
Multi-Service Access Device For Channelized Interfaces
Telecom Standard Product Data Sheet
Production
Proprietary and Confidential to PMC-Sierra, Inc., and for its Customers’ Internal Use
Document ID: PMC-1990823, Issue 4
570
Figure 22 A 63 Byte Packet Data Structure
Bit 31 Bit 16
Bit 15 Bit 0
Word 1
Byte 1/SOP
Byte 2
Byte 3
Byte 4
Word 2
Byte 5
Byte 6
Byte 7
Byte 8
Word 3
Byte 9
Byte 10
Byte 11
Byte 12
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
Word 15
Byte 57
Byte 58
Byte 59
Byte 60
Word 16
Byte 61
Byte 62
Byte 63/EOP
Unused
Both the start of the packet and the end of the packet must be identified by the TSOP/RSOP and
TEOP/REOP signals. When the first section of a packet is transferred over the interface, the
TSOP/RSOP signals will be high for Byte 1 of the packet only. The TMOD[1:0] pins will
indicate how many bytes of the final word are valid.
Bits 31 to 24 form the first transmitted byte and bit 31 can be configured to be the bit which is
transmitted first. This is the desired set-up for byte-synchronous POS. Bit 24 can also be
configured to be the first transmitted bit of this byte. This is the desired set-up for bit-
synchronous HDLC.
14.8.1 Limitation When Using Externally Generated FCS in STS-48c Mode
When the S/UNI-MACH48 is set up in STS-48c POS mode (TX48C is logic 1) and FCS bytes
are passed through the transmit POS-PHY L3 interface, the overall throughput is reduced. The
maximum bandwidth throughput will be reduced by a maximum of 2 bytes per packet. The
overall effect will depend on the length of the packets + FCS bytes being transferred through the
POS-PHY L3 interface. If the packet + FCS length is evenly divisible by 4 bytes, no bandwidth
is lost for that packet.
14.8.2 Limitations on Small Packets
The S/UNI-MACH48 cannot handle packet payloads (excluding FCS) which are smaller than 4
bytes.
In the receive direction, the RCFP and RTDP should be programmed such that the minimum
packet size is at least 4 bytes + FCS. Packet payloads of size 3 bytes or less are tagged by the
RCFP and RTDP packet processors and marked as minimum length violations. For
configurations where the FCS is passed through the PL3 bus, the FCS byte count should be
treated as payload bytes for these minimum packet size calculations.
In the transmit direction, the minimum packet payload size permitted is 4 bytes. The exception to
this is when the packet is aborted by use of the TERR pin. Aborted packets can be less than the 4
byte minimum. The S/UNI-MACH48 will extend these packets to 4 bytes before tagging with
the appropriate abort sequence.