ISDN Layer 2
ISDN Layer 2 is meant to provide physical addressing on the network. Because multiple logical devices can exist in a single physical device, it is necessary to correctly identify the source and/or destination process or logical entity when transmitting or receiving data. In communication with the ISDN switch, an identifier must be issued by the switch. This is known as a Terminal Endpoint Identifier (TEI). The service provider has the option of creating a specific profile for your implementation. Should this be the case, the service provider will assign a SPID for each of your bearer channels. The use of SPIDs is optional.
• Terminal Endpoint Identifier (TEIs) A terminal endpoint can be any ISDN-capable device attached to an ISDN network. The TEI is a number between 0 and 127, where 0-63 is used for static TEI assignment, 64-126 are used for dynamic assignment, and 127 is used for group assignments. The TEI provides the physical identifier, and the service access point identifier (SAPI) carries the logical identifier. In North America, Layer 1 and Layer 2 are activated at all times. In Europe, the activation does not occur until the call setup is sent. This delay conserves switch resources.
• Service Profile Identifiers (SPIDs) are another key part of the ISDN BRI Layer 2. They are used only in BRI implementations. The SPID specifies the services to which you are entitled from the switch and defines the feature set that you ordered when the ISDN service was provisioned. The SPID is a series of characters manually entered into the router's configuration to identify the router to the switch. This is different from the TEI. The TEI address is dynamically assigned. The SPID is statically assigned to the router based on information provided by the service provider. If needed, two SPIDs are configured, one for each channel of the BRI. Usually, the SPID includes the ten-digit phone number of each B channel followed by four additional digits assigned by the service provider. SPID requirements are dependent on both the software revision and the switch. Many switch manufacturers are moving away from SPIDs, as they have already done in Europe. SPIDs are required only in the U.S., and then are used only by certain switches.