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IDT Switch Core
Notes
PES48H12G2 User Manual
Switch Core
Port 0 IFB
VC 0
Port 1 IFB
VC 0
Port 13
Port 15 IFB
VC 0
Port Arbitration
Each egress port does port arbitration among multiple ingress ports for packets. Each egress port
contains a port arbiter. Ingress port(s) that wish to transfer one or more packets to the egress port partici-
pate in arbitration. Prior to participating in port arbitration, each ingress port does packet ordering. Based on
this, each ingress port selects zero, one, or multiple packets as candidates for transfer towards the EFBs.
Port arbitration is done according to the configuration of the egress port's VC Capability Structure. The
PES48H12G2 ports operating in upstream switch port or downstream switch port mode support port arbitra-
tion using Hardware Fixed Round-Robin (default). The arbitration algorithm is programmed independently
for each of the ports via the VC Capability Structure located in the port's configuration space.
Cut-Through Routing
The PES48H12G2 utilizes a combined input and output buffered cut-through switching architecture to
forward PCIe TLPs between switch ports. Cut-through means that while a TLP is being received on an
ingress link, it can be simultaneously routed across the switch and transferred on the egress link. The entire
TLP need not be received and buffered prior to starting the routing process (i.e., store-and-forward). This
reduces the latency experienced by packets as they are transferred across the switch.
Typically, cut-through occurs when a TLP is received on an ingress link whose bandwidth is greater than
or equal to the bandwidth of the egress link. For example, a TLP received on a x4 Gen2 port and destined
to a x1 Gen2 port is cut-through the switch. This rule ensures that the ingress link has enough bandwidth to
prevent 'underflow' of the egress link. In addition to this, the PES48H12G2 does "adaptive cut-through",
meaning that packets are cut-through even if the egress link bandwidth is greater than the ingress link
bandwidth. In this case, the cut-through transfer starts when the ingress port has received enough quantity
of the packet such that the packet can be sent to the egress link without underflowing this link. The ingress
Port 0 Egress Arbitration
TC/
VC 0
VC
Port
Map
Arbitration
Port 13
Port 15 Egress Arbitration
TC/
VC 0
VC
Port
Map
Arbitration
Figure 3.2 Architectural Model of Arbitration
3 - 6
EFB
VC
Arbitration
VC 0
(not
applicable)
EFB
VC
Arbitration
VC 0
(not
applicable)
April 5, 2013
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