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11-4
C
11: ATM N
HAPTER
ETWORKS
GFC
VPI
VCI
VCI
HEC
User Data
Figure 11-2 UNI ATM Cell Structure
These fields make up an ATM cell:
Generic Flow Control (GFC)— Controls the flow of traffic across the
user-network interface (UNI) and into the network. The mechanisms for
using this field are still under development.
Virtual Path Identifier (VPI) and Virtual Channel Identifier (VCI) — The
VPI and VCI are addressing identifiers used to route cell traffic. Because of
their routing significance, the VPI and VCI addressing identifiers are
described further in the next section.
Payload Type Indicator (PTI) — A 3-bit field that indicates if the cell
contains user information or connection-associated layer management
information.
Cell Loss Priority (CLP) — Indicates that, under congested conditions, cells
with this bit set are discarded by the network before cells with the CLP bit
clear.
Header Error Control (HEC) — Used by the physical layer for detection of
bit errors in the cell header.
Following the HEC field is the 48-byte cell information field containing the
user data.
VPI
1
VCI
2
3
PTI
CLP
4
5
6
.
.
.
.
.
.
.
53
GFC — Generic Flow Control (on U
VPI — Virtual Path Identifier
VCI — Virtual Channel Identifier
PTI — Payload Type Identifier
CLP — Cell Loss Priority
HEC — Header Error Control
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