Info

1860 and 1980

660 and 540

The group I forward signals represent the digits of the called party number. Group II signals indicate the category of the calling party, and group III signals, which are used in some national networks only, represent the digits of the calling party's number.

In R2 and CCITT-R2 signaling, the incoming exchange controls the signaling sequence. A group A signal requests a particular next forward signal, or indicates that register signaling has ended. Group A signals can be sent by intermediate exchanges, and by the terminating local exchange.

The group B backward signals are sent by the terminating local exchange only. They acknowledge a forward signal, and convey call-charging instructions, and called-party status.

The incoming and outgoing exchanges must know the type of the signal that is being received. To accomplish this, a R2 register signaling sequence follows certain rules:

• The first signal received by an incoming exchange is a group I signal.

• The outgoing exchange interprets received backward signals as group A signals, until it receives a group A signal that indicates that the next backward signal will be a group B signal. The receipt of a group B signal always ends the signal sequence.

4.4.4 National R2 Interregister Signaling Sequences

We now explore a few register signaling sequences in a national network. In these examples, the meanings of the forward and backward register signals, which vary somewhat from country to country, are as listed in Tables 4.4-2 and 4.4-3.

Figure 4.4-4 shows the signaling on trunk T, for a call from subscriber S! to S2, whose subscriber number is 34-5678. Exchange X has received the called number from S,. It seizes trunk T, and sends a seizure signal. The exchange also connects an outgoing register (OR) to the trunk, and orders it to send 1-3 (the first digit of the called number). When exchange Z receives the seizure signal, it connects an incoming register (IR) to the trunk. The register receives the 1-3, and acknowledges with A-l (send next digit). The acknowledgment of the first digit indicates to exchange X that an incoming register has been connected to the trunk (this is why R2 signaling does not include a proceed-to-send signal). Exchange X sends the subsequent digits, and exchange Z acknowledges the second through fifth digits with A-l. On receipt of the sixth digit (1-8), exchange Z knows that the called number is complete, and acknowledges with A-3, which requests the calling party category, and indicates that the next backward signal is a group B signal. After receiving calling category (II-2), exchange Z sends a group B signal that contains information on the called party status and on charging. The group B signal ends the interregister signaling.

Table 4.4-2 Example of national R2 forward interregister signals.

Group I: Digits in the called number

1-1 digit 1 1-2 digit 2 1-3 digit 3 1-4 digit 4 1-5 digit 5 1-6 digit 6 1-7 digit 7 1-8 digit 8 1-9 digit 9 1-10 digit 0

1-15 end of called number

I-11 through 1-14 are not used

Group II: Calling party category

II-1 Operator with trunk-offering II-2 Subscriber

II-3 Pay-phone

II-4 through 11-15 are not used

Group III: Digits in the calling number

III-l digit 1 III-2 digit 2 III-3 digit 3 III-4 digit 4 III-5 digit 5 III-6 digit 6 III-7 digit 7 III-8 digit 8 III-9 digit 9 111-10 digit 0

III-15 end of calling number III-ll through 111-14 are not used

End-to-end Interregister Signaling. Now consider the signaling for a call from S, to S2 that is routed via intermediate exchange Y—see Fig. 4.4-5. The subscriber number of S2 is again 34-5678. In some countries, R2 register signaling is link-by-link. This means that exchange Y receives the entire called number from exchange X, then seizes trunk T2, and sends the number to exchange Z.

However, in most national networks, R2 register signaling is end-to-end. In this mode, outgoing register (OR) in originating exchange X communicates successively with incoming registers (IR) in exchanges Y and Z. The initial register signaling is between exchanges X and Y. Having received the called

Table 4.4-3 Example of R2 national backward interregister signals.

Group A

A-1 Send next digit of called number. A-2 Resend first digit of called number.

A-3 Send calling line category, and prepare to receive a group B signal

A-4 Congestion

A-5 Send calling line category

A-7 Send next digit of calling number

A-6 Send next digit of calling number

A-8 Digit n of called number received; resend digit (n-1)

A-9 Digit n of called number received; resend digit (n-2)

A-10 through A-15 are not used

Group B

B-l Called subscriber idle, charge

B-2 Called subscriber busy

B-5 Called subscriber idle, do not charge

B-6 Called party idle, call to be held under control of called subscriber

B-7 Vacant number received

Local Exchange X

Exchange Y

Local Exchange Z

Seizure

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