Data Communications and Networking 3rd Chapter 1#

Data Communications  and Networking 3rd semester Chapter 1#

 

LECTURE : 01

 DATA COMMUNICATIONS:

 Data communications are the exchange of data between two devices via some form of transmission medium such as a wire cable. 

. The effectiveness of a data communications system depends on four fundamental characteristics:

1. Delivery. 

 The system must deliver data to the correct destination. Data must be received by the intended device

 or user and only by that device or user.

2. Accuracy.

The system must deliver the data accurately. Data that have been altered in transmission and left

 uncorrected are unusable.

3. Timeliness.

 The system must deliver data in a timely manner. Data delivered late are useless. In the case of video

 and audio, timely delivery means delivering data as they are produced, in the same order that they are

 produced, and without signifi cant delay. 

 4. Jitter. 

Jitter refers to the variation in the packet arrival time. It is the uneven delay in the delivery of audio or

 video packets. For example, let us assume that video packets are sent every 30 ms.

 Components:

                          A data communications system has five components:

 1. Message.

 The message is the information (data) to be communicated. Popular forms of information include text, numbers, pictures, audio, and video. 

2. Sender.

 The sender is the device that sends the data message. It can be a computer, workstation, telephone handset, video camera, and so on.

 3. Receiver.

 The receiver is the device that receives the message. It can be a computer, workstation, telephone handset, television, and so on.

4. Transmission medium.

 The transmission medium is the physical path by which a message travels from sender to receiver.

For Example: of transmission media include twisted-pair wire, coaxial cable, fiber-optic cable, and radio waves.

Data Representation

 Information today comes in different forms such as text, numbers, images, audio, and video.

1.Text:

 In data communications, Text is represented as a bit pattern, a sequence of bits (0s or 1s). Different sets of bit patterns have been designed to represent text symbols. Each set is called a code, and the process of representing symbols is called coding. 

2.Numbers:

 Numbers are also represented by bit patterns. However, a code such as ASCII is not used to represent numbers; the number is directly converted to a binary number to simplify mathematical operations. Appendix B discusses several different numbering systems.

3.Image:

 Images are also represented by bit patterns. In its simplest form, an image is composed of a matrix of pixels (picture elements), where each pixel is a small dot. The size of the pixel depends on the resolution.

For example, an image can be divided into 1000 pixels or 10,000 pixels. 

In the second case, there is a better representation of the image (better resolution), but more memory is needed to store the image.

4. Audio:

Audio refers to the recording or broadcasting of sound or music. Audio is by nature different from text, numbers, or images. It is continuous, not discrete. Even when we use a microphone to change voice or music to an electric signal, we create a continuous signal.

5.Video:

  Video refers to the recording or broadcasting of a picture or movie. Video can either be produced as a continuous entity (e.g., by a TV camera), or it can be a combination of images, each a discrete entity, arranged to convey the idea of motion.

 Data Flow :

 Data flow (simplex, half-duplex, and full-duplex)

1. Simplex: 

 In simplex mode, the communication is unidirectional, as on a one-way street. Only one of the two devices on a link can transmit; the other can only receive 

For Example:Keyboards and traditional monitors are examples of simplex devices. The key board can only introduce input; the monitor can only accept output.

USE:The simplex mode can use the entire capacity of the channel to send data in one direction.

2. Half-Duplex:

 In half-duplex mode, each station can both transmit and receive, but not at the same time. When one device is sending, the other can only receive, and vice versa.

For Exaple: The half-duplex mode is like a one-lane road with traffic allowed in both direc tions. When cars are traveling in one direction, cars going the other way must wait.

USE:The half-duplex mode is used in cases where there is no need for communication in both directions at the same time;

3.Full-Duplex:

 In full-duplex mode (also called duplex), both stations can transmit and receive simultaneously.

 The full-duplex mode is like a two-way street with traffic flowing in both directions at the same time. In full-duplex mode, signals going in one direction share the capacity of the link with signals going in the other direction.

USE:The full-duplex mode is used when communication in both directions is required all the time.

 NETWORKS :

 A network is the interconnection of a set of devices capable of communication. In this definition, a device can be a host (or an end system as it is sometimes called) such as a large computer, desktop, laptop, workstation, cellular phone, or security system.

A device in this definition can also be a connecting device such as a router, which connects the network to other networks, a switch, which connects devices together, a modem (modulator-demodulator), which changes the form of data, and so on.

 Network Criteria: 

A network must be able to meet a certain number of criteria. The most important of these are performance, reliability, and security.

1.Performance: 

 Performance can be measured in many ways, including transit time and response time. Transit time is the amount of time required for a message to travel from one device to another. Response time is the elapsed time between an inquiry and a response.

Performance is often evaluated by two networking metrics: throughput and delay. We often need more throughput and less delay.

2.Reliability:  

 In addition to accuracy of delivery, network reliability is measured by the frequency of failure, the time it takes a link to recover from a failure, and the network’s robustness in a catastrophe.

3. Security:

 Network security issues include protecting data from unauthorized access, protecting data from damage and development, and implementing policies and procedures for recovery from breaches and data losses.

Physical Structures:

Type of Connection :

A network is two or more devices connected through links. A link is a communications pathway that transfers data from one device to another. 

1. Point-to-Point:

 A point-to-point connection provides a dedicated link between two devices. The entire capacity of the link is reserved for transmission between those two devices. 

 Most point-to-point connections use an actual length of wire or cable to connect the two ends, but other options, such as microwave or satellite links, are also possible.

For Example When we change television channels by infrared remote control, we are establishing a point-to-point connection between the remote control and the television’s control system

 2. Multipoint:

 A multipoint (also called multidrop) connection is one in which more than two specific devices share a single link 

 In a multipoint environment, the capacity of the channel is shared, either spatially or temporally. If several devices can use the link simultaneously, it is a spatially shared connection. If users must take turns, it is a timeshared connection.

 Physical Topology:

The term physical topology refers to the way in which a network is laid out physically. Two or more devices connect to a link; two or more links form a topology. The topology of a network is the geometric representation of the relationship of all the links and linking devices (usually called nodes) to one another.

. Mesh Topology

.Star Topology

.Bus Topology

.Ring Topology

1.Mesh Topology:

In a mesh topology, every device has a dedicated point-to-point link to every other device. The term dedicated means that the link carries traffic only between the two devices it connects.

To find the number of physical links in a fully connected mesh net work with n nodes, we first consider that each node must be connected to every other node. Node 1 must be connected to n – 1 nodes, node 2 must be connected to n – 1 nodes, and finally node n must be connected to n – 1 nodes.

Advantages:A mesh offers several advantages over other network topologies. First, the use of dedicated links guarantees that each connection can carry its own data load, thus eliminating the traffic problems that can occur when links must be shared by multiple devices. Second, a mesh topology is robust.

Disadvanges:The main disadvantages of a mesh are related to the amount of cabling and the number of I/O ports required. First, because every device must be connected to every other device, installation and reconnection are difficult.

2.Star Topology:

 In a star topology, each device has a dedicated point-to-point link only to a central con troller, usually called a hub. The devices are not directly linked to one another. Unlike a mesh topology, a star topology does not allow direct traffic between devices.  A star topology is less expensive than a mesh topology.

Advantages:Other advantages include robustness. If one link fails, only that link is affected. All other links remain active.

Disadvantages:One big disadvantage of a star topology is the dependency of the whole topology on one single point, the hub. If the hub goes down, the whole system is dead.

3.Bus Topology:

 The preceding examples all describe point-to-point connections. A bus topology, on the other hand, is multipoint. One long cable acts as a backbone to link all the devices in a network.

 Nodes are connected to the bus cable by drop lines and taps. A drop line is a connection running between the device and the main cable. A tap is a connector that either splices into the main cable or punctures the sheathing of a cable to create a contact with the metallic core.

Advantages of a bus topology include ease of installation. Backbone cable can be laid along the most efficient path, then connected to the nodes by drop lines of various lengths. In this way, a bus uses less cabling than mesh or star topologies.

 Disadvantages include difficult reconnection and fault isolation. A bus is usually designed to be optimally efficient at installation. It can therefore be difficult to add new devices.

4.Ring Topology:

  In a ring topology, each device has a dedicated point-to-point connection with only the two devices on either side of it. A signal is passed along the ring in one direction, from device to device, until it reaches its destination.

AdvantagesA ring is relatively easy to install and reconfigure. Each device is linked to only its immediate neighbors (either physically or logically). To add or delete a device requires changing only two connections.

unidirectional traffic can be a disadvantage. In a simple ring, a break in the ring (such as a disabled station) can disable the entire network.