Data Communication

Data encoding: the process of transforming input data or signals into signals that can be transmitted

Signal generation: generating appropriate electro-magnetic signals to be transmitted over a transmission medium

Synchronization: timing of signals between the transmitter and receiver ; when a signal begins and when it ends; duration of each signal


Data Communication Channels

•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.
•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.
•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.

Data Communication Terminology

•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 same as symbol rate and is a measure of the number of line changes which occur every second. Each symbol can represent or convey one (binary encoded signal) or several bits of data. 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 data 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.

• Protocols
–A protocol is a set of rules which governs how data is sent from one point to another. In data communications, there are widely accepted protocols for sending data. Both the sender and receiver must use the same protocol when communicating.
–BY CONVENTION, THE LEAST SIGNIFICANT BIT IS TRANSMITTED FIRST (Network order) 

•ASYNCHRONOUS PROTOCOLS
–Asynchronous systems send data bytes between the sender and receiver. Each data byte is preceded with a start bit, and suffixed with a stop bit. These extra bits serve to synchronize the receiver with the sender.
–Transmission of these extra bits (2 per byte) reduce data throughput. Synchronization is achieved for each character only. When the sender has no data to transmit, the line is idle and the sender and receiver are NOT in synchronization. Asynchronous protocols are suited for low speed data communications. 

•SYNCHRONOUS PROTOCOLS
–Synchronous protocols involve sending timing information along with the data bytes, so that the receiver can remain in synchronization with the sender. When the sender has no data to transmit, the sender transmits idle flags (a sequence of alternating 0's and 1's) to maintain sender/receiver synchronization. Data bytes are packaged into chunks called packets, with address fields being added at the front (header) and checksums at the rear of the packet.

 Communication Network

A communication network is a collection of devices connected by some communications media
•Example devices are:

• –mainframes, minicomputers, supercomputers 
• –workstations, personal computers 
• –printers, disk servers, robots 
• –X-terminals 
• –Gateways, switches, routers, bridges 
• –Cellular phone, Pager, TRS 
• –Refrigerator, Television, Video Tape Recorder