This interface is designed where possible to comply with a number of recommendations, including the following,

CCITT X.25 International Telegraph and Telephone Consultative Committee, Geneva.

NET2 CEPT Bern, published by BSI in the UK.

X.25 is a CCITT recommendation which describes an interface to be provided by a packet switching network. This document is not intended as an X.25 tutorial, but as a guide to the options which are used on the UK network.

In the first part of this guide you will find an overview of the UK service, information about installation and configuring your equipment, facilities and options available. The remainder of the document contains more detailed information about the UK X.25 implementation.

How To Access the network

When you Order an X.25 line, a dataline is provided into your premises, the Modem is normally supplied by BT. You will need to connect this to a computer or terminal which supports the X.25 protocol and is compatible with UK. Most major computer manufacturers supply hardware and software for this purpose.

X.25 allows many simultaneous calls (known as virtual circuits) to be connected over one physical circuit.

X.25 lines can make calls to, or receive calls from,

* other X.25 lines.

* X.25 dial lines (X.32).

* Asynchronous terminals - using public or private PADs (Packet assembler-Disassembler).

* Other protocols, such as SNA when using compatible software of using the appropriate translation gateway.

&notes& = yTemporary notes for reviewers

This diagram was drawn at the height of Public Network expansion, I need to check wich sites still exist.

I also need to check which are UK(b) sites.

Configuring users equipment

Your equipment may have to be configured before it will work with UK, refer to your equipment suppliers instructions for information about how to do this. You may find the following values useful in order to get started:

Note: some suppliers may refer to the service as 'Public Network'.

Interface: your equipment should be configured as a DTE.

Use a V.24, V.35 or V.11 interface depending on the type of circuit you have ordered.

Level 2: Link initiation - UK(a) = 'DTE initiated SABM start'

UK(b) = 'DCE initiated'

Timer T1: 3 seconds

Timer T2: 0.5 second

Timer T3: 60 seconds

Timer T4: 12 seconds

N1 = 4320

N2 = 20

Window size = K = 7

Level 3: NUA - This will be supplied by UK when you order your


PVC or SVC - use SVC

Logical channels: Start at the following values:

Type of circuit Logical Channel Group Number (LCGN)

bothway 4

incoming 2

outgoing 6

Some manufacturers require the value to be entered as

a number as follows: LCN + LCGN*256

for example to enter 10 bothway channels use

1024 to 1033

Facility options should match the values your requested

on the UK order form.

International Access

UK is connected to a large number of packet switching networks around the world. The call is setup in the usual way by calling the Address (NUA) of the line you wish to access. The first 4 digits of the address is known as the DNIC, this is used to specify the country or network you wish to call. In general UK uses the following DNICs.

2342 for UK UK(a)

2343 for UK UK(b)

2344 for UK in Europe

3106 for UK in USA and rest of world.

Other networks can be called by using their DNICs.

The facilities that can be specified on a call request will depend on the type of gateway:

Countries on the UKb Global Network

End-to-end service within UK gives maximum facilities for UK(b) users.

Austria France New Zealand Switzerland
Belgium Germany Norway UK
Canada Italy Spain USA
Denmark Japan Sweden
Finland Netherlands

Countries accessible via the Enhanced Gateway

Provides many of the facilities as above, but is not end-to-end managed by British Telecom.

Antigua Bermuda Guatemala Peru Trinidad & Tobago
Antilles Cayman Islands Honduras Philippines Turks & Caicos
Argentina Chile Hong Kong Puerto Rico US Virgin Islands
Austria Columbia Hungary Saudi Arabia Uruguay
Bahamas Egypt Lebanon South Korea
Bahrain Greece N Marianas Taiwan
Barbados Guam Panama Tortola

Countries accessible via the Standard Gateway

This service is similar to the former International Public Network and provides a basic X.25 service to over 140 networks in over 100 countries worldwide.

Brazil Iceland Singapore
Bulgaria Ivory Coast South Africa
China Jamaica Thailand
Costa Rica Kuwait Togo
Cuba Liechtenstein Tunisia
Cyprus Luxembourg Turkey
Djibouti Malaysia United Arab Emirates
Dominican Republic Macau USSR
Faroe Island Mauritius Vanuatu
French Antiles Mexico Venezuela
French Guyana Namibia Yugoslavia
French West Indies New Caledonia Zimbabwe
Gabon Niger
Gambia Papua N.G.
Greenland Polynesia
Ireland Portugual
India Qatar
Indonesia Reunion Island
Israel San Marino
Iraq Senigal


Charges are made up of the following parts:

* Dataline rental.

* Call charges - Data.

- Connect time.

See **** for details of the charges.

&notes& = yTemporary notes for reviewers

The usages charges are calculated differently for the two networks:

UK(a) - The user is charged per segment, which is a 64 byte unit, it costs the same to send a segment whether it contains one byte or 64 bytes.

UK(b) - The user is charged per byte, regardless of how efficiently bytes are packed into segments.

The charges will be set to give the same average charge for the two networks, however users who pack their data more efficiently into segments will be worse off when they move from UK(a) to UK(b), users who send lots of short packets will be better off.

Calls from UK(a) to UK(b) will be charged at the UK(a) rate.

Calls from UK(b) to UK(a) will be charged at the UK(b) rate.

New international charge rates apply to UK(b) users.


Packet switching networks are inherently secure, because of the error correction and because, once the call is in the network it has the choice of many internode trunk lines depending on availability and traffic loading of the various available routes.

Like the Telephone network UK can be 'open' in that calls can be received from any user on the network. Of course the recipient of the call can choose to accept or clear the call, they may choose to prompt for a password and/or validate the call on the basis of the calling address.

This is the most flexible in that the user has complete control over passwords etc. If you have even more stringent security requirements then there are many options listed in this document, for example:

See the relevant section in this document for details of these and other facilities available.

Overview of service options

BASIC SERVICEX.2 (1984) UK(a)UK(b) interworking
REFCLASS nownowR5a R5bR6
Permanent Virtual Circuit (PVC) T
Switched Virtual Circuit (SVC)

X.2 LEVEL 2 PER USER SUBSCRIPTION X.2 (1984) UK(a)UK(b) interworking
Multilink procedures1.21 Axxx xx
Extended frame sequence numbering1.30 ATp pp

X.2 LEVEL 3 PER USER SUBSCRIPTION X.2 (1984) UK(a) UK(b) interworking
Extended packet sequence numbering..1.1 A .
Non-standard default window sizes...1.2 A T
Non-standard default packet sizes...1.3 A T
Default throughput class assignment.1.4 A p
Flow control parameter negotiation..1.5 E
Throughput class negotiation........1.6 E p
Packet Retransmission...............1.7 A . . . .
Incoming calls barred1.8 E T p
Outgoing calls barred1.9 E T p
One-way logical channel outgoing1.10 E
One-way logical channel incoming1.11 E
Closed user group (CUG)1.12 E
CUG with outgoing access1.13 A p p
CUG with incoming access1.14 A p p
Incoming calls barred within CUG1.15 A .
Outgoing calls barred within CUG1.16 A .

X.2 LEVEL 3 PER USER SUBSCRIPTION X.2 (1984) UK(a) UK(b) interworking
Bilateral closed user group.........1.17 A . . . . .
Bilateral CUG with outgoing access..1.18 A . . . . .
Reverse charge acceptance1.19 A . . . .
Fast select acceptance1.20 E
Charging information1.22 A p . . . .
Direct call1.23 FS . . . . .
Hunt group (multiline)1.24 A p
with adr replacement . . . p
On-line facility registration1.25 A . .
D-bit modification1.26 A T . . . .
Local charge prevention1.27 A .
Call redirection on out of order1.28 A p
on busy
systematic. . . p
Network user identification1.29 A .
RPOA selection1.31 A .


X.2 LEVEL 3 PER CALL FACILITY X.2 (1984) UK(a) UK(b) interworking
Closed user group selection2.1 E
Bilateral closed user group select2.2 A . . . . .
Reverse charging2.3 A . .
RPOA selection2.4 A .
Flow control parameter negotiation2.5 E
Fast select2.6 E
Throughput class negotiation2.7 E .
Abbreviated address calling2.8 FS . p p
Charging information2.9 A p . . . .
Transit delay selection and indicati2.10 E . p p p
Call redirection notification2.11 A . . .
Called line address modified
notification2.12 A . . .
Network user identification (NUI)2.13 A .
Closed user group with outgoing
access selection2.14 A .
DTE facilities.
DTE facility marker.
calling address extension .
called address extension........ .
minimum throughput class........ .
end to end transit delay .
expedited data negotiation. .

PROCEDURE CHANGES AT CCITT 84 X.2 (1984) UK(a) UK(b) interworking
Registration Packet type .
Diagnostic Packet.
Packet sizes 2048 and 4096 .
Increase in maximum facility field .
Interrupt packet maximum data size
32 octets. .
non-zero DTE causes.
new network clear causes . . . . .
CUG extensions. . . . .
Fast select extensions . . . .


= Supported, see details under relevant section.

| = Part Indicates that an explanatory note is required, due


to significant difference between the CCITT recommendation and the software implementation. See relevant section.

T = Technically the equipment could support this but it is not being marketed due to difficulty in supporting it.

FS Indicates that the facility is referred to in the recommendation, but is for further study.

A Additional CCITT facility

E Essential CCITT facility

R5a = Requires release 5 XCOM at both ends of the call.

Expected date on network 1Q 1991

This is a new release of software in the Packet switching nodes which provides new facilities such as:

* On-line registration.

* Extended Interrupt Packet.

* Diagnostic Packet.

R5b = Requires LOGII and release 5 XCOM at both ends of the call.

Expected date on network 3Q 1991

This is a new internal protocol in the network between the switching nodes and the network supervisor , which provides new facilities such as:

* enhanced routing.

* more powerful user validation.

* CRN.


R6 = Requires release 6 XCOM at both ends of the call.

Expected date on network 1Q 1992

This is a new release of software in the Packet switching nodes which provides new facilities such as:

* D-bit support.

* Transit Delay Selection and Indication.

&notes& = y

Temporary notes for reviewers

I know marketing don't want to mention software releases, and I can see why, but what should I do? If we only claim support of the options currently available then the new service will not look very impressive.

Also Marketing will have to decide which facilities we are going to offer, some facilities will incur extra engineering and administration costs, some facilities should be discouraged for technical reasons.

* UKb uses an internal protocol which is very different than X.25, therefore a protocol conversion has to be done at each end of the call. (UKa is very similar to X.25 all through the network).

* Each software slot runs its own copy of its software, using a lot of memory.

* Code is written in UKb Engine Assembly language then translated into 68000 code.

* Centralised routing could delay call setups.

Other QOS issues:

* The Turbo has much less redundancy than UKa (only the GP cards and power supply are backed up, not line cards or backplane), although less complexity could improve the reliability.

* Centralised routing, effects call setup times and network traffic.