Administrative issues such as security, data backups, and data ownership may be compromised in a large peer-to-peer network.

10-2-2 Dedicated client/server network. In a dedicated client/server network, one computer is designated the server, and the rest of the computers are clients. As the network grows, additional computers can be designated servers. Generally, the designated servers function only as servers and are not used as a client or workstation. The servers store all the network’s shared files and applications programs, such as word processor documents, com- pilers, database applications, spreadsheets, and the network operating system. Client com- puters can access the servers and have shared files transferred to them over the transmission medium.

Figure 25 shows a dedicated client/server-based network with three servers and three clients (users). Each client can access the resources on any of the servers and also the re- sources on other client computers. The dedicated client/server-based network is probably

Introduction to Data Communications and Networking

Hub

Client 1 Client 2 Client 3

Dedicated file server Dedicated print server Dedicated mail server Printer

FIGURE 25 Dedicated client/server network

the most commonly used computer networking model. There can be a separate dedicated server for each function (i.e., file server, print server, mail server, etc.) or one single general- purpose server responsible for all services.

In some client/server networks, client computers submit jobs to one of the servers. The server runs the software and completes the job and then sends the results back to the client computer. In this type of client/server network, less information propagates through the network than with the file server configuration because only data and not applications programs are transferred between computers.

In general, the dedicated client/server model is preferable to the peer-to-peer client/server model for general-purpose data networks. The peer-to-peer model client/server model is usu- ally preferable for special purposes, such as a small group of users sharing resources.

10-3 Network Topologies

Network topology describes the layout or appearance of a network—that is, how the com-

puters, cables, and other components within a data communications network are intercon- nected, both physically and logically. The physical topology describes how the network is actually laid out, and the logical topology describes how data actually flow through the network.

In a data communications network, two or more stations connect to a link, and one or more links form a topology. Topology is a major consideration for capacity, cost, and reli- ability when designing a data communications network. The most basic topologies are

point to point and multipoint. A point-to-point topology is used in data communications

networks that transfer high-speed digital information between only two stations. Very of- ten, point-to-point data circuits involve communications between a mainframe computer and another mainframe computer or some other type of high-capacity digital device. A two- point circuit is shown in Figure 26a.

A multipoint topology connects three or more stations through a single transmission medium. Examples of multipoint topologies are star, bus, ring, mesh, and hybrid.

Bus Ring (f) (d) (c) Bus (b) (a) (e) Hub

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10-3-1 Star topology. A star topology is a multipoint data communications net- work where remote stations are connected by cable segments directly to a centrally located computer called a hub, which acts like a multipoint connector (see Figure 26b). In essence, a star topology is simply a multipoint circuit comprised of many two-point circuits where each remote station communicates directly with a centrally located computer. With a star topology, remote stations cannot communicate directly with one another, so they must re- lay information through the hub. Hubs also have store-and-forward capabilities, enabling them to handle more than one message at a time.

10-3-2 Bus topology. A bus topology is a multipoint data communications circuit that makes it relatively simple to control data flow between and among the computers be- cause this configuration allows all stations to receive every transmission over the network. With a bus topology, all the remote stations are physically or logically connected to a sin- gle transmission line called a bus. The bus topology is the simplest and most common method of interconnecting computers. The two ends of the transmission line never touch to form a complete loop. A bus topology is sometimes called multidrop or linear bus, and all stations share a common transmission medium. Data networks using the bus topology generally involve one centrally located host computer that controls data flow to and from the other stations. The bus topology is sometimes called a horizontal bus and is shown in Figure 26c.

10-3-3 Ring topology. A ring topology is a multipoint data communications net- work where all stations are interconnected in tandem (series) to form a closed loop or cir- cle. A ring topology is sometimes called a loop. Each station in the loop is joined by point- to-point links to two other stations (the transmitter of one and the receiver of the other) (see Figure 26d). Transmissions are unidirectional and must propagate through all the stations in the loop. Each computer acts like a repeater in that it receives signals from down-line computers then retransmits them to up-line computers. The ring topology is similar to the bus and star topologies, as it generally involves one centrally located host computer that controls data flow to and from the other stations.

10-3-4 Mesh topology. In a mesh topology, every station has a direct two-point communications link to every other station on the circuit as shown in Figure 26e. The mesh topology is sometimes called fully connected. A disadvantage of a mesh topol- ogy is a fully connected circuit requires n(n 1)/2 physical transmission paths to in- terconnect n stations and each station must have n 1 input/output ports. Advantages of a mesh topology are reduced traffic problems, increased reliability, and enhanced security.

10-3-5 Hybrid topology. A hybrid topology is simply combining two or more of the traditional topologies to form a larger, more complex topology. Hybrid topologies are sometimes called mixed topologies. An example of a hybrid topology is the bus star topol- ogy shown in Figure 26f. Other hybrid configurations include the star ring, bus ring, and virtually every other combination you can think of.

10-4 Network Classifications

Networks are generally classified by size, which includes geographic area, distance between stations, number of computers, transmission speed (bps), transmission me- dia, and the network’s physical architecture. The four primary classifications of net- works are local area networks (LANs), metropolitan area networks (MANs), wide

Introduction to Data Communications and Networking

Table 2 Primary Network Types

Network Type Characteristics

LAN (local area network) Interconnects computer users within a department, company, or group

MAN (metropolitan area network) Interconnects computers in and around a large city WAN (wide area network) Interconnects computers in and around an entire country GAN (global area network) Interconnects computers from around the entire globe Building backbone Interconnects LANs within a building

Campus backbone Interconnects building LANs Enterprise network Interconnects many or all of the above

PAN (personal area network) Interconnects memory cards carried by people and in computers that are in close proximity to each other

PAN (power line area network, Virtually no limit on how many computers it can interconnect sometimes called PLAN) and covers an area limited only by the availability of power

distribution lines

area networks (WANs), and global area networks (GANs). In addition, there are three

primary types of interconnecting networks: building backbone, campus backbone, and enterprise network. Two promising computer networks of the future share the same acronym: the PAN (personal area network) and PAN (power line area network, sometimes called PLAN). The idea behind a personal area network is to allow people to transfer data through the human body simply by touching each other. Power line area networks use existing ac distribution networks to carry data wherever power lines go, which is virtually everywhere.

When two or more networks are connected together, they constitute an internetwork or internet. An internet (lowercase i) is sometimes confused with the Internet (uppercase

I). The term internet is a generic term that simply means to interconnect two or more net-

works, whereas Internet is the name of a specific worldwide data communications net- work. Table 2 summarizes the characteristics of the primary types of networks, and Figure 27 illustrates the geographic relationship among computers and the different types of net- works.

10-4-1 Local area network. Local area networks (LANs) are typically privately

owned data communications networks in which 10 to 40 computer users share data re- sources with one or more file servers. LANs use a network operating system to provide two-way communications at bit rates typically in the range of 10 Mbps to 100 Mbps and higher between a large variety of data communications equipment within a relatively small geographical area, such as in the same room, building, or building complex (see Figure 28). A LAN can be as simple as two personal computers and a printer or could contain dozens of computers, workstations, and peripheral devices. Most LANs link equipment that are within a few miles of each other or closer. Because the size of most LANs is limited, the longest (or worst-case) transmission time is bounded and known by everyone using the network. Therefore, LANs can utilize configurations that otherwise would not be possible.

LANs were designed for sharing resources between a wide range of digital equip- ment, including personal computers, workstations, and printers. The resources shared can be software as well as hardware. Most LANs are owned by the company or organization

Introduction to Data Communications and Networking

Metropolitan area network

Multiple buildings or entire city

Wide area network

Entire country Global area network

Entire world

Personal area network

Between people and computers

Local area network

Single building

FIGURE 27 Computer network types

that uses it and have a connection to a building backbone for access to other departmental LANs, MANs, WANs, and GANs.

10-4-2 Metropolitan area network. A metropolitan area network (MAN) is a high-speed network similar to a LAN except MANs are designed to encompass larger areas, usually that of an entire city (see Figure 29). Most MANs support the trans- mission of both data and voice and in some cases video. MANs typically operate at

Introduction to Data Communications and Networking CD-ROM/WORM FAX machine File/application/ print server Hub/repeater To building backbone Router or switch Patch panel Wall jack LAN Scanner Laptop PC Workstation NOS client software NOS client software NOS server

FIGURE 28 Local area network (LAN) layout

speeds of 1.5 Mbps to 10 Mbps and range from five miles to a few hundred miles in length. A MAN generally uses only one or two transmission cables and requires no switches. A MAN could be a single network, such as a cable television distribution net- work, or it could be a means of interconnecting two or more LANs into a single, larger network, enabling data resources to be shared LAN to LAN as well as from station to station or computer to computer. Large companies often use MANS to interconnect all their LANs.

A MAN can be owned and operated entirely by a single, private company, or it could lease services and facilities on a monthly basis from the local cable or telephone company. Switched Multimegabit Data Services (SMDS) is an example of a service of- fered by local telephone companies for handling high-speed data communications for MANs. Other examples of MANs are FDDI (fiber distributed data interface) and ATM (asynchronous transfer mode).

10-4-3 Wide area network. Wide area networks (WANs) are the oldest type of data

communications network that provide relatively slow-speed, long-distance transmission of data, voice, and video information over relatively large and widely dispersed geographical areas, such as a country or an entire continent (see Figure 30). WANs typically interconnect cities and states. WANs typically operate at bit rates from 1.5 Mbps to 2.4 Gbps and cover a distance of 100 to 1000 miles.

WANs may utilize both public and private communications systems to provide serv- ice over an area that is virtually unlimited; however, WANs are generally obtained through service providers and normally come in the form of leased-line or circuit-switching tech- nology. Often WANs interconnect routers in different locations. Examples of WANs are

Introduction to Data Communications and Networking

FIGURE 29 Metropolitan area network (MAN)

ISDN (integrated services digital network), T1 and T3 digital carrier systems, frame relay, X.25, ATM, and using data modems over standard telephone lines.

10-4-4 Global area network. Global area networks (GANs) provide connects be-

tween countries around the entire globe (see Figure 31). The Internet is a good example of a GAN, as it is essentially a network comprised of other networks that interconnects virtu- ally every country in the world. GANs operate from 1.5 Mbps to 100 Gbps and cover thou- sands of miles

10-4-5 Building backbone. A building backbone is a network connection that nor- mally carries traffic between departmental LANs within a single company. A building back- bone generally consists of a switch or a router (see Figure 32) that can provide connectiv- ity to other networks, such as campus backbones, enterprise backbones, MANs, WANs, or GANs. Phoenix metropolitan area Manufacturing facility Research facility Shipping facility Headquarters building Service provider MAN

Introduction to Data Communications and Networking Service provider WAN Seatle, WA Tempe, AZ San Diego, CA Oriskany, NY Router Router Router Router Router Miami, FL

FIGURE 30 Wide area network (WAN)

10-4-6 Campus backbone. A campus backbone is a network connection used to carry traffic to and from LANs located in various buildings on campus (see Figure 33). A campus backbone is designed for sites that have a group of buildings at a single location, such as corporate headquarters, universities, airports, and research parks.

A campus backbone normally uses optical fiber cables for the transmission media be- tween buildings. The optical fiber cable is used to connect interconnecting devices, such as

Loa Angeles, CA – USA

Sidney, Australia

Rome, Italy London, England GAN

Introduction to Data Communications and Networking

Hub

Building backbone optical fiber cables

PC Wall jacks LAN Hub Workstation LAN Switch Router To WAN, MAN, or campus Patch cables Patch panels

FIGURE 32 Building backbone

Router/switch Building 1 Router/switch Router/switch LAN LANs Fiber cables LANs Router/switch Building 2 Building 3 WAN WAN

FIGURE 33 Campus backbone

bridges, routers, and switches. Campus backbones must operate at relatively high trans- mission rates to handle the large volumes of traffic between sites.

10-4-7 Enterprise networks. An enterprise network includes some or all of the previ- ously mentioned networks and components connected in a cohesive and manageable fashion.

Introduction to Data Communications and Networking