DATA Communication AND Networks Notes PDF

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Data communication & networks

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1. Introduction to Networks and Networking Concepts: Definition of networking concepts: A network is simply defined as something that connects things together for a specific purpose. The term network is used in a variety of contexts, including telephone, television, computer, or even people networks. Computer network is any collection of independent computers that communicate with one another over a shared network medium. A computer network is a collection of two or more connected computers. Or It is a collection of two or more computers that are linked in order to share resources (such as printers and CDs), exchange files, or allow electronic communications. Protocols are rules that govern how devices communicate and share information across a network. or Protocol is an agreed-upon format for transmitting data between two devices. Examples of protocols include: IP – Internet Protocol HTTP - Hyper Text Transfer Protocol SMTP – Simple Mail Transfer Protocol Need for a network File sharing: A network makes it easy for everyone to access the same file and prevents people from accidentally creating different versions. Hardware sharing: With a network, several computers can share the same printer, scanner, CD-ROM drive, hard drive etc. Although you might need a more expensive hardware to handle the added workload, it's still cheaper to use network hardware than to connect a separate hardware to every computer. User Communication: A network allows employees to share files, view other people's work, and exchange ideas more efficiently. In a larger office, you can use e-mail and instant messaging tools to communicate quickly and to store messages for future reference. Organization: A variety of scheduling software is available that makes it possible to arrange meetings without constantly checking everyone's schedules. This software usually includes other helpful features, such as shared address books and to-do lists. Remote access: Having your own network allows greater mobility while maintaining the same level of productivity. With remote access in place, users are able to access the same files, data, and messages even when they're not in the office. This access can even be given to mobile handheld devices. Data protection: A network makes it easier to back up all of your company's data on an offsite server, a set of tapes, CDs, or other backup systems. (Of course, another aspect of data protection is data security. Page 1

Data communication & networks

Network models:  Peer-to-Peer: In this architecture, all hosts on the network can both request and provide data and services. For example, two Windows XP workstations configured to share files would be considered a peer-to-peer network. Advantage: Peer-to-peer networks are very simple to configure. Disadvantages:  Data is difficult to manage and back-up, as it is spread across multiple devices.  Security is equally problematic, as user accounts and permissions must be configured individually on each host.  Client/Server: In this architecture, hosts are assigned specific roles. Clients request data and services stored on servers. An example of a client/server network would be Windows XP workstations accessing files off of a Windows 2003 server. Advantage:  Data and services are centrally located on one or more servers, consolidating the management and security of that data. Disadvantage:  The server can present a single point of failure.  Mainframe/Terminal: In this architecture, a single device (the mainframe) stores all data and services for the network. Advantage:  Centralized management and security of data.  Additionally, the mainframe performs all processing functions for the dumb terminals that connect to the mainframe. The dumb terminals perform no processing whatsoever, but serve only as input and output devices into the mainframe. In simpler terms, the mainframe handles all thinking for the dumb terminals. A dumb terminal typically consists of only a keyboard/mouse, a display, and an interface card into the network. Types of network:  Local-area networks (LANs): A computer network that spans a relatively small area. Most LANs are confined to a single building or group of buildings.  Metropolitan-area networks (MANs): Data network designed for a town or city  Wide-area networks (WANs): A computer network that spans a relatively large geographical area. Typically, a WAN consists of two or more local-area networks (LANs). Computers connected to a wide-area network are often connected through public networks, such as the telephone system. They can also be connected through leased lines or satellites. The largest WAN in existence is the Internet. Page 2

Data communication & networks

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Network topologies: Topology is the shape of a local-area network (LAN) or other communications system. There are three principal topologies used in LANs. A network topology is the geometric arrangement of nodes and cable links in a LAN. Bus topology: All devices are connected to a central cable, called the bus or backbone. Ring topology: All devices are connected to one another in the shape of a closed loop, so that each device is connected directly to two other devices, one on either side of it. Star topology: All devices are connected to a central hub. Star networks are relatively easy to install and manage, but bottlenecks can occur because all data must pass through the hub. Mesh topology: Devices are connected with many redundant interconnections between network nodes. In a true mesh topology every node has a connection to every other node in the network. 2. Network design essentials Network Design: Good network design includes:  Analyzing network requirement  Selecting a network topology  Selecting equipment to fit that topology Designing a Network Layout:  Topology refers to physical layout including computers, cables, and other resources. It determines how components communicate.  Basic network design is referred to as topology, layout, diagram, and map  Physical topology refers to arrangement of cabling  Logical topology refers to how data travels between computers on the network. Network may use one physical topology but a different logical topology to pass data. Topology affects network’s performance, growth potential and determines type of equipment to purchase and how to manage network. When designing a network, you must understand different topologies, consider growth and security requirements. Good design grows and adapts as needs change. Standard Topologies: Today’s network designs are based on three topologies:  Bus consists of series of computers connected along a single cable segment  Star connects computers via central connection point or hub  Ring connects computers to form a loop Page 3

Data communication & networks a) Bus topology: It is the simplest topology where components connect via backbone or single cable segment. The major weakness is single cable break can halt entire network

Sending the Signal: All computers, regardless of topology, communicate by addressing data to one or more computers and transmitting it across cable as electronic signals. Data is broken into packets and sent as electronic signals that travel on the cable. Only the computer to which the data is addressed accepts it. Bus Communications: In bus topology, only one computer can send information at a time. Network performance slows as more computers are placed on the bus. Bus is a passive topology:  Computers only listen for data being sent; not responsible for moving data to next computer  Failure of one computer has no effect on rest of network In active topology, computers regenerate signals; move data through network. Signal Bounce:  Signals move from point of transmission to both ends of any bus  Something must stop signals when they reach end of bus to avoid signal bounce as shown in the figure below.

 Terminator attached to end of cable absorbs electronic signal, prevents signals from bouncing as shown in the figure below.

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Data communication & networks

Terminated bus network Cable Failure:  Cable break means bus network is no longer terminated  Without termination, signals bounce and halt all network activity as shown in the figure below.

Cable break Bus Network Expansion:  Easy to expand bus network by using Ethernet 10Base2 (thinnet) and BNC barrel connectors.  Longer network segments can cause attenuation or weakening of signal.  Repeater regenerates incoming signals to eliminate signal attenuation and does not correct incoming errors.  Bus topology not so popular because of single cable failure and troubleshooting and management problems. b) Star Topology: It is the dominant topology in today’s networks. It connects computers to central hub that receives and transmits signals to all devices. Only computer to which packet is addressed processes it.

It offers centralization of resources, but requires more cable and has single point of failure.

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Data communication & networks  If the hub fails, network is down, but failure of single computer or cable does not affect network.  Easy to troubleshoot. c) Ring Topology: Computers are attached in a circle with no termination necessary  Signals travel in one direction around ring  Each computer receives signal and passes it along

Electronic token passes around ring with computer able to communicate only when it has token.  May be physically wired as a star with central hub passing token in a circle Ring Network: Some networks use dual counter-rotating rings for speed and redundancy  Fiber Distributed Data Interface (FDDI), One computer failing can bring down single-ring network unless it has smart hub that automatically removes failed computer from ring  When one ring fails, dual ring network uses secondary ring and continues to work. Shares network resources equally. d) Wireless Topologies: They eliminate cables. Wireless LANs use centralized device similar to hub to control communication  Use star topology  Signals travel through one central device Hubs Central point of concentration for star network, as shown in the figure below

Hub connection

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Data communication & networks Hubs May be active or passive; Active hub, also called a multiport repeater, regenerates signal and passes it along while passive hub is simply central connection point, with no amplification or regeneration. Hybrid hubs maximize network’s efficiency by interconnecting different types of cables and topologies. Switches  Central connecting point for star topology network.  Determines destination of message and sends it only to destination port  Provide full bandwidth to each station on network and handle several conversations at once  They are more expensive than hubs and provide better performance  It is a device of choice Variations on the Major Topologies: Three variations of major network topologies are combinations of topologies i. Mesh Topology:  Most fault tolerant topology  Offers multiple connections to each device  Uses intricate cabling configuration; every device connected to every other device in network  Expensive to implement  Internet is mesh topology with multiple paths to key junction points as shown in the figure below

ii. Star Bus Topology  Uses bus backbone  Interconnects two or more hubs as shown below

iii.Star Ring Topology  Wired as star  Handles traffic like ring  Can have several outer hubs connected to inner hub  Single computer failure does not affect network

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Data communication & networks

Selecting a Topology: When selecting network topology, consider many factors such as the advantages and disadvantages of each topology. Constructing a Network Layout: i. First step in network design is evaluating underlying requirements Important questions to consider include:  How many clients will be attached?  How many servers will be attached?  What are company’s plans for expansion?  What kind of applications will run?  Will this be peer -to-peer or server-based network?  How much fault tolerance do applications require?  How much money is available to build network? ii. The next step is to sketch a basic network layout  Obtain blueprints of building  Mark all planned locations of network resources  Use third-party application, such as netViz, to map network for example:

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Simple network layout diagram Include enough detail for technician to use diagram for troubleshooting problems Keep network diagram up to date

3. Network Communications and Protocols:  Communication networks and services: Protocols and telegrams: Communications protocols are required to exchange the data between two or more devices/participants reliably. When we send a letter, we need to indicate the sender’s address as well as the destination address. This is also Page 8

Data communication & networks true when sending digital letters. Protocols help distinguish between the control data and the proper message. This is achieved by defining a certain format for the data telegram, which specifies the beginning and the end of the telegram as well as the control data and the proper messages. Usually, telegrams consist of the following basic components: The data field containing the proper message is preceded by the header information. In multi-point connections (networks), the header generally contains the source and destination address of the message or indicates its contents. Additionally, the message control data, such as message length (e.g. number of bits), is included here. The data field is succeeded by the data security section. This component provides the error checking information generated by the transmitter, helping the recipient to check whether the message is correct.(access to telegram) Addressing and communication services: When a communication system comprises more than two participants and a message must be passed to a particular participant, direct addressing of this participant must be made possible. The simplest way is to assign an address statically to each device. Static address assignment can be implemented on the hardware level via switches or on the software level. Another possibility is to assign the addresses dynamically. To enable the participants to perform their communication tasks, various services are available. The number of services provided is a criterion for assessing communication systems. There are two different types of communication services: a. Connection-oriented services transmit data over a communication channel which links the transmitter and the recipient directly by using their addresses. b. Connectionless communication services do not provide a direct connection between the transmitter and the recipient. Typical examples for connectionless services are the following:  Multicasting: message directed to a group of participants  Broadcasting: message directed to all participants. Applications and Layered architectures:  OSI reference model: The International Standards Organization (ISO, Est. 1947) developed the Open Systems Interconnection (OSI) model. The purpose of OSI model does not specify the protocols to be used to perform networking tasks. It shows how to facilitate communication between different systems without requiring changes to the logic of the underlying hardware and software. Page 9

Data communication & networks The OSI model is the layered framework for the design of network system that allows communication between all types of computer systems. It consists of seven separate but related layers as shown below.

APPLICATION PRESENTATION SESSION TRANSPORT NETWORK DATA LINK PHYSICAL All People Seem To Need Data Processing (top down) Please Do Not Throw Sausage Pizza Away (bottom up) 1. The Physical Layer Physical layer is the lowest layer of the OSI model. It is concerned with transmitting raw bits over a communication channel. It is concerned with transmitting and receiving bits. It deals with the electrical and mechanical specifications of interface and transmission media. It also deals with procedures and functions required for transmission. The device that operates at this layer is a repeater.

Functions of Physical layer: i. Physical characteristics of interface and Media- The design issue of physical layer considers the characteristics of interface between devices and transmission media. ii. Representation of bits- Physical layer encodes the bit stream into electrical or optical signal. iii. Data rate- Physical layer defines the duration of bit which is called the data rate. iv. Synchronization of bits- physical layer synchronizes the transmission rate and receiving rate. 2. The Data Link Layer It receives messages, called frames, from upper layers. A primary function of the Data Link layer is to disassemble these frames into bits for transmission and then to reconstruct the frames from the bits received. Data link layer implements physical addressing. The device that operates at this level is bridge. Other responsibilities\functions of data link layer: i. Framing - The frames received from network layer is divided into manageable data units called frame. ii. Physical addressing - If the frames are to be sent to different systems in the network, the data link later adds a header to the frame to define the sender and/or receiver. Page 10

Data communication & networks iii. Flow Control - Flow control determines the amount of data that can be transmitted in a given time period. When the rate of data reception is less than the rate of data produced by the sender, the data link layer imposes control mechanism to avoid overwhelming the receiver. iv. Error control - Error control detects errors in received frames and requests retransmission of frames. The data link layer adds reliability to the physical layer by adding mechanisms to detect and retransmit damaged or lost frames. Error control is normally achieved through a trailer to the end of the frame. v. Access control - When multiple devices are connected to the same link, data link layer will determine which device has the control over the link at any given time. 3. The Network Layer The Network layer handles communication with devices on logically separate networks that are connected to form internetworks. Because internetworks can be large and can be constructed of different types of networks, the Network layer utilizes routing algorithms that guide packets from their source to their destination networks. Functions of Network layer: i. Logical addressing - The physical addressing implemented by the data link layer handles the addressing problem locally. When a network passes the network boundary, an addressing system is needed to distinguish source and destination, network layer performs this function. The network layer adds a header to the packet coming from the upper later that, among other things, includes the logical addresses of the sender and receiver. ii. Routing - Network layer route or switch (through switching techniques) the packets to its final destination in an internetwork. Switching techniques are mechanisms for moving data from one network segment to another which are: Circuit switching, Message switching and Packet switching. 4. The Transport Layer The transport layer is responsible for process-to-process delivery of the entire message. It ensures that the whole message arrives complete and in order, overseeing both error control and flow control at source-to-destination level. Functions of transport layer: i. Service-point/port addressing - computer performs several processes simultaneously. For this reason, source-to-destination delivery means from a specific process on one computer to a specific process on the other. The transport layer header must therefore include a type of address called a service-point address. Thus the transport layer gets the entire m...