Overview
Integrated Services Digital Network (ISDN) evolved from IDN. It provides end-to-end digital connectivity and supports an extensive range of services, covering both voice and non-voice services.
ISDN furnishes a finite set of standard multi-purpose UNIs. The ITU-T I.412 recommendation specifies two types of UNIs: basic rate interface (BRI) with bandwidth of 2B + D and primary rate interface (PRI) with Bandwidth of 30B + D or 23B + D. Where:
B channel is a user channel, transmitting user information such as voice and data with a transmission rate of 64 kbps.
D channel is a control channel, transmitting the public channel signaling. These signals control the calls on the B channel of the same interface. The rate of D channel is 16 kbps (BRI) or 64 kbps (PRI). The ITU-T Q.921 is a data link layer protocol of D channel. It defines the rule for Layer 2 information interchange through D channel from the user to a network interface and supports the access of a Layer-3 entity. The ITU-T Q.931 is a network layer protocol of D channel. It provides a measure for creating, maintaining, and terminating network connections between communication application entities. Call control (CC) is a further encapsulation of Q.931, which forwards the message from the network side to CC for CC to perform information interchange with higher layer applications such as DCC.
Figure 17: ISDN D channel protocol stack
The ISDN protocol proposed by the ITU-T provides different services in different areas, forming the ISDN protocols that are suitable for different regions, such as NTT in Japan, ETSI in Europe, NI, AT&T, and ANSI in North America.
Besides the default DSS1 ISDN protocol, the router supports the basic calling function of NTT, ETSI, AT&T, ANSI, NI, NI2, Q.SIG, and 5ESS protocols, but does not support the supplementary functions of these protocols. Additionally, DSS1, Q.SIG, NI2, and 5ESS support network side operation.
NI protocol used in North America is only applied to BRI interfaces. The ISDN network uses service profile identification (SPID) as the ID of different services, and the switch provides the service to the terminal user according to the SPID. Each B channel corresponds to a SPID. The user can proceed with normal calling and disconnection process only after having employed the SPID to perform the SPID handshake interaction. After the Q.921 establishes link successfully and before the Q.931 calling processing starts, the user needs to obtain SPID to interact with the switch to perform the Layer 3 (Q.931) initialization, and then the user can start normal calling and disconnect process, otherwise, the calling will fail.
By far, you can obtain the SPID on one BRI interface over the ISDN in North America in any of the following ways:
Manually enter the SPID consisting of 9 to 20 digits.
14-digit SPID (Generic SPID Format). The former 10 digits are entered by the user, and the latter 4 digits can only be 0101.
Allocate by Stored Program Control Switching System (SPCS) through automated SPID selection regulation.
The former two ways to obtain SPID are regarded as static configuration methods, and the third one is taken as dynamic negotiation method. If the user does not specify a SPID in static method, the system will adopt dynamic method by default.