ISDN Call Setup and Data Link Protocols

Call setup differs between ISDN and modems. With a telephone call and with analog modems, DTMF tones are sent across the analog local loop to the telco. The telco switch at the local CO interprets the dialed digits and sets up the call. However, with ISDN, there is no analog local loop over which the analog DTMF tones can be sent.

ISDN devices send and receive signaling messages to and from the local ISDN switch to which it is connected. In telco terminology, signaling refers to any type of request to establish a circuit. So, punching keys on your telephone is considered signaling to set up a circuit over an analog local line. Instead of DTMF tones, ISDN defines a set of messages that are sent over the D channel to the local CO. As a result, the PSTN sets up a circuit to the ISDN device whose phone number was put inside the signaling message. Figure 15-9 outlines the process and the result.

Figure 15-9 D Channel Call Setup Signaling and Resulting B-Channel Call

Figure 15-9 D Channel Call Setup Signaling and Resulting B-Channel Call

The service provider can use anything it wants to set up the call inside its network. ITU Q.931 messages are used for signaling between the ISDN device and the CO; typically, Signaling System 7 (SS7) is used between the two telco switches—the same protocol used inside phone company networks to set up circuits for phone calls.

When the call is established, a 64-kbps circuit exists between a B channel on each of the two routers in the figure. The routers can use High-Level Data Link Control (HDLC), but they typically use PPP as the data-link protocol on the B channel from end to end. As on leased lines and dialed circuits using modems, the switches in the phone company do not interpret the bits sent inside this circuit—they just help create a serial bit stream in each direction.

The D channel remains up all the time so that new signaling messages can be sent and received. Because the signals are sent outside the channel used for data, this is called out-of-band signaling.

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