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UNIT 9 NETWORK CONCEPTS
Structure
9.0 Objectives
9.1 Introduction
9.2 Definitions and Basic Concepts
9.2.1 What is a Network? 9.2.2 Advantages of Networking 9.2.3 Circuit and Packet-based Networks 9.2.4 Understanding Bandwidth
9.3 Network Topologies
9.3.1 Physical Topologies : Bus, Star, Ring 9.3.2 Logical Topologies : Ethernet, Token Ring, FDDI, ATM 9.3.3 Ethernet and its Varieties
9.4 LANs, MANs, WANs
9.4.1 Client/Server Network 9.4.2 Peer-to-peer Network
9.5 Network Components 9.5.1 Hardware : Network Devices 9.5.2 Software : Network Operating Systems
9.6 Wired and Wireless Communication Channels
9.6.1 Wired Channels: Twisted-pair Wire, Coaxial Cable, Fiber-optic Cable 9.6.2 Wireless Channels : Radio Link, Microwave Link, Satellite Communication
9.7 Network Security
9.8 Summary
9.9 Keywords
9.10 Answers to Self Check Exercises
9.11 References and Further Reading
9.0 OBJECTIVES
After reading the Unit, you will be able to :
understand the key concepts related to networking, benefits of networks and what networks are made of;
comprehend physical and logical topologies of networks;
differentiate various types of networks, communication channels and network devices; and
understand the importance of network security.
9.1 INTRODUCTION
The rapid growth in Information and Communication Technologies (ICT) has resulted in proliferation of computer networks in all walks of our lives today. We are fast moving towards an information society which critically depends 239
Introduction to Communication Technology
on networked environment for its sustenance. In recent years there has been a dramatic increase in the number of professions where an understanding of concepts underlying telecommunications and networking is essential for success.
The Unit aims to provide key network concepts in an easy-to-understand manner with illustrations wherever necessary. It covers a wide range of topics which include concepts related to bandwidth, network topologies, protocols, types of networks, network devices, communication media and network security. It is hoped that the information presented in this Unit will enable the learner to grasp the essentials relating to networks and their practical use.
9.2 DEFINITIONS AND BASIC CONCEPTS
9.2.1 What is a Network?
A network is a system of computers or communication devices (often called nodes) connected by media links that can communicate with one another and share applications and data. A node (computer, printer or any other device) is capable of sending and/or receiving data generated by other nodes on the network. The media links connecting the devices are called communication channels.
The characteristic feature of a network is its capability of sharing data and other resources between several users. All networks have the same basic goal: To ensure that data is shared quickly, reliably and accurately.
9.2.2 Advantages of Networking
A stand-alone computer can be a useful tool for individual use but its value increases many- fold when connected to a network.
Some of the benefits of networking are as follows :
i) Sharing of software and data : In most organisations people make use of the same software and need access to the same data. Instead of buying separate copies of software, network version of the software permitting a certain number of users can be loaded onto a file server and accessed simultaneously by multiple users. And also access to the same data using shared database on a server is cost-effective and avoids updating problems. ii) Sharing of peripheral devices : Peripheral devices such as laser printers, scanners, disk drives and others can be connected to a network allowing users to share these resources. iii) e-mail : One of the important uses of a network is electronic mail. It facilitates efficient communication amongst the users of the network. iv) Backup: Backing up i.e., making copies of vital data at regular intervals is essential to avoid data loss on failure of a storage device or natural disaster. In a networked environment, backup operations are more reliable and secure. v) Access to databases : Networks allow access to numerous databases available online through Internet.
Introduction to
Communication Technology 9.3^ NETWORK TOPOLOGIES
Network can be laid out or arranged in different ways. The term topology refers to the way a network is laid out, either physically or logically. Two or more devices connect to a link; two or more links form a topology.
9.3.1 Physical Topologies : Bus, Star, Ring
A physical topology is simply the connection arrangement representing relationship of all the links and linking devices (nodes) to each other. There are three basic physical topologies : bus, star and ring. Bus topology : In a bus topology all communication devices are connected to a common channel. One long cable acts as a backbone to link all the devices in the network (see Fig. 9.1 a). Each device transmits messages to other devices. If some of those messages collide, the device waits and tries to transmit again. Advantages of a bus topology include ease of installation and disadvantages include difficult reconfiguration and fault isolation. A fault or break in the bus cable stops all transmission. Star topology : In a star topology each device has a dedicated point-to-point link only to a central controller usually called a hub or a switch (see Fig. 9. b). The devices are not directly linked to each other. The controller acts as an exchange. If one device wants to send data to another, it sends the data to the controller, which then relays the data to the other connected device. Networks built on star topologies are more robust and reliable than those built on bus topologies. Here, if one link fails only that link is affected. All other links remain active. Ring topology : In a ring topology all devices are connected in a continuous loop. Each device has a dedicated point-to-point line configuration only with the two devices on either side of it (see Fig. 9.1 c). A signal is passed along the ring in one direction, from device to device, until it reaches its destination. When a device receives a signal intended for another device, its repeater regenerates the bits and passes them along. The advantage of ring network is that messages flow only in one direction. Thus there is no danger of collisions.
C 1 C 2 C 3
(a) Bus topology
C 1 C 2 C 3^ Network Concepts
(b) Star topology
(c) Ring Topology Fig. 9.1: Physical Topologies
The disadvantage is that, if a connection is broken the entire network stops working.
9.3.2 Logical Topologies : Ethernet, Token Ring, FDDI, ATM
Logical topologies layout the rules for data transmission on the network. In contrast to physical topologies which can be expressed through concrete pieces of equipment such as wiring types and communication devices; logical topologies are largely abstract and represent rules of the road (network). The rules are developed into specific standards for implementation. There are four main logical topologies : Ethernet, Token Ring, FDDI, ATM.
It should be remembered that at the base of a network system is the physical topology. On the top of that is the logical topology. And on the top of the logical topology are network protocols. Just as a logical topology is, a protocol is a set of rules for sending and receiving data across a network. An example of a protocol is TCP/IP (Transmission Control Protocol/Internet Protocol) which is a popular protocol on Internet.
Let us now consider in more details the logical topologies.
Ethernet : Ethernet was first developed by the Xerox in the 1970s. The Institute of Electrical and Electronic Engineers (IEEE) modified the specifications for
C1, C2, C
Communication devices
C
C
C
names. Some of the members of the Ethernet family which one comes across^ Network Concepts are :
i) Thick Ethernet : This refers to 10 Base 5 Ethernet discussed earlier and in also called Thicknet. It uses thick coaxial cable. The maximum length of coaxial cable in a Thick Ethernet segment is 500 meters.
ii) Thin Ethernet : This refers to 10 Base 2 Ethernet and is also called Thinnet. It uses thin coaxial cable and has a maximum segment length of 185 meters.
iii) Fast Ethernet : Fast Ethernet supports a data rate of 100 Mbps and has the following versions :
100 Base-T: It uses two pairs of unshielded twisted pair (UTP). The maximum cable length between the concentrator (hub) and the workstation is 20 meters.
100 Base-T4 : It uses four pairs of UTP and like 100 Base-T has a maximum cable length of 20 meters between the concentrator and the workstation.
100 Base–Fx : This refers to Fast Ethernet running on fiber optic cable. Since fiber optic cable can carry data much further than copper wire, 100 Base-Fx does not have the constraint of maximum cable length.
iv) Gigabit Ethernet : This standard supports data rates of 1 Gbps (gigabit per second) over an Ethernet network.
The growth path of Ethernet standard has been spectacular. Starting with data rate of 10Mbps it moved to 100 Mbps (Fast Ethernet) and then to 1000 Mbps (Gigabit Ethernet). Efforts are on to develop the standard to support data rate of 10 Gps (gigabits per second).
9.4 LANS, MANS AND WANS
Networks which consist of various combinations of computers, storage devices and communication devices may be divided into three main categories, differing primarily in their geographical range. These are:
Local Area Networks (LANs).
Metropolitan Area Networks (MANs).
Wide Area Networks (WANs).
i) Local Area Network (LAN): A LAN connects computers and devices in a limited geographical area. It is usually privately owned and links the devices in a single office, building or campus (see Fig.9.2). Depending upon the needs of an organisation and type of technology, a LAN can be as simple as two PCs and a printer in an office or several computers and peripherals.
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Introduction to Communication Technology
Workstati
Workstation 1
Hub/Swit
Fig. 9.2: Schematic of a Typical LAN (Star Topology) LANs are designed to allow resources to be shared between the PCs or workstations and have the following characteristics : They occupy only one physical location and hence the word local is used to describe them. They can be peer networks (i.e., there is no central computer), or they may be client/server networks (i.e., a central computer called a server, has most of the network’s resources and is accessed by clients or users’ computers). They have high-speed data transfer rates. All data is part of the local network. Currently, LAN size is limited to a few kilometers. In addition to size, LANs are distinguished from other networks by their transmission media and topology. ii) Metropolitan Area Network (MAN): A MAN is designed to extend over an entire city. It may be a single network or it may be a means of connecting a number of LANs into a larger network so that resources of the individual LANs may be shared (see Fig.9.3.).
Fig. 9.3: Schematic of a MAN
Workstation 2
Workstation 1
Serv
Hub/Switch
N/W Printer
Introduction to Communication Technology
In a client/server network computers are divided into clients and servers. Clients are computers (workstations) that request data and servers are computers used to supply data. The server is usually a dedicated, powerful computer while clients are usually less powerful and connect only to the server through the network (see Fig.9.5). Different servers may be used to manage different tasks. A file server is a computer that acts like a disk drive, storing the programs and data files shared by users on the LAN. A database server is a computer in a LAN which stores data but does not store programs.
Client 2
Client 3
Fig. 9.5: Client/Server Architecture
The benefits of client/server architecture include :
Centralised management of network resources.
Capacity to enforce rigorous controls on security and file access.
Capability to secure and backup data from the server.
Capability to scale up i.e., increase the size of the network.
The disadvantages of this architecture are the high cost of implementation (because of cost of the server) and the server acting as a single point of failure. If the server fails, the network is down. Therefore, many servers are embedded with fault tolerant features which further increase their cost.
There is only one basic criterion for client/server networking: no client (workstation) on the network will ever need to use any resources on any other workstation. In other words, workstations connected to the network see only servers —they never see one another. A client/server network is a one-to- many scheme with the server being the one and the workstations being the many. Client/server architecture is widely used in building large networks. Client/server networks are reliable and secure.
9.4.2 Peer-to-peer Network^ Network Concepts
In a peer-to-peer LAN, all computers on the network communicate directly with one another without relying on a server (see Fig.9.6). Peer-to-peer networks support many-to-many scheme with each peer capable of connecting to all other peers. In peer-to-peer architecture all user workstations also handle some server functions. For example one machine with a large hard disk drive may be used to store some of the users’ files. Another system connected with a printer may share the printer with other workstations.
Fig. 9.6: A Schematic of Pear-to-pear Network
Peer-to-peer networks are less expensive than client/server networks and are easier to install. However, total lack of centralised control and inadequate security makes them highly unreliable.
Peer-to-peer network architecture is suitable only for small networks (typically with not more than 25 computers). With increase in the number of computers the network slows down drastically.
9.5 NETWORK COMPONENTS
Networks are made of several components which can be broadly grouped into two categories :
Hardware-related components : network devices and software-related components : network operating system (NOS).
9.5.1 Hardware : Network Devices
Some of basic network devices are :
Workstation (client) : A workstation or a client is the computer terminal
loaded for client machines to function, a network-server must have a network^ Network Concepts operating system to provide networking support for multiple simultaneous users as well as administrative, security and management functions. Unlike a single-user operating system, NOS must acknowledge and respond to requests from many workstations, managing such details as network access and communications, resource allocation and sharing, data protection and error control.
The following network operating systems are well known :
i) Novell NetWare : This network operating system was developed by Novell, Inc. and is the oldest compared to newer network operating systems. NetWare is complex and difficult to manage operating system. It is not well-suited for being an application server or server that runs network service applications such as databases. It allows users to share files and system resources such as hard disks and printers. NetWare has good directory services. (Directory services refer to a set of tools that enables network administrators to provide users access to specific resources regardless of where on the network the user logs on). NetWare supports proprietory network protocol Internetworking Packet Exchange/Sequenced Packet Exchange (IPX/SPX). It does not provide native support for TCP/IP, the protocol which Internet uses. NetWare’s position has deteriorated as a result of increase in popularity of TCP/IP based networking.
ii) Microsoft Windows NT Server : Windows NT Server operating system, sometimes referred to as simply NT is an high-end NOS which has a rich set of utilities and tools. It is a completely self-contained operating system with a built-in graphical user interface (GUI). Windows NT Server supports TCP/IP natively. NT Server also works well for single-server networks that have to connect to Internet because NT Server has a suite of tools to handle networked Internet connections.
iii) UNIX : Unlike NetWare and Windows NT, UNIX is not a monolithic operating system owned by a single corporation. Instead, it is represented by a plethora of manufacturers (Sun Microsystem’s Solaris and the Santa Crux Operation’s SCO UNIX to have as examples). UNIX has evolved into a complex powerful operating system, because it is written in the C language, is more portable – that is, less machine specific – than many other operating systems. Unix uses TCP/IP network protocol natively, TCP/IP was created on and for UNIX systems. UNIX is widely used as a network operating system, especially in conjunction with the Internet. LINUX—the new version of UNIX named after its author Linus Torvalds is making great inroads as a server operating system especially with Internet related companies. Many Internet Service Providers are choosing LINUX as the preferred NOS. All the three discussed network operating systems viz. Novell NetWare, Windows NT, UNIX/LINUX are designated for client/server networks. However, they can also be used to build peer-to-peer networks.
Introduction to
Communication Technology 9.6^ WIRED AND WIRELESS
COMMUNICATION CHANNELS
Communication devices use analog electromagnetic signals representing data to transmit information from one device to another. Electromagnetic signals can travel through vacuum, air or other transmission media (wire, fiber optics etc.). A communication channel is the path—transmission medium—over which information travels in a communication system from its source to its destination. Channels are also called links, lines, or media. Communication channels which use a physical medium for transmission (twisted pair wire, coaxial cable, fiber- optic cable) are called wired channels. Communication channels which do not require any physical medium for transmission (radio, microwave and communication satellite) are called wireless channels. The basis for all communication channels, both wired and wireless, is the electro magnetic spectrum. The spectrum covers frequencies for voice, radio waves, infrared light, visible light, ultraviolet light and X, gamma and cosmic rays.
9.6.1 Wired Channels : Twisted-pair Wire, Coaxial Cable and
Fiber-optic Cable
Wired communication channels use the following physical media :
- Twisted-pair wire
- Coaxial cable
- Fiber-optic cable These media are also called guided media since they provide a conduit from one device to another, A signal travelling along any of these media is directed and contained by the physical limits of the medium. Twisted-pair and coaxial cable use metallic (copper) conductors that accept and transport signals in the form of electrical current. Optical fiber is a glass or plastic cable that accepts and transports signals in the form of light. Let us look more closely at each of the medium. i) Twisted-pair Wire : Twisted-pair wire comes in two types : Unshielded Twister Pair (UTP) and Shielded Twisted Pair (STP). Unshielded Twisted Pair (UTP) is the most common type and also used in telephone lines. A twisted pair consists of two conductors (copper) each with its own coloured plastic insulation and twisted around each other. Twisted pair configuration reduces interference from electrical field as compared to parallel pair configuration. Unshielded twisted pair is currently the cable standard for most networks. It is relatively inexpensive, easy to install, very reliable, and easy to maintain and expand. UTP support a maximum data rate of 155 Mbps. Shielded Twisted Pair (STP) wire has a metal foil or braided mesh covering that encases each pair of insulated conductors. The metal casing prevents the penetration of electromagnetic noise and the quality of transmission improves. In all other respects it resembles UTP.
Introduction to Communication Technology
regulated by government authorities. These bands are rated from very low frequency (VLF) to extremely high frequency (EHF). Radio link, microwave link and satellite communication utilise frequencies in the radio spectrum for data communication. Figure 9.7 illustrates the bands of the radio spectrum.
VLF LF MF
VLF – Very low frequency VHF – Very high frequency LF – Low frequency UHF – Ultra high frequency MF – Middle frequency SHF – Super high frequency HF – High frequency EHF – Extremely high frequency Fig.9.7: Radio Spectrum for Wireless Channels
Radio link: Radio link also called broadcast radio deals with transmission of data over long distance. A transmitter is required to send messages and a receiver to receive them. Depending upon the type of the service, it uses a range of frequencies (3kHz to 30MHz). In the lower frequencies of radio spectrum several broadcast radio bands are reserved for conventional AM/FM radio, broadcast television and private radio services. Radio link can support a bandwidth up to 2 Mbps. It is easy to install and involves low recurring costs. Microwave link: Microwave link also called microwave radio utilises point- to-point radio transmissions at frequencies in the super-high frequency (SHF) and extremely high frequency (EHF) bands. Microwaves do not follow the curvature of the earth and therefore require line-of-sight transmission and reception equipment. Microwave dishes which contain transceivers (sending and receiving devices) and antennas are set up on towers or buildings to establish the link. Microwave stations need to be placed at some distances (a few kilometers) from each other with no obstruction in between. The size of the dish varies with the distance. A string of microwave relay stations is used with each station receiving incoming messages, boosting the signal strength, and relaying the signal to the next station. Microwave link supports a bandwidth up to 45Mbps and is widely used in data communication. Satellite communication: To overcome line-of-sight constraint of microwave earth stations, communication satellites (microwave ‘sky stations’) are used. Communication satellites are microwave relay stations in orbit around the earth. Transmitting a signal from a ground station to a satellite is called uplinking, the reverse is called downlinking. Geosynchronous satellites are most commonly used in data communication. A geosynchronous satellite is placed in geostationary earth orbit (nearly 36, km directly above the equator) where it travels at the same speed as the earth and appears to an observer on the ground to be stationary. Consequently, microwave earth stations are always able to beam signals to a fixed location above. The orbiting satellite has solar powered transceivers to receive the signals, amplify them and re-transmit them to another earth station.
3kHz 30kHz 300kHz 3MHz 30MHz 300MHz 3GHz 30GHz 300GHz
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User terminals in case of satellite communications are called very small aperture^ Network Concepts terminals (VSATS). A schematic of communication between two users through satellite communication is shown in Fig.9.8.
Satell
Uplinking (Hop 1)
Fig. 9.8: VSAT to VSAT Communication
In VSAT to VSAT communication there is master earth station (MES) also called hub which primarily acts as a signal amplifier. Signal from VSAT goes to the hub where it is amplified and sent to VSAT2 through satellite (hop 1 for uplinking and hop 2 for downlinking). The two hops in the communication introduce 250 ms (a quarter of a second) delay which is a major constraint in voice communication.
Satellite communication provides transmission capabilities to and from any location on earth, no matter how remote. This advantage makes high quality communication available to less developed regions without requiring huge investment in ground-based infrastructure. Satellite communication supports high bandwidths capable of carrying large amounts of data and ensures low error rates.
9.7 NETWORK SECURITY
With proliferation of data networks providing a broad range of applications, network security plays an increasingly important role. Network security may be defined as a system of safeguards for protecting information on the network against a host of threats including unauthorised access, systems failure, fraud and other disasters resulting in damage or loss.
Network security issues broadly include :
Privacy: Preventing others from being able to comprehend data. Computer programs are available for hiding information by putting it into secret
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iv) Circuit switching is more reliable and efficient mode of data^ Network Concepts communication than packet switching. v) Proxy server is a component of firewall. vi) On a network, a server is used to store applications and data, often databases. vii) A local area network (LAN) covers and entire city. viii) On an Ethernet network all nodes listen to the traffic on the network and try to send data only when it’s quiet. ix) Protocols are rules that govern a communication exchange. x) Optical fibers allow data communication at low bandwidths.
- Multiple Choice Questions
i) The ___________ is the physical path over which a message travels. a) protocol b) medium c) signal d) all of the above ii) The connections that give the highest possible bandwidth are. a) twisted –pair wires b) coaxial cable c) fiber-optic cable iii) Only a ___________ network has all the computers connected to a central computer. a) ring b) star c) bus d) peer-to-peer iv) _____________ signal refers to waves continuously varying in strength and or quantity. a) analog b) digital c) modulated v) The performance of a data communication network depends on _____. a) the number of users b) the transmission media c) the hardware and software d) all of the above vi) Viruses are a network _________ issue. a) performance b) reliability
Introduction to Communication Technology
c) security d) all of the above vii) The bandwidth in digital communication is measured in ________. a) bits per second b) hertz c) cycles per second viii) Microwaves are electromagnetic radiation of _________ frequencies. a) high b) low c) medium ix) The information to be communicated in a data communication system is the __________. a) medium b) protocol c) message d) transmission x) A network node is a ________________connected to the network. a) computer b) communication device c) printer d) all of the above
9.8 SUMMARY
The goal of this Unit has been to provide a very broad overview of major network concepts and give some insight into the basic elements governing a network. Starting with the definition of a network, circuit-switching and packet switching which are crucial to network understanding have been dealt with. The concept of bandwidth and its role in accessing data have been explained. Network topologies — both physical and logical have been covered with details on varieties of Ethernet which one comes across frequently. Network components comprising networking devices and network operating systems have been explained. Transmission media have been described categorising them into wired and wireless communication channels. At the end, the Unit discusses out issues pertaining to network security.
9.9 KEYWORDS
ATM : Asynchronous Transfer Mode. A standard for high-speed network developed by ATM Forum. ATM transmits data in the form of 53-byte packets called cells.
