PM5381-BI PMC-Sierra, Inc., PM5381-BI Datasheet - Page 93

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

10.7.8
10.7.9
Proprietary and Confidential to PMC-Sierra, Inc., and for its Customers’ Internal Use
Document ID: PMC-2000489, Issue 4
In the event of a FIFO overflow caused by the FIFO being full while a packet is being received,
the packet is marked with an error so it can be discarded by the system. Subsequent bytes
associated with this now aborted frame are discarded. Reception of PPP/HDLC data resumes
when a Start of Packet is encountered and the FIFO level is below a programmable Reception
Initialization Level.
POS Byte Destuffing
The byte destuffing algorithm searches the Control Escape character (0x7D). These characters,
listed in the table below are added for transparency in the transmit direction and must be
removed to recover the user data. When the Control Escape character is encountered, it is
removed and the following data byte is XOR’ed with 0x20. Therefore, any escaped data byte will
be processed properly by the S/UNI-2488.
Table 3 Byte Destuffing
POS FCS Check
The FCS Generator performs a CRC-CCITT or CRC-32 calculation on the whole POS frame,
after byte destuffing and data descrambling 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
polynomial g(X) = 1 + X + X
X
are marked as such and should be discarded by the system.
Figure 15 CRC Decoder
Original
7E (Flag Sequence)
7D (Control Escape)
Aborted Packet
Message
32
. The first FCS bit transmitted is the coefficient of the highest term. . Packets with FCS errors
+
D
5
0
+ X
2
12
+ X
+ X
Escaped
7D-5E
7D-5D
7D-7E
4
g
+
+ X
16
1
. The CRC-32 is four bytes in size and has a generating
5
+ X
7
D
+ X
1
8
S/UNI-2488 Telecom Standard Product Datasheet
+ X
10
g
+
+ X
2
11
. . .
+ X
12
+ X
g
+
n-1
16
+ X
22
D
+ X
n-1
23
+ X
Released
26
+
93