Unit 2204 Stage 1 Network Security

BSC IN INFORMATION TECHNOLOGY – Stage II Semester 1

BIT 2204: NETWORK SYSTEMS ADMINISTRATION
COURSE OUTLINE

Purpose of the Course:

The course equips the student with the necessary skills in administer different network platforms in terns of network users and shared resources.

Expected Learning Outcomes:

Upon completion of the course, the student should be able to:

• Outline the roles, responsibilities and ethics in network administration.

• Install, configure, troubleshoot and mange different network operating systems.

• Configure DHCP, DNS and WINS on a Windows server.

• Create user accounts, and implement group policies in a network environment.

• Explain network security concepts

Course Content:

Week
1

← Introduction to networks: definitions, terminologies used, advantages and disadvantages of computer networks, peer to peer networks and server based networks.

← Classification of computer networks: local area networks (LANs), metropolitan area networks (MANs) and wide area networks (WANs)

Week 2

← Transmission media: coaxial cable, Twisted pair (UTP, STP), Fiber optic cable, wireless transmission (satellite, radio and infrared transmission)

← Network topologies: Bus, star, ring, star-bus, star-ring.

Week 3

← Switching techniques: packet switching: ATM, frame relay, X.25, Circuit switching: Dial-up, ISDN, leased lines and T carrier line, Message switching.

Week 4

← Digital communication: modulation and encoding techniques, modems, Bits and Bytes, Broadband and baseband, serial and parallel transmission.

Week 5

← The OSI reference model: the seven layers, functions, the protocols of the seven layers. Overview of the TCP/IP protocol..

Week 6

← Access Methods: CSMA/CD, CSMA/CA, Token Bus, Token Ring, DQBD, FDDI and B-ISDN.

Week 7

← Networking devices: repeaters, Hubs, bridges, Switches, Routers and gateways.

Week 8

← Using a suitable platform such as Linux/Windows NT in system administration: Introduction to network administration: definitions, roles, functions, responsibilities and ethics.

← Network host configuration options: Network adapter card (NIC) options, network connections options, and Internet options.

Week 9

← Server configuration options: Installation options, services, configurable components.

Week 10

← Installing windows server 2003. Directory services: Introduction, Microsoft Active directory (AD) service, Lightweight Directory (ADAM) Services, configuring AD service.

Week 11

← Dynamic Host Configuration Protocol (DHCP): Scopes, DHCP clients, DHCP servers, relay servers, troubleshooting DHCP.

Week 12

← Name services: Introduction, Domain Name Services (DNS), DNS daemons, clients, servers, reverse DNS, Windows Internet Name Service (WINS).

Week 13

← Resource sharing: User groups and policies, access control, file sharing, creating user accounts, configuring shared resources.

Week 14

← Network troubleshooting commands, network performance and network security: software tools that help to solve network connectivity (ping, net watcher/viewer, tracert), TCP/IP network service protocols (DHCP, ICMP, SNMP), installing SNMP service, monitoring network performance, optimization of network, network security.

Mode of Delivery:

• Lectures.

• Practical lab sessions.

• Group discussions.

Instructional Materials and/or Equipment:

• Computer installed with a Server Operating Systems

Course Assessment:

• Continuous Assessment 30 %

• End of Semester Examination. 70 %

Core Reading Material:

• Local area networks: Hodson, Peter: Letts Educational: ISBN-1-65805-230-0

• Business data communications: William Sterllings: prentice Hall:ISBN 0-13-016093-8

• Burgess, M.: Principles of Network and System Administration, 2nd Edition; Wiley higher education, 2007.

• Chappell, A. and Tittel, E.: Guide to TCP/IP, 3rd Edition; Thomson, 2007.

• McCann, B. and Dinicolo, D.: MCSE Guide to Managing a Microsoft Windows Server 2003 Network, Enhanced; Thomson, 2005

• McCann, B. and Wright, B.: MCSE Guide to Planning a Microsoft Windows Server 2003 Network, Enhanced; Thomson, 2005
Lesson 1

Introduction to networks:

Definition:
A computer network consists of a collection of computers, printers and other equipment that is connected together so that they can communicate with each other

Advantages of computer networks

Speed.
Networks provide a very rapid method for sharing and transferring files. Without a network, files are shared by copying them to floppy disks, then carrying or sending the disks from one computer to another. This method of transferring files in this manner is very time-consuming.

Cost.
The network version of most software programs are available at considerable savings when compared to buying individually licensed copies. Besides monetary savings, sharing a program on a network allows for easier upgrading of the program. The changes have to be done only once, on the file server, instead of on all the individual workstations.

Centralized Software Management.
One of the greatest benefits of installing a network is the fact that all of the software can be loaded on one computer (the file server). This eliminates that need to spend time and energy installing updates and tracking files on independent computers throughout the building.

Resource Sharing.
Sharing resources is another area in which a network exceeds stand-alone computers. Most organizations cannot afford enough laser printers, fax machines, modems, scanners, and CD-ROM players for each computer. However, if these or similar peripherals are added to a network, they can be shared by many users.

Security.
Files and programs on a network can be designated as "copy inhibit," so that you do not have to worry about illegal copying of programs. Also, passwords can be established for specific directories to restrict access to authorized users.
Electronic Mail. E-mail aids in personal and professional communication. Electronic mail on a LAN can enable staff to communicate within the building having not to leave their desk.Â
Flexible Access. Networks allow users to Access their files from computers throughout the firm. Users can save part of their work on a public access area of the network, then go home after work to finish their work. Also, workgroup software (such as Microsoft BackOffice) allows many users to work on a document or project concurrently.

Disadvantages of computer networks.
Security Issues: One of the major drawbacks of computer networks is the security issues involved. If a computer is a standalone, physical access becomes necessary for any kind of data theft. However, if a computer is on a network, a computer hacker can get unauthorized access by using different tools. In case of big organizations, various network security softwares are used to prevent the theft of any confidential and classified data.
Rapid Spread of Computer Viruses: If any computer system in a network gets affected by computer virus, there is a possible threat of other systems getting affected too. Viruses get spread on a network easily because of the interconnectivity of workstations. Such spread can be dangerous if the computers have important databases which can get corrupted by the virus.
Costs
Although a network will generally save money over time, the initial costs can be substantial, and the installation may require the services of a technician.

Requires Administrative Time.
Proper maintenance of a network requires considerable time and expertise. Many organizations have installed a network, only to find that they did not budget for the necessary administrative support.

File Server May Fail.
Although a file server is no more susceptible to failure than any other computer, when the files server "goes down," the entire network may come to a halt. When this happens, the entire organization may lose access to necessary programs and files.

Types of network configuration
Broadly speaking, there are two types of network configuration, peer-to-peer networks and client/server networks.
Peer-to-peer networks are more commonly implemented where less then ten computers are involved and where strict security is not necessary. All computers have the same status, hence the term 'peer', and they communicate with each other on an equal footing. Files, such as word processing or spreadsheet documents, can be shared across the network and all the computers on the network can share devices, such as printers or scanners, which are connected to any one computer.

Client/server networks are more suitable for larger networks. A central computer, or 'server', acts as the storage location for files and applications shared on the network. Usually the server is a higher than average performance computer. The server also controls the network access of the other computers which are referred to as the 'client' computers. Typically, end-users in an organization will use the client computers for their work and only the network administrator (usually a designated staff member) will have access rights to the server.

Summary comparison between Peer-to-Peer and Client/Server Networks.

|Peer-to-Peer Networks vs Client/Server Networks |
|Peer-to-Peer Networks |Client/Server Networks |
|Easy to set up |More difficult to set up |
|Less expensive to install |More expensive to install |
|Can be implemented on a wide range of operating systems |A variety of operating systems can be supported on the client |
| |computers, but the server needs to run an operating system that |
| |supports networking |
|More time consuming to maintain the software being used (as |Less time consuming to maintain the software being used (as most of|
|computers must be managed individually) |the maintenance is managed from the server) |
|Very low levels of security supported or none at all. These |High levels of security are supported, all of which are controlled |
|can be very cumbersome to set up, depending on the operating|from the server. Such measures prevent the deletion of essential |
|system being used |system files or the changing of settings |
|Ideal for networks with less than 10 computers |No limit to the number of computers that can be supported by the |
| |network |
|Does not require a server |Requires a server running a server operating system |
|Demands a moderate level of skill to administer the network |Demands that the network administrator has a high level of IT |
| |skills with a good working knowledge of a server operating system |

Types of networks
The three basic types of networks include: LAN, MAN and WAN.
1) Local Area Networks (LANs) A local area network (LAN) is a group of computers and network communication devices interconnected within a geographically limited area, such as a building or campus, expanding not more than a mile apart to other computers.
Local area networks first appeared in the 1970s. it has since then become widespread in commercial and academic environments

LANs are characterized by the following: ➢ They transfer data at high speeds. ➢ They exist in a limited geographical area. ➢ Their technology is generally less expensive. ➢ Error rates are low. ➢ Local area networks, generally called LANs, are privately owned networks within a single building or campus of up to a few kilometers in size. ➢ They are widely used to connect personal computers and workstations in company offices and factories to share resources (e.g., printers) and exchange information. ➢ LANs are distinguished from other kinds of networks by three characteristics: (1) their size, (2) their transmission technology, and (3) their topology.

Classification of interconnected processors by scale.

Primary functions of a LAN provides access to hardware and software resources that will allow users to perform one or more of the following; ➢ To provide access to hardware and software resources that will allow users to perform one or more of the following activities: ➢ File serving - A large storage disk drive acts as a central storage repository. ➢ Print serving - Providing the authorization to access a particular printer, accept and queue print jobs, and providing a user access to the print queue to perform administrative duties. ➢ Video transfers - High speed LANs are capable of supporting video image and live video transfers. ➢ Manufacturing support - LANs can support manufacturing and industrial environments. ➢ Academic support – In classrooms, labs, and wireless ➢ E-mail support ➢ Interconnection between multiple systems

2) Metropolitan Area Networks
A metropolitan area network or MAN is a data network intended to serve an area the size of a large city. Often used by local libraries and government agencies to connect to citizens and private industries.
It is characterized by the following; 1. The network size falls intermediate between LAN and WAN. Many MANs covers an area the size of a city, although in some cases MANs may be as small as a group of buildings. 2. A MAN is not generally owned by a single . Communication links and equipments are generally owned by either a consortium of users or a single network provider who sells the service to the users. The level of service provided to each user therefore must be negotiated and some performance guarantee be specified. 3. A MAN offers a high-speed network to allow sharing of regional resources [similar to a LAN]
3) Wide Area Networks (WANs)

A wide area network (WAN) interconnects LANs. A WAN may be located entirely within a state or country, or it may be interconnected around the world.

WANs are characterized by the following: • They exist in an unlimited geographical area. • They are more vulnerable to errors due to the distances data travels. • They interconnect multiple LANs. • They are more sophisticated and complex than LANs. • Their technology is expensive. ➢ WANs can be further classified into two categories: 1) Enterprise WANs. An enterprise WAN is a WAN that connects the widely separated computer resources of a single organization. An organization with computer operations at several distant sites can employ an enterprise WAN to interconnect the sites. An enterprise WAN can use a combination of private and commercial network services but is dedicated to the needs of a particular 2) Global WANs. A global WAN interconnects networks of several corporations or organizations. An example of a global WAN is the Internet.

➢ WANs are often a natural outgrowth of the need to connect geographically separate LANs into a single network. For instance, a company might have several branch offices in different cities. Every branch would have its own LAN so that branch employees could share files and other resources, and all the branches together would be part of a WAN, a greater network that enables the exchange of files, messages, and application services between cities. ➢ Much of the complexity and expense of operating a WAN is caused by the great distances that the signal must travel to reach the interconnected segments. WAN links are often slower and typically depend on a public transmission medium leased from a communications service provider.

Lesson 2
Basic Network Media

Basic Network Media

In this second chapter, we look deeper into the physical aspects of a network to learn about the cables and circuitry that connect one computer to another.
Network Cabling
Building on our understanding of the different network topologies that connect computers, we focus next on the cables that connect them. In this lesson, we examine the construction, features, and operation of each type of cable, and the advantages and disadvantages of each.
After this lesson, student should be able to: • Determine which type of cabling is best for any networking situation. • Define terms related to cabling, such as shielding, crosstalk, attenuation, and plenum. • Identify the primary types of network cabling. • Distinguish between baseband and broadband transmissions and identify appropriate uses for each.
Primary Cable Types
The vast majority of networks today are connected by some sort of wiring or cabling that acts as a network transmission medium that carries signals between computers. Many cable types are available to meet the varying needs and sizes of networks, from small to large.
Only three major groups of cabling connect the majority of networks: 1. Coaxial cable 2. Twisted-pair (unshielded and shielded) cable 3. Fiber-optic cable
The next part of this lesson describes the features and components of these three major cable types. Understanding their differences will help you determine which type of cabling is appropriate in a given context.
Coaxial Cable
In its simplest form, coaxial cable consists of; 1. a core of copper wire surrounded by insulation, Surrounding the core is a dielectric insulating layer that separates it from the wire mesh.
The core of a coaxial cable carries the electronic signals that make up the data. This wire core can be either solid or stranded. If the core is solid, it is usually copper. 2. a braided metal shielding, The braided wire mesh acts as a ground and protects the core from electrical noise and crosstalk. (Crosstalk is signal overflow from an adjacent wire. 3. An outer cover. A nonconducting outer shield—usually made of rubber, Teflon, or plastic—surrounds the entire cable. [pic] Coaxial cable showing various layers
Note:
Shielding protects transmitted data by absorbing stray electronic signals, called noise, so that they do not get onto the cable and distort the data.
Coaxial cable is more resistant to interference and attenuation than twisted-pair cabling. Attenuation is the loss of signal strength that begins to occur as the signal travels farther along a copper cable.
[pic]

Types of Coaxial Cable
There are two types of coaxial cable: • Thin (thinnet) cable • Thick (thicknet) cable
Which type of coaxial cable you select depends on the needs of your particular
network.
Thinnet Cable Thinnet cable is a flexible coaxial cable about 0.64 centimeters (0.25 inches) thick. Because this type of coaxial cable is flexible and easy to work with, it can be used in almost any type of network installation.
Thinnet coaxial cable can carry a signal for a distance of up to approximately 185 meters (about 607 feet) before the signal starts to suffer from attenuation.
Thicknet Cable Thicknet cable is a relatively rigid coaxial cable about 1.27 centimeters (0.5 inches) in diameter. Figure below shows the difference between thinnet and thicknet cable. Thicknet cable is sometimes referred to as Standard Ethernet because it was the first type of cable used with the popular network architecture Ethernet. Thicknet cable's copper core is thicker than a thinnet cable core.
The thicker the copper core, the farther the cable can carry signals. This means that thicknet can carry signals farther than thinnet cable. Thicknet cable can carry a signal for 500 meters (about 1640 feet). Therefore, because of thicknet's ability to support data transfer over longer distances, it is sometimes used as a backbone to connect several smaller thinnet-based networks.
Thinnet vs. Thicknet Cable As a general rule, the thicker the cable, the more difficult it is to work with. Thin cable is flexible, easy to install, and relatively inexpensive. Thick cable does not bend easily and is, therefore, harder to install. This is a consideration when an installation calls for pulling cable through tight spaces such as conduits and troughs. Thick cable is more expensive than thin cable, but will carry a signal farther.
[pic]

Coaxial-Cable Connection Hardware
There are several important components in the BNC family, including the following: • The BNC cable connector The BNC cable connector is either soldered or crimped to the end of a cable. It is used to make the connections between the cable and the computers. [pic] Figure BNC cable connector • The BNC T connector Figure below shows a BNC T connector. This connector joins the network interface card (NIC) in the computer to the network cable.
[pic]

• The BNC barrel connector Figure below shows a BNC barrel connector. This connector is used to join two lengths of thinnet cable to make one longer length.
[pic]
• The BNC terminator Figure below shows a BNC terminator. A BNC terminator closes each end of the bus cable to absorb stray signals. Otherwise, as we saw in previous chapter "Introduction to Networking," the signal will bounce and all network activity will stop.
[pic]

NOTE
[pic]
The origin of the acronym "BNC" is unclear, and there have been many names ascribed to these letters, from "British Naval Connector" to "Bayonet Neill-Councelman." Because there is no consensus on the proper name and because the technology industry universally refers to these simply as BNC-type connectors, in this book we will refer to this family of hardware simply as BNC.
Coaxial-Cable Grades and Fire Codes
The type of cable grade that you should use depends on where the cables will be laid in your office. Coaxial cables come in two grades: • Polyvinyl chloride (PVC) grade • Plenum grade
Polyvinyl chloride (PVC) is a type of plastic used to construct the insulation and cable jacket for most types of coaxial cable. PVC coaxial cable is flexible and can be easily routed through the exposed areas of an office. However, when it burns, it gives off poisonous gases.
A plenum is the shallow space in many buildings between the false ceiling and the floor above; it is used to circulate warm and cold air through the building. Fire codes give very specific instructions about the type of wiring that can be routed through this area, because any smoke or gas in the plenum will eventually blend with the air breathed by everyone in the building.
Plenum-grade cabling contains special materials in its insulation and cable jacket. These materials are certified to be fire resistant and produce a minimum amount of smoke; this reduces poisonous chemical fumes. Plenum cable can be used in the plenum area and in vertical runs (for example, in a wall) without conduit. However, plenum cabling is more expensive and less flexible than PVC cable.
NOTE
[pic]
You should consult your local fire and electrical codes for specific regulations and requirements for running networking cable in your office.
Coaxial-Cabling Considerations
Consider the following coaxial capabilities when making a decision about which type of cabling to use.
Use coaxial cable if you need a medium that can: • Transmit voice, video, and data. • Transmit data for greater distances than is possible with less expensive cabling. • Offer a familiar technology with reasonable data security.
Twisted-Pair Cable
In its simplest form, twisted-pair cable consists of two insulated strands of copper wire twisted around each other.
A number of twisted-pair wires are often grouped together and enclosed in a protective sheath to form a cable. The total number of pairs in a cable varies. The twisting cancels out electrical noise from adjacent pairs and from other sources such as motors and transformers.
[pic]

Unshielded Twisted-Pair (UTP) Cable
UTP, using is the most popular type of twisted-pair cable and is fast becoming the most popular LAN cabling. The maximum cable length segment is 100 meters, about 328 feet.
[pic]
Traditional UTP cable consists of two insulated copper wires. UTP specifications govern how many twists are permitted per foot of cable; the number of twists allowed depends on the purpose to which the cable will be put.
The 568A Commercial Building Wiring Standard of the Electronic Industries Association and the Telecommunications Industries Association (EIA/TIA) specifies the type of UTP cable that is to be used in a variety of building and wiring situations. The objective is to ensure consistency of products for customers. These standards include five categories of UTP: • Category 1 This refers to traditional UTP telephone cable that can carry voice but not data transmissions. Most telephone cable prior to 1983 was Category 1 cable. • Category 2 This category certifies UTP cable for data transmissions up to 4 megabits per second (Mbps). It consists of four twisted pairs of copper wire. • Category 3 This category certifies UTP cable for data transmissions up to 16 Mbps. It consists of four twisted pairs of copper wire with three twists per foot. • Category 4 This category certifies UTP cable for data transmissions up to 20 Mbps. It consists of four twisted pairs of copper wire. • Category 5 This category certifies UTP cable for data transmissions up to 100 Mbps. It consists of four twisted pairs of copper wire.
Most telephone systems use a type of UTP. In fact, one reason why UTP is so popular is because many buildings are prewired for twisted-pair telephone systems. As part of the prewiring process, extra UTP is often installed to meet future cabling needs. If preinstalled twisted-pair cable is of sufficient grade to support data transmission, it can be used in a computer network. Caution is required, however, because common telephone wire might not have the twisting and other electrical characteristics required for clean, secure, computer data transmission.
One potential problem with all types of cabling is crosstalk.
Crosstalk is defined as signals from one line interfering with signals from another line.) UTP is particularly vulnerable to crosstalk, but the greater the number of twists per foot of cable, the more effective the protection against crosstalk.
[pic]
Shielded Twisted-Pair (STP) Cable
STP cable uses a woven copper-braid jacket that is more protective and of a higher quality than the jacket used by UTP. Figure below shows a two-twisted-pair STP cable. STP also uses a foil wrap around each of the wire pairs. This gives STP excellent shielding to protect the transmitted data from outside interference, which in turn allows it to support higher transmission rates over longer distances than UTP.
[pic]
Twisted-Pair Cabling Components.
While we have defined twisted-pair cabling by the number of twists and its ability to transmit data, additional components are necessary to complete an installation. As it is with telephone cabling, a twisted-pair cable network requires connectors and other hardware to ensure proper installation.
Connection hardware
Twisted-pair cabling uses RJ-45 telephone connectors to connect to a computer. These are similar to RJ-11 telephone connectors. Although RJ-11 and RJ-45 connectors look alike at first glance, there are crucial differences between them.
[pic]
The RJ-45 connector is slightly larger and will not fit into the RJ-11 telephone jack. The RJ-45 connector houses eight cable connections, while the RJ-11 houses only four.
Several components are available to help organize large UTP installations and make them easier to work with.
Distribution racks and rack shelves Distribution racks and rack shelves can create more room for cables where there isn't much floor space. Using them is a good way to organize a network that has a lot of connections.
Expandable patch panels These come in various versions that support up to 96 ports and transmission speeds of up to 100 Mbps.
Jack couplers These single or double RJ-45 jacks snap into patch panels and wall plates and support data rates of up to 100 Mbps.
Wall plates these support two or more couplers.
[pic]

Twisted-Pair Cabling Considerations
Use twisted-pair cable if: • Your LAN is under budget constraints. • You want a relatively easy installation in which computer connections are simple.
Do not use twisted-pair cable if: • Your LAN requires a high level of security and you must be absolutely sure of data integrity. • You must transmit data over long distances at high speeds.
Fiber-Optic Cable
In fiber-optic cable, optical fibers carry digital data signals in the form of modulated pulses of light. This is a relatively safe way to send data because, unlike copper-based cables that carry data in the form of electronic signals, no electrical impulses are carried over the fiber-optic cable. This means that fiberoptic cable cannot be tapped, and its data cannot be stolen.
Fiber-optic cable is good for very high-speed, high-capacity data transmission because of the purity of the signal and lack of signal attenuation.
Fiber-Optic Cable Composition
An optical fiber consists of an extremely thin cylinder of glass, called the core, surrounded by a concentric layer of glass, known as the cladding. The fibers are sometimes made of plastic. Plastic is easier to install, but cannot carry the light pulses for as long a distance as glass.
Because each glass strand passes signals in only one direction, a cable includes two strands in separate jackets. One strand transmits and one receives. A reinforcing layer of plastic surrounds each glass strand, and Kevlar fibers provide strength. See Figure below for an illustration of fiber-optic cable. The Kevlar fibers in the fiber-optic connector are placed between the two cables. Just as their counterparts (twisted-pair and coaxial) are, fiber-optic cables are encased in a plastic coating for protection.
[pic]
Fiber-optic cable transmissions are not subject to electrical interference and are extremely fast, currently transmitting about 100 Mbps with demonstrated rates of up to 1 gigabit per second (Gbps). They can carry a signal—the light pulse—for many miles.
Fiber-Optic Cabling Considerations
Use fiber-optic cable if you: • Need to transmit data at very high speeds over long distances in very secure media.
Do not use fiber-optic cable if you: • Are under a tight budget. • Do not have the expertise available to properly install it and connect devices to it.
NOTE
[pic]
Pricing for fiber-optic cable is competitive with high-end copper cabling. Fiber-optic cable has become increasingly easier to work with, and polishing and terminating techniques now require fewer parts and less expertise than just a few years ago.

Selecting Cabling
To determine which cabling is the best for a particular site you need to answer the following questions: • How heavy will the network traffic be? • What level of security does the network require? • What distances must the cable cover? • What are the cable options? • What is the budget for cabling?
The better the cable protects against internal and external electrical noise, the farther and faster the cable will carry a clear signal. However, the better the speed, clarity, and security of the cable, the higher the cabling cost.
Cabling Considerations
As with most network components, there are trade-offs with the type of cable you purchase. If you work for a large organization and choose the least expensive cable, the accountants might initially be pleased, but you might soon notice that the LAN is inadequate in both transmission speed and data security.
Which cabling you select will depend on the needs of a particular site. The cabling you purchase to set up a LAN for a small business has different requirements from those of a larger organization, such as a major banking institution. considerations that affect cabling price and performance.
Installation Logistics
How easy is the cable to install and work with? In a small installation where distances are short and security isn't a major issue, it does not make sense to choose thick, cumbersome, and expensive cable.
Shielding
The level of shielding required will affect cable cost. Almost every network uses some form of shielded cable. The noisier the area in which the cable is run, the more shielding will be required. The same shielding in a plenum-grade cable will be more expensive as well.
Crosstalk
Crosstalk and noise can cause serious problems in large networks where data integrity is crucial. Inexpensive cabling has low resistance to outside electrical fields generated by power lines, motors, relays, and radio transmitters. This makes it susceptible to both noise and crosstalk.
Transmission Rates
Transmission rates are measured in megabits per second. A standard reference point for current LAN transmission over copper cable is 100Â Mbps. Fiber-optic cable transmits at more than 1 Gbps.
Cost
Higher grades of cables can carry data securely over long distances, but they are relatively expensive; lower-grade cables, which provide less data security over shorter distances, are relatively inexpensive.
Signal Attenuation
Different cable types have different rates of attenuation; therefore, cable specifications recommend specific length limits for the different types. If a signal suffers too much attenuation, the receiving computer will be unable to interpret it. Most networks have error-checking systems that will generate a retransmission if the signal is too weak to be understood. However, retransmission takes time and slows down the network.

Cable Comparison Summary
|Characteristics |Thinnet coaxial |Thicknet coaxial |Twisted-pair (UTP & STP)|Fiber-optic Cable |
| |(10Base2) Cable |(10Base5) Cable | | |
| | | |(10BaseT) Cable1 | |
|Cable cost |More than UTP |More than thinnet |UTP: Least expensive |More than thinnet, but |
| | | |STP: More than thinnet |less than thicknet |
|Usable cable length (Usable cable |185 meters (about 607 |500 meters (about 1640 |UTP and STP: 100 meters |2 kilometers (6562 feet) |
|length can vary with specific network |feet) |feet) |(about 328 feet) | |
|installations. As technology improves,| | | | |
|usable cable length also increases. ) | | | | |
|Transmission rates |4-100 Mbps |4-100 Mbps |UTP: 4-100 Mbps |100 Mbps or more ( > |
| | | |STP: 16-500 Mbps |1Gbps) |
|Flexibility |Fairly flexible |Less flexible than |UTP: Most flexible |Less flexible than |
| | |thinnet |STP: Less flexible than |thicknet |
| | | |UTP | |
|Ease of installation |Easy to install |Moderately easy to |UTP: Very easy; often |Difficult to install |
| | |install |preinstalled | |
| | | |STP: Moderately easy | |
|Susceptibility to interference |Good resistance to |Good resistance to |UTP: Very susceptible |Not susceptible to |
| |interference |interference |STP: Good resistance |interference |
|Special features |Electronic support |Electronic support |UTP: Same as telephone |Supports voice, data, and|
| |components are less |components are less |wire; often preinstalled|video |
| |expensive than |expensive than |in buildings | |
| |twisted-pair cable |twisted-pair cable |STP: Supports higher | |
| | | |transmission rates than | |
| | | |UTP | |
|Preferred uses |Medium to large sites |Linking thinnet networks |UTP: smaller sites on |Any size installation |
| |with high security needs | |budget. STP: Token Ring |requiring speed and high |
| | | |in any size |data security and |
| | | | |integrity |

Lesson 3

Network topologies and transmission media

Def:
Topology refers to the way in which the network of computers is connected. Each topology is suited to specific tasks and has its own advantages and disadvantages. The choice of topology is dependent upon type and number of equipment being used, planned applications and rate of data transfer required, response time, and cost. Topology can also be defined as the geometrically interconnection pattern by which the stations (nodes/computers) are connected using suitable transmission media (which can be point-to-point and broadcast). Various commonly used topologies are discussed in the following sections.

Mesh Topology

In this topology each node or station is connected to every other station as shown in the figure below The key characteristics of this topology are as follows: Key Characteristics: • Fully connected • Robust – Highly reliable • Not flexible • Poor expandability

[pic]

Two nodes are connected by dedicated point-point links between them. So the total number of links to connect n nodes = n(n-1)/2. Media used for the connection (links) can be twisted pair, co-axial cable or optical fiber. With this topology there is no need to provide any additional information that is from where the packet is coming, along with the packet because two nodes have a point-point dedicated link between them. And each node knows which link is connected to which node on the other end.
Mesh Topology is not flexible and has a poor expandability as to add a new node n links have to be laid because that new node has to be connected to each of the existing nodes via dedicated link as shown in Figure below. For the same reason the cost of cabling will be very high for a larger area. And due to these reasons this topology is rarely used in practice.

[pic]

Bus Topology
In Bus Topology, all stations attach through appropriate hardware interfacing known as a tap, directly to a linear transmission medium, or bus as shown in Figure below. Full-duplex operation between the station and the tap allows data to be transmitted onto the bus and received from the bus. A transmission from any station propagates the length of the medium in both directions and can be received by all other stations. At each end of the bus there is a terminator, which absorbs any signal, preventing reflection of signal from the endpoints. If the terminator is not present, the endpoint acts like a mirror and reflects the signal back causing interference and other problems.

[pic]
Fig: Bus topology

Key Characteristics of this topology are: • Flexible • Expandable • Moderate Reliability • Moderate performance

A shared link is used between different stations. Hence it is very cost effective. One can easily add any new node or delete any node without affecting other nodes; this makes this topology easily expandable. Because of the shared medium, it is necessary to provide some extra information about the desired destination, i.e. to explicitly specify the destination in the packet, as compared to mesh topology. This is because the same medium is shared among many nodes. As each station has a unique address in the network, a station copies a packet only when the destination address of the packet matches with the self-address. This is how data communications take place among the stations on the bus.

Bus topology LANs have been supplanted by star and ring topology ❑ Characteristics of the Bus Topology • requirement to regulate which station on the medium may transmit at any point in time ➢ Medium Access Protocol (MAC) • Another design issue : Signal balancing ➢ When two stations exchange data over a link, the signal strength of the transmitter must be adjusted to be within certain limits.
Baseband Coaxial Cable • Most popular medium for bus • Using digital signaling, usually Manchester or differential Manchester encoding • The nature of the digital signals is such that the entire frequency spectrum of the cable is consumed. ➢ Hence, it is not possible to have multiple channels (FDM) on the cable • Use of a special 50 ohm cable rather than the standard CATV 75 ohm ➢ suffering less intense reflections from the insertion capacitance of the taps ➢ Providing better immunity against low-frequency electromagnetic noise
Engineering trade-offs involving data rate, cable length, number of taps, etc. ➢ The lower the data rate, the longer the cable can be. ❑ the individual pulses of a digital signal last longer in the presence of interference.

10BASE5 (10 Mbps, baseband, 500m cable length) and 10BASE2 LANs Specifications

[pic]

• The thinner cable is more flexible; thus it is easer to bend around corners • The thinner cable suffers greater attenuation and lower noise resistance than the thicker cable. ➢ Thus it supports fewer taps over a shorter distance ➢ To extend the length of the network, repeaters may be used

Transmission Techniques for Coaxial Cable Bus LANs

[pic]

Ring Topology
In the ring topology, the network consists of a set of repeaters joined by point-to-point links in a closed loop as shown in Figure below. The repeater is a comparatively simple device, capable of receiving data on one link and transmitting them, bit by bit, on the other link as fast as they are received, with no buffering at the repeater. The links are unidirectional; that is data are transmitted in one direction only and all are oriented in the same way. Thus, data circulate around the ring in one direction (clockwise or counterclockwise).

[pic]

Each station attaches to the network at a repeater and can transmit data onto the network through that repeater. As with the bus and tree, data are transmitted in frames.

As a frame circulates past all the other stations, the destination station recognizes its address and copies the frame into a local buffer as it goes by. The frame continues to circulate until it returns to the source station, where it is removed. Because multiple stations share the ring, medium access control is needed to determine at what time each station may insert frames.
How the source knows whether it has to transmit a new packet and whether the previous packet has been received properly by the destination or not. For this, the destination change a particular bit (bits) in the packet and when the receiver sees that packet with the changed bit, it comes to know that the receiver has received the packet.
This topology is not very reliable, because when a link fails the entire ring connection is broken. But reliability can be improved by using wiring concentrator, which helps in bypassing a faulty node and somewhat is similar to star topology.

• A ring consists of a number of repeaters, with unidirectional transmission links • Each repeater regenerates and retransmits each bit • Three functions ➢ data insertion ➢ data reception ➢ data removal • Because the ring is a closed loop, data will circulate infinitely unless removed. ➢ A frame may be removed by the addressed repeater ➢ Alternatively, each frame could be removed by the transmitting repeater after it has made one trip around the loop

Description • A ring consists of a number of repeaters, with unidirectional transmission links • Each repeater regenerates and retransmits each bit • Three functions ➢ data insertion ➢ data reception ➢ data removal • Because the ring is a closed loop, data will circulate infinitely unless removed. ➢ A frame may be removed by the addressed repeater ➢ Alternatively, each frame could be removed by the transmitting repeater after it has made one trip around the loop

Two main Purposes of repeater 1) To contribute to the proper functioning of the ring by passing on all the data 2) To provide an access point for attached stations to send and receive data 3) Corresponding to these two purposes are two states: the listen and transmit states

Listen state • Performing necessary functions for 1 bit time in repeater : ➢ Scan passing bit stream for pertinent patterns – Addresses of attached devices ➢ Copy each incoming bit and send it to the attached station ➢ Modify a bit as it passes by – For example, indicating that the frame has been copied

Transmit State • Receiving bits from the station and retransmitting them one its outgoing link • During the period of transmission, bits may appear on the incoming ring link : having two possibilities ➢ Bits from the same frame, so can check the bits as a form of ACK ➢ More than one frame on the ring at the same time, while transmitting, so buffering frames to be transmitted later
Bypass State • A bypass relay is activated, so that signals propagate past the repeater with no delay other than medium propagation; two benefits: ➢ Providing partial solution to the reliability problem ➢ Improving performance by eliminating repeater delay

Ring benefits • Other benefits provided by the ring that are not shared by the bus topology : The most important benefit or strength of the ring ➢ The use of point-to-point communication links – Because the transmitted signal is regenerated at each node, greater distance can be covered than with baseband bus – The ring can accommodate optical fiber links that provide very high rates and excellent electromagnetic interference (EMI) characteristics – The maintenance of point-to-point lines are simpler than multipoint lines ➢ The fault isolation and recovery are simpler than for bus/tree ➢ The potential throughput of the ring

Potential ring problems • Cable vulnerability • Repeater failure • Perambulation ➢ for locating the failure : “pocket full of keys” problem • Installation headaches • Size limitations : A limit to the number of repeaters • Initialization and recovery : when a frame is garbled by a transient line error • Timing jitter : having to do with the clocking or timing

Star-Ring Architecture • Overcoming some of the problems of the ring and allowing the construction of large local networks • Having the interrepeater link: ring wiring concentrator ➢ Easy to isolate a fault ➢ Easy to add a new repeater • The bypass relay associated with each repeater can be moved into the ring wiring concentrator • Alleviating the perambulation and installation problems • Permitting rapid recovery from a cable or repeater failure • Still having problems ➢ a single failure could, at least temporarily, disable the entire network ➢ limit of the number of repeaters because of jitter problem ➢ concentrating a lot of cable into a single site

So, how to attack those problems ? ➢ considering a local network consisting of multiple rings by a bridge – routing data frames one ring subnetwork to another • Five function of the bridge ➢ Input filtering ➢ Input buffering ➢ switching ➢ Out buffering ➢ Output transmission

Multiple bridges interconnected by high-speed link

STAR Topology
In the star topology, each station is directly connected to a common central node as shown in the fig below. Typically, each station attaches to a central node, referred to as the star coupler, via two point-to-point links, one for transmission and one for reception.
[pic]
Key features: ← High Speed ← Very Flexible ← High Reliability ← High Maintainability

In general, there are two alternatives for the operation of the central node.

← One approach is for the central node to operate in a broadcast fashion. A transmission of a frame from one station to the node is retransmitted on all of the logically a bus; a transmission from any station is received by all other stations, and only one station at a time may successfully transmit. In this case the central node acts as a repeater. ← Another approach is for the central node to act as a frame-switching device. An incoming frame is buffered in the node and then retransmitted on an outgoing link to the destination station. In this approach, the central node acts as a switch and performs the switching or routing function. This mode of operation can be compared with the working of a telephone exchange, where the caller party is connected to a single called party and each pair of subscriber who needs to talk have a different connection.

Very High speeds of data transfer can be achieved by using star topology, particularly when the star coupler is used in the switch mode. This topology is the easiest to maintain, among the other topologies. As the number of links is proportional to n, this topology is very flexible and is the most preferred topology.

Twisted Pair and Optical Fiber Star LANs • Several benefits of UTP cables ➢ no installation cost with UTP, because the wire is already there ➢ Easy to extending LAN coverage • The star wiring using UTP ➢ The most popular approach • The length of link ➢ limited to about 100 m ➢ in case of optical fiber, can extend up to about 500 m

Twisted Pair Star Topology • Physically a star, logically a bus : A transmission from any one station is received by all other stations

Two-level Star Topology

Hub and Switches • Recently, popularity of LAN switch for high-speed LANs • Distinction between LAN hubs and switches

Shared Medium Bus ← Sharing the total capacity of 10 Mbps LAN Bus

Shared Medium Hub

← using star wiring arrangement to stations to the hub

LAN Switch

❑ Features of LAN switches • Attached devices for a bus LAN or a hub LAN don’t require the change for converting to the LAN switch • Each attached device has a dedicated capacity equal to that of the entire original LAN • The LAN switch scales easily ❑ Two types of LAN switches • Store-and-forward switch ➢ Accepts a frame on input line ➢ Buffers it briefly ➢ Routes it to appropriate output line ➢ So, involving a delay • Cut-through switch ➢ Just checking the destination address in MAC frame ➢ some risk of propagating bad frames because the switch is not able to check the CRC

Choice of Topology
Factors to consider include reliability, flexibility/expandability, and performance • Bus is most flexible • Ring provides high throughput, but reliability problems ➢ a single link and repeater failure could disable the entire network • Star can be high speed for short distances, but has limited expandability

Transmission Media Options • Twisted pair : digital signaling • Optical fiber : analog signaling • Baseband coax : digital signaling ➢ transmission of signals without modulation ➢ The original Ethernet scheme makes use of baseband coax. cable • Broadband coax : analog signaling (broadband implies the use of analog signaling) ➢ Uses FDM to carry multiple channels ➢ Can be used over longer distances : some tens of kilometers ➢ Inherently unidirectional, due to amplifier limitations ➢ More expensive and more difficult to install and maintain than baseband coaxial cable ➢ No longer made

Choice of Transmission Medium • Capacity: Can it support expected network traffic? • Reliability: Can it meet requirements for availability? • Types of data supported: Is it well-suited to the applications involved? • Environmental scope: Can it provide services over the range of the environments required?

Topology and transmission media are interrelated. For example, all the important criteria of a network such as reliability, expandability and performance depend on both the topology and the transmission media used in the network. As a consequence, these two aspects are interrelated.

Various transmission media, which are used for different topologies. • Twisted pair is suitable for use in star and ring topologies
Cat 3: voice grade UTP, data rate up to 10 Mbps
Cat 5: data grade UTP, data rate up to 100 Mbps • Coaxial cable is suitable for use in bus topology
Baseband coaxial cable supports data rates of 20 Mbps at distances up to 2 Km. • Fiber optics is suitable for use in ring and star topology
-Gigabit data rates and longer distances. • Unguided media are suitable for star topology

Structure Cabling System

❑ Standard that aids in the development if cabling plans, standards have been issued that specify the cabling types and layout for commercial buildings ❑ Generic wiring schemes • Cabling scheme for wiring within a building • Include cabling for all applications, including LANs, voice, video, etc • Vendor and equipment independent • Designed to encompass entire building, so that equipment can be easily relocated • Provide guidance for pre-installation in new buildings and renovations

❑ Two standards • EIA/TIA-568, ISO 11801 ❑ Based on the use of a hierarchical, star-wired cable layout

Elements of a structured Cabling Layout

Structured Cabling Hierarchy

Cable Distances Specified in EIA-568-A

Wireless Transmission Media[pic]

A wireless transmission media uses radio waves and infrared light waves as a medium for their signals to be able to transfer data, that way they will not be forced to be constricted by cords and will in turn have a larger range of consumers that they can service. Examples of wireless transmission media are infrared, broadcast radio,cellular radio, communications satellite, and a microwave.

[pic]
How it Works
Wireless transmission media enables computers to send data using radio waves or infrared light. ← Infrared is a wireless transmission medium that sends signals using infrared light waves.

← Broadcast radio is a wireless transmission medium that distributes radio signals through the air over long distances such as between cities, regions, and countries, and short distances such as within an office or home. ← Cellular radio is a form of broadcast radio that is used widely for mobile communications, specifically wireless modems and cellular phones. ← Microwaves are radio waves that provide a high-speed signal transmission. ← A communications satellite is a space that recieves microwave signals from an earth-based station, amplifies the signals, and broadcasts the signals back over a wide area to any number of earth-based stations.

Advantages • It is convenient • It is very versatile in that you can easily move from place to place with no restraints • It usually runs a little faster than a standard connection
Disadvantages
• It is a little bit more expensive to change a regular connection to a wireless one • It is easier to hack into a wireless network than a standard connection.

Lesson 4

Wide Area Networks (quick review)
Introduction
← Metropolitan area networks (MANs) typically span from 3 to 30 miles and connect backbone networks (BNs), and LANs.

← Wide area networks (WANs) connect BNs and MANs across longer distances, often hundreds of miles or more.

← Most organizations cannot afford to build their own MANs and WANs, so they rent or lease circuits from common carriers

WANs
What is a WAN?: A communications system used to interconnect geographically remote computer systems. ← WANs can be

o Private networks: The switching and communications equipment is owned by an organization

o Public networks: The switching and communications equipment is leased from a regulated carrier.

← Because of the distances involved, the LAN/MAN technologies can not be applicable and switching is used.

What is Switching?: The act of activating links to form a path through a network.
The Telephone Network ← Many countries have government agencies that regulate data and voice communications.

o The United States agency is the Federal Communications Commission (FCC).

o The Kenyan Agency is the Communication Commission of Kenya (CCK).

← A common carrier is an organization that sells or leases communications services and facilities to the public.

o Common carriers also provide local telephone services (called a local exchange carrier (LEC)); one providing long distance services is called an interexchange carrier (IXC).

Switching techniques:
Circuit Switching
Circuit switching was designed in 1878 in order to send telephone calls down a dedicated channel. This channel remained open and in use throughout the whole call and could not be used by any other data or phone calls.
In circuit switching, three steps are required for communication: ← Connection establishment

o Required before data transmission.

← Data transfer

o Can proceed at maximum speed.

← Connection termination

o Required after data transmission is over.

o For deallocation of network resources.

← The telephone message is sent in one go, it is not broken up. The message arrives in the same order that it was originally sent.

← In modern circuit-switched networks, electronic signals pass through several switches before a connection is established.

← During a call, no other network traffic can use those switches.

← The resources remain dedicated to the circuit during the entire data transfer and the entire message follows the same path.

← Circuit switching can be analogue or digital

← With the expanded use of the Internet for voice and video, analysts predict a gradual shift away from circuit-switched networks.

← A circuit-switched network is excellent for data that needs a constant link from end-to-end. For example real-time video.

Circuit Switching Advantages: ← Circuit is dedicated to the call – no interference, no sharing

← Guaranteed the full bandwidth for the duration of the call

← Guaranteed Quality of Service

Disadvantages: ← Inefficient – the equipment may be unused for a lot of the call, if no data is being sent, the dedicated line still remains open

← There is an initial delay meaning that it takes a relatively long time to set up the circuit

← During a crisis or disaster, the network may become unstable or unavailable.

← It was primarily developed for voice traffic rather than data traffic.

Circuit Switched Services
Dial-up Technology
Dialup is simply the application of the Public Switched Telephone Network (PSTN) to carry data on behalf of the end user. It involves a customer premises equipment (CPE) device sending the telephone switch a phone number to direct a connection to. ← The oldest and simplest MAN/WAN approach.

← Uses the Public Switched Telephone Network (PSTN).

← Provided by common carriers like AT&T and Ameritech, Telcomm Kenya.

← This is what you are using when you use your modem to dial-up and connect to your ISP.

← The two basic types in use today are: Plain Old Telephone System (POTS) and Integrated Services Digital Network (ISDN).

Circuit Switched Services: Basic Architecture ← Uses a cloud architecture, meaning that users connect to a network and what happens inside of the network “cloud” is hidden from the user.

← A user using a computer and a modem dials the number of a another computer and creates a temporary circuit between the two.

← When the communications session is completed, the circuit is disconnected.

Plain Old Telephone Service (POTS
The regular phone lines used in voice calls are referred to as Plain old telephone service (POTS). ← POTS-based data communications just uses regular dial-up phone lines and a modem.

o The modem is used to call another modem. Once a connection is made, data transfer can begin.

o POTS is most commonly used today to connect to the Internet by calling an ISP’s access point.

← Wide Area Telephone Services (WATS) are another type of POTS that are essentially wholesale long distance services used for both voice and data.

o Users buy so many hours of call time per month (e.g., 100 hours per month).

Integrated Services Digital Network (ISDN) ← First offered in the late 1970s, acceptance slowed due to a lack of standardization and relatively high costs.

← International standard for transmitting data over digital lines

o Established by the ITU

← Narrowband ISDN, combines voice, video, and data over the same digital circuit.

← ISDN provides digital dial-up lines that work much like analog lines.

o Since the line is digital, an “ISDN modem” which sends digital transmissions is used.

Dedicated Circuit Services ← Dedicated circuits involve leasing circuits from common carriers to create point to point links between organizational locations.

← These points are then connected together using special equipment such as routers and switches.

← Dedicated circuits are billed at a flat fee per month for which the user has unlimited use of the circuit.

← Dedicated circuits therefore require more care in network design than dialed circuits.

← The three basic dedicated circuit architectures are ring, star, and mesh architectures.

Students to make short notes on the following dedicated circuit services: (not more than 2 fullscaps) 1. T-Carriers

2. DSL (Digital Subscriber Lines)

3. Synchronous Optical Network (SONET)

Packet Switching
Modern form of long-distance data communication. — Network resources are not dedicated.

— A link can be shared

[pic]

← In packet-based networks, the message gets broken into small data packets. These packets are sent out from the computer and they travel around the network seeking out the most efficient route to travel as circuits become available. This does not necessarily mean that they seek out the shortest route.

← Each packet may go a different route from the others.

← Each packet is sent with a ‘header address’. This tells it where its final destination is, so it knows where to go.

← The header address also describes the sequence for reassembly at the destination computer so that the packets are put back into the correct order.

← One packet also contains details of how many packets should be arriving so that the recipient computer knows if one packet has failed to turn up.

← If a packet fails to arrive, the recipient computer sends a message back to the computer which originally sent the data, asking for the missing packet to be resent.

Packets handled in two ways 1. Datagram

2. Virtual circuit

Datagram Approach
Basic concept: — No route is established beforehand.

— Each packet is transmitted as an independent entity and can take any practical route and may arrive out of order

— Does not maintain any history and hence Up to receiver to re-order packets and recover from missing packets

— Analogy: Postal system.

Virtual Circuit Approach
Similar in concept to circuit switching. — A route is established before packet transmission starts.

— All packets follow the same path.

— The links comprising the path are not dedicated. Different from circuit switching in this respect.

— Analogy: Telephone system.

How it works? — Preplanned route established before any packets sent

— Packet forwarded from one node to the next using store-and-forward scheme.

— Only the virtual circuit number need to be carried by a packet.

— Each intermediate node maintains a table.

— Created during route establishment.

— Used for packet forwarding.

— 􀂾No dynamic routing decision is taken by the intermediate nodes.

Packet Switching Advantages: ← Bandwidth used to full potential

← Devices of different speeds can communicate

← Not affected by line failure (rediverts signal)

← Availability – do not have to wait for a direct connection to become available

← During a crisis or disaster, when the public telephone network might stop working, e-mails and texts can still be sent via packet switching

Packet Switching Disadvantages ← Under heavy use there can be a delay

← Data packets can get lost or become corrupted

← Protocols are needed for a reliable transfer

← Not so good for some types data streams e.g real-time video streams can lose frames due to the way packets arrive out of sequence.

Comparing Virtual Circuits v Datagram • Virtual circuits

— Network can provide sequencing and error control

— Packets are forwarded more quickly

• No routing decisions to make

— Less reliable

• Loss of a node loses all circuits through that node

• Datagram

— No call setup phase

• Better if few packets

— More flexible

• Routing can be used to avoid congested parts of the network

Comparing Packet Vs Circuit Switching

Students to make short notes on the following areas of application of packet switched networks: 1. X.25

2. Frame Relay

Message Switching:
In Message Switching, each message is treated as a separate entity. Each message contains addressing information, and at each switch this information is read and the transfer path to the next switch is decided. Depending on network conditions, a conversation of several messages may not be transferred over the same path.
Each message is stored (usually on hard drive due to RAM limitations) before being transmitted to the next switch. Because of this it is also know as a 'store-and-forward' network. Email is a common application for Message Switching. A delay in delivering email is allowed unlike real time data transfer between two computers.
The advantages to Message Switching are: — Data channels are shared among communication devices improving the use of bandwidth.

— Messages can be stored temporarily at message switches, when network congestion becomes a problem

— .Priorities may be used to manage network traffic.

— Broadcast addressing uses bandwidth more efficiently because messages are delivered to multiple destinations.

The only real disadvantage to Message Switching is it's not suitable for real time applications such as data communication, video or audio.

[pic]
-----------------------
Data rate 10 Mbps 10 Mbps
Maximum segment length 500 m 185 m
Network span 2500 m 1000 m
Nodes per segment 100 30
Node spacing 2.5m 0.5 m
Cable diameter 1 cm 05 cm

10BASE5 10BASE2

Digital signalling

Entire bandwidth consumed by signal – no FDM

Bidirectional

Bus topology

Distance: up to a few kilometers

Analog signaling (requires RF modem)

FDM possible-multiple channels for data, video, audio

Unidirectional

Bus or tree topology

Distance: up to tens of kilometer

Baseband Broadband

Packet Switched • Bandwidth dynamically allocated on as-needed basis

• May have concurrent transmissions over physical channel

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GX?¸í@Wj¡½[pic]03ìÜDzÇìǚNj|‹|‹|‹fìfì²ìfQìQìQì(h­5“hbxe6?B*[pic]CJOJQJaJph+h­5“hbxe5?B*[pic]CJOJQJ\?aJphh­5“hihîB*[pic]OJQJphh­5“hbxeB*[pic]OJQJph.h­5“hbxe5?>*[pic]B*[pic]CJOJQJ\?aJMay have delays and congestion • More cost-effective, offer better performance

Circuit Switched • Bandwidth guaranteed

• Circuit capacity not reduced by other network traffic

• Circuit costs independent of amount of data transmitted, resulting in wasted bandwidth