11.5 SIGNALING

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In a telephone network, before the conversation takes place, a circuit has to be established. Lot of information is to be exchanged between the subscriber terminal and the switch, and between the switches for the call to materialize and later get disconnected. This exchange of information is known as signaling.

Four T1 carriers are multiplexed to obtain T2 carrier. Note that the data rate of T2 carrier is 6.312 Mbps and not 4 × 1.544 Mbps (=6.176 Mbps). The extra bits are added for synchronization. Similarly, extra bits are added to form T3 and T4 carriers. The digital hierarchy is standardized by ITU-T to provide high capacity trunks between large switches.

Signaling is used to indicate/exchange the following information:

Calling and called party numbers
Availability/non-availability of network resources such as trunks
Availability of the called party
Billing information
Network information such as busy trunks, faulty telephones etc.
Routing information as regards how the call has to be routed.
To provide special services such as calling card facility, toll-free numbers, called party, paying the telephone bill etc.

In the telephone network, there are three types of signaling.

In-band signaling: When a subscriber lifts his telephone, he gets a dial tone which is fed by the switch. The subscriber dials the called number and the switch interprets the number and finds out to which switch the called subscriber is connected. The switch establishes a connection to the other switch and the other switch checks whether the called subscriber is available. If she is available, the path is established and the conversation takes place. When the caller puts back the telephone, the circuit is freed. Prior to the conversation and after the conversation, the information exchanged is the signaling information. Normally, this signaling information is exchanged in the same communication link in which the conversation takes place. This signaling is known as in-band signaling. In-band signaling is simple, but creates problems because the tones corresponding to the signaling fall in the voice band and cause disturbances to the speech.

The signaling information exchanged between the subscriber and the switch consists of dialed digits and the various tones such as dial tone, busy tone, ring back tone, etc. This signaling information is carried on the local loop using the in-band signaling mechanism.

Channel-Associated Signaling (CAS): Between two switches, separate channels are used for signaling and information transfer as shown in Figure 11.6. For instance, when two switches are connected using an E1 link, one time slot is used for signaling. This results in substantial savings as traffic channels are not used for transferring the signaling information.

Common channel signaling (CCS): Another mechanism for signaling is to have a separate communication network for exchanging signaling information. When two network elements have to exchange signaling information, they use this independent network, and the actual voice conversation takes place using the voice trunks. This mechanism (though it appears complex and calls for additional infrastructure) is extremely efficient and is now being widely used.

An ITU-T standard called Signaling System No. 7 (SS7) is used for common channel signaling. SS7 uses concepts of data communications, and we will discuss the details of SS7 in a later portion of the book (in Chapter 26, "Signaling System No. 7").

Two switches in the PSTN exchange signaling information using dedicated time slots in the trunks. This is known as channel associated signaling.

Note

In common channel signaling (CCS), a separate data communication network is used to exchange signaling information. CCS is much more efficient and reliable compared to Channel-Associated Signaling. An ITU-T standard called Signaling System No. 7 (SS7) is used for CCS. SS7 is now used in PSTN, ISDN, and mobile communication systems.

Summary

In this chapter, the architecture of PSTN is presented. The PSTN consists of subscriber terminals, local loops, switches, and trunks. The local loop is a dedicated link between the subscriber terminal and the switch. Currently, twisted copper pair is used as the local loop, but optical fiber-based local loops are likely to be installed in the future. The switches are interconnected through trunks. Normally digital trunks are used. The basic digital trunk supports 30 voice channels and is called an E1 trunk. The PSTN operates in circuit switching mode: a connection is established between two subscribers and, after the conversation, the circuit is disconnected. The signaling information is exchanged between the subscriber and the switch as well as between switches. The signaling used in PSTN is in-band signaling and channel associated signaling.

References

Ray Horak. Communications Systems and Networks, Third Edition. Wiley-Dreamtech India Pvt. Ltd., 2002. Chapter 5 of this book is on PSTN.
50th Anniversary Commemorative Issue of IEEE Communications Magazine. May 2002. This issue traces the major developments in communications.

Questions

What are the network elements of PSTN? Explain the function of each network element.
What is the difference between pulse dialing and DTMF dialing?
Explain circuit-switching operation.
What are the different types of signaling used in PSTN?

Exercises

1.	Two switches need to be interconnected to carry 60 voice channels. Calculate the number of T1 carriers required.
2.	Design a circuit to generate/decode DTMF tones. You can use a chip such as MITEL 8880.
3.	Study the characteristics of an integrated circuit (IC) that does PCM coding and decoding.
4.	The present switches are hardware driven. Nowdays, "soft switches" are being developed. Prepare a technical paper on soft switches.

Answers

1.	Each T1 carrier supports 24 voice channels. To support 60 voice channels, three T1 carriers are required. Two T1 carriers are not sufficient as they can support only 48 voice channels. However, when 3 T1 carriers are used, 12 voice channels remain free. It is a good practice to keep some spare capacity for future expendability.
2.	You can see the datasheet of the IC 8880 from MITEL. This is a DTMF chip, and you can use the reference circuit given in the data sheet.
3.	The IC 44233 generates PCM-coded speech.
4.	The PSTN is based on circuit-switching. The switches contain (a) the line cards that interface with the subscriber equipment; and (b) processor-based hardware/software to switch the calls and establish a circuit between the two subscribers. Now the trend is to use packet switching for voice calls using the computer networking protocols. Switches that handle packet- switched voice are called the soft switches.

Projects

Simulate a DTMF generator on a PC. The telephone keypad has to be simulated through software. When you click on a digit, the corresponding tones have to be generated (the tone frequencies are given in Figure 11.2) and played through the sound card of your PC.
Develop a telephone tapping software package. You can connect a voice/ data modem in parallel to the telephone line. When someone calls your telephone, the complete conversation has to be recorded on the PC to which the modem is connected. You can use the Telephony API (TAPI) to develop the software.

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