Best CAS Practices for VoIP
One of the protocols for digital telephone connection and transmission of voice and data over a digital line is through using the Integrated Services Digital Network (ISDN), a set of communication standards. In the ISDN, you will find two types of channels. Bearer or simply B, channels responsible for carrying voice signal and Data or just D channel is responsible for signaling control.
Signaling is the dialog language for the communication between various parts of the telecommunication network. Signaling is simply a way of conveying network-related information between switching nodes, as well as between the end office switches and their subscribers.
Signaling is used to do the following:
- Monitor the conditions of network lines/trunks;
- Manage and maintain the telecom networks;
- Request service from the central office switch;
- Offer your central office switch with the information necessary to route a telephone call;
- Alert destination address of incoming call (ringing);
- Offer status information and call supervision for billing;
- Authenticate the identities of the calling and called parties;
- Avoid network misuse and minimize the risk of denial-of-service (DOS) attacks; and
- Avert snooping or alteration of message content.
There are two major types of signaling paths: common channel signaling (CCS) and channel associated signaling (CAS).
What is channel associated signaling (CAS) for VoIP?
According to Revolvy, CAS is a type of signaling, also referred to as per-trunk signaling (PTS), where control/signaling signals, including those for coordinating and adjoining frames, are transmitted, in the same channel, together with voice and data signals. CAS is an alternative to CCS (common channel signaling) where a set of voice-and-data channels use a distinct channel that is used only for control signals.
Difference between CAS and CCS
In CAS, signaling is transmitted with voice calls over the same channel. Pulse code modulation (PCM) technique is employed here to digitize all forms of analog data (such as voices, music, full-motion video and telemetry) which is then transmitted well within the PCM frame.
In CCS, signaling is transferred over diverse channels other than the channel over which voice calls are carried. CCS is the most used signaling technique in all the telephone exchanges and between exchanges (PBXs). CCS usually needs only one signaling channel for up to about 1000 traffic channels.
Advantages of CAS
- CAS is less expensive to implement;
- CAS can be used on any transmission medium;
- Unlike CCS where a single link can control thousands of voice circuits, CAS has alternative routes. Therefore if one link fails, there are alternative routes that can be used to transmit data and voice.
- CAS has inherent testing of speech path by call set-up signaling, as such, it does not need elaborate Continuity Test procedures.
Disadvantages of CAS
In comparison with CCS, CAS has the following disadvantages:
- Limited signaling information: CAS is restricted by the volume of information that can be signaled using the voice channel.
- Inefficient use of resources: CAS systems are inefficient because they require either endless signaling or, in the case of digital CAS, at regular intervals even without new signals.
- Signaling is restricted to call set-up and release phases only. This implies that signaling cannot occur during the call connection stage.
- High exposure to fraud: phone fraudsters can build boxes to play call setup and teardown tones.
- Prone to interferences between signaling tones used by the network and frequencies of human speech patterns.
- Low speed: call setup and teardown is slower when using CAS.
According to John G. van Bosse’s book, Signaling in Telecommunication Networks, CAS possibly leads to lower available bandwidth for the payload. This is because CAS use user bandwidth to perform signaling functions.
Quality of Service
VoIP phone services from reputable cloud solution providers are Quality of Service (QoS)-enabled. This guarantees that packet traffic for a voice or other media connection will not be delayed or dropped due to interference from other lower priority traffic.
QoS manages the following VoIP issues:
- Latency: delay of packet delivery
- Jitter: variations in delay of packet delivery
- Packet loss due to too much traffic
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