DATA COMMUNICATIONS....
Data Communications

Data Communications is the transfer of data or information between a source and a receiver. The source transmits the data and the receiver receives it. The actual generation of the information is not part of Data Communications nor is the resulting action of the information at the receiver. Data Communication is interested in the transfer of data, the method of transfer and the preservation of the data during the transfer process.

In Local Area Networks, we are interested in "connectivity", connecting computers together to share resources. Even though the computers can have different disk operating systems, languages, cabling and locations, they still can communicate to one another and share resources.

The purpose of Data Communications is to provide the rules and regulations that allow computers with different disk operating systems, languages, cabling and locations to share resources. The rules and regulations are called protocols and standards in Data Communications.

TYPES OF SIGNAL:

• ANALOGUE SIGNAL

            Analogue Signal on the other hand is like the human voice. It is formed by continuously varying voltage levels that create a wave that can be grasped by an analogue transmitter like microphone. These are typically represented by their characteristics of wave.

• DIGITAL SIGNAL

              Digital Transmission is the transmission of binary electrical or light pulses that only have two possible states 0, and 1. This is the language of computers. Graphically they are represented as a square wave.

Bit (binary digit)

A bit (short for binary digit) is the smallest unit of data in a computer. A bit has a singlebinary value, either 0 or 1. Although computers usually provide instructions that can test and manipulate bits, they generally are designed to store data and execute instructions in bit multiples 

called bytes. In most computer systems, there are eight bits in a byte. The value of a bit is usually stored as either above or below a designated level of electrical charge in a single capacitor within a memory device.

Half a byte (four bits) is called a nibble. In some systems, the term octet is used for an eight-bit unit instead of byte. In many systems, four eight-bit bytes or octets form a 32-bit word. In such systems, instruction lengths are sometimes expressed as full-word (32 bits in length) or half-word (16 bits in length).

In telecommunication, the bit rate is the number of bits that are transmitted in a given time period, usually a second.

Memory capacity and data storage capacity for computers are measured in bytes. File sizes are also measure in bytes (one byte is 8 bits, remember). However, a byte is small (it can hold only one character) so we use larger units:

A kilobyte (KB) is approximately 1,000 bytes. But it is NOT exactly 1,000 bytes; it is 1,024 bytes. Why a strange number like 1,024? Because 1,024 is exactly 10000000000 in binary; a nice multiple of two is very handy for the computer. So remember: When the computer tells you that your file takes up 40 kilobytes, it is actually using 40,960 bytes (not 40,000). But you can think of a kilobyte as “roughly 1,000 bytes,” which is how it got its name. This web page file is approximately 20 KB in size.

Similarly, you can think of a megabyte (MB) as approximately a million bytes, but it is precisely 1,048,576 bytes (1,024 x 1,024). The MS Word application takes up about 13 MB on the computer’s hard disk (depending on version). A typical personal computer may have 512 MB of memory.

A gigabyte (GB) is approximately one billion bytes (1,073,741,824 exactly). The root word for the giga is the same one our word giant comes from, so gigabyte should technically be pronounced with a soft g—but the pronunciation with either hard or soft g is acceptable. The storage capacity of a typical hard disk is measures in the tens or hundreds of GB.

In case you are wondering, a trillion bytes is a terabyte, but PC capacities haven’t gotten into that range yet.

Data Transmission Modes

• Data travel over a network in various

characteristics: form, amount of data : type,

direction and timing :

– Analog vs digital

– Bandwidth

– Serials Vs Parallel

– Transmission timing

– Transmission directions

– Circuit switched, Packet‐Switched, and Broadcast

Connections

Or the other hand, transmission modes is a given transmission on a communications channel between two machines can occur in several different ways. The transmission is characterised by:

• the direction of the exchanges
• the transmission mode: the number of bits sent simultaneously
• synchronisation between the transmitter and receiver

Simplex, half-duplex and full-duplex connections

There are 3 different transmission modes characterised according to the direction of the exchanges:

• A simplex connection is a connection in which the data flows in only one direction, from the transmitter to the receiver. This type of connection is useful if the data do not need to flow in both directions (for example, from your computer to the printer or from the mouse to your computer...).
  
• A half-duplex connection (sometimes called an alternating connection or semi-duplex) is a connection in which the data flows in one direction or the other, but not both at the same time. With this type of connection, each end of the connection transmits in turn. This type of connection makes it possible to have bidirectional communications using the full capacity of the line.
  
• A full-duplex connection is a connection in which the data flow in both directions simultaneously. Each end of the line can thus transmit and receive at the same time, which means that the bandwidth is divided in two for each direction of data transmission if the same transmission medium is used for both directions of transmission.
  

Serial and parallel transmission

The transmission mode refers to the number of elementary units of information (bits) that can be simultaneously translated by the communications channel. In fact, processors (and therefore computers in general) never process (in the case of recent processors) a single bit at a time; generally they are able to process several (most of the time it is 8: one byte), and for this reason the basic connections on a computer are parallel connections.

Parallel connection

Parallel connection means simultaneous transmission of N bits. These bits are sent simultaneously overN different channels (a channel being, for example, a wire, a cable or any other physical medium). Theparallel connection on PC-type computers generally requires 10 wires.

These channels may be:

• N physical lines: in which case each bit is sent on a physical line (which is why parallel cables are made up of several wires in a ribbon cable)
• one physical line divided into several sub-channels by dividing up the bandwidth. In this case, each bit is sent at a different frequency...

Since the conductive wires are close to each other in the ribbon cable, interference can occur (particularly at high speeds) and degrade the signal quality...

Serial connection

In a serial connection, the data are sent one bit at a time over the transmission channel. However, since most processors process data in parallel, the transmitter needs to transform incoming parallel data into serial data and the receiver needs to do the opposite.

These operations are performed by a communications controller (normally a UART (Universal Asynchronous Receiver Transmitter) chip). The communications controller works in the following manner:

• The parallel-serial transformation is performed using a shift register. The shift register, working together with a clock, will shift the register (containing all of the data presented in parallel) by one position to the left, and then transmit the most significant bit (the leftmost one) and so on:
  
• The serial-parallel transformation is done in almost the same way using a shift register. The shift register shifts the register by one position to the left each time a bit is received, and then transmits the entire register in parallel when it is full:
  

synchronous transmission

  

Data transfer method in which a continuous stream of data signals is accompanied by timing signals (generated by an electronic clock) to ensure that the transmitter and the receiver are in step (synchronized) with one another. The data is sent in blocks (called frames or packets) spaced by fixed time intervals. In contrast, asynchronous transmission works in spurts and must insert a start bit before each datacharacter and a stop bit at its termination to inform the receiver where it begins and ends. Most network protocols (such as Ethernet, SONET, Token Ring) use synchronous transmission whereas asynchronous transmission is used commonly for communications over telephone lines.

asynchronous transmission 

Data transfer method in which signals are sent in spurts, spaced by varying time intervals. Each data character is preceded by a start-bit and is followed by an end-bit to inform the receiving equipment where the character begins and ends. This arrangement is in contrast to synchronous transmission where data is sent in continuous blocks of characters (called 'frames' or 'packets') spaced by fixed time intervals. Asynchronous transmission is used commonly for communications over telephone lines, whereas most network protocols (such as Ethernet, SONET, Token Ring) use synchronous transmission.