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2-10 Cisco BPX 8600 Series Reference
Functional Description
Figure 2-10 STI Header
The most important fields in all three ATM cell header types are the Virtual Path Identifier (VPI) and
a Virtual Circuit Identifier (VCI). The VPI identifies the route (path) to be taken by the ATM cell
while the VCI identifies the circuit or connection number on that path. The VPI and VCI are
translated at each ATM switch, they are unique only for a given physical link.
A 4-bit Generic Flow Control (GFC) field in the UNI header is intended to be used for controlling
user access and flow control. At present, it is not defined by the standards committees and is
generally set to all zeros.
A 3-bit Payload Type Indicator (PTI) field indicates the type of data being carried in the payload.
The first bit is a “0” if the payload contains user information and is a “1” if it carries connection
management information. The second bit indicates if the cell experienced congestion over a path. If
the payload is user information, the third bit indicates if the information is from Customer Premises
Equipment. The PTI field is identical for UNI/NNI/STI.
HCF
VPI
VPI VCI
VCI Payload class
CC
FR
PTI
CLP
HCS
HCF: Header Control Field, a 01 indicates an STI Cell
VPI/VCI: Virtual Path/Virtual Channel Identifiers, same
as UNI and NNI.
Payload Class:
0001 Non-Timestamped Data/Constant BIt Rate
0010 High Priority/Variable Bit Rate
0011 Voice/Constant BIt Rate
0100 Bursty Data A/Variable BIt Rate
0101 Time-Stamped Data/Constant BIt Rate
0110 Bursty Data B/Variable BIt Rate
CC: Congestion Control
00: No report 10: Congestion
01: Uncongested 11: Severe Congestion
PTI, bits 4,3, and 2:
bit 4 = 0, user data cell;
bit 4 = 1, connection management cell
bit 3 = 0, No congestion experienced
bit 3 = 1, Congestion experienced
bit 2 = 0, for user data cell, indicates CPE information
bit 2 = 1, not used
PTI Description
Bits
432
010 User Data Cell congestion experienced, SDU Type 0 (CPE information)
100 Connection Management Cell, OAM F5 Segment Flow Related cell
101 Congestion Management Cell, OAM F5 End-to-End Flow related cell
110 Connection Management Cell, reserved for future use.
111 Connection Management Cell, reserved for future use.
F: ForeSIght Forward Congestion
Indication (FFCI).
Set to 1 if FECN in Frame is a 1.
or if incoming cell FFCI is a 1, or
egress queue experiences congestion.
R: Reserved
PTI: Payload Type Indicator
CLP: Cell Loss Priority. Same as for UNI
or NNI. The CLP bit is set to 1 if the
DE is set for a frame, or if the first
FastPacket in a frame has its CLP set.
STI Header
H8149
000 User Data Cell no congestion experienced SDU Type 0 (CPE information)
001 User Data Cell no congestion experienced SDU Type 1
011 User Data Cell congestion experienced, SDU Type 1
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