Telecommunications - Configuration of X.25 Networks

 

Quality of service parameters

General

All delays are specified for normal busy hour load and expressed, where appropriate, in terms of:

the mean value

the 95% probability value ie, the time interval in which 95% of packets sent to the network will be responded to by the network.

Delays across the network are measured from the receipt at the Exchange (PSE) of the last bit of the packet from the DTE to the insertion of the first bit of the packet into the output queue of the destination PSE. All figures given only apply to calls wholly set up within the National network.


Call request delay

The call set-up time is defined as the time from the receipt of the originating exchange of the last bit of the call request packet to the placing of the first bit of the call connected packet on the output queue of the calling DTE, but excluding the time from the placing of the first bit of the incoming call packet on the output queue to the called DTE to the receipt by the destination exchange of the last bit of the call accepted packet.

The network has been designed to offer a mean call set-up time for a call request packet of length 38 bytes, including the time taken to progress and switch the packet and to record accounting and statistical information, routed over a connection consisting of 5 * 48 Kbit links in tandem, of less than 400ms and for 95% of such calls of less than 480ms.


DATA packet transfer delay

The delay of the DATA packet across UK may include one or several retransmissions for error correction. This delay is variable and depends on the routing, the trunk speed, traffic loading etc.

The network has been designed to provide a mean delay of a DATA packet of (3+128) octets across UK of less than 360ms and the 95% probability of less than 400ms.


Established Call Clear

The network may clear a call during the DATA TRANSFER phase because of congestion by sending a CLEAR INDICATION packet to the DTE. The network has been designed to ensure the probability of this occurrence should not exceed 6E-6 per second.

 

Spontaneous Resets

The network may reset the call because of congestion by sending the appropriate indication to the DTE. The network has been designed to ensure the probability of this occurrence should not exceed 6E-6 per second.


Timers

T10 UK value: 60 secs

started: DCE sends Restart

stopped: DTE sends Restart conf

action: send Restart local procedure error

T11 UK value: 180 secs

started: DCE sends Incoming call

stopped: call accept or clear

action: send clear local procedure error

and remote procedure error to distant end

T12 UK value: 90 secs

started: DCE sends Reset indication

action: SVC sends clear indication to local and remote DTEs with clearing cause of local and remote procedure error respectively

T13 UK value: 180 secs

non CCITT timers

UK value: 90 secs

started: DCE receives restart request

UK value: 180 secs

started: DCE receives call request

UK value: 90 secs

started: DCE receives Reset request

UK value: 90 secs

started: DCE receives Clear request

T20 UK value 180 secs

started when DTE sends a Restart request

terminated when: DTE receives restart conf or restart

action on expirey: DTE retransmits the restart request and restarts T20 up to maximum or R20 times

T21 UK value 200 secs

started when DTE sends a Call request

terminated when: DTE receives call connected,incoming call,

clear indication or clear

action on expirey: DTE transmits a clear request packet

T22 UK value 180 secs

started when DTE sends a Restart request

terminated when: DTE receives reset conf or reset ind.

action on expirey: DTE transmits the reset request and restarts T22 up to a maximum of R22 times.

T23 UK value 180 secs

started when DTE sends a Clear request

terminated when: DTE receives clear confirmation or clear ind action on expirey: DTE transmits the clear request and restarts T23 up to a maximum of R23 times.

T24 optional use

default value: 60 secs

window status transmission timer

started when DTE sends a packet with a P(R).

terminated when:

action on expiry: DTE transmits a RR or RNR packet reflecting the current window condition and restarts T24

T25 optional use

default value: 150 secs

window rotation timer

started when DTE sends a DATA packet.

terminated when: there are no outstanding DATA packets in the window action on expirey: DTE retransmits all DATA packets in the window and restarts T25 up to a maximum of R25 times

T26 default value: 180 secs

interrupt response timer

started when DTE sends a INTERRUPT packet.

terminated when: DTE receives an INTERRUPT CONFIRMATION packet action on expirey: DTE transmits a reset request

T27 default value: 60 secs

reject response timer

started when DTE sends a REJECT packet.

terminated when: DTE receives the first retransmitted DATA packet. action on expirey: DTE retransmits the REJECT packet and restarts T27 upto a maximum of R27 times.

T28 CCITT 1984 ONLY

default value: 300 secs

registration request response timer

started when DTE sends a REGISTRATION REQUEST packet.

terminated when: DTE receives a REGISTRATION CONFIRMATION packet. action on expirey: DTE retransmits the REGISTRATION REQUEST packet and restarts T28 upto a maximum of R28 times.


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