DATA TRANSMISSION BASICS
When we enter data into a computer via a keyboard, each selected keyed element, an alphabetic or numeric character, for example is encoded by the electronics within the keyboard into an equivalent binary-coded pattern, using one of the standard coding schemes that are used for the interchange of information. In order to represent all the characters on a keyboard with a unique pattern, 7 or 8 bits are utilized. The use of 7 bits means that 128 different elements can be represented, while 8 bits can represent 256 elements. A similar procedure is followed on output, except in this case the printer will decode each recived binary-coded pattern to print the corresponding character. We refer to the coded bit patterns for each character as codewords.
The two most widely used codes that have been adopted for this function are the Extended Binary Coded Decimal Interchange Code (EBCDIC) and the American Standards Committee for Information Interchange (ASCII). EBCDIC is an 8-bit code that is used with most equivalent manufactured by IBM. As such it is a proprietary code but, owing to the widespread use of IBM equipment in the computer industry, it is frequently used.
The ASCII code is the same as that defined by the ITU-T, known as International Alphabet number 5(IAS), and also that used by the International Organization for Standardization known as ISO 645. ASCII is a 7-bit code.
Both coding schemes cater for all the normal alphabetic numeric and punctuation characters, collectively referred to as printable characters, plus a range of additional control characters-also known as non-printable characters.
A) TRANSMISSION MODE
The term transmission mode is used to define the direction of signal flow between two linked devices. There are three types of transmission modes: simplex, half-duplex, and full-duplex. In the simplex mode, the communication is unidirectional as on a one-way street. Only one of the two stations on a link can transmit: the other can only receive. Keyboards and traditional monitors are both example of simplex devices.
The keyboard can only introduce input; the monitor can only accept output. In the half-duplex mode, each station can both transmit and receive. but not at the same time. When one device is sending, the other can only receive, and vice versa. In full-duplex mode, both stations can transmit and receive simultaneously.
simplex, half-duplex and full-duplex communications.

                 
A transmission mode defines the way in which a group of bits goes from one device to another. It also defines whether bits may travel in both directions simultaneously or whether devices must take turns sending and receiving. There are two categories of transmission.
(1) Parallel transmission
(2) Serial transmission

B) Parallel Transmission
Binary data, consisting of 1 s and 0 s, may be organized into groups of n bits each. Parallel transmission means that a group of bits is transmitted simultaneously by using a separate line (wire) for each bit as shown in Figure.

Parallel transmissions are commonly used when the distance between the two devices are short; for example, communication between a computer and peripheral devices. Speed of the transmission of the bits in parallel transmission is higher than in serial transmission, because bits group are sent simultaneously. Parallel transmission is more expensive than single transmission for long distances: For long distance, wires should be thicker to maintain the signal degradation.

C) Serial Transmission
Serial transmission means that a group of bits is transmitted one by one using one line (wire) for all bits as shown in Figure.Serial transmission is cheaper and more reliable than parallel transmission over long distance. It is also slower because the bits are sent one at a time.

There are two ways to provide serial communication:

• Asynchronous transmission
• Synchronous transmission

Asynchronous transmission means that bits are divided into small groups (usually bytes) and sent independently. The sender can send the groups at any time and the receiver never knows when they will arrive (somewhat like a visit from a long-lost relative). Asynchronous means that "asynchronous at the bvte level- but the bits are still synchronized; their durations are the same.

In asynchronous transmission, we send one start bit (0) at the beginning and one or more stop bits (Is) at the end of each byte. There may be a gap between each byte. When the receiver detects a start bit, it sets a timer and begins counting bits as they come in. After n bits, the receiver looks for a stop bit. As soon as it detects the stop bit, it ignores any received pulses until it detects the next start bit.
In synchronous transmission, the bit stream is combined into longer "frames", which may contain multiple bytes. We send bits one after another without start/stop bits or gaps. It is the responsibility of the receiver to group the bits.

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