Common Channel Signaling Systems

Common channel signaling (CCS) differs from CAS in that all channels use a common channel and protocol for call setup. Using E1 as an example, a signaling protocol, such as ISDN Q.931, would be deployed in time slot 17 to exchange call-setup messages with its attached telephony equipment, as seen in Figure 2-36.


Examples of CCS signaling are as follows:

• Proprietary implementations Some PBX vendors choose to use CCS for T1 and E1 and implement a proprietary CCS protocol between their PBXs. In this implementation, Cisco devices are configured for Transparent Common Channel Signaling (T-CCS) because the Cisco devices do not understand proprietary signaling information.

• Integrated Services Digital Network (ISDN) ISDN uses Q.931 in a common channel to signal all other channels.

• Q Signaling (QSIG) Like ISDN, QSIG uses a common channel to signal all other channels.

• Digital Private Network Signaling System (DPNSS) DPNSS is an open standard developed by British Telecom for implementation by any vendor who chooses to use it. DPNSS also uses a common channel to signal all other channels.

• Signaling System 7 (SS7) SS7 is an out-of-band network implemented and maintained by various telephone companies and used for signaling and other supplemental services.

The following discussions elaborate on various CCS implementations. Note that proprietary implementations are not discussed because they vary widely among vendors.


ISDN

ISDN (Integrated Services Digital Network) is an access specification to a network. You may have studied ISDN as an access method for dial-up data systems. Because it is a digital system, ISDN makes connections rapidly.

ISDN can be implemented in two different ways: BRI (Basic Rate Interface) and PRI (Primary Rate Interface). BRI features two bearer (B) channels, while PRI supports 23 (for T1) or 30 (for E1) B channels. Each implementation also supports a data (D) channel, used to carry signaling information (CCS).

The following are benefits of using ISDN to transmit voice:

• Each B channel is 64 kbps, making it perfect for G.711 PCM.

• ISDN has a built-in call control protocol known as ITU-T Q.931.

• ISDN can convey standards-based voice features, such as call forwarding.

• ISDN supports standards-based enhanced dial-up capabilities, such as Group 4 fax and audio channels.

Figure 2-37 shows the architecture of an ISDN network. The B channel carries information, such as voice, data, and video, at 64 kbps. The D channel carries call signaling between customer premises equipment (CPE) and the network, usually as the Q.931 protocol but sometimes as the QSIG protocol.


BRI operates using the average local copper pair. It uses two B channels and one signaling channel, which is written as 2 B + D.

PRI implemented on T1 uses 23 B channels and one signaling channel, which is written as 23 B + D. PRI implemented on E1 uses 30 B channels and one signaling channel, which is represented as 30 B + D.

ISDN's Q.931 protocol, which operates at Layer 3 of the OSI (Open System Interconnection) model, uses a standard set of messages to communicate, as illustrated in Figure 2-38.


These standard messages cover the following areas:

• Call establishment Initially sets up a call. Messages travel between the user and the network. Call establishment events include alerting, call proceeding, connect, connect acknowledgment, progress, setup, and setup acknowledgment.

• Call information phase Data sent between the user and the network after the call is established. This allows the user to, for example, suspend and then resume a call. Events in the call information phase include hold, hold acknowledgment, hold reject, resume, resume acknowledgment, resume reject, retrieve, retrieve acknowledgment, retrieve reject, suspend, suspend acknowledgment, suspend reject, and user information.

• Call clearing Terminates a call. The following events occur in the call-clearing phase: disconnect, release, release complete, restart, and restart acknowledgment.

• Miscellaneous messages Negotiates network features (supplementary services). Miscellaneous services include congestion control, facility, information, notify, register, status, and status inquiry.

QSIG

The QSIG (Q Signaling) protocol is based on the ISDN Q.931 standard and provides signaling for private integrated services network exchange (PINX) devices. Figure 2-39 shows how different QSIG operations map to the OSI model.


DPNSS

British Telecom and selected PBX manufacturers originally developed the Digital Private Network Signaling System (DPNSS) in the early 1980s. It was developed and put into use before the ISDN standards were completed because customers wanted to make use of digital facilities as soon as possible.

DPNSS operates over standard ISDN physical interfaces and is described in four documents:

• BTNR 188: Digital Private Networking Signalling System No 1, Issue 6, January 1995.

• BTNR 188-T: Digital Private Networking Signalling System No 1: Testing Schedule.

• BTNR 189: Interworking between DPNSS1 and other Signalling Systems, Issue 3, March 1988.

• BTNR 189-I: Interworking between DPNSS1 and ISDN Signalling Systems, Issue 1, December 1992.

SIGTRAN

SIGTRAN, as illustrated in Figure 2-40, is a signaling protocol defined in RFC 2719 and RFC 2960. It describes the way the IP protocol carries SS7 messages in a VoIP network. SIGTRAN relies on the Stream Control Transport Protocol at Layer 4 of the TCP/IP protocol stack.


Using SIGTRAN, a service provider may interconnect a private VoIP network to the public switched telephone network (PSTN) and ensure that SS7 signals are conveyed end to end.