Channel Associated vs. Common Channel Signaling

The Role of Signaling in a Network

To establish a connection, a network exchanges control information between its devices. This process, called signaling, includes establishing, maintaining, monitoring, and releasing connections. It also detects an off-hook condition, carries address digits, and reports whether a line is busy. Two basic transport methods are used: a channel associated with the user circuit or a common signaling channel.

Channel Associated Signaling (CAS)

Channel Associated Signaling is a method in which information for a traffic channel is sent over the same channel as user traffic or over a signaling resource permanently assigned to that channel. The signaling and user circuit remain tied to a specific channel.

How CAS Works

In the European E1 digital hierarchy, timeslot TS16 can carry a 64 kbit/s signaling channel. In a CAS arrangement, its bits are assigned to signaling for 30 telephone channels. In the North American T1 hierarchy, this was often done using robbed-bit signaling, in which the least significant bit of a voice sample was replaced with signaling information in selected frames.

In the G.704 E1 multiframe, frame 0 carries the multiframe alignment pattern. Frames 1 through 15 carry ABCD signaling bits for channel pairs 1 and 16, 2 and 17, through 15 and 30. Each telephone channel therefore receives four 500 bit/s signaling streams. The meaning of the A, B, C, and D bits depends on the CAS variant. Line signaling commonly represents seizure, answer, and clear, while interregister signaling carries address digits.

Advantages and Disadvantages of CAS

• Advantage: Shared-path observation: When signaling uses the same path as user traffic, some faults can appear during call setup. This is not a complete continuity test for every CAS variant.
• Advantage: Limited fault scope: In an arrangement with a resource assigned to one channel, a fault usually affects that channel. A shared timeslot such as TS16 can affect a group of channels.
• Disadvantage: Delay: Older CAS systems send information, especially address digits, sequentially, which lengthens call setup.
• Disadvantage: Resource use: In-band systems occupy voice bandwidth with signaling, while robbed-bit systems assign some sample bits to signaling.
• Disadvantage: Limited information range: Many variants mainly carry line states and address digits, which makes supplementary services harder to implement.

Common Channel Signaling (CCS)

Common Channel Signaling logically separates signaling from traffic channels. One shared signaling link or network carries labeled messages for many circuits. This separation allows the signaling transport and user channels to be designed independently, but it requires coordination between the two paths.

How CCS Works

One of the major CCS systems is Signaling System No. 7 (SS7). In SS7, signaling messages are exchanged between signaling points, commonly processor-controlled exchanges. A 64 kbit/s signaling link can serve many traffic channels; its practical capacity depends on traffic load and network configuration. The system supports both circuit-related signaling and messages unrelated to a single call.

SS7 separates message transport from service-specific processing. The Message Transfer Part (MTP) labels and reliably transfers messages between signaling points, while the ISDN User Part (ISUP) handles circuit-related call control. Other user parts support additional services. Duplicate links and alternative routes can keep signaling available when one link fails.

Advantages and Disadvantages of CCS

• Advantage: Fast call setup: Messages sent over a separate link can shorten call setup compared with many CAS systems. The actual time depends on the protocol, route, and network load.
• Advantage: Resource use: One signaling link can serve many traffic channels, leaving their bandwidth for user data. Voice channels do not need robbed-bit signaling.
• Advantage: Information range: CCS messages can carry addressing, call state, and service information. SS7 supports PSTN, ISDN, mobile networks, and network management functions.
• Disadvantage: No inherent traffic-channel test: A separate signaling path means that call setup does not by itself confirm traffic-channel continuity. Separate checking procedures may be needed.
• Disadvantage: Concentrated fault impact: A failure of a shared link can disable the channels it serves. SS7 limits this risk with redundant links and diversion of signaling traffic to alternative routes.

At a Glance: CAS vs. CCS