Transmission of binary data across a channel can be
done in either serial or parallel mode. In parallel mode, multiple bits are
sent with each clock pulse but serial mode sent one bit with each clock tick. Serial
transmission can accomplish in three ways: asynchronous, synchronous, and
isochronous.
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| Transmission modes |
Parallel Transmission
In parallel transmission,
multiple bits are sent with each clock tick. Binary data is organized into groups
and send through the link. To send n-bits simultaneously we need a n-line
channel. The following figure illustrate parallel transmission through a 8-line
channel,
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| Parallel Transmission |
The advantage of the parallel transmission is speed. However
it is expensive.
Serial Transmission
In serial mode, 1 bit is sent with each clock tick. Here we need only one communication line. Normally communication within devices occur in parallel mode, so conversion devices are required at the interface between the sender and the channel and between the channel and the receiver as shown below,
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| Serial Transmission |
Serial transmission can be possible in three ways: asynchronous,
synchronous, and isochronous.
Asynchronous Transmission
In
asynchronous transmission, timing of the signal is not important. Ordering of
the received signal is the responsibility of receiver. To inform the arrival of
a new group of bits, one extra bit (start bit, usually 0) is used along with
each byte. To notify end of the byte one or more extra bits are used (stop
bits, usually 1s). By this method each byte is increased at least 2 bits. There
may be some gap between each byte. Asynchronous
here means asynchronous at the byte level, but the bits are synchronized.
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| Asynchronous transmission |
Synchronous Transmission
In synchronous transmission, bits are combined into frames. Each frame may contain multiple bytes. Bits in the frame are send without any gap between them, so here does not require start and stop bits. It is the responsibility of the receiver to group the bits. The advantage of synchronous transmission is speed, that is synchronous transmission is faster than asynchronous transmission. There is no gap between the bits, even though there may be some gap between frames. |
| Synchronous transmission |
Isochronous transmission
In the
case of real time systems, even the gaps between the frames are unacceptable. For
example, TV images are broadcast at the rate of 30 images per second and they
must be viewed at the same rate. If each image is sent by using one or more
frames, there should be no delays between frames. The isochronous transmission
guarantees no gaps between frames and send data as continuous stream.
Next : Multiplexing
Previous : Analog and digital transmission
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