Data Transmission: Analog & Digital

Data Transmission: Analog and Digital

Analog Transmission:

Dominated the last 100 years and is here for a while yet networks designers made use of the existing telephone network which was aimed at voice transmission. This is actually very poor for computer networking for example, 2 computers connected by a direct cable can achieve a data rate of up to 100Mbps with very low error rate using phone lines, 56Kbps is the maximum transmission speed with a relatively high error rate. It is approximately 10 orders of magnitude worse the cost of bus ticket town versus a moon landing is same order of magnitude

Modems:

Phone lines deal with frequencies of 300 to 3000 Hz. A computer outputs a serial stream of bits (1’s,0’s). A Modem is a device that accepts such a bit stream and converts it to an analog signal, using modulation it also performs the inverse conversion. These two computers can be connected using two modems and phone line.

Using a modem, a continuous signal (tone) is sent in the range 1000 to 2000Hz. To transmit information, this carrier signal is modulated. Its amplitude, frequency, phase or a combination can be modulated.

This diagram illustrates the carrier signal always present between two modems.

One common form of modulation Frequency Modulation (FM)

Frequency Modulation:

FM Modulation Wave

This is the most common form of modulation used for data transmission. In FM the carrier signed is modulated between two different frequencies (say 1200Hz and 2200Hz) without affecting the amplitude in accordance with the digital signal that it must send.

 The serial interface between the modem and computer is governed by the RS232 standard (also known as CCITT V24 standard).

Digital Transmission:

Digital transmission takes place in the form of pulses representing bits (1’s and 0’s). This is the type of communication used internally in computers. The high-speed trunks linking central phone exchanges use digital transmission. It has a lower error rate than analog transmission.

The local loop (from phone to exchange) is still analog. This must be converted at the exchange to digital. A device called a Codec (coder/decoder) does this. It samples the analog signal 8000 times per second and encodes the signal digitally by representing each sample as a binary number. The technique used is called Pulse Coded Modulation or PCM

Types of Transmission:

There are three types of transmission-

1.      Simplex: information is transmitted in one direction only and the roles of transmitter and the receiver are fixed. This form is not used for conventional data transmission.

2.      Half Duplex (HDX): transmission is allowed in both directions but in only one direction at a time. Data communication systems that use the telephone network usually transmit in HDX

3.      Full Duplex (FDX): sender/receiver can transmit and receive from each other at the same time. In order to transmit in FDX, the user usually has private direct lines.

Multiplexing:

With high bandwidth channels it is possible to share the channel so that a number of users can use the channel at the same time. This is called multiplexing.

For example, 4 users could each operate terminals at 10,000bps connected to mainframe over a single 40,000bps line as shown in the figure below. The 4 lines from the terminals are connected to a multiplexer which is connected to another multiplexer by the 40,000bps line. The second multiplexer de-multiplexes the signal onto 4 separate lines for the computer. This gives us efficient line usage and saves memory.

Multiplexing is also used for voice transmission where optic fibres and microwave cable can handle from 8000 to 16000 simultaneous conversations. There are a number of types of multiplexing. Two common ones are Time Division Multiplexing (TDM) and Frequency Division Multiplexing (FDM).