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

circuit.

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

Tariffs

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.

Security

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:

• Validation - The network can be configured only to allow calls to or from certain addresses, which you can define at subscription time, this is available on UK(b) only.
• Closed User Group - this is similar to the validation method above but it requires you to specify the closed user group in the facility field of the call packet, this allows the user to belong to more than one closed user group. This is available on both UK(a) and UK(b).
• Redirection - allows calls to be routed to another line if the called line is out of order or has all virtual circuits busy.
• Hunt Group (Multiline) - Allows your incoming calls to be distributed over several physical lines.
• Dial backup - Allows the Telephone network to be used in case of a fault on your dataline.
  
  

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

Overview of service options

BASIC SERVICE	X.2 (1984)		UK(a)	UK(b)				interworking
	REF	CLASS	now	now	R5a	R5b	R6
Permanent Virtual Circuit (PVC)			T
Switched Virtual Circuit (SVC)

X.2 LEVEL 2 PER USER SUBSCRIPTION	X.2 (1984)		UK(a)	UK(b)				interworking
OPTIONS	REF	CLASS	now	now	R5a	R5b	R6
Multilink procedures	1.21	A	x	x	x	x	x
Extended frame sequence numbering	1.30	A	T	p	p	p

X.2 LEVEL 3 PER USER SUBSCRIPTION	X.2 (1984)	UK(a)	UK(b)				interworking
OPTIONS	REF CLASS	now	now	R5a	R5b	R6
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 barred	1.8 E	T	p
Outgoing calls barred	1.9 E	T	p
One-way logical channel outgoing	1.10 E
One-way logical channel incoming	1.11 E
Closed user group (CUG)	1.12 E
CUG with outgoing access	1.13 A	p					p
CUG with incoming access	1.14 A	p					p
Incoming calls barred within CUG	1.15 A	.
Outgoing calls barred within CUG	1.16 A	.

X.2 LEVEL 3 PER USER SUBSCRIPTION	X.2 (1984)	UK(a)	UK(b)	interworking
OPTIONS	REF CLASS	now	now	R5a	R5b	R6
Bilateral closed user group.........	1.17 A	.	.	.	.	.
Bilateral CUG with outgoing access..	1.18 A	.	.	.	.	.
Reverse charge acceptance	1.19 A		.	.	.	.
Fast select acceptance	1.20 E
Charging information	1.22 A	p	.	.	.	.
Direct call	1.23 FS	.	.	.	.	.
Hunt group (multiline)	1.24 A						p
with adr replacement		.	.	.			p
On-line facility registration	1.25 A	.	.
D-bit modification	1.26 A	T	.	.	.	.
Local charge prevention	1.27 A	.
Call redirection on out of order	1.28 A						p
on busy
systematic		.	.	.			p
Network user identification	1.29 A	.
RPOA selection	1.31 A	.

-------------------------------------------------------------------------------------------+

X.2 LEVEL 3 PER CALL FACILITY	X.2 (1984)	UK(a)	UK(b)	interworking
OPTIONS	REF CLASS	now	now	R5a	R5b	R6
Closed user group selection	2.1 E
Bilateral closed user group select	2.2 A	.	.	.	.	.
Reverse charging	2.3 A		.	.
RPOA selection	2.4 A	.
Flow control parameter negotiation	2.5 E
Fast select	2.6 E
Throughput class negotiation	2.7 E	.
Abbreviated address calling	2.8 FS	.	p	p
Charging information	2.9 A	p	.	.	.	.
Transit delay selection and indicati	2.10 E	.	p	p	p
Call redirection notification	2.11 A	.	.	.
Called line address modified
notification	2.12 A	.	.	.
Network user identification (NUI)	2.13 A	.
Closed user group with outgoing
access selection	2.14 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
OPTIONS	REF CLASS	now	now	R5a	R5b	R6
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			.	.	.	.

KEY

= Supported, see details under relevant section.

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

P

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.

* CLAMN.

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.