May 8, 2011, 11:01 a.m.
posted by who
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.
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 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 operate slightly differently for analog and digital 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:
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 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.
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).
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 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.
Three new signals are introduced in Figure: Release-guard, Busy-flash, and Forward-transfer.
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.
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 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.
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.
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.
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.
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.
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.
The following inter-register group sequence rules are used to identify the signal's group:
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.