IDT Switch Description
Revision 1.5
Integrated Device Technology, Inc.
CPS-16/12/8 User Manual
3 - 4
July 10, 2012
3.4.2 Output Buffer to Transmit Buffer and Transmit Buffer to Line
3.4.2.1 Unicast Packets
If a tx port fails in an attempt to transmit a priority N packet (call it PA) due to CRC error, and there is a
priority >N Packet (call it PB) available at the port’s retransmit buffer, then that higher priority packet PB
must be the next one sent. Specifically, if tx port A transmits a packet of priority N (call it PA) but receives a
“not accepted” response, then the next packet transmitted after receiving the response must be a packet of
priority > N if one is available in the retransmit buffer. Since the “not accepted” result is due to CRC error,
then the original packet PA will be retransmitted up to X times (See port operation register, bit 3-1, number
of retransmissions). Before each of the X attempts, the retransmit buffer will be reviewed to verify that there
are no priority > N packets available. If after X attempts the original packet PA is still not accepted, then PA
will be discarded and transmission of the next packet in tx port A’s retransmit buffer will be attempted. If the
“not accepted” cause by the traffic flow control such as the endpoint is busy or endpoint input buffer is full,
then PA will remain in retransmit buffer and keep retransmit until PA pass through. The following is standard
behavior:
1) The retransmit buffer must have room for at least one maximum size packet of each of the four
priorities. If the highest priority packet in the retransmit buffer is priority N, then the retransmit buffer
must reserve space sufficient to store at least S maximum size packets, where S = 3-N. The mini-
mum size retransmit buffer that can meet this requirement is 4 times the maximum packet size.
2) If the highest priority packet in the retransmit buffer has priority N, and a packet of priority > N is
being offered to the retransmit buffer by the switch's TXBUFs, then the retransmit buffer must
accept the packet from the TXBUFs. This is true even if the tx port is blocked by a packet of priority
N or less.
3) If TX/RX BUFs has lower priority packet A in buffer and continuously received higher priority
packet which reach the port bandwidth, then the lower priority packet will continuously hold off until
higher priority packet bandwidth drop below the port bandwidth. The same rule apples to all ports.
Also, the following behavior is standard:
1)
Not blocking with the same priority.
If rx port B receives a packet (call it PC) of priority M
(where M <= N) targeted for tx port A, but port A cannot receive it due to the conflict resulting from
PA described above, then PC will remain in the input buffer. All subsequent packets received at rx
port B of priority M or higher targeted for other tx port will still switch over. For the subsequent
packet with priority < M targeted for ANY tx port will delayed until packet PB can be forwarded to
the switch.
3.4.2.2 Multicast Packets
In general, multicast packets will be treated similarly to unicast packets. Specifically, if tx port A transmits a
multicast packet of priority N (call it PA) but receives a “not accepted” response, then the next packet trans-
mitted after receiving the response must be a packet (unicast OR multicast) of priority > N if one is available
in the retransmit buffer. If the “not accepted” cause by the CRC error, then the original packet PA will be
retransmitted up to X times (See port operation register, bit 3-1, number of retransmissions). Before each of
the X attempts, the retransmitted buffer will be reviewed to verify that there are no priority > N packets avail-
able. If after X attempts the original packet PA is still not accepted, then PA will be discarded and transmis-
sion of the next packet in tx port A’s retransmit buffer will be attempted. If the “not accepted” cause by the
traffic flow control such as the endpoint is busy or input buffer is full, then PA will remain in retransmit buffer
and keep retransmit until PA pass through. The following is standard behavior:
1) The retransmit buffer must have room for at least one maximum size packet of each of the four
priorities. If the highest priority packet in the retransmit buffer is priority N, then the retransmit buffer
must reserve space sufficient to store at least S maximum size packets, where S = 3-N. The mini-
mum size retransmit buffer that can meet this requirement is 4 times the maximum packet size. It is
not necessary to distinguish between unicast and multicast packets in filling these buffers. They
may be filled with 4 unicast packets, 4 multicast packets, or any combination of unicast and multi-
cast packets.