a code called Morse code. The codes were transmitted as pulses along a wire or as radio-frequency bursts in the case of wireless telegraph. Then the develop- ment of data communications slowed, but during the last few decades data communications have expanded rapidly as computers have become tools for everyone in both business and residential environments. Introduction to Telecommunications Network Engineering 6.1.1 Computer Communications Modern computers manipulate bits, binary symbols, of electrical energy. When a computer communicates with another computer it sends these bits along a cable between them. This is relatively easy if the computers are within the same room or a building. If the distance is longer, a telecommuni- cations network is required that provides an end-to-end communications channel. Data communications can be accomplished by means of many vari- ous alternatives, some of which we discuss in the following sections. 6.1.2 Serial and Parallel Data Communications In a transmission network only one channel is usually allocated for one end- to-end connection in each direction. Let us use as an example source of data a simple American Standard Code for Information Interchange (ASCII) termi- nal. We press keys on the keyboard and each keystroke generates a 7-bit binary word (8 bits with parity) corresponding to the letter or number of the key pressed. For example character a corresponds to the binary sequence 1000011 (the first bit on the left) [1]. If we have only one channel available, we have to send bits of this word in turn (first bit on the left) to the channel; such a case represents serial data transmission (see Figure 6.1). When serial transmission is used between a computer and its peripheral device, a parallel clock signal may be used for timing. In serial transmission Bit 1 Parallel tranmission: Bit 2 Bit 3 Complete 8-bit word is sent Bit 4 at a time. Bit 5 Is not suitable for long- Bit 6 distance transmission because it: Bit 7 Bit 8 - requires many wires or channels and - requires addi