Telecommunications - Level 3 X.25 - packet types

In X.25 (1980) there are 14 types of packet (more defined in ' 84)



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.


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