Common Channel Signaling (CCS)

CCS refers to the situation in which the signaling capacity is provided in a common pool, with the capacity being used as and when necessary. The signaling channel can usually carry signaling information for thousands of traffic circuits.

In North America, signaling can be placed on its own T1 carrier even though it only takes up one timeslot. This means that two physical networks, "speech" and "signaling," can have different routings. (Please refer to Chapter 5 for a description of carriers and timeslots.) Alternatively, the signaling might exist on a carrier with other user traffic, depending on the network operator.

Outside of North America, the signaling is placed in its own timeslot on an E1 (that is, logically rather than physically separated). The other timeslots on E1 are for user traffic—apart from TS0, which is used for synchronization. E1 systems tend to use the TS16 timeslot for signaling; some core network equipment ignores TS16, expecting it to be used for signaling traffic because it has historically been the timeslot for digital CAS signaling.

The only CCS systems that have been implemented to date are Signaling Systems No. 6 and No. 7 (SS6 and SS7). The ITU for the international network originally standardized SS6, but they saw limited deployment. AT&T nationalized SS6 for the North American network and called it Common Channel Interoffice Signaling (CCIS) No. 6. SS6 saw a limited deployment after the mid-1970s because it had far less bandwidth and a much smaller packet size than SS7. In addition, its evolutionary potential was severely limited because it was not a layered protocol architecture.

CCS systems are packet-based, transferring over 200 bytes in a single SS7 packet, as opposed to a few bits allocated to act as indicators in digital CAS. The signaling information is transferred by means of messages, which is a block of information that is divided into fields that define a certain parameter or further sub-field. The signaling system's specifications (Recommendations and Standards) define the structure of a message, including its fields and parameters.

Because CCS is packet-based and there is not a rigid tie between the signaling and the circuits it controls, it can operate in two distinct ways. These two distinct ways are circuit-related signaling and non-circuit-related signaling.

Circuit-Related Signaling

Circuit-related signaling refers to the original functionality of signaling, which is to establish, supervise, and release trunks. In other words, it is used to set up, manage, and clear down basic telephone service calls. Circuit-related signaling remains the most common mode of signaling. As it is with CAS, signaling capacity is not pre-allocated for each traffic circuit. Rather, it is allocated as it is required. Each signaling message is related to a traffic circuit. Because no dedicated relationship exists between the circuits and the signaling, it is necessary to identify the traffic circuit to which a particular signal message refers. This is achieved by including a circuit reference field in each signaling message.

Non-Circuit-Related Signaling

Non-circuit-related signaling refers to signaling that is not related to the establishment, supervision, and release of trunks. Due to the advent of supplementary services and the need for database communication in cellular networks and Intelligent Networks, for example, signaling is no longer exclusively for simply setting up, managing, and clearing down traffic circuits. Non-circuit-related signaling allows the transfer of information that is not related to a particular circuit, typically for the purpose of transmitting both the query and response to and from telecommunication databases. Non-circuit-related signaling provides a means for transferring data freely between network entities without the constraint of being related to the control of traffic circuits.

Common Channel Signaling Modes

A signaling mode refers to the relationship between the traffic and the signaling path. Because CCS does not employ a fixed, deterministic relationship between the traffic circuits and the signaling, there is a great deal of scope for the two to have differing relationships to each other. These differing relationships are known as signaling modes.

There are three types of CCS signaling modes:

• Associated

• Quasi-associated

• Non-associated

SS7 runs in associated or quasi-associated mode, but not in non-associated mode. Associated and quasi-associated signaling modes ensure sequential delivery, while non-associated does not. SS7 does not run in non-associated mode because it does not have procedures for reordering out-of-sequence messages.

Associated Signaling

In associated mode, both the signaling and the corresponding user traffic take the same route through the network. Networks that employ only associated mode are easier to design and maintain; however, they are less economic, except in small-sized networks. Associated mode requires every network switch to have signaling links to every other interconnected switch (this is known as a fully meshed network design). Usually a minimum of two signaling links are employed for redundancy, even though the switched traffic between two interconnected switches might not justify such expensive provisioning. Associated signaling mode is the common means of implementation outside of North America. Figure 1-4 illustrates the associated concept.

Quasi-Associated Signaling

In quasi-associated mode, signaling follows a different route than the switched traffic to which it refers, requiring the signaling to traverse at least one intermediate node. Quasi-associated networks tend to make better use of the signaling links; however, it also tends to create a more complex network in which failures have more potential to be catastrophic.

Quasi-associated signaling can be the most economical way of signaling for lightly loaded routes because it avoids the need for direct links. The signaling is routed through one or more intermediate nodes. Signaling packets arrive in sequence using quasi-associated signaling because the path is fixed for a given call (or database transaction) at the start of a call (or transaction). Figure 1-5 shows the quasi-associated signaling mode, which is the common means of implementation within North America.

Non-Associated Signaling

Because the path is not fixed at a given point in time in non-associated mode, the signaling has many possible routes through the network for a given call or transaction. Therefore, the packets might arrive out of sequence because different routes might have been traversed.

SS7 does not run in non-associated mode because no procedures exist for reordering out-of-sequence messages. Associated and quasi-associated signaling modes assure sequential delivery, while non-associated signaling does not. Quasi-associated mode is a limited case of non-associated mode, in which the relative path is fixed.