Summary

CCS has evolved to address the limitations of the CAS signaling method. CCS has the following advantages over CAS:

• Much faster call set-up time

• Greater flexibility

• Capacity to evolve

• More cost effective than CAS

• Greater call control

Most CCS calls can be set up in half the time it takes to set up CAS calls. CCS achieves greater call control because no contention exists between signaling and user traffic as it does with in-band CAS. Because the subscriber cannot generate particular signals intended for inter-switch (core network) signaling, CCS offers a greater degree of protection against fraud than analog CAS methods.

CCS has the following disadvantages in comparison to CAS:

• CCS links can be a single point of failure—a single link can control thousands of voice circuits, so if a link fails and no alternative routes are found, thousands of calls could be lost.

• There is no inherent testing of speech path by call set-up signaling, so elaborate Continuity Test procedures are required.

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 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. illustrates the associated concept.

Channel Associated Signaling

The key feature that distinguishes Channel Associated Signaling (CAS) from CCS is the deterministic relationship between the call-control signals and the bearers (voice circuits) they control in CAS systems. In other words, a dedicated fixed signaling capacity is set aside for each and every trunk in a fixed, pre-determined way.

Channel Associated Signaling (CAS) is often still used for international signaling; national systems in richer nations almost exclusively use Common Channel Signaling (CCS). CCS is replacing CAS on international interfaces.

CAS can be implemented using the following related systems:

• Bell Systems MF, R2, R1, and C5.

• Single-frequency (SF) in-band and out-of-band signaling

• Robbed bit signaling

The following sections discuss these methods in context with the type of signal, either address or supervisory.

Address Signals

Multifrequency systems, such as the Bell System MF, R2, R1, and C5, are all types of address signals used by CAS.

Multifrequency

The CAS system can be used on either analog Frequency Division Multiplexed (FDM) or digital Time Division Multiplexed (TDM) trunks. MF is used to signal the address digits between the switches.

MF simultaneously sends two frequencies, from a choice of six, to convey an address signal. The switch indicates to the switch on the other end of a trunk that it wishes to transmit address digits by sending the KP (start pulsing) signal, and indicates the end of address digits by sending the ST (end pulsing) signal. The timing of MF signals is a nominal 60 ms, except for KP, which has a nominal duration of 100 ms. A nominal 60 ms should be between digits.

As with the MF address signaling, SF is sent switch to switch. A trunk is initially on-hook at both ends. One of the switches sends a forward off-hook (seizure) to reserve a trunk. The receiving switch indicates that it is ready to receive address digits, (after connecting a digit received by the line by sending a wink signal. When the originating switch receives the wink signal, it transmits the digits of the called party number. When a call is answered, the called parties switch sends an off-hook signal (answer). During the conversation phase, both ends at each trunk are off-hook. If the calling a party clears the call, it sends a clear-forward signal; likewise, when the called party hangs up, it sends a clear-backward signal.

SF uses an in-band tone. In-band systems send the signaling information within the user’s voice frequency range (300 Hz to 3400 Hz). A major problem with in-band supervisory signaling, however, is its susceptibility to fraud. The hacker quarterly magazine “2600″ was named for the infamous 2600 Hz tone, which could be used by the public to trick the phone system into giving out free calls. The subscriber could send supervisory tone sequences down his telephone’s mouthpiece using a handheld tone generator. This enabled the subscriber to instruct switches and, in doing so, illegally place free telephone calls.

The other major problem with in-band signaling is its contention with user traffic (speech). Because they share the same frequency bandwidth, only signaling or user traffic can be present at any one time. Therefore, in-band signaling is restricted to setting up and clearing calls down only because signaling is not possible once a call is in progress.

Limitations of CAS

We discuss the general disadvantages of CAS for the purpose of reinforcing the concepts and principles we have introduced thus far. CAS has a number of limitations, including:

• Susceptibility to fraud

• Limited signaling states

• Poor resource usage/allocation

The following sections discuss these limitations in more detail.

Susceptibility to Fraud

CAS employing in-band supervisory signaling is extremely susceptible to fraud because the subscriber can generate these signals by simply using a tone generator down a handset mouthpiece. This type of device is known as a blue box; from the beginning of the 1970s, it could be purchased as a small, handheld keypad. Blue box software was available for the personal computer by the beginning of the 1980s.

Limited Signaling Information

CAS is limited by the amount of information that can be signaled using the voice channel. Because only a small portion of the voice band is used for signaling, often CAS cannot meet the requirements of today’s modern networks, which require much higher bandwidth signaling.

Inefficient Use of Resources

CAS systems are inefficient because they require either continuous signaling or, in the case of digital CAS, at regular intervals even without new signals.

In addition, there is contention between voice and signaling with in-band CAS. As a result, signaling is limited to call set-up and release phases only. This means that signaling cannot take place during the call connection phase, severely imposing technological limits on the system’s complexity and usefulness.