Data communication circuits can be configured in a huge number of arrangements depending on the specifics of the circuit, such as how many stations are on the circuit, type of transmission facility, distance between the stations, how many users at each station and so on. Data communication circuits can however be classified as either two point or multipoint. A two-point configuration involves only two stations, whereas a multipoint configuration involves more than two stations. Regardless of configuration, each station can have one or more computers, computer terminals or workstations. A two point circuit involves the transfer of digital information from a mainframe computer and a personal computer, two mainframe computers, two personal computers or two data communication networks. A multipoint network is generally used to interconnect a single mainframe computer to many personal computers or to interconnect many personal computers. Coming to transmission modes, there are three modes of transmission for data communication circuits namely;
- Simplex: In a simplex mode, the transmission of data is always unidirectional. Information will be sent always only in one direction Simplex lines are also called receive-only, transmit-only, or one-way-only lines. A best example of simplex mode is Radio and Television broadcasts.
- Half-Duplex: In the half-duplex mode, data transmission is possible in both the directions but not at the same time. When one device is sending, the other can only receive, and vice-versa. These communication lines are also called two-way-alternate or either-way lines.
- Full Duplex: In the full-duplex mode, the transmissions are possible in both directions simultaneous, but they must be between the same two stations. Full-duplex lines are also called two-way simultaneous duplex or both-way lines. A good example for full-duplex transmission is a telephone
Parallel & Serial Transmissions
There are two types of data transmission modes. These are:
- Parallel Transmission: In parallel transmission, bits of data flow concurrently through separate communication lines. Parallel transmission is shown in figure below. The automobile traffic on a multi-lane highway is an example of parallel transmission. Inside the computer binary data flows from one unit to another using parallel mode. If the computer uses 32-bk internal structure, all the 32-bits of data are transferred simultaneously on 32-lane connections. Similarly, parallel transmission is commonly used to transfer data from computer to printer. The printer is connected to the parallel port of computer and parallel cable that has many wires is used to connect the printer to computer. It is very fast data transmission mode.
- Serial Transmission: In serial data transmission, bits of data flow in sequential order through single communication line. The flow of traffic on one-lane residential street is an example of serial data transmission mode. Serial transmission is typically slower than parallel transmission, because data is sent sequentially in a bit-by-bit fashion. Serial mouse uses serial transmission mode in computer.
Synchronous & Asynchronous Transmissions
- Synchronous Transmission: In synchronous transmission, large volumes of information can be transmitted at a time. In this type of transmission, data is transmitted block-by-block or word-byword simultaneously. Each block may contain several bytes of data. In synchronous transmission, a special communication device known as synchronized clock’ is required to schedule the transmission of information. This special communication device or equipment is expensive.
- Asynchronous Transmission: In asynchronous transmission, data is transmitted one byte at a ‘time’. This type of transmission is most commonly used by microcomputers. The data is transmitted character-by-character as the user types it on a keyboard. An asynchronous line that is idle (not being used) is identified with a value 1, also known as ‘Mark’ state. This value is used by the communication devices to find whether the line is idle or disconnected. When a character (or byte) is about to be transmitted, a start bit is sent. A start bit has a value of 0, also called a space state. Thus, when the line switches from a value of 1 to a value of 0, the receiver is alerted that a character is coming.