CAS






CAS

CAS exists in many varieties that operate over various analog and digital facilities. The analog facilities are either two- or four-wire and the digital facilities are either North American T1 or European E1. This section discusses Bell System MF, CCITT No. 5, R1, and R2 CAS systems.

The main areas of discussion for each CAS system are supervision signaling and address signaling over analog and digital facilities. Bell System uses in-band MF for address signaling. For supervision signaling it uses Single Frequency for analog and a/b bits for digital trunks. CCITT No. 5 was designed for analog trunks and uses different MF signals for supervision and address signaling. In-band tone detection is used to detect and interpret the MF signals.

It is important to cover a few points before proceeding with a discussion of CAS systems. When a call is placed from Exchange A toward Exchange B, Exchange A is considered the outgoing exchange and Exchange B the incoming exchange.

One-way trunks are trunks on which only Exchange A or Exchange B can initiate a call. Exchanges A and B can initiate a call over two-way trunks. Double seizures can occur over twoway trunks when both exchanges try to seize the trunk at the same time, however. When this occurs, mechanisms such as timers are used to detect and resolve such events.

Three groups of signals are present in channel-associated interexchange signaling systems:

  • Supervision Signals These signals represent events that occur on a trunk and can be specific to the CAS variant. Signals include seizure, wink, and answer; they also are referred to as line signals.

  • Address Signals These signals typically represent the digits dialed or called party number and, in some instances, other information. In this chapter, address signals are based on MF signaling and can be system- or variant-specific.

  • Tones and Announcements These include tones such as ringing and busy tones and announcements such as, "The number you have dialed is no longer in service."

One more concept to cover before moving forward is that of service circuits. Service circuits are used in most exchanges to send and receive address signals and tones, as well as to play announcements. These circuits are typically system-specific; the processor connects a path from the trunk to the appropriate service circuit inside the switch. The pools of service circuits are temporarily used to send and receive tones or to play announcements.

Bell System MF Signaling

This section introduces the MF signaling systems developed by Bell System in the 1950s. The Bell System is still used today in local networks in the United States and is nearly identical to the R1 signaling system discussed later in this chapter.

With Bell System MF signaling, which you can use on one-way or two-way trunks, supervision and address signaling are signaled link-by-link. Supervision signaling is accomplished through a Single Frequency tone for analog facilities and through robbed-bit signaling for digital facilities. Address information is sent through MF tones.

Supervision Signaling

Supervision signals are continuously sent by endpoint exchanges indicating the state of the trunk. This is known as continuous two-state signaling. States can be different at each endpoint of the trunk. MF signaling is used to indicate on-hook and off-hook states, as listed in Figure.

Supervision Signals

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


Supervision signals operate slightly differently for analog and digital trunks.

Analog Trunks

A Single Frequency 2600 Hz tone is used to indicate trunk state between exchanges over analog facilities. This tone is applied in-band over the trunk and is turned off when a call is in progress or established. Therefore, the state is on-hook or idle when the tone is present and off-hook or in use when the tone is absent. The supervision signals for the Single Frequency method are illustrated in Figure.

Forward and Backward Supervision Signals for a Call


In Figure, assume that Switch A sends the forward signals and Switch B sends the backward signals. Switch A sends a forward seizure or off-hook signal to Switch B on a chosen trunk. Then, Switch B sends a backward wink or proceed-to-send to Switch A and waits for address signaling or dialed digits. After the digits are sent and the call is answered, Switch B sends a backward answer or off-hook to Switch A, enabling an end-to-end voice path.

In this case, the calling party hangs up first and a clear-forward is sent from Switch A to Switch B. When the called party hangs up, a clear-back signal is sent by Switch B.

Two important aspects of this signaling method need to be discussed:

  • First, Bell System MF does not have backward signaling for connections that fail during setup. Therefore, the exchange where the call failed must connect an announcement server indicating to the calling party that a problem occurred.

    The signaling system then relies on the calling party to release or drop the call so that clear-forward procedures can be initiated.

  • Second, no release guard-type signal exists, and timers are used after trunks are released. Therefore, after an exchange releases a trunk, it initiates a timer for approximately 1 second. After this timer expires, the exchange assumes that the trunk was released at the other end and is available for use.

Digital Trunks

The digital trunks most commonly used today are either T1 or E1 facilities (as described in the "Physical LayerMTP L1" section of Chapter 4). With digital trunks, bits are robbed from specific frames and are used for signaling purposes. This discussion focuses on T1 digital trunks.

T1 has two types of framing formats: Super Frame (SF) and Extended Superframe (ESF). The least significant bits are robbed from frames 6 and 12 for SF and frames 6, 12, 18, and 24 for ESF. These bits are referred to as the Sa and Sb bits for SF, and the Sa, Sb, Sc, and Sd bits for ESF. Robbing these bits has a negligible effect on voice quality.

The SF signaling bitsSa and Sbare equal to each other and provide two-state, continuous supervision signaling. Bit values of zero are used to indicate on-hook, and bit values of 1 are used to indicate off-hook.

Address Signaling

Address signaling is used to indicate the called and calling number as well as to identify the start and end of the address information. In the Bell System MF method, address signals are a combination of two voice-band frequencies chosen from six different frequencies, as illustrated in Figure.

Bell System MF Address Signals

Signal

Frequencies in Hz

Digit 1

700 and 900

Digit 2

700 and 1100

Digit 3

900 and 1100

Digit 4

700 and 1300

Digit 5

900 and 1300

Digit 6

1100 and 1300

Digit 7

700 and 1500

Digit 8

900 and 1500

Digit 9

1100 and 1500

Digit 0

1300 and 1500

KP (start)

1100 and 1700

ST (end)

1500 and 1700


The address signaling sequence is initiated with a KP or start-of-pulsing signal and terminated with an ST or end-of-pulsing signal. Two important timing intervals exist:

The KP signal's duration is from 90 to 110 ms, and the ST signal's duration is from 61 to 75 ms. The silent interval between signals also is from 61 to 75 ms. Figure demonstrates supervision and address signaling sequences.

Supervision and Address Signaling Sequences


Address signaling uses two other key information digits. The codes in this information (or I bits) indicate the calling number or Automatic Number ID (ANI), as well as operator services (see Figure).

Address Signaling Codes

I-Codes

Information

I = 00

Calling number is available.

I = 02

Calling number is not available.

I = 06

Hotel room identification required.

I = 10

Test call.


The information codes are sent after the KP signal and before the called party number. I codes 02 and 06 identify that operator assistance is required to proceed with these calls.

CCITT No. 5 Signaling

The CCITT adopted the CCITT No. 5 signaling system in the 1960s for use in international networks. This signaling system is still used today, usually on long international trunks and, in some cases, over transoceanic and satellite links. This signaling system was designed to operate over analog trunks equipped with Time Assignment Speech Interpolation (TASI). TASI is similar to voice activity detection (VAD), in that it enables unused bandwidth (silences or pauses in speech) to be used for other phone conversations. Link-by-link and in-band signaling are used for both supervision and address signaling.

Supervision Signaling

Supervision signaling is accomplished by two frequencies, sent either individually or in combination. CCITT No. 5 uses compelled supervision signaling, whereby the signaling tone is left on until an acknowledgment is received.

The two in-band frequencies are f1, which equals 2400 Hz, and f2, which equals 2600 Hz. The combination of f1 and f2 produces a composite signal; these signals and frequencies are listed in Figure.

CCITT No. 5 Supervision Signals

Direction

Signal Type

Frequency

Forward

Seizure

f1

Backward

Proceed-to-send

f2

Backward

Answer

f1

Forward

Acknowledgment

f1

Backward

Clear-back

f2

Forward

Acknowledgment

f1

Forward

Clear-forward

f1 and f2

Backward

Release-guard

f1 and f2

Backward

Busy-flash

f2

Forward

Acknowledgment

f1

Forward

Forward-transfer

f2


Three new signals are introduced in Figure: Release-guard, Busy-flash, and Forward-transfer.

  • Release-guard This signal is used by the incoming exchange to acknowledge a clear-forward from the outgoing exchange. It also indicates to the outgoing exchange that the trunk is now available for an incoming call.

  • Busy-flash This signal is used by the incoming exchange to indicate to the outgoing exchange that call setup cannot be extended toward the destination.

  • Forward-transfer This signal is used on calls for operator services.

Address Signaling

In CCITT No. 5, address signaling is based on the combination of two frequencies, as illustrated in Figure. The address signaling sequence starts with KP1 for national numbers and KP2 for international numbers. Codes 11 and 12 are used to connect international operator services.

CCITT No. 5 Address Signals

Signal

Frequencies in Hz

Digit 1

700 and 900

Digit 2

700 and 1100

Digit 3

900 and 1100

Digit 4

700 and 1300

Digit 5

900 and 1300

Digit 6

1100 and 1300

Digit 7

700 and 1500

Digit 8

900 and 1500

Digit 9

1100 and 1500

Digit 0

1300 and 1500

Code 11

700 and 1700

Code 12

900 and 1700

KP1

1100 and 1700

KP2

1300 and 1700

ST

1500 and 1700


R1

The CAS system known as R1 is available in the International Telecommunication Union Telecommunication Standardization Sector (ITU-T) Q.310 to Q.332 specifications. This signaling system is almost identical to Bell System MF signaling and, therefore, is not further discussed.

R2

R2 signaling is a CAS system developed in the 1960s that is still in use today in Europe, Latin America, Australia, and Asia. Originally known as multi-frequency code (MFC) signaling, R2 signaling exists in several country versions or variants and in an international version called CCITT-R2.

R2 signaling operates over two- or four-wire analog and digital trunks and does not operate over TASI-equipped trunks or satellite links. R2 signaling is more suitable for relatively short international trunks. One of the differentiating aspects of this system compared to R1 is its register or inter-register signaling.

This section focuses on supervision and inter-register signaling for CCITT-R2 and National R2 signaling systems.

Supervision Signaling on Analog Trunks

For the purposes of supervision signaling on analog trunks, this section covers operation over four-wire trunks. The transmission path is divided into two parts: a 300- to 3400 Hz voice-band and a 3825Hz narrow-band for signaling. In this method, filters separate the signaling tone from the voice path. This is considered out-of-band signaling, even though signaling is over the same facility.

CCITT-R2 uses the tone-on-idle signaling supervision method; National R2 uses pulse signaling.

CCITT-R2

This method is commonly used on one-way trunks, is tone-on-idle, and provides two-state signaling. The forward and backward signals and transition states are similar to Bell System MF signaling and are illustrated in Figure.

CCITT-R2 Supervision Signals

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

Tone-off to tone-on

Backward

Blocking

Tone-on to tone-off


National R2

National R2 signaling has many country variants. Most versions of National R2 use pulse outof-band supervision signals, however. Examples of National R2 supervision signals are illustrated in Figure.

Examples of National R2 Supervision Signals

Direction

Signal Type

Pulse Duration in ms

Forward

Seizure

150

Forward

Clear-forward

600

Backward

Answer

150

Backward

Clear-back

600

Backward

Release-guard

600

Backward

Blocking

Continuous


Supervision Signaling on Digital Trunks

R2 signaling operates over E1 digital facilities (described in the "Physical LayerMTP L1" section of Chapter 4). E1 has 32 time-slots, numbered TS0 to TS31, whereby TS1TS15 and TS17TS31 are used to carry voice encoded with pulse code modulation (PCM), or to carry 64 kbps data.

Sixteen consecutive frames are in the SF format, and they are numbered 0 to 15. TS16 in frame 0 is used for SF alignment, and TS16 in the remaining frames (115) is used for trunk signaling. Four status bits are used from TS16 for signaling. They are called a, b, c, and d.

In the case of CCITT-R2 signaling, only the a and b bits are used. The c and d bits are set to 0 and 1, respectively. An idle state is denoted when a and b are equal to 1 and 0. Signaling is continuous. For two-way trunks, the supervision roles for forward and backward signaling vary on a call-by-call basis. Figure illustrates the R2 supervision signal, transition, and direction used on digital trunks.

R2 Supervision Signaling on Digital Trunks

Direction

Signal Type

Transition

Forward

Seizure

a,b: 1,0 to 0,0

Forward

Clear-forward

a,b: 0,0 to 1,0

Backward

Seizure acknowledgment

a,b: 1,0 to 1,1

Backward

Answer

a,b: 1,1 to 0,1

Backward

Clear-back

a,b: 0,1 to 1,1

Backward

Release-guard

a,b: 0,1 to 1,0


Inter-Register Signaling

The concept of address signaling in R2 is slightly different from that used in the other CAS systems previously discussed. In the case of R2, the exchanges are considered registers, and the signaling between these exchanges is called inter-register signaling. Inter-register signaling uses forward and backward in-band MF signals to transfer called and calling party numbers as well as the calling party category.

In this case, signaling is compelled because the registers in the outgoing and incoming exchanges hold the signal until an acknowledgment is received. The signals consist of two voice-band frequencies and are listed in Figure.

CCITT-R2 and National R2 Inter-Register Signal Frequencies

Signal

Forward Frequency in Hz

Backward Frequency in Hz

Digit 1

1380 and 1500

1140 and 1020

Digit 2

1380 and 1620

1140 and 900

Digit 3

1500 and 1620

1020 and 900

Digit 4

1380 and 1740

1140 and 780

Digit 5

1500 and 1740

1020 and 780

Digit 6

1620 and 1740

900 and 780

Digit 7

1380 and 1860

1140 and 660

Digit 8

1500 and 1860

1020 and 660

Digit 9

1620 and 1860

900 and 660

Digit 0

1740 and 1860

780 and 660

Not used

1380 and 1980

1140 and 540

Not used

1500 and 1980

1020 and 540

Not used

1620 and 1980

900 and 540

Not used

1740 and 1980

780 and 540

End of #

1860 and 1980

660 and 540


Groups for Inter-Register Signaling

In R2 signaling, the forward and backward signals can have different meanings depending on which group is used. Three groups of forward signals and two groups of backward signals exist. The forward groups are I, II, and III, and the backward groups are A and B.

  • Group I These forward signals represent the called party number or dialed digits.

  • Group II These forward signals identify the calling party category.

  • Group III These forward signals represent the digits of the calling party number.

  • Group A These backward signals indicate if the signaling ended or if a particular forward signal is required.

  • Group B These backward signals are sent by the terminating switch to acknowledge a forward signal, or to provide call charging and called party information.

The following inter-register group sequence rules are used to identify the signal's group:

  • The initial signal received by the incoming exchange is a Group I signal.

  • Outgoing exchanges consider backward signals as Group A signals.

  • Group A signals received by outgoing exchanges are used to identify whether the next signal is a Group B signal.

  • Group B signals always indicate an end-of-signaling sequence.

Feature Support

The end-to-end information and status that National R2 signaling provides enable support for several features. These features include free calls, called party hold, malicious call tracing, and release on failed connections.



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