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Data Communications - IM2

Essentially, there are four modes of transmission for data communicati...
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Electronics Engineering (CR 061)

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Academic year: 2022/2023
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Samar State University

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LESSON CONTENT

  1. Circuit Configurations

Data communications networks can be generally categorized as either two-point or multi-point. A twopoint configuration involves only two locations or stations, whereas a multi-point configuration involves three or more stations. Regardless of the configuration, each station can have one or more computers, computer terminals, or workstations.

A two-point circuit involves the transfer of digital information between a mainframe computer and a personal computer, two mainframe computers, two personal computers, or two data communications networks. A multi-point network is generally used to interconnect a single mainframe computer (host) to many personal computers or to interconnect many personal computers.

  1. Transmission Modes

A given transmission on a communications channel between/among two or more machines can occur in several different ways. The transmission is characterized by:

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

Essentially, there are four modes of transmission for data communications circuits: simplex, half duplex, full duplex, and full/full duplex.

2 Simplex. In the simplex (SX) mode, data transmission is unidirectional; information can be sent in only one direction. Simplex lines are also called receive-only, transmit-only, or one-way-only lines. 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...). Commercial radio broadcasting is also an example of simplex transmission, as information is propagated in only one direction—from the broadcasting station to the listener.

Figure 2 Simplex Connection

2 Half duplex. In the half-duplex (HDX) mode, data transmission is possible in both directions but not at the same time. Half-duplex communications lines are also called two-way-alternate, eitherway lines, alternating connection, or semi duplex. Citizens band (CB) radio is an example of halfduplex transmission because to send a message, the push-to-talk (PTT) switch must be depressed, which turns on the transmitter and shuts off the receiver. To receive a message, the PTT switch must be off, which shuts off the transmitter and turns on the receiver. This type of

connection makes it possible to have bidirectional communications using the full capacity of the line.

Figure 2 Half-duplex connection

2 Full duplex. In the full-duplex (FDX) mode, transmissions are possible in both directions simultaneously, but they must be between the same two stations. 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. Full-duplex lines are also called two-way simultaneous, duplex, or both-way lines. A local telephone call is an example of full-duplex transmission. Although it is unlikely that both parties would be talking at the same time, they could if they wanted to.

Figure 2 Full-duplex connection

2 Full/full duplex. In the full/full duplex (F/FDX) mode, transmission is possible in both directions at the same time but not between the same two stations (i., one station is transmitting to a second station and receiving from a third station at the same time). Full/full duplex is possible only on multipoint circuits. The U. postal system is an example of full/full duplex transmission because a person can send a letter to one address and receive a letter from another address at the same time.

  1. 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. Moreover, there are two ways to move binary bits from one place to another: transmit all bits of a word simultaneously or send only one bit at a time. These methods are referred to, respectively parallel transfer and serial transfer.

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.

Figure 3 Serial data transmission

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:

Obviously, the principal trade-off between parallel and serial data transmission is speed versus simplicity. Data transmission can be accomplished much more rapidly using parallel transmission; however, parallel transmission requires more data lines. As a general rule, parallel transmission is used for short-distance data communications and within a computer, and serial transmission is used for long-distance data communications.

  1. Synchronous and asynchronous transmission

Given the problems that arise with a parallel-type connection, serial connections are normally used. However, since a single wire transports the information, the problem is how to synchronize the transmitter and receiver, in other words, the receiver can not necessarily distinguish the characters

(or more generally the bit sequences) because the bits are sent one after the other. There are two types of serial transmission that address this problem:

4 An asynchronous connection, in which each character is sent at irregular intervals in time (for example a user sending characters entered at the keyboard in real time). So, for example, imagine that a single bit is transmitted during a long period of silence... the receiver will not be able to know if this is 00010000, 10000000 or 00000100...

To remedy this problem, each character is preceded by some information indicating the start of character transmission (the transmission start information is called a START bit) and ends by sending end-of-transmission information (called STOP bit, there may even be several STOP bits). However, when huge volumes of information must be transmitted, the start and stop bits represent a significant percentage of the bits transmitted. We call that percentage the overhead transmission.

4 In a synchronous connection, the transmitter and receiver are paced by the same clock. The receiver continuously receives (even when no bits are transmitted) the information at the same rate the transmitter send it. This is why the transmitter and receiver are paced at the same speed. In addition, supplementary information is inserted to guarantee that there are no errors during transmission.

During synchronous transmission, the bits are sent successively with no separation between each character, so it is necessary to insert synchronization elements; this is called character- level synchronization.

The special synchronization codes at the beginning and end of a block represent very small percentage of the total number of bits being transmitted, especially in relation to the number of start and stop bits used in asynchronous transmission. Synchronous transmission is therefore much faster than asynchronous transmission because of the lower overhead.

An important consideration in synchronous transmission is how the receiving station keeps track of the individual bits and bytes, especially when the signal is noisy, since there is no clear separation between them. This is done by transmitting the data at a fixed, known, precise clock rate. The number of bits can be counted to keep track of the number of bytes or characters transmitted. For every 8 bits counted, 1 byte is received. The number of received bytes is also counted.

Synchronous transmission assumes that the receiver knows or has a clock frequency identical to that of the transmitter clock. Usually, the clock at the receiver is derived from the received signal, so that it is precisely the same frequency as, and in synchronization with, the transmitter clock.

The main disadvantage of synchronous transmission is recognizing the data at the receiver, as there may be differences between the transmitter and receiver clocks. That is why each data transmission must be sustained long enough for the receiver to distinguish it. As a result, the transmission speed cannot be very high in a synchronous link.

Example: A block of 256 sequential 12-bit data words is transmitted serially in 0 s. Calculate (a) the time duration of 1 word, (b) the time duration of 1 bit, and (c) the speed of transmission in bits per second.

Solution:

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Data Communications - IM2

Course: Electronics Engineering (CR 061)

95 Documents
Students shared 95 documents in this course

University: Samar State University

Was this document helpful?
LESSON CONTENT
1. Circuit Configurations
Data communications networks can be generally categorized as either two-point or multi-point. A
twopoint configuration involves only two locations or stations, whereas a multi-point configuration
involves three or more stations. Regardless of the configuration, each station can have one or more
computers, computer terminals, or workstations.
A two-point circuit involves the transfer of digital information between a mainframe computer and a
personal computer, two mainframe computers, two personal computers, or two data
communications networks. A multi-point network is generally used to interconnect a single
mainframe computer (host) to many personal computers or to interconnect many personal
computers.
2. Transmission Modes
A given transmission on a communications channel between/among two or more machines can
occur in several different ways. The transmission is characterized by:
• the direction of the exchanges
• the transmission mode: the number of bits sent simultaneously • synchronization between
the transmitter and receiver
Essentially, there are four modes of transmission for data communications circuits: simplex, half
duplex, full duplex, and full/full duplex.
2.1 Simplex. In the simplex (SX) mode, data transmission is unidirectional; information can be sent
in only one direction. Simplex lines are also called receive-only, transmit-only, or one-way-only
lines. 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...). Commercial
radio broadcasting is also an example of simplex transmission, as information is propagated in
only one direction—from the broadcasting station to the listener.
Figure 2.1 Simplex Connection
2.2 Half duplex. In the half-duplex (HDX) mode, data transmission is possible in both directions but
not at the same time. Half-duplex communications lines are also called two-way-alternate,
eitherway lines, alternating connection, or semi duplex. Citizens band (CB) radio is an example
of halfduplex transmission because to send a message, the push-to-talk (PTT) switch must be
depressed, which turns on the transmitter and shuts off the receiver. To receive a message, the
PTT switch must be off, which shuts off the transmitter and turns on the receiver. This type of

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