
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
97
HCS Generator
The HCS Generator performs a CRC-8 calculation over the first four header octets. A parallel
implementation of the polynomial, x8+x2+x+1, is used. The coset polynomial, x6+x4+x2+1, is
added (modulo 2) to the residue. The HCS Generator optionally inserts the result into the fifth
octet of the header.
11.25.2 TTDP Packet Processor
The TTDP Frame Processor provides rate adaptation by transmitting flag sequences (0x7E) or
Idle sequences (0xFF) between packets, provides FCS generation and insertion, performs byte/bit
stuffing and data scrambling, and provides performance monitoring functions.
PPP/HDLC Frame Generator
The PPP/HDLC Frame Generator generates packets whose format is shown in Figure 8. Flags or
Idle sequences are inserted whenever the Transmit FIFO (TXSDQ) is empty and there is no data
to transmit. When there is enough data to be transmitted, the block operates normally; it removes
packets from the Transmit FIFO and transmits them. In addition, FCS generation, error insertion,
bit stuffing (for DS3) or byte stuffing, and scrambling can be optionally enabled.
In the event of a FIFO underflow caused by the TXSDQ FIFO being empty while a packet is
being transmitted, the packet is aborted by transmitting the Abort Sequence. The PPP Abort
Sequence consists of an Escape Control character (0x7D) followed by the Flag Sequence (0x7E).
The bit synchronous HDLC Abort Sequence is 11111111. Bytes associated with this aborted
frame are still read from the FIFO but are discarded and replaced with the Flag Sequence in the
outgoing data stream. Transmission of data resumes when a Start of Packet is encountered in the
FIFO data stream.
The POS Frame Generator also performs Inter Packet Gapping. This operation consists of
inserting a programmable number of Flag and Idle Sequence characters between each PPP/HDLC
Frame transmission. This feature allows one to control the system effective data transmission rate
if required.
FCS Generator
The FCS Generator performs a CRC-CCITT or CRC-32 calculation on the whole POS frame,
before byte stuffing and data scrambling. A parallel implementation of the CRC polynomial is
used. The CRC algorithm for the frame checking sequence (FCS) field is either a CRC-CCITT or
CRC-32 function. The CRC-CCITT is two bytes in size and has a generating polynomial g(X) =
1 + X
X + X
transmitted is the coefficient of the highest term. When transmitting a packet from the Transmit
FIFO, the FCS Generator appends the result after the last data byte, before the closing flag. Note
that the Frame Check Sequence is the one's complement of the CRC register after calculation
ends. FCS calculation and insertion can be disabled.
5
+ X
2
+ X
12
+ X
4
+ X
16
. The CRC-32 is four bytes in size and has a generating polynomial g(X) = 1 +
5
+ X
7
+ X
8
+ X
10
+ X
11
+ X
12
+ X
16
+ X
22
+ X
23
+ X
26
+ X
32
. The first FCS bit