![](http://datasheet.mmic.net.cn/380000/-PD98501_datasheet_16745028/-PD98501_270.png)
CHAPTER 4 ATM CELL PROCESSOR
270
Preliminary User’s Manual S14767EJ1V0UM00
4.1.3.1 AAL-5 SAR sublayer function
When ATM Cell Processor transmits a cell, it adds a trailer to the variable-length data, as well as padding, so that
their overall length becomes a multiple of 48 bytes, thereby generating an AAL-5 PDU. When ATM Cell Processor
receives a cell, it builds up a CPCS PDU from cells received and store it in the SDRAM. ATM Cell Processor checks
the trailer if the error has occurred and informs the result to V
R
4120A RISC Processor with Rx indication.
Figure 4-3. AAL-5 Sublayer and ATM Layer
User Data
0-64kbyte
Padding
CPC
S-UU
CPI
Length
CRC-32
trailer
0-47 bytes 1 byte
1 byte
2 bytes
4 bytes
segment
segment
segment
48 bytes
48 bytes
48 bytes
(a) Padding field: Field of 0 to 47 bytes that is inserted between the user data and the trailer to adjust the
length of the resulting packet to a multiple of 48 bytes. Angel writes zeros to all its bits.
(b) CPCS-UU field: Used to transfer user information. The value set in the packet descriptor by the
host is written in this field.
(c) CPI field: The use of this field has yet to be finalized. According to the current specifications, all its bits must
be set to zeros. ATM Cell Processor, however, writes the value set in the packet descriptor into this field, as
it does to the CPCS-UU field.
(d) Packet length (Length) field: Indicates the user data length in bytes, in binary notation.
(e) CRC-32 field: The CRC-32 value calculated for the range from the user data to the end of the Length field
is set in this field. Generation polynomial is following
G(x) = 1+x+x
2
+ x
4
+ x
5
+ x
7
+ x
8
+ x
10
+ x
11
+ x
12
+ x
16
+ x
22
+ x
23
+ x
26
+ x
32