In X.25 (1980) there are 14 types of packet (more defined in ' 84)
- call
- data
- RR
- RNR
- REJ
- RESET
- RESET CONFIRMATION
- INTERRUPT
- INTERRUPT CONFIRMATION
- CLEAR REQUEST
- CLEAR CONFIRMATION
- RESTART REQUEST
- RESTART CONFIRMATION
5 - Call Packet
5.1 CALL REQUEST PACKET
INCOMING CALL PACKET
CALL ACCEPTED PACKET
CALL CONNECTED PACKET
call is now up and data can be sent
The call request packet is really the same as an incoming call packet, it is known as a call request packet when it goes from the customers equipment to the network and an incoming call packet when it goes from the network to the customers equipment.
DATA PACKET
distinguishes packet type, in the case of data packets is coded as follows:
more data bit - used when packet is full to say next packet is continuation
of this one
Note: an even number packet type always represents a DATA packet.
rr packet
rnr packet
REJ packet
RESET REQUEST PACKET
| RESET INDICATION PACKET |
| RESET CONFIRMATION PACKET |
An SVC can be reset at any time by either DTE or by the network.
Reset by DTE
DTE | DCE DCE | DTE |
<-- RESET INDICATION 00dd | <-- RESET REQUEST 00dd | |
RESET CONFIRMATION | RESET CONFIRMATION |
Reset by Network
DTE | DCE DCE | DTE |
RESET INDICATION ccdd | RESET INDICATION ccdd | |
RESET CONFIRMATION | RESET CONFIRMATION |
5.7 INTERRUPT PACKET
| INTERRUPT CONFIRMATION PACKET |
5.8 CLEAR REQUEST PACKET
| CLEAR INDICATION PACKET |
| CLEAR CONFIRMATION PACKET |
An SVC can be cleared at any time by either DTE or by the net-
work.
Clear by DTE
DTE | DCE DCE | DTE |
CLEAR INDICATION 00dd | CLEAR REQUEST 00dd | |
CLEAR CONFIRMATION | CLEAR CONFIRMATION |
Clear by Network
DTE | DCE DCE | DTE |
CLEAR INDICATION ccdd | CLEAR INDICATION ccdd | |
CLEAR CONFIRMATION | CLEAR CONFIRMATION |
restart REQUEST PACKET
| RESTART INDICATION PACKET |
| RESTART CONFIRMATION PACKET |
5.9.1 Packet Level initialisation
Once the link is established then the packet level is initialised. This can
be done by either end,
Restart by network:
customer network
RESTART IND 07FA | cause 07 network operational |
RESTART CONF | diagnostic FA line status change |
Restart by customer:
customer network
RESTART REQ 00xx
RESTART CONF
Restart collision:
RESTART REQ
RESTART IND
This ensures level 3 starts in a tidy state with all calls cleared, and keeps
level 2 and level 3 independent.
Following a link level failure, calls should not be made or accepted until
a Restart exchange has occurred. see section on level 2/3 interface.
Restart - Restart Conf exchanges can be done at any time and will have the
effect of clearing all calls on the link.
Valid Restart Causes for the network to send are as follows:
Hex | cause |
01 | local procedure error |
03 | Network Congestion |
07 | Network Operational |
Note: RESTART and RESTART CONF are the only packet types which affect all calls on the link and so they always use GFI=10, LCN=0, see section on logical channel numbers for details.