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.

Multifrequency (MF) signaling can still be found in traces within the United States, and it is still often found on international interfaces. On international interfaces outside of North America, MF is still used via the CCITT System 5 (C5) implementation. C5 is quite similar to Bell MF and was developed jointly by Bell Laboratories and the British Post Office [102]. R2 is the MF system that was deployed outside North America and is still used in less developed nations. R2 was developed by CEPT (which later became ETSI; see Chapter 2) and was previously known as Multifrequency Compelled (MFC) signaling. The CCITT later defined an international version; see Chapter 2 for additional information regarding the international version [102].

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.

Table 1-3 shows the tone combinations for Bell System MF, R1, and C5. R2 tone combinations are not shown.

Table 1-3. Tones Used to Create MF Signals

Digit	Frequencies
	700	900	1100	1300	1500	1700
1	+	+
2	+		+
3		+	+
4	+			+
5		+		+
6			+	+
7	+				+
8		+			+
9			+		+
0				+	+
KP			+			+
ST					+	+
11 [*]	+					+
12 [*]		+				+
KP2 [*]				+		+

^[*] = Used only on CCITT System 5 (C5) for international calling.

As stated, many international trunks still use C5. Signal KP2 indicates that the number is an international number; by inference, KP indicates that the number is a national number. International operators also use codes 11 and 12. More details on C5 are available in ITU-T Q.152. Supervision signals for MF systems are performed on FDM trunks by the use of Single Frequency (SF), which we describe in the following section.

For circuit supervision, both Bell System MF and R1 use Single Frequency (SF) on FDM trunks and employ robbed bit signaling on TDM controlled trunks. C5 uses a different set of MF tones for supervisory signaling.

Supervisory Signals

Single frequency systems, robbed bit signaling, and digital signaling are all types of supervisory signals used by CAS.

Single Frequency(SF)

Single Frequency (SF) was used for supervisory signaling in analog CAS-based systems. North America used a frequency of 2600 Hz (1600 Hz was previously used), and Great Britain used 2280 Hz (as defined in British Telecom's SSAC15 signaling specification). When in an on-hook state, the tone is present; when in an off-hook state, the tone is dropped.

Table 1-4 details the tone transitions Bell System MF and R1 use to indicate the supervision signals. C5 uses a combination of both one and two in-band signaling tones, which are not presented here.

Table 1-4. Bell System MF and R1 Supervision Signaling

Direction	Signal Type	Transition
Forward	Seizure	On-hook to off-hook
Forward	Clear-forward	Off-hook to on-hook
Backward	Answer	On-hook to off-hook
Backward	Clear-back	Off-hook to on-hook
Backward	Proceed-to-send (wink)	Off-hook pulse, 120–290 ms

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.

Subscriber Line Signaling A regular subscriber line (that is analog) still uses in-band access signaling. For example, DTMF is used to signal the dialed digits and the frequencies used are within the voice band (see Table 1-1). You can prove that DTMF uses in-band signaling by using a device, such as a computer, to generate the tones for each digit (with correct pauses). Simply play the tones from the computer speaker down the mouthpiece of a touch-tone telephone. This allows you to dial a number without using the telephone keypad. Because the signaling is sent down the mouthpiece, you can be certain that it traveled within the user's voice frequency range.

FDM analog systems nearly always reserve up to 4000 Hz for each circuit, but only use 300–3400 Hz for speech; therefore, signaling is sent above the 3400 Hz (and below 4000 Hz). This is known as out-of-band signaling and is used in R2 for supervisory signaling. Unlike with in-band signaling, no contention exists between user traffic and signaling. North America uses a frequency of 3700 Hz, and CCITT (international) uses 3825 Hz. Table 1-5 details the tone transitions that indicate the supervision signals used in R2 and R1.

Table 1-5. R2 Supervision Signaling

Direction	Signal Type	Transition
Forward	Seizure	Tone-on to tone-off
Forward	Clear-forward	Tone-off to tone-on
Backward	Answer	Tone-on to tone-off
Backward	Clear-back	Tone-off to tone-on
Backward	Release-guard	450 ms tone-off pulse
Backward	Blocking	Tone-on to tone-off

R2 does not use a proceed-to-send signal; instead, it includes a blocking signal to stop the circuit that is being seized while maintenance work is performed on the trunk. The release guard signal indicates that the trunk has been released after a clear-forward signaling, thereby indicating that the trunk can be used for another call.

Digital

Supervisory signaling can be performed for R2 on digital TDM trunks. On an E1 facility, timeslot 16 is set aside for supervisory signaling bits (TS16). These bits are arranged in a multiframe structure so that specific bits in the multiframe's specific frames represent the signaling information for a given TDM audio channel. See Chapter 5, "The Public Switched Telephone Network (PSTN)," for explanation of facilities and timeslots.

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.