The 'Nervous System' of the Network

For any communication to happen (like a simple phone call), a network needs a way to exchange control information. This process, called signaling, is the network's nervous system. It handles tasks like detecting an off-hook phone, transmitting dialed digits, and indicating if a line is busy. There are two fundamentally different philosophies for how to send these control signals: associated with the channel or on a separate, common channel.

Channel Associated Signaling (CAS)

Channel Associated Signaling is a method where the signaling information for a traffic channel is sent either over that same channel or on a dedicated signaling channel that is permanently associated with it. Think of it like passing a note to the person you are talking to (the control message travels along the same path as the conversation).

How CAS Works

In digital systems like the North American T1 hierarchy, this was often done using robbed-bit signaling, where a bit was periodically stolen from the voice data itself to carry signaling status. In the European E1 hierarchy, a dedicated timeslot (TS16) was used, with its bits divided up to serve the signaling needs of the 30 voice channels.

Advantages and Disadvantages of CAS

• Advantage - Inherent Link Testing: Because signaling uses the same path as the voice data, successfully setting up a call implicitly confirms the continuity of the entire traffic circuit.
• Advantage - Localized Failures: A fault on one signaling path typically affects only its associated traffic channel, not the entire system.
• Disadvantage - Slow: Transmitting signaling information, especially address digits, was very slow. Transmitting a single digit could take 150 ms, making call setup times long.
• Disadvantage - Inefficient: It inefficiently uses resources, either by degrading voice quality (bit robbing) or by occupying bandwidth for signaling on every single channel, even when the channel is idle.
• Disadvantage - Limited Vocabulary: The amount of information that could be sent was extremely limited, often just basic states (on-hook, off-hook, digits). This made it impossible to implement advanced services.

Common Channel Signaling (CCS)

Common Channel Signaling revolutionized telecommunications by completely separating the signaling network from the traffic (voice/data) network. All signaling messages for a large number of traffic channels are routed over a separate, shared, high-speed data network. Think of it as having a dedicated, high-speed courier service just for control messages, leaving the main roads completely free for user traffic.

How CCS Works

The most prevalent CCS system is Signaling System No. 7 (SS7). In SS7, signaling is performed using data packets called messages, which are exchanged between processor-controlled exchanges. A single 64 kbit/s signaling link can control up to a thousand or more traffic channels.

Advantages and Disadvantages of CCS

• Advantage - Fast Call Setup: Because signaling is sent over a high-speed data link, call setup times are dramatically reduced from many seconds to often less than one second.
• Advantage - Efficient: A single signaling link serves hundreds of traffic channels, freeing up the entire bandwidth of those channels for user data. No more bit robbing means better voice quality.
• Advantage - Flexible & Powerful: The message-based nature of CCS allows for a rich and extensive set of information to be exchanged. This flexibility is what enabled all modern telecom services like Caller ID, text messaging (SMS), 800 numbers, mobile roaming, and more.
• Disadvantage - No Inherent Link Testing: Since the signaling path is separate, setting up a call does not test the traffic channel's integrity. Separate continuity tests might be required.
• Disadvantage - Centralized Point of Failure: A failure of the common signaling link could potentially disable all traffic channels it controls. This risk is mitigated in practice by using highly redundant, duplicated signaling links routed over physically diverse paths.

At a Glance: CAS vs. CCS