NEC Network Controller uPD98502 - Manual (Page 234)

CHAPTER 4 ATM CELL PROCESSOR
234 Preliminary User’s Manual S15543EJ1V0UM
Figure 4-4. ATM Cell
VCI
GFC
Segment
48 byte
VPI
VPI
PTI
HEC
CLP
header
1
2
3
4
5
6
7
8
bit
The function of each field in the header is as follows:
(a) GFC (General Flow Control) field: Used for flow control. At transmission, the value set in the packet
descriptor is written into this field. At reception, this field is ignored.
(b) VPI/VCI fields: VPI (Virtual Path Identifier), VCI (Virtual Channel Identifier) are routing fields which indicate
the routing path. ATM Cell Processor supports a total of 64 VPI/VCI combinations for transmission and
reception. For transmission, the 24-bit value set in the Tx VC table is written into these fields. For
reception, only the VPIs/VCIs registered in the reception lookup table are allowed for reception.
(c) PTI (Payload Type Indication) field: 3-bit field indicating whether the cell payload is user data or
management data. It also contains congestion information.
PTI Usage
000 user data cell, no congestion, SDU type = 0
001 user data cell, no congestion, SDU type = 1
010 user data cell, congestion occurred, SDU type = 0
011 user data cell, congestion occurred, SDU type = 1
100 OAM F5 flow cell
101 OAM F5 flow cell
110 reserved for future use
111 reserved for future use
Here,
SDU type = 0: all segment except last cell of AAL-PDU
SDU type = 1: last cell of AAL-PDU. In this segment, trailer is included.
However, setting SDU type = 1 by user is prohibited.
OAM F5 flow cell: The cells for Operation, Administration and Maintenance.
(d) CLP (Cell Loss Priority) field: Indicates whether this cell is to be discarded preferentially if the network is
congested. A CLP value of 1 indicates that the cell is to be discarded preferentially. ATM Cell Processor
sets the appropriate value in this field according to the CLPM field of the packet descriptor.
(e) HEC (Header Error Control) field: Used for cell delineation, header error detection and correction. This field
is processed in TC sublayer.