11.1 PSTN NETWORK ELEMENTS

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11.1 PSTN NETWORK ELEMENTS

The elements of PSTN are shown in Figure 11.1. The PSTN consists of:

• Subscriber terminals

• Local loops

• Switches (or exchanges)

• Trunks

Figure 11.1: Elements of PSTN.

11.1.1 The Subscriber Terminal

In its simplest form, the subscriber terminal is the ordinary telephone with a keypad to dial the numbers. There are two types of dialing: (a) pulse dialing; and (b) DTMF dialing.

The Public Switch Telephone Network (PSTN) consists of subscriber terminals, local loops, switches or exchanges, and trunks.

Pulse dialing: In pulse dialing, when a digit is dialed, a series of pulses is sent out. When the user dials 1, 1 pulse is transmitted to the exchange, when 2 is dialed, 2 pulses are sent, and so on; when 0 is dialed, 10 pulses are sent. The exchange uses a pulse counter to recognize the digits. Since pulses are likely to be distorted over the medium due to attenuation, pulse recognition accuracy is not very high. Many old switches and telephones support only pulse dialing, though slowly pulse dialing is becoming outdated.

DTMF dialing: DTMF stands for Dual Tone Multi Frequency. DTMF dialing is also known as tone dialing or speed dialing. When a digit is dialed, a combination of two sine waves is sent. The various combinations of tones are shown in Figure 11.2. When 1 is dialed, a combination of 697Hz and 1209Hz is sent from the terminal to the exchange. A DTMF recognition chip is used at the exchange to decode the digits. DTMF recognition is highly accurate and is becoming predominant. Most present-day telephones support DTMF.

Figure 11.2: DTMF digits.

The two types of dialing supported by the subscriber terminals are (a) pulse dialing and (b) tone dialing or DTMF dialing. In pulse dialing, for each digit, a series of pulses is sent to the switch. In tone dialing, for each digit, a combination of two sine waves is sent.

Note 

DTMF dialing is more reliable as compared to pulse dialing. Pulses are likely to get distorted due to transmission impairments, and so pulse dialing is not always reliable. On the other hand, tone detection is very reliable, and hence DTMF dialing is now extensively used.

11.1.2 Local Loop

The local loop is a dedicated link between a subscriber terminal and the switch. Present local loop uses twisted-pair copper wire as the local loop. In the future, fiber is being planned to provide high bandwidth services to subscribers. In remote and rural areas, where laying the cable is costly or infeasible (due to terrains such as hills etc.), radio is used. This wireless local loop (WLL) has many advantages: fast installation, low maintenance costs. Moreover, it obviates the need for digging below ground. Hence WLL deployment is also catching up, even in urban areas.

Local loop is the dedicated link between the subscriber terminal and the switch. Twisted pair copper wire is the most widely used medium for local loop. Nowdays, wireless local loop is gaining popularity. In the future, optical fiber will be used as the local loop to support very high data rates.

11.1.3 Switch

In earlier days, mechanical and electromechanical switches (strowger and crossbar switches) were used extensively. Present switches use digital technology. These digital switches have the capacity to support several thousand to a few million telephones.

To cater to large areas, the switching system is organized as a hierarchy as shown in Figure 11.3. At the lowest level of the hierarchy, the switches are called end offices or local exchanges. Above that, there will be toll exchanges (class 4 switches), primary (class 3) switches, secondary (class 2) switches, and regional (class 1) switches.

Figure 11.3: Hierarchical switching system of PSTN.

In a city, an exchange is designated as a toll exchange and acts as the gateway for all long distance calls. Similarly, a few gateway switches carry calls from one nation to another. However, the billing for subscribers is always done by the parent exchange (the exchange to which the subscriber is connected).

In PSTN, the switching system is organized as a hierarchical system. The local switches are connected to toll exchanges. Toll exchanges are connected to primary switches which are in turn connected to secondary and regional switches.

Note 

The switch to which a subscriber is connected is called the parent switch. Billing is always done by the parent switch.

11.1.4 Trunks

Trunks interconnect the switches. Based on traffic considerations as well as administrative considerations, the interconnection between the switches through trunks is decided. Nowdays, trunks are mostly digital: speech is converted to PCM format, multiplexed, and transmitted through the trunks. The trunks can be T1 or E1 links if the switches are of small capacity (say, 512 ports). Depending on which switches are connected, the trunks are categorized as intracity trunks and intercity trunks.

In the following sections, we will study the important technical aspects of local loops, switches, and trunks.

The switches are interconnected through trunks. Most of the trunks are digital and use PCM format for carrying the voice traffic. E1 trunks carry 30 voice channels.

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