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CHAPTER 4 ATM CELL PROCESSOR
Preliminary User’s Manual S15543EJ1V0UM
233
4.1.3.1 AAL-5 SAR sublayer function
When ATM Cell Processor transmits a cell in AAL-5 mode, it adds a trailer to the variable-length data, as well as
padding, so that its overall length becomes a multiple of 48 bytes, thereby generating an AAL-5 PDU. When ATM Cell
Processor receives cells, it stores them in the SDRAM in order to assemble a CPCS PDU. ATM Cell Processor
verifies the trailer of the assembled CPCS PDU. If the errors have occurred, the ATM Cell Processor informs the
result to V
R
4120A RISC Processor with Rx indication.
Figure 4-3. AAL-5 Sublayer and ATM Layer
User Data
0 to 64 kbytes
Padding
CPCS-
UU
CPI
Length
CRC-32
trailer
0 to 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. ATM Cell Processor 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
4.1.3.2 ATM layer function
(1) Traffic classes
ATM Cell Processor supports 3 traffic classes; CBR (Constant Bit Rate), VBR (Variable Bit Rate) and UBR
(Undefined Bit Rate).
(2) Generation a cell
ATM Cell Processor generates a cell by adding 5 bytes header to the segment as the figure shown below.