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Technical Guide to
Call Setup etc

2 April '96
Version 2.0


  1. The BT ChargeCard.
  2. Jargon Notes.
  3. The Cashless Services system.
  4. Cashless Services Processing Unit (CSPU).
  5. Cashless Services Database (CSDB).
  6. End Notes.

The BT ChargeCard:-

Ok here we go, I'm sure your all familiar with the wonderful device (hee, hee) that is the BT Chargecard but here is a brief summary. As far as I know this is all correct, But I might be wrong about some stuff .

Automatic calls are set up in the following way:

Dude dials the access code - 144.
A voice message of some bitch says 'Please dial your account number, blah, blah, etc, etc'.
Dude dials 12 digits on the keypad.
The system validates the account number/PIN pair. The bitch says 'Please dial the telephone number you require.' Dude enters the destination number.

The bitch can speak in five languages. The selection codes are:-
*1 English (default)
*2 Welsh
*3 French
*4 German
*5 Spanish
Thease can be changed at anytime during setup.

Three levels of service are offered to customers: - FULL SERVICE allows calls to any destination in the world (what phun....). - NATIONAL SERVICE allows inland calls only (O.K if you live in the U.K). - SPECIFIED NUMBERS ONLY SERVICE allows calls only to one number specified in advance (boring).

To make a follow on call don't hang up, dial ## and then the number you want. To change a PIN fone 0800 345 144.
To make operator connected calls dial *0 after entering 144, the account number and PIN.

All calls are fully itemised on the home or office telephone bill.

Jargon Notes:-

ALE : Analogue local exchange.
DMSU: Digital main switching unit.
CSDB: Cashless services database.
DLE : Digital local exchange.
CSPU: Cashless services processing unit. TIBS: Telephone input billing system.
AMS : Account mangement system.

The Cashless Services System:-

    Phone ---->---- ALE ---->---- DLE ---->--- DSMU ---->
                                               |  |
                                               |  |
    District                                   CSPU
    Account           Private Circuit           |
    Mangement -------------------------------- CSDB
    System                                      |
    Terminals                                  TIBS ----> Billing

Ok this could be complicated to new comers. The cashless services system is essentially an overlay intelligent network. It comprises of 2 items of equipment:

  1. The CSPU. Thease are located at DMSU sites and provide the exchange functions of the system.
  2. The CSDB. A central intelligent database used to keep records of customer accounts and to validate account numbers and PINs before a call is allowed.

A call is set up in the following way:

The call can originate from any MF4 capable telephone. When the dude dials 144 , the call is routed to the nearest digital node. This digital node adds three digits identifiying the oringnating charge group before passing the call on to the DMSU and ultimately to the CSPU.

The call terminates on the CSPU, and is effectivly a free call. There now exists a through transmission path between the caller and the CSPU. The CSPU can communicate with the caller by using voice messages, and the caller can communicate with the CSPU by using MF tones.

When the call terminates on the CSPU, the CSPU sends out a one second burst of 1600 Hz tone. If the originating telephone is a payphone, this triggers the payphone to send forward it's identity to the CSPU using MF tones (9 digits). Ordinary telephones do not respond to the tone. The tone is followed by a message from the CSPU's voice guidence system which asks the dude to dial his /her account number and PIN. The CSPU then receives MF digits from the dude and stores them. Voice messages are reapeated if the caller does not respond.

When the CSPU has received the 12 digits representing the account number and PIN, it must validate these numbers before allowing the call set up to continue. In order to do this the CSPU sends a message over an X.25 communications link to the CSDB. The CSDB is a four processor non-stop intelligent database. The incoming PIN is encrypted and compared with the encrypted PIN stored against the account number given. As these validations are against the dude's actual records as stored in the database this is known as positive validation. It is also the most secure method of gaurding against hacking. Validation takes about 100ms and when it is completed the CSDB sends back a message to the CSPU telling it to allow the call and giving it details of the dude's account. The CSPU then sends a voice message to the dude, telling him/her to dial the telephone number required. If validation should fail, the CSDB tells the CSPU to give the customer the appropriate message explaining what is wrong.

At the end of the call, the CSPU sends a further message to the CSBD. This is the billing message and contains all of the deatails necessary to construct an itemised billing record; for example, start time, duration, originating charge group, destination number etc. The CSDB then performs a billing calculation and allocates the appropriate call charge before passing the call record into the existing BT billing system via the TIBS computer in the normal way.

In order to process calls, the CSDB must contain records of everyone who has a Chargecard. This data is entered via the AMS. Each district has a Chargecard office with one or more AMS terminals. These terminals are CSS compatible for ease of use and allow Districts to maintain paperless records of all customers in a paperless form. New customers can be added, details changed or enquiries made all in real time via the AMS.

The Cashless Services Processing Unit:-

The CSPU is an all digital device. Calls are routed into and out of the CSPU over 30 channel PCM systems using TS16 signalling. Calls are not decoded to analogue at any time. Each incoming channel has a corresponding outgoing channel, so no switching capability is required. The CSPU is made up of 5 different parts :- the exchange interface, the call control, the data switch, the alarm subsystem and the record collector.

The record colector consists of a pair of DEC Microvax computers running in hot standby mode. It provides system operators with facilites for control and mangement of the CSPU. It also provides the X.25 signalling facilities between the CSPU and the CSDB.

Each CSPU can handle 720 erlangs of traffic and is designed to grow as demand increases.

The Cashless Services Database:-

The CSDB is divided up into various subsystems.

The card mangement subsystem is the most complex. It interacts with terminals in each BT district to allow customer details to be reviewed, added or changed on-line. Changes entered by the districts go through a checking process before being added or changed on-line. Changes entered by the districts go through a checking process before being added to the main customer and accounts file.

The validation subsystem has the task of checking account numbers and PINs before calls are allowed to proceed. Validation messages are sent from the CSPU via the X.25 network, and the incoming account numbers and PINS are checked against records in the validation file (a subset of the main file). Validation takes about 100ms to complete. This activity takes priority over other CSDB functions to ensure best possible performance from the system.

The pricing subsystem deals with the billing messages sent from the CSPU at the end of the call. The call details are extracted and the call is priced in accordence with the current pricing tables held in the CSDB. An itemised record is then assembled and is sent to the approprite billing system. All calls are archived on optical disc.

As you'd imagine hacking and phreaking is a huge problem on this type of service. So there is a fraud mangement subsystem and fraud supervisor to constantly monitor fraud levels and activities, taking appropriate action as required. So they can catch your sorry ass (well attempt at least...).

End Notes :-

Greets and thanx go out ta Darkcyde, Krew-L-T, whole o' MED krew, Kry0 and HaiZe :)

To contact me i can be mailed on Beyond Phear.

Remember to use PGP if it's something important or incriminating. Here's my public key :-

Version: 2.6.2