A communications channel is a pathway
over which information can be conveyed. It may be defined by a
physical wire that connects communicating devices, or by a radio,
laser, or other radiated energy source that has no obvious physical
presence. Information sent through a communications channel has a
source from which the information originates, and a destination to
which the information is delivered. Although information originates
from a single source, there may be more than one destination,
depending upon how many receive stations are linked to the channel
and how much energy the transmitted signal possesses.
In a digital communications channel, the information is represented by individual data bits, which may be encapsulated into multibit message units. A byte, which consists of eight bits, is an example of a message unit that may be conveyed through a digital communications channel. A collection of bytes may itself be grouped into a frame or other higher-level message unit. Such multiple levels of encapsulation facilitate the handling of messages in a complex data communications network.
Any communications channel has a direction associated with it:
In a digital communications channel, the information is represented by individual data bits, which may be encapsulated into multibit message units. A byte, which consists of eight bits, is an example of a message unit that may be conveyed through a digital communications channel. A collection of bytes may itself be grouped into a frame or other higher-level message unit. Such multiple levels of encapsulation facilitate the handling of messages in a complex data communications network.
Any communications channel has a direction associated with it:
The message source is the transmitter,
and the destination is the receiver. A channel whose direction of
transmission is unchanging is referred to as a simplex channel. For
example, a radio station is a simplex channel because it always
transmits the signal to its listeners and never allows them to
transmit back.
A half-duplex channel is a single physical channel in which the direction may be reversed. Messages may flow in two directions, but never at the same time, in a half-duplex system. In a telephone call, one party speaks while the other listens. After a pause, the other party speaks and the first party listens. Speaking simultaneously results in garbled sound that cannot be understood.
A half-duplex channel is a single physical channel in which the direction may be reversed. Messages may flow in two directions, but never at the same time, in a half-duplex system. In a telephone call, one party speaks while the other listens. After a pause, the other party speaks and the first party listens. Speaking simultaneously results in garbled sound that cannot be understood.
A full-duplex channel allows
simultaneous message exchange in both directions. It really consists
of two simplex channels, a forward channel and a reverse channel,
linking the same points. The transmission rate of the reverse channel
may be slower if it is used only for flow control of the forward
channel.
Next topic will be on Types of Transmission
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