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4. Internal Switching Fabric > Packet Queuing 98
Tsi574 User Manual
June 6, 2016
Integrated Device Technology
www.idt.com
4.4.5 Input Queuing Model for the Broadcast Buffer
The broadcast buffer receives data from only one source - the multicast work queue.
The broadcast buffer operates in strict First-In, First-Out (FIFO) order. The broadcast buffer does not
use watermarks.
The broadcast buffer always operates in store-and-forward mode. For more information, refer to
“Store-and-Forward Mode” on page 85 .
4.4.5.1 Broadcast Buffer Ingress Flow Control
Like the multicast work queue, the broadcast buffer operates as a bounded buffer. Packets can begin at
any point within the bounded buffer. For more information on the operation of the broadcast buffer,
refer to
“Multicast” on page 101 .
The broadcast buffer has space for 1 maximum sized (276 byte) packet, or up to 8 smaller packets. Up
to 8 packets can be accepted, provided that their individual sizes, rounded up to the nearest multiple of
8 bytes, sum to less than 280 bytes.
The broadcast buffer ingress arbitration operates on the following two rules:
• If there is not sufficient space for 8 more bytes of data to be received, the multicast work queue is
signalled that no more packet data can be accepted by the broadcast buffer.
• If there is sufficient space for 8 more bytes of data to be received, the multicast work queue is
signalled that more packet data can be accepted by the broadcast buffer.
4.4.6 Output Queuing Model for Multicast
Both the multicast work queue and the broadcast buffer operate in First-In First-Out order. No packet
reordering is performed.
The multicast work queue always allows packets to cut-through to the broadcast buffers. This reduces
the latency of multicast operations.
Broadcast buffers always operate in store-and-forward mode. This ensures that packet transmission to
the egress port is never delayed by packet replication.
4.4.7 ISF Bandwidth
The ISF delivers full 10 Gbits/s of bandwidth in both transmit and receive directions. This is sufficient
to handle a 4x Serial RapidIO port operation at 3.125 Gbits/s. ISF packets can be sent back-to-back,
without interruption.
Delays due to arbitration only occur for the first packet to be sent after a period when no packets were
available for transmission. After the first packet has been sent, following packets can be sent back to
back.
IDT recommends that multicast packets within a system all have the same priority.