WHAT IS DATA COMMUNICATION
Data transmission and data reception or, more broadly, data communication or digital communications is the transfer and reception of data in the form of a digital bitstream or a digitized analog signal over a point-to-point or point-to-multipoint communication channel.
Data communications (DC) is the process of using computing and communication technologies to transfer data from one place to another, or between participating parties.
DC enables the movement of electronic or digital data between two or more network nodes, regardless of geographical location, technological medium or data contents.
Data communications incorporate several techniques and technologies to enable any form of electronic communication. It’s a very broad way to refer to all of this networked technology on any platform or in any digital environment.
Some common types of data communications technologies include telecommunications, computer networking and radio/satellite communication. Data communication usually requires the existence of a transportation or communication medium between the nodes wanting to communicate with each other, such as copper wire, fiber optic cables, or wireless signals.
For example, a common example of data communications is a computer connected to the Internet via a Wi-Fi connection, which uses a wireless medium to send and receive data from one or more remote servers.
Some devices/technologies used in data communications are known as data communication equipment (DCE) and data terminal equipment (DTE). DCE is used at the sending node, and DTE is used at the receiving node
Data communication refers to the exchange of data between a source and a receiver via form of transmission media such as a wire cable. Data communication is said to be local if communicating devices are in the same building or a similarly restricted geographical area.
The meanings of source and receiver are very simple. The device that transmits the data is known as source and the device that receives the transmitted data is known as receiver. Data communication aims at the transfer of data and maintenance of the data during the process but not the actual generation of the information at the source and receiver.
Components of data communication system
A Communication system has following components:
1. Message: It is the information or data to be communicated. It can consist of text, numbers, pictures, sound or video or any combination of these.
2. Sender: It is the device/computer that generates and sends that message.
3. Receiver: It is the device or computer that receives the message. The location of receiver computer is generally different from the sender computer. The distance between sender and receiver depends upon the types of network used in between.
4. Medium: It is the channel or physical path through which the message is carried from sender to the receiver. The medium can be wired like twisted pair wire, coaxial cable, fiber-optic cable or wireless like laser, radio waves, and microwaves.
5. Protocol: It is a set of rules that govern the communication between the devices. Both sender and receiver follow same protocols to communicate with each other.
Digital Data Communication Theory
One of the great benefits of digital technology is the ability to communicate vast amounts of information over networks. This very textbook you are reading was transmitted in digital form over the electronic network we call the Internet: a feat nearly impossible with any sort of analog electronic technology. The main benefit of digital data communication in industrial control is simple: no longer must we dedicate a single pair of wires to each and every variable we wish to measure and control in a facility as is necessary with analog (4-20 mA) signaling. With digital signaling, a single pair of wires or coaxial cable is able to convey a theoretically unlimited number of data points.
This benefit comes at a price, though: in order to communicate multiple variables (data points) over a single channel (wire pair), we must transmit and receive those signals one at a time. This means a digital communications system will necessarily exhibit some degree of time delay in acquiring, transmitting, receiving, and interpreting a signal. Analog systems, by contrast, are virtually instantaneous. Thus, we see a contrast between analog and digital communication pitting channel capacity against speed:
|Only one signal per channel
|Many signals per channel possible
With modern electronic technology it is possible to build digital communication systems that are so fast, the time delays are negligible for most industrial processes, which renders the second comparison (instantaneous versus time-delayed) moot. If time is no longer an issue, the advantage that digital communication has over analog in terms of channel usage makes it the superior choice.
Another important advantage of digital data communication for industrial processes is increased noise immunity. Analog data is continuous by nature: a signal of 11.035 milliamps has a different meaning than a signal of 11.036 milliamps, because any measurable increment in signal represents a corresponding increment in the physical variable represented by that signal. A voltage value in a 0-5 volt digital signaling system of 0.03 volts, however, means the exact same thing as a voltage value of 0.04 volts: either one is still interpreted as a “0” or “low” state. Any amount of electrical noise imposed on an analog signal corrupts that signal to some degree. A digital signal, however, may tolerate a substantial amount of electrical noise with no corruption whatsoever.
Not surprisingly, though, the noise immunity enjoyed by digital signals comes with a price: a sacrifice in resolution. Analog signals are able to represent the smallest imaginable changes because they are continuously variable. Digital signals are limited in resolution by the number of bits in each data “word.” Thus, we see another contrast between analog and digital data representation:
|Corrupted by any amount of noise
|Immune to certain (limited) amounts of noise
With modern digital electronic technology, however, the “limited resolution” problem is almost nonexistent. 16-bit converter chipsets are commonly available today for input/output (I/O) modules on digital systems, providing a resolution of 216 (65536) counts, or ± 0.00153%, which is good enough for the vast majority of industrial measurement and control applications.
This section will focus on serial data transmission, as opposed to parallel. In order to transmit digital data in parallel form, the number of wires scales directly with the number of bits in each data “word.” For example, if a 16-bit ADC chip were to communicate its data to some other digital device using a parallel network, it would require a cable with 16 wires (plus a common “ground” wire) at minimum. Since this approach undercuts the “fewer wires” advantage that digital communications theoretically enjoys over analog communication, parallel data transmission is rarely seen in industry except for within the internal construction of a digital device (e.g. a parallel data bus inside a personal computer, or inside a PLC or DCS rack).
In serial communications systems, digital data is sent over a wire pair (or fiber optic cable, or radio channel) one bit at a time. A 16-bit digital “word” (two bytes in length) then will require a succession of 16 bits transmitted one after the other in time. How we represent each bit as an electrical signal, how we arrange those bits in time to group them into meaningful “words,” and how multiple devices share access to a common communications channel, is our next subject of exploration: the technical details of serial data communication.
Digital Data Communication Techniques
DATA TRANSMISSION MODES
Transmission of digital data through a transmission medium can be performed either in serial or in parallel mode. In the serial mode, one bit is sent per clock tick, whereas in parallel mode multiple bits are sent per clock tick. There are two subclasses of transmission for both the serial and parallel modes, as shown in Fig
Parallel transmission involves grouping several bits, say n, together and sending all the n bits at a time shows.
how parallel transmission occurs for n = 8. This can be accomplishes with the help of eight wires bundled together in the form of a cable with a connector at each end. Additional wires, such as request (req) and acknowledgement (ack) are required for asynchronous transmission.Primary advantage of parallel transmission is higher speed, which is achieved at the expense of higher cost of cabling. As this is expensive for longer distances, parallel transmission is feasible only for short distances.
Serial transmission involves sending one data bit at a time. Figure shows how serial transmission occurs. It uses a pair of wire for communication of data in bit-serial form.Since communication within devices is parallel, it needs parallel-to-serial and serial-to-parallel conversion at both ends.Serial mode of communication widely used because of the following advantages:•Reduced cost of cabling: Lesser number of wires is required as compared to parallel connection•Reduced cross talk: Lesser number of wires result in reduced cross talk•Availability of suitable communication media•Inherent device characteristics: Many devices are inherently serial in nature•Portable devices like PDAs, etc use serial communication to reduce the size of the connectorHowever, it is slower than parallel mode of communication.There are two basic approaches for serial communication to achieve synchronization of data transfer between the source-destination pair. These are referred to as – asynchronous and synchronous. In the first case, data are transmitted in small sizes, say character by character, to avoid timing problem and make data transfer self-synchronizing, as discussed later. However, it is not very efficient because of large overhead. To overcome this problem, synchronous mode is used. In synchronous mode, a block with large number of bits can be sent at a time. However, this requires tight synchronization between the transmitter and receiver clocks.
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