Data communication systems are the exchange of data between two devices via some form of transmission medium such as a wire cable. For data communications to occur, the communicating devices must be part of a communication system made up of a combination of hardware (physical equipment) and software (programs).
# The effectiveness of a data communication system depends on four fundamental characteristics:
- Delivery
- Accuracy
- Timeliness
- Jitter.
# Components of a communication system are as given below:
- Message
- Sender
- Receiver
- Transmission medium
- Protocol.
Fig: Simplified Data Communication Model
Consider that the input device and transmitter are components of a personal computer. The user of the PC wishes to send a message to another user-for example,” The meeting scheduled for March 25 is canceled” (m). The user activates the electronic mail package on the PC and enters the message via the keyboard (input device). The character string is briefly buffered in main memory.
We can view it as a sequence of bits (g) in memory. The personal computer is connected to some transmission medium, such as a local network or a telephone line, by an (1) device (transmitter), such as a local network transceiver or a modem. The input data are transferred to the transmitter as a sequence of voltage shifts [g(t)] representing bits on some communications bus or cable. The transmitter is connected directly to the medium and converts the incoming stream [g(t)] into a signal [s(t)] suitable for transmission. The transmitted signal s(t) presented to the medium is subject to a number of impairments.
Thus, the received signal r(t) may differ to some degree from s(t). The receiver will attempt to estimate the original s(t), based on r(t) and its knowledge of the medium, producing a sequence of bits gl(t). These bits are sent to the output personal computer, where they are briefly buffered in memory as a block of bits (g). In many cases, the destination system will attempt to determine if an error has occurred and, if so, will cooperate with the source system to eventually obtain a complete, error-free block of data. These data are then presented to the user via an output device, such as a printer or a screen. The message (m’), as viewed by the user, will usually be an exact copy of the original message (m).
Now consider a telephone conversation. In this case, the input to the telephone is a message (m) in the form of sound waves. The sound waves are converted by the telephone into electrical signals of the same frequency. These signals are transmitted without modification over the telephone line. Hence, the input signal g(t) and the transmitted signal s(t) are identical. The signal s(t) will suffer some distortion over the medium, so that r(t) will not be identical to s(t). Nevertheless, the signal r(t) is converted back into a sound wave with no attempt at correction or improvement of signal quality.
Thus ‘m’ is not an exact replica of m. However, the received sound message is generally comprehensible to the listener. The discussion so far does not touch on other key aspects of data communications, including data-link control techniques for controlling the flow of data and detecting and correcting errors, and multiplexing techniques for transmission efficiency.
Communication Model
The fundamental purpose of a communications system is the exchange of data between two parties. We begin our study with a simple model of communications, illustrated by the block diagram in Fig-l(a).
Fig-1(a):General block diagram
Fig-l(b)presents one particular example, which is the communication between a workstation and a server over a public telephone network. Another example is the exchange of voice signals between two telephones over the same network.
(b)Example
Fig 1: Simplified Communications Model
The key elements of the communication model are:
# Source:
This device generates the data to be transmitted; examples are telephones and personal computers.
# Transmitter:
Usually, the data generated by a source system are not transmitted directly in the form in which they were generated. Rather, a transmitter transforms and encodes the information in such a way as to produce electromagnetic signals that can be transmitted across some sort of transmission system. For example, a modem takes a digital bit stream from an attached device such as a personal computer and transforms that bit stream into an analog signal that can be handled by the telephone network. Transmission system. This can be a single transmission line or a complex network connecting source and destination.
# Receiver:
The receiver accepts the signal from the transmission system and converts it into a form that can be handled by the destination device. For example, a modem will accept an analog signal coming from a network or transmission line and convert it into a digital bit stream.
# Destination:
Takes the incoming data from the receiver. This simple narrative conceals a wealth of technical complexity. To get some idea of the scope of this complexity, Table 1.1 lists some of the key tasks that must be performed in a data communications system. The list is somewhat arbitrary: Elements could be added; items on the list could be merged; and some items represent several tasks that are performed at different “levels” of the system. However, the list as it stands is suggestive of the scope of this book.
