IBM ATM OC-3c - Manual (Page 26)

2-14 C HAPTER 2: N ETWORK L AYER C ONCEPTS
The ATM Layer and Cell Structure
This section describes the cell structure, and how the
ATM Layer uses the information stored in the cell
header to perform each of its tasks.
The ATM Layer’s primary responsibility is to manage
the sending and receiving of cells between the user
and the network.
The ATM Layer accepts the user data and control
information from the ATM Adaptation Layer, adds the
cell header, and passes the resulting 53 byte cell to
the physical layer.
In addition, it also receives cells from the physical
layer, strips off the cell header and passes the
remaining 48 bytes to the higher layer protocols.
The ATM cell has 48 bytes of payload (information to
be carried) and five bytes of header information,
making the cell 53 bytes in length.
The cell header contains the information used by the
network to forward each cell to its destination. The
ATM cell structure is shown in Figure 2-10.
Figure 2-10 ATM Cell Structure
The ATM cell header consists of the following fields:
Generic Flow Control (GFC) — Provides local
functions, such as flow control over the
User-to Network Interface (UNI). The value encoded in
the GFC is not carried end-to-end and can be
overwritten by the ATM Switch.
Virtual Path Identifier (VPI) and Virtual Channel
Identifier (VCI) — The VPI/VCI values allow the
network to associate a cell with a given connection,
so that the cell can be switched to its destination.
Payload Type Identifier (PTI) — The PTI is used to
indicate whether the cell contains user information,
or management information. The management
information is used for resource and network
congestion management.
Cell Loss Priority (CLP) — The purpose of the Cell
Loss Priority (CLP) bit in the ATM cell is to indicate that
cells with this bit set should be discarded before cells
which do not have the CLP bit set. Cells can be
discarded based on CLP condition and according to
the network load. When the network overloads, a
discard mechanism, based on the value of the CLP bit
in the cell header, may come into operation.
Header Error Check (HEC) — The HEC field is used
for detecting bit errors in the cell header. It is also
used for cell delineation, defining where the cell
begins in a SONET frame.