the high-speed port from maximizing its output bandwidth. If other ports
are also sending to the same destination, the high speed ingress ports could suffer a drop in throughput.
Congestion Example
In this congestion example the following parameters are true:
•
Port 0 is currently sending Packet #1 to port 2
•
Packet #2, also destined for port 2, starts to arrive on port 1
Packet #2 must wait for the Packet #1 to finish before it has access to port 2. Some or all of Packet #2
must be buffered.
Ti
p
In store and forward mode, the incoming packet is not sent to the ISF until the whole packet is
received.
Ti
p
In cut-through mode, the incoming packet is forwarded through the switch as soon as the
routing information is received.