MSN Overview

Overview

One of the big advantages of ISDN over an analogue service is the ability to assign multiple phone numbers to one line.  In Europe (and other countries using the EuroISDN specification) this is implemented using Multiple Subscriber Numbering.

The MSN service is provided to you by your ISDN service provider. It allows you to have several phone numbers for your ISDN line. For example, your PTT might give you a range of numbers like 810 3500, 810 3501,…, 810 3509.

The use of MSN allows you to assign different numbers to the various features (virtual devices) that your DIVA product can provide. For instance, you could have an analogue fax modem on one number (810 3502), a Dial-Up Networking connection to a RAS Server on another number (810 3500), etc.

According to the Euro-ISDN specification, the numbers do not have to be in sequence. Therefore, some providers will allocate non-contiguous MSN numbers like 810 3500, 810 3632, 810 4592, 810 7653 etc.

Incoming Calls: Directing calls to specific devices using MSN

Whenever you wish to receive calls from more than one type of remote device or service, you will need to tell the difference between these calls, so that, for instance,  fax calls are answered by your fax software and not by your remote access software.

The easiest way to do this is to assign different phone numbers on your ISDN line for the different uses.  This destination number (the 'Called Party Number' or 'CPN') is then provided on the incoming ISDN call so that your different items of ISDN equipment (or different pieces of ISDN software) can decide whether the call is for them.

If the only incoming calls will be for one application you do not need to worry about this and one number is enough.

You should not confuse this 'destination number' with the 'origination number' (the caller's number provided by the service, known as the 'Calling Line Identity', or 'CLI').  The caller's number can also be optionally provided on the incoming call, and some ISDN equipment can make use of this information (see below).

Outgoing Calls: Assigning MSN numbers to identify specific callers

The EuroISDN standard allows an outgoing call to present the originating number to the recipient by a feature known as Calling Line Identification, commonly abbreviated to CLI.

This facility allows ISDN recipients to identify a caller, and this can be used for security and billing purposes, for example, or to direct incoming calls to a call centre or a RAS server. To further identify specific calling devices, which may be sharing an ISDN base number, MSN numbers can be allocated to the calling device to uniquely identify that device.

The way to do this is to assign the MSN numbers allocated to an ISDN line to the various users and devices sharing the line. This MSN number then becomes the originating number of the call (the 'calling line identification' or 'CLI') and is presented on the outgoing ISDN call. The ISDN equipment (or ISDN software) at the other end of the call can then specifically identify the calling party, as opposed to simply identifying the base ISDN number.