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:
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Bell Systems MF, R2, R1, and C5.
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Single-frequency (SF) in-band and out-of-band signaling
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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:
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Susceptibility to fraud
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Limited signaling states
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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.
Tags: SS7, C7, signaling, network, telecommunication
