Data Communications

Data Communications

We share information when we communicate. This sharing can take place on a local or remote level. Local communication takes place face to face between persons, whereas remote communication takes place over a distance. Telecommunication refers to long-distance communication (tele is Greek for "far"). It encompasses telephony, telegraphy, and television.
The term "data" refers to information given in whatever format the parties originating and using the data have agreed upon.
The exchange of data between two devices across a transmission channel such as a wire cable is known as data communications. The communicating devices must be part of a communication system that consists of a combination of hardware (physical equipment) and software for data communications to take place (programs).

The delivery, accuracy, timeliness, and jitter properties of a data communications system determine its effectiveness.

  1. Data must be delivered to the proper location by the system. Only the specified device or user can get data.

  2. Perfection Data must be delivered accurately by the system. Data that has been tampered with and left incorrect during transmission is useless.

  3. Accuracy of information. Data must be delivered quickly via the system. Data that arrives late is of no use. In the case of video and audio, timely delivery refers to delivering data as soon as it is created, in the same sequence as it is created, and with minimal delay.Real-time transmission is the term for this type of distribution.

The variance in packet arrival time is referred to as jitter. It is the delivery of audio or video packets with an uneven delay. Assume that video packets are transmitted every 3D milliseconds. If some packets come with a 3D-ms delay while others arrive with a 4D-ms delay, the video quality will be inconsistent.

There are five parts to a data transmission system (see Figure 1).

  1. The Message
    The information (data) to be delivered is the message. Text, numbers, photos, audio, and video are all common types of information.

  2. sender
    The device that sends the data packet is known as the sender. It might be anything from a computer to a workstation to a phone handset to a video camera.


  1. Recipient
    The device that receives the message is referred to as the receiver. It might be anything from a computer to a workstation to a phone handset to a television.

  2. Medium of transmission
    A message's transmission medium is the physical path it takes to get from sender to receiver. Twisted-pair wire, coaxial cable, fiber-optic cable, and radio waves are examples of transmission media.Follow the protocol. A protocol is a set of rules that control the transmission of data.

Data Representation

Text, numbers, photos, audio, and video are all examples of information today.

Text is represented as a bit pattern, or a sequence of bits, in data transmissions (Os or Is). To represent written symbols, many sets of bit patterns have been created. The process of representing symbols is called coding, and each set is termed a code.

Bit patterns can also be used to represent numbers.

Bit patterns are also used to represent images. An image is made up of a matrix of pixels (picture elements) in its most basic form, with each pixel being a little dot. The resolution determines the size of the pixel.

The recording or broadcasting of sound or music is referred to as audio. Audio, unlike words, numbers, or images, is created by nature. It's a continual process, not a single one.

The recording or broadcasting of a picture or movie is referred to as video. Video can be created as a single continuous image (e.g., from a TV camera) or as a collection of discrete images organised to portray the illusion of motion.

Data Flow
As indicated in Figure 2, communication between two devices can be simplex, half-duplex, or full-duplex.

Figure 2: Data flow (simplex, half duplex and full duplex)

Communication is unidirectional in simplex mode, as it is on a one-way street. On a link, only one of the two devices can send, while the other can only receive (see Figure 2a). Simplex devices include keyboards and standard monitors. The monitor can only accept output; the keyboard can only introduce input. The simplex mode can transfer data in one direction while using the complete capacity of the channel.

Each station can transmit and receive in half-duplex mode, but not at the same time. When one device is sending, the other is only capable of receiving, and vice versa (see Figure 2b).The half-duplex mode is similar to a one-lane road with both directions of traffic allowed. Cars heading in one direction must wait for cars travelling in the opposite direction. In a half-duplex transmission, whichever of the two devices is broadcasting at the time takes over the entire capacity of the channel. CB (citizens band) radios and walkie-talkies are both half-duplex systems. For either direction, the channel's complete capacity can be used.

Both stations can transmit and receive data at the same time in full-duplex mode (see Figure2c). The full-duplex mode works similarly to a two-way street, with traffic moving in both directions at the same time. When running in full-duplex mode Either the link must have two physically separate transmission pathways, one for sending and the other for receiving, or the channel's capacity must be shared by signals moving in both directions. The telephone network is a good example of full-duplex communication. When two people are talking on the phone, they can both talk and listen at the same time. The channel's capacity, on the other hand, must be split between the two directions.


A network is a collection of devices (also known as nodes) linked together through communication links. A node can be a computer, printer, or any other device that can send and/or receive data generated by other network nodes.

Distributed Processing 
The majority of networks employ distributed processing, which divides a task across numerous computers. Separate computers (typically a personal computer or workstation) handle a subset of a process instead of a single huge machine handling all elements.

Criteria for Networks
A network must be able to meet a set of requirements. Performance, dependability, and security are the most crucial.

The number of users, the kind of transmission media, the capabilities of the linked hardware, and the efficiency of the software all influence the performance of a network. Throughput and latency are two networking metrics that are frequently used to assess performance. More throughput and less delay are frequently required. These two criteria, however, are frequently in conflict. If we try to send more data to the network, we may increase throughput, but we will also increase latency due to network congestion.

Network dependability is assessed by the frequency of failure and the time it takes for a link to recover from a failure, in addition to delivery accuracy.

Preserving data from illegal access, protecting data from harm and development, and adopting rules and procedures for recovering from breaches and data losses are all security challenges.

Physical Structures

We need to establish some network attributes before we can talk about networks. Connection Type
A network is made up of two or more devices that are linked together. A link is a communication channel that allows data to be sent from one device to another. Point-to-point and multipoint connections are the two types of connections available.

Figure 3: Types of connection: point to point and multipoint

A dedicated link between two devices is provided by a point-to-point connection. Transmission between those two devices takes up the full bandwidth of the link. The majority of point-to-point connections employ a physical cable or wire to connect the two ends, although alternative options, such as microwave or satellite links, are also available (see Figure 3a). When you use an infrared remote control to change the channels on your television, you're creating a point-to-point link between the remote control and the television's control system.

A multipoint (also known as multidrop) connection is one that allows more than two devices to share a single link (see Figure 1.3b). The capacity of the channel is shared in a multipoint context, either spatially or temporally. It's a spatially shared connection if multiple devices can use it at the same time. It's a timeshared connection if users have to take turns.

Physical Topology

The phrase "physical topology" refers to how a network is physically laid out.
A link connects two or more devices; a topology is formed by two or more links.
There are four different topologies that can be used: mesh (see Mesh), star (see Star), bus (see Bus), and ring (see Ring)

Figure 4: categories of topology

Every device in a mesh topology has its own dedicated point-to-point link to every other device. The term "dedicated" refers to the fact that the link exclusively transports data between the two devices it links. To calculate the number of physical links in an n-node mesh network, we first assume that each node must be connected to every other node. Node 1 must be connected to n - I nodes, node 2 to n - 1 nodes, and finally node n to n - 1 nodes. We require n(n - 1) physical connections. However, if each physical link can communicate in both directions (duplex mode), the total number of links can be reduced by two. In other words,

n(n -1) /2 duplex-mode links are required in a mesh topology.

Figure 5 : A fully connected mesh topology (five devices)

For starters, using dedicated links ensures that each connection can carry its own data load, eliminating traffic issues that can arise when links must be shared by multiple devices. Second, a mesh topology is resistant to failure. If one link fails, the entire system is not rendered inoperable. Finally, there is the benefit of privacy or security. Only the intended recipient sees a message that travels along a dedicated line. Physical barriers prevent other users from accessing messages. Finally, point-to-point links facilitate fault identification and isolation. Traffic can be routed to avoid suspected problem links.

The main drawbacks of a mesh are related to the amount of cabling required and the number of I/O ports. For starters, installation and reconnection are difficult because every device must be connected to every other device. Second, the sheer size of the wiring may be larger than the available space (in walls, ceilings, or floors). Finally, the hardware necessary to connect each link (I/O ports and cables) can be prohibitively expensive.

A mesh topology is typically implemented in a limited capacity, such as as a backbone connecting the main computers of a hybrid network that may also include other topologies. A practical application of a mesh topology is the connection of telephone regional offices, in which each regional office must be linked to every other regional office.

Star Topology

Figure 6 A star topology connecting four stations

In a star topology, each device has a dedicated point-to-point link to a central controller, which is commonly referred to as a hub. The devices are not connected in any way. A star topology, unlike a mesh topology, does not allow direct traffic between devices. When one device wants to send data to another, it sends the data to the controller, who then relays the data to the other connected device (see Figure 6).

A mesh topology is more expensive than a star topology. Each device in a star requires only one link and one I/O port to connect to any number of others.
It is simple to install and configure.

Far less cabling is required.
Another advantage is its toughness. Only one link is affected if it fails. All other links remain operational.

One significant disadvantage of a star topology is its reliance on a single point, the hub. If the hub fails, the entire system fails.

Local-area networks employ the star topology (LANs)
A star topology with a central hub is commonly used in high-speed LANs.

Bus Topology 

Topology of Buses
All of the preceding examples are of point-to-point connections. In contrast, a bus topology is multipoint. A single long cable serves as the network's backbone, connecting all devices (see Figure.7)

Figure 7 A bus topology connecting three stations

Drop lines and taps connect nodes to the bus cable. A drop line is a connection that connects the device to the main cable. A tap is a type of connector.

The ease of installation is one of the benefits of a bus topology.
A bus topology requires less cabling than a mesh or star topology.

It entails difficult reconnection as well as fault isolation. A bus is typically designed to be as efficient as possible during installation. As a result, adding new devices can be difficult. Signal reflection at the taps can degrade signal quality. The number and spacing of devices connected to a given length of cable can be limited to control this degradation. Furthermore, a fault or break in the bus cable halts all transmission.

One of the first topologies used in the design of early local area networks was the bus topology. A bus topology can be used in Ethernet LANs.

Ring Topology

Figure 8 A ring topology connecting six stations

Each device in a ring topology has a dedicated point-to-point connection with only the two devices on either side of it. A signal is passed from device to device along the ring in one direction until it reaches its destination. A repeater is built into each device in the ring. When one device receives a signal intended for another, its repeater regenerates the bits and forwards them (see Figure 8).

A ring is simple to install and reconfigure. Each device is only connected to its immediate neighbours (either physically or logically). Adding or removing a device requires only two connections to be changed.
Furthermore, fault isolation is simplified. A signal is typically circulating in a ring at all times. If a device does not receive a signal within a certain time frame, it can generate an alarm. The alarm notifies the network operator of the problem's location.
one of the initial topologies used in the design of early local area networks A bus topology can be used in Ethernet LANs.

Unidirectional traffic can be inconvenient.
A break in the ring (such as a disabled station) in a simple ring can disable the entire network. This flaw can be overcome by employing a dual ring.

When IBM introduced its local-area network Token Ring, ring topology was common.

Hybrid Topology

A hybrid network can exist. As shown in Figure 9, we can have a main star topology with each branch connecting several stations in a bus topology.

Figure 9 A hybrid topology: a star backbone with three bus networks