Information Technology Reference
In-Depth Information
Figure22-3 AAL5 Process
AAL5 CPCS - PDU frame = n x 48 bytes
Control
(CPI and UU)
Payload
length
User
Data
CRC-32
0-47
bytes
2 bytes
2 bytes
4 bytes
0-65535 bytes
AAL5 CPCS PDU trailer
SAR PDU payload = 48 bytes
AAL5 SAR PDU 1 cell payload
Higher layers
Frame
Convergence
sublayer
CS PDU
trailer
CS PDU payload
SAR
sublayer
SAR PDU
payload
SAR PDU
payload
SAR PDU
payload
SAR PDU
payload
AT M
layer
0
payload
0
0
1
ATM cell
At the destination router, the reverse process is being applied. The destination router can easily find
which cell is the end of the AAL5 packet with a special bit set to 1 in the cell header.
The whole process is usually implemented in hardware and works efficiently. For now, let's focus on
what can go wrong. Two main problems that can arise are described here:
One or more cells are corrupted at the destination by either the transmitter or a device in the ATM
network. The only field in the cell that performs a type of cyclic redundancy check (CRC) is the
Header Checksum field (HEC). As the name suggests, it checks only the cell header.
One or more cells could be discarded in the provider's network.
Let's examine the impact of those two problems at the destination router and determine how we can
detect them:
If one cell is corrupted, the number of cells is still the same. The CPCS-PDU frame is reassembled
with the correct size. The router checks whether the length field is indeed correct. But, because one
cell is corrupted, the whole frame will be trivially corrupted. Therefore, the CRC field of the AAL5
CPCS-PDU frame will be different from the one that was originally sent.
If one cell is missing at destination, both the size and the CRC will be different from the ones that
are contained in the CPCS-PDU frame.
You will see that the CRC error counter of the show interface is equal to the number of input
error. On some end systems, such as the LANE modules of the Catalyst 5000, only the input
error counter increases. Focusing on the input errors is then recommended. As a rule of
thumb, if you are not running a recent release, checking the output of sh controller is also
recommended because it gives more physical details on the counters of the ATM card itself .
Note
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