Data Communication Model

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Network

The user of the PC wishes to send a message to another user-for example, “Good Day” (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 transceiver of 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, before it reaches the receiver. 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 g(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).

datacommunication_model

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 or m. However, the received sound message is generally comprehensible to the listener.

Some of key tasks to be performed by a Data Communications
System are listed below.