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© Copyright Brian Brown, 1992-2001. All rights reserved.

DATA COMMUNICATIONS, Part 1
The objective of this section is to

• explain terminology associated with data communications
• outline the common data communication circuits available
• discuss data communications equipment

At the end of this section you should be able to

• define a number of common terms related to data communications
• list the basic components of a network
• describe how the basic components of a network function

Data Transmission Signals

Analogue Signals
Analogue signals are what we encounter every day of our life. Speech is an analogue signal, and varies in amplitude (volume) and frequency (pitch).

The main characteristics of analogue signals are,

• Amplitude
  This is the strength of the signal. It can be expressed a number of different ways (as volts, decibels). The higher the amplitude, the stronger (louder) the signal.

  
  Fig 7.1: Signal Amplitude
  
  

• Frequency
  This is the rate of change the signal undergoes every second, expressed in Hertz (Hz), or cycles per second. A 30Hz signal changes thirty times a second. In speech, we refer to it as the number of vibrations per second, as the air that we expel out of our mouth pushes the surrounding air at a specific rate.

  
  Fig 7.2: Signal Frequency

  One cycle is defined as a point on the wave to the next identical point. The number of cycles there are per second determine the frequency of the signal.

Digital Signals
Digital signals are the language of modern day computers. Digital signals normally comprise only two states. These are expressed as ON or OFF, 1 or 0 respectively.

The following diagram shows a digital signal.

Fig 7.3: Digital Signal showing two states, 0 and 1

Data Transmission Circuits
The following is a discussion on the THREE main types of transmission circuits, simplex, half duplex and full duplex.

• Simplex
  Data in a simplex channel is always one way. Simplex channels are not often used because it is not possible to send back error or control signals to the transmit end. An example of a simplex channel in a computer system is the interface between the keyboard and the computer, in that key codes need only be sent one way from the keyboard to the computer system.

  
  Fig 7.4: Simplex (one-way) channel

• Half Duplex
  A half duplex channel can send and receive, but not at the same time. Its like a one-lane bridge where two way traffic must give way in order to cross. Only one end transmits at a time, the other end receives.

  
  Fig 7.5: Half-Duplex channel

• Full Duplex
  Data can travel in both directions simultaneously. There is no need to switch from transmit to receive mode like in half duplex. Its like a two lane bridge on a two-lane highway.

  
  Fig 7.6: Full-Duplex channel

PARALLEL AND SERIAL DATA
Data may be transmitted between two points in two different ways. Lets consider sending 8 bits of digital data (1 byte).

• Parallel
  Each bit uses a separate wire. The organisation looks likes,

  
  Fig 7.7: Parallel Interface

  To transfer data on a parallel link, a separate line is used as a clock signal. This serves to inform the receiver when data is available. In addition, another line may be used by the receiver to inform the sender that the data has been used, and its ready for the next data.

  
  Fig 7.8: Parallel Interface timing

• Serial
  Each bit is sent over a single wire, one after the after. The organisation looks like,

  
  Fig 7.9: Serial Interface

  No signal lines are used to convey clock (timing information). There are two ways in which timing information is encoded with the signal so that the sender and receiver are synchronised (working on the same data at the same time).

  If no clock information was sent, the receiver would misinterpret the arriving data (due to bits being lost, going too slow).

Parallel transmission is obviously faster, in that all bits are sent at the same time, whereas serial transmission is slower, because only one bit can be sent at a time. Parallel transmission is very costly for anything except short links.

Data transmission protocols
There are TWO main methods of sending data, asynchronous and synchronous.

• Asynchronous Serial Transmission
  Because no signal lines are used to convey clock (timing) information, this method groups data together into a sequence of bits (five - eight), then prefixes them with a start bit and a stop bit.

  The purpose of the start and stop bits was introduced for the old electromechanical tele-typewriters. These used motors driving cams which actuated solenoids which sampled the signal at specific time intervals. The motors took a while to get up to speed, thus by prefixing the first bit with a start bit, this gave time for the motors to get up to speed, and thus acted as a reference point for the start of the first bit.

  At the end of the character sequence, a stop bit was used to allow the motors/cams etc to get back to normal, in addition, it was also need to fill in time in case the character was an end of line, when the tele-type-writer would need to go to the beginning of a new-line. Without the stop character, the machine could not complete this before the next character arrived.

  
  Fig 7.10: Asynchronous signal showing start and stop bits

• Synchronous Serial Transmission
  In synchronous transmission, the line idle state is changed to a known character sequence (7E), which is used to synchronise the receiver to the sender. The start and stop bits are removed, and each character is combined with others into a data packet. The data packet is prefixed with a header field, and suffixed with a trailer field which includes a checksum value (used by the receiver to check for errors in sending).

  
  Fig 7.11: Synchronous data

  The header field is used to convey address information (sender and receiver), packet type and control data.

  The data field contains the users data (if it can't fit in a single packet, then use multiple packets and number them) or control data. Generally, it has a fixed size.

  The tail field contains checksum information which the receiver uses to check whether the packet was corrupted during transmission.

Data Communications Terminology
The following is an explanation of common terms applicable to data communications.

Channel
A channel is a portion of the communications medium allocated to the sender and receiver for conveying information between them. The communications medium is often subdivided into a number of separate paths, each of which is used by a sender and receiver for communication purposes.

Baud Rate
Baud rate is the reciprocal of the shortest signal element (a measure of the number of line changes which occur every second). For a binary signal of 20Hz, this is equivalent to 20 baud (there are 20 changes per second).

Bits Per Second
This is an expression of the number of bits per second. Where a binary signal is being used, this is the same as the baud rate. When the signal is changed to another form, it will not be equal to the baud rate, as each line change can represent more than one bit (either two or four bits).

Bandwidth
Bandwidth is the frequency range of a channel, measured as the difference between the highest and lowest frequencies that the channel supports. The maximum transmission speed is dependant upon the available bandwidth. The larger the bandwidth, the higher the transmission speed.

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© Copyright Brian Brown, 1992-2001. All rights reserved.