Atm Cell - NEC uPD98502 User Manual

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.
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.
000
001
010
011
100
101
110
111
Here,
SDU type = 0:
SDU type = 1:
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.
234
CHAPTER 4 ATM CELL PROCESSOR
Figure 4-4. ATM Cell
header
PTI
user data cell, no congestion, SDU type = 0
user data cell, no congestion, SDU type = 1
user data cell, congestion occurred, SDU type = 0
user data cell, congestion occurred, SDU type = 1
OAM F5 flow cell
OAM F5 flow cell
reserved for future use
reserved for future use
all segment except last cell of AAL-PDU
last cell of AAL-PDU. In this segment, trailer is included.
However, setting SDU type = 1 by user is prohibited.
Preliminary User's Manual S15543EJ1V0UM
Segment
48 byte
Usage
For