Home | WAN | WAN Terminology | Frame Relay | The Internet 101 | ISDN

What is a SPID? How come my ISDN device won't work without one?

SPIDs are Service Profiles IDs

SPIDs are used to identify what sort of services and features the switch provides to the ISDN device. Currently they are used only for circuit-switched service (as opposed to packet-switched). Annex A to ITU recommendation Q.932 specifies the (optional) procedures for SPIDs. They are most commonly implemented by ISDN equipment used in North America.

When a new subscriber is added, the Telco personnel allocate a SPID just as they allocate a directory number. In many cases, the SPID number is identical to the (full ten digit) directory number. In other cases it may be the directory number concatenated with various other strings of digits, such as digits 0100 or 0010, 1 or 2 (indicating the first or second B channel on a non-centrex line), or 100 or 200 (same idea but on a centrex line) or some other, seemingly arbitrary string. Some people report SPIDs of the form 01nnnnnnn0 for AT&T custom and 01nnnnnnn011 for NI-1, where n is the seven digit directory number.
It is all quite implementation dependent.

The subscriber needs to configure the SPID into their terminal (i.e. computer or telephone, etc., not their NT-1 or NT-2) before they will be able to connect to the central office switch.

When the subscriber plugs in a properly configured device to the line, Layer 2 initialization takes place, establishing the basic transport mechanism. However if the subscriber has not configured the given SPID into their ISDN device, the device will not perform layer 3 initialization and the subscriber will not be able to make calls. This is, unfortunately, how many subscribers discover they need a SPID.

Once the SPID is configured, the terminals go through an initialization/identification state which has the terminal send the SPID to the network in a Layer 3 Information message whereby the network responds with an INFO message with the EID information element (i.e.). Thereafter the SPID is not sent again to the switch. The switch may send the EID or the Called Party Number (CdPN) in the SETUP message to the terminal for the purpose of terminal selection.

SPIDs should not be confused with TEIs (terminal endpoint identifiers). TEIs identify the terminal at Layer 2 for a particular interface (line). TEIs will be unique on an interface, whereas SPIDs will be unique on the whole switch and tend to be derived from the primary directory number of the subscriber. Although they are used at different layers, they have a 1-to-1 correspondence so mixing them up isn't too dangerous. TEIs are dynamic (different each time the terminal is plugged into the switch) but SPIDS are not. Following the initialization sequence mentioned above the 1-to-1 correspondence is established. TEIs are usually not visible to the ISDN user so they are not as well known as SPIDs.

The "address" of the layer 3 message is usually considered to be the Call Reference Value (also dynamic but this time on a per call basis) as opposed to the SPID, so the management entity in the ISDN device's software must associate EID/CdPN on a particular TEI and Call Reference Number to a SPID.

There are some standards that call for a default Service Profile, where a terminal doesn't need to provide a SPID to become active. Without the SPID however, the switch has no way of knowing which terminal is which on the interface so for multiple terminals an incoming call would be offered to the first terminal that responded, rather than to a specific terminal.