The Telephone Network

The public switched telephone network (PSTN) is the world-wide telephone system and usually this network system uses the digital technology. In the past, it was used for voice communication only but now it is playing very important role for data communication in the computer network such as in the Internet. There are different types of telephone lines that are used for data communication in the network. These are discussed below.

1. Dial-Up Lines

It is a temporary connection that uses one or more analog telephone lines for communication. Modem is used at the sending end as well as at the receiving end. A telephone number is dialed from the sending end through modem and the modem at the receiving end answers the call. In this way, connection is established for data communication between computers or electronic devices. In dial up line connection, the cost of data communication is very low. The mobile users can also access the Internet through this connection.

2. Dedicated Lines

It is a permanent connection that is used to establish connection between two devices permanently. It is better than dial-up line connection because dedicated lines provide a constant connection. These types of connections may be digital or analog. The data transmission speed, of digital lines is very fast as compare to analog dedicated line. The data transmission speed is also measured in bits per second (bps). In dial-up and dedicated lines, it is up to 56 Kbps. The dedicated lines are mostly used for business purposes. The most important digital dedicated lines are described below.

1) ISDN Lines

ISDN stands for Integrated Services Digital Network. It is a set of standards used for digital transmission over telephone line. The ISDN uses the multiplexing technique to carry three or more data signals at once through the telephone line. It is because the data transmission speed of ISDN line is very fast. In ISDN line, both ends of connections require the ISDN modem and a special telephone set for voice communication. Its data transmission speed is up to 128 Kbps.

2) DSL

DSL stands for Digital Subscriber Line. It is another digital line. In DSL, both ends of connections require the network cards and DSL modems for data communication. The data transmission speed and other functions are similar as ISDN line. DSL transmits data on existing standard copper telephone wiring. Some DSLs provide a dial tone, which allows both voice an4 data communication.

3) ADSL

The ADSL (Asymmetric Digital Subscriber Line) is another digital connection. It is faster than DSL. ADSL is much easier to install and provides much faster data transfer rate. Its data transmission speed is from 128 Kbps up to 10 Mbps. This connection is ideal for Internet access.

4) Cable Television Line

The Cable Television (CATV) line is not a standard telephone line. It is a dedicated line used to access the Internet. Its data transmission speed is 128 Kbps to 3 Mbps.

A cable modem is used with the CATV. it provides a high speed Internet connections through the cable television network. A cable modem sends and receives digital data over the cable television network.

To access the Internet using the CATV network, the CATV Company installs a splitter inside your house. From the splitter, one part of the cable runs to your television and other part connects to the cable modem. A cable modem usually is an external device, in which one end of a cable connects to a CATV wall outlet while the other end plugs into a port (such as on an Ethernet card) in the system unit.

5) T-Carrier Lines

It is very fast digital line that can carry multiple signals over a single communication line whereas a standard dialup telephone line carries only one signal. T-carrier lines use multiplexing so that multiple signals share the line. T-carrier lines provide very fast data transfer rates. The T-carrier lines are very expensive and large companies can afford these lines. The most popular T-carrier lines are:

1. TI Line
2. T3 Line
   
   

i) T1 Line

The most popular T-carrier line is the Ti line (dedicated line). Its data transmission speed is 1.5 Mbps. Businesses often use Ti lines to connect to the Internet. Many ISPs use Ti, lines to connect to the Internet backbone. Another type of TI line is the fractional TI line. It is slower than TI line but it is less expensive. The home and business users use this line to connect to the Internet and share a connection to the Ti line with other users.

ii) T3 Line

Another most popular and faster T-carrier line is T3 line. Its data transmission speed is 44 Mbps. It is more expensive than Ti line. The main users of T3 line are telephone companies and ISPs. The Internet backbone itself also uses T3 lines.

6) Asynchronous Transfer Mode (ATM)

It is very, fast data transmission connection line that can carry data, voice, video, multimedia etc. Telephone networks, Internet and other network use ATM. In near future, ATM will become the Internet standard for data transmission instead of T3 lines. Its data transmission speed is from 155 Mbps to 600 Mbps.