PM5381-BI PMC-Sierra, Inc., PM5381-BI Datasheet - Page 104

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PM5381-BI

Manufacturer Part Number
PM5381-BI
Description
ATM UNI, Single Channel ATM PHY Interface for 2488.32Mbps
Manufacturer
PMC-Sierra, Inc.
Datasheet

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10.15.5 POS FCS Generator
10.15.6 POS Byte Stuffing
Proprietary and Confidential to PMC-Sierra, Inc., and for its Customers’ Internal Use
Document ID: PMC-2000489, Issue 4
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.
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.
Figure 19 CRC Generator
An error insertion mechanism is provided for system diagnosis purposes. Error insertion is
performed by inverting the resulting FCS value, before transmission. This should cause an FCS
Error at the far end.
The PPP Frame generator provides transparency by performing byte stuffing. This operation is
done after the FCS calculation. Two characters are being escaped, the Flag Sequence (0x7E) and
the Escape Character itself (0x7D). When a character is being escaped, it is XORed with 0x20
before transmission and preceded by the Control Escape (0x7D) character.
Table 7 Byte Stuffing
Original
7E (Flag Sequence)
5
D
2
+ X
0
+ X
LSB
12
4
+ X
+ X
5
g
+
16
1
+ X
. The CRC-32 is four bytes in size and has a generating polynomial g(X) = 1 +
7
+ X
8
D
+ X
1
Parity Check Digits
10
Escaped
7D-5E
+ X
g
+
11
2
+ X
. . .
12
+ X
16
S/UNI-2488 Telecom Standard Product Datasheet
g
+ X
+
n-1
22
+ X
23
D
n-1
+ X
MSB
26
+ X
+
32
. The first FCS bit
Message
Released
104

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