System information

Troubleshooting ATM Switching Environments 21-437
The ATM Adaptation Layer
Table 21-2 Fields in an ATM Cell Header
The ATM Adaptation Layer
The AAL accepts frames from higher OSI layers and adapts them to the 48-byte segments that are
placed into the Payload field of ATM cells. The ATM layer accepts the 48-byte segments, adds the
5-byte header, and produces ATM cells to be transferred to the physical layer, as illustrated in
Figure 21-10.
Figure 21-10 ATM Adaptation Layer Functions
Header Field Name Location in Header Description
GFC
1
1 GFC = generic flow control. For a network-to-node (NNN) interface, there is no GFC field. These 4 bits are part of the VPI field.
First 4 bits of
Byte 1
Controls the flow of traffic across the user network interface and
thus into the ATM network.
VPI
2
2 VPI = virtual path identifier
Second 4 bits of Byte 1
and the first 4 bits of
Byte 2
Identifies a particular VPC
3
. A VPC is a group of virtual
connections carried between two points and may involve several
ATM links. VPIs provide a way to bundle traffic heading to the
same destination.
3 VPC = virtual path connection
VCI
4
4 VCI = virtual channel identifier
Second 4 bits of Byte
2, Byte 3, and the first
4 bits of Byte 4
Identifies a particular VCC
5
. A VCC is a connection between two
active, communicating ATM entities. The VCI consists of a
concatenation of several ATM links.
5 VCC = virtual channel connection
PT
6
6 PT = payload type
The fifth, sixth, and
seventh bits of Byte 4
Indicates the type of information in the payload field. ATM cells
carry different types of information that may require different
handling by the network or terminating equipment.
CLP
7
7 CLP = cell loss priority
The eighth bit of
Byte 4
Indicates the cell loss priority set by the user. This bit indicates
the eligibility of the cell for discard by the network under
congested conditions. If the bit is set to 1, the cell may be
discarded by the network if congestion occurs.
HEC
8
8 HEC = header error control
Byte 5 Contains an error-correcting code calculated across the previous
four bytes of the header. The HEC detects multiple-bit header
errors and can be used to correct single-bit errors. The HEC
provides protection against incorrect delivery of messages caused
by address errors. The HEC does not provide any protection for
the payload field itself.
AAL process
48
bytes
User data
53-byte
ATM cells
To
physical
layer
48
bytes
ATM process
48
bytes
5-byte
header
48
bytes
5-byte
header
Data link layer
Layer 2: