Network Design

Advances in networking technology have been great and many over the past two decades. From early technology that transmitted at 4 Mbps to current technology that transmits at 100 Gbps, and from thick Ethernet to wireless, technology continues to evolve. Telephones have also experienced an astonishing evolution, going from voice only to now supporting voice, data, and video. The equipment used to interconnect our networks has also continued to change. Maintaining a working knowledge of the hardware that forms the fabric of our networks is essential. This section will relay a basic explanation of the networking components that you may encounter while designing telecommunications systems.

Network Interface Adapters

Every network computer must have at least one network interface adapter to provide the link between the computer and the network. It could be integrated in the motherboard or inserted in the expansion slots. Industry Standard Architecture (ISA), Peripheral Component Interconnect (PCI), or PC Card buses are a few of the many types of adapters. A computer's network interface adapter is associated with the specific data link-layer protocol that the network is using and is usually connected with a cable jack, such as RJ-45 for twisted-pair cables, BNC or AUI for coaxial cables, or ST or SC for fiber-optic cables, but it can also be a wireless transmitter of some sort.

Understanding Network Interface Adapter Functions

In a sending machine, the network interface adapter is responsible for creating frames with the data (datagram) received from the network layer. In the receiving machine, the network interface adapter reads the contents of the incoming frame and passes the data to the appropriate network layer protocol.

The network interface adapter implements the physical-layer encoding scheme that converts the binary data into electrical voltage (or light pulses or radio signals) and then converts the received signal back into binary data.

It transmits and receives data one frame at a time, so it uses built-in buffers to store data temporarily. It is also responsible for converting data from parallel communication within the computer to serial communication over the media.

In addition, the network interface adapter is responsible for implementing the media access control (MAC) mechanism (CSMA/CD or CSMA/CA) that the data link-layer protocol uses to regulate access to the network medium.

Figure 2.12 Network Interface Card

Source: User: Barcex. [Photo of a FORE Systems network interface card]. Used under Creative Commons Attribution-Share Alike 3.0 Unported license.

Network Hubs

A hub or concentrator is a device used to connect all the computers on a star or ring network and is associated with the physical link-layer protocol that the network is using. Ethernet hubs are most common because Ethernet is the most popular data link-layer protocol. Token ring multistation access units (MAUs) are hubs, too, and other protocols, such as FDDI, can also use hubs.

An Ethernet hub is a multiport repeater, meaning that it amplifies all of the signals that pass it to counteract the effects of attenuation. A large network can have many hubs interconnected through a specialized port called an uplink port. When data enter a hub through any of its ports, the hub amplifies the signal and transmits it through all the other ports.

It seems feasible to create very large networks just by using many repeaters or hubs, but in practice, the addition of hubs or repeaters is governed by the 5-4-3 Rule. The rule states that in a collision domain, no two nodes may be separated by more than five repeaters or hubs, four segments, and three populated segments. A populated segment is an Ethernet segment with at least one machine directly connected to it.

Another function of the Ethernet hub is to provide a crossover circuit that connects the transmit pins to the receive pins for each connection between two computers. The uplink port in the hub is the one port that has no crossover circuit. A crossover cable is a UTP cable that has the transmit pins on one end of the cable wired to the receive pins on the other end, thus eliminating the need for a crossover circuit in the hub. Therefore, two computers can be connected directly to each other without a hub by using a crossover cable.

Hubs are physical-layer devices, meaning that they usually do not interpret a signal. However, some expensive hubs have data-processing capabilities and can provide store and forward services using their buffers. Some "intelligent" hubs include management features. These intelligent hubs can be used to monitor the operations of the hub's ports along with Simple Network Management Protocol (SNMP).

Although they look similar, the MAU used in token ring networks is different from the Ethernet hub. A MAU is a passive device and does not work as a repeater. Moreover, a MAU does not transmit a packet to all of the connected computers simultaneously. Instead, it transmits to one computer at a time in one direction in the ring, until the packet reaches the originator, where the packet is removed.

Ports in MAUs where no running computers are connected are not included in the ring and are said to be in the loopback state. MAUs have no uplink port to interconnect, but they do have a different mechanism called ring in and ring out ports, which are used to connect one MAU to another.

Network hub connections are illustrated in figure 2.13.

Figure 2.13 Repeaters Connecting Five Segments of a Network

Network Connections

In the simplest network, two computers are connected to each other, perhaps only with a crossover cable, and they share data. With a hub, some cables, and some network interface adapters, a group of computers can be turned into an effective local area network. However, other types of hardware devices will soon be required as the network grows larger—as efficiency decreases with increased traffic. By using such special devices, the size of the network can be increased without decreasing its efficiency.

Bridging

Bridging is a technique used to connect networks at the data link layer to provide packet filtering, meaning that it propagates only the packets that are destined for the other side of the network. When a large LAN experiences excessive collisions or delays due to high traffic levels, a bridge can reduce the traffic by splitting the network in half.

a. Connecting LANs with a Bridge

A bridge can be used to join two existing LANs or to split one LAN into two segments. It is a physical unit, typically a box with two ports, and it can interpret the data link-layer protocol header. The bridge does packet filtering, in which it reads the destination MAC address in the packet and allows the packet to go to the other side of the bridge only when the packet is destined to go there. If a computer on one segment (one side of the bridge) sends a packet to a computer in the same segment (the same side of the bridge), then the bridge does not allow that packet to cross it. The use of a bridge thus cuts the unnecessary traffic in the network.

Figure 2.14 Bridge Operation

b. Bridges and Collisions

A collision domain is an area in a network where only one packet can function at a time, but multiple packets collide. When a new hub is added to an existing network, the computers attached to that new hub become part of the same collision domain. On the other hand, when a bridge is added to an existing network, computers on each side of the bridge create separate collision domains. Smaller collision domains bring about fewer collisions, fewer retransmissions, and improved efficiency.

c. Bridges and Broadcasts

There are three types of message in a network:

· unicast message

· multicast message

· broadcast message

Unicast is a communication between a single sender and a single receiver over a network. An earlier term, point-to-point communication, is similar in meaning to unicast. Most network traffic is unicast traffic.

Multicast is a communication between a single sender and multiple receivers on a network. In multicasting, one computer sends data to a specific group of computers on the network simultaneously.

Broadcast is to cast or throw forth something in all directions at the same time. In computer broadcasting, a packet is addressed to all machines on the network.

A broadcast domain is the group of computers that receive such broadcast messages. Although a bridge divides a collision domain into multiple collision domains, it cannot divide a broadcast domain into multiple broadcast domains (a router or a brouter can handle this).

d. Transparent Bridging

Transparent bridging is a technique that bridges in the Ethernet network use to automatically compile hardware addresses (MAC addresses) in an internal address table. When the bridge receives a frame and reads the destination MAC address in the data link-layer protocol header, it compares that address with the table. If the address is not among the listed addresses, the bridge allows that packet to pass to the other side of the bridge.

If multiple bridges are installed for redundancy, then there is a potential danger of getting the broadcast packets forwarded endlessly. This is called a bridge loop. To avoid this, bridges communicate among themselves, using a protocol called a spanning tree algorithm (STA), in which one bridge is selected to process the packet while all other bridges in the segment remain idle until the processing bridge fails. The use of a spanning tree algorithm is illustrated in figure 2.12.

Figure 2.15 Using the Spanning Tree Algorithm

e. Source Route Bridging

Token ring networks do not use the spanning tree algorithm to protect a communication from bridge loop. Instead, they use a different technique called source route bridging, in which they add a route designator in broadcast messages (All Rings Broadcast—ARB) and use that route designator to avoid sending the packet to the same bridge twice and to determine which bridge provides the most efficient route through the network to a given destination.

f. Bridge Types

The simplest bridge used to connect two similar sections is called a local bridge. It does not modify the data in the packet; it simply reads the MAC address in the data link-layer packet header, compares it with the table, and either forwards it to the other section or discards it.

A translation bridge is a data link-layer device that connects network segments using dissimilar network media or protocols. The functions of these bridges are more complicated as they strip off the data link-layer frame to repackage it in a new format. These bridges can connect an Ethernet segment to a token ring segment or a 100Base-TX to a 100Base-T4.

A remote bridge uses some form of WAN link, such as a modem connection or leased telephone line, to connect two network segments at distant locations. This reduces the amount of traffic passing over the WAN link.

Switching

A switch is a network connection device that forwards an incoming packet only to the port that provides access to the destination system. Switches may look like hubs, but they differ in that hubs forward incoming packets out through all of their ports. A switch essentially converts a LAN from a shared network medium to a dedicated one. A bridge reduces unnecessary traffic congestion on the network, but a switch practically eliminates it. A standard switched network is illustrated in figure 2.16.

Another advantage of switching is that each pair of computers using a switch has the full bandwidth of the network. In addition, full-duplex operations are available in switch, effectively doubling the throughput of the network. Replacing all routers with switches is not recommended, however, because a switch still relays a broadcast to every other computer's packets, increasing the number of unnecessary packets processed by each system. This situation can be addressed by using a virtual LAN (VLAN)—a broadcast domain created by one or more switches.

With a VLAN, subnets can be created on a switched network. When a computer on a particular subnet transmits a broadcast message, the packet goes only to the computers in the subnet, rather than to all computers in the network. Communication between subnets can be either routed or switched, but all traffic within a VLAN is switched. Layer 3 switching is a variation on the VLAN concept that minimizes the amount of routing needed between the VLANs. A switch using VLAN software is illustrated in figure 2.17.

Three types of switching technologies exist:

· cut-through

· store-and-forward

· fragment-free

Cut-through switches, in what is called matrix switching or crossbar switching, forward packets immediately by reading the destination address in the data link-layer protocol header. They are inexpensive, but the major drawback with this technique is that the switch can forward incomplete or damaged frames, resulting in more unwanted traffic and bandwidth utilization.

A store-and-forward switch, on the other hand, waits until the entire packet arrives, using a memory buffer that stores the incoming data and verifies the data by performing a frame check sequence (FCS). It is more expensive. This type of switching enables the switch to drop frames that are incomplete or damaged, thus reducing unwanted traffic.

This technique is similar to that of a cut-through switch in that it forwards a part of the frame before receiving the entire frame. The difference is that the switch waits for 64 bytes to be received before forwarding the first part of the frame. This is done to check for collisions that can be detected in the first 64 bytes of the frame. This switch does not check the FCS in the frame.

Figure 2.16 A Standard Switched Network for Two LANs

Figure 2.17 A Switch Using VLAN Software

Routing

A router connects two networks, forming an internetwork. Routers operate at the network-layer protocol of the OSI model—the boundaries of all LANs. On a large internetwork, such as the Internet, a packet may have to pass through many routers to its destination, connecting distant networks using WAN links.

A router has an internal table called a routing table that contains information about the networks around it. The router uses this routing table to determine where to send each packet. When a packet has to pass multiple networks on its way to the destination, each router that processes it is called a hop. Finding out the most efficient way to transmit a packet by minimizing the number of hops is one major function of all routers. The router does this by maintaining a value in its routing table called a metric that specifies the relative efficiency of each route.

The process of building a routing table can be either manual or automatic. Static routing is the process of creating a routing table manually, which may be practicable in a small network with few routers. However, manual build-up is unworkable in a large network with hundreds of routers. Dynamic routing is the process of building a routing table automatically, using a specialized protocol in which the system registers, updates, and shares routing information on the fly, without the network administrator's participation. Routers are mostly hardware, but there are also software-based routers. Routers can be small and cost as little as $100 each, or they can be large and very expensive, as much as $100,000 each. Even a computer with two or more network interface cards, called a multihomed computer, can be used as a private router.

Firewalls

A firewall is a network security device. Firewalls are used in protecting local area networks when they connect to the Internet or any other untrusted source, such as incoming traffic from unauthorized or unknown sources. Firewalls can be thought of as routers with an additional set of security rules, which are defined and configured based on the identified security requirements associated with them. What separates a firewall from a standard router is that packets are not automatically forwarded by the firewall from one network to the next. When the firewall first examines a packet, it checks the source and destination addresses, the protocol type, the port, and other data within the packet in accordance with security rules. If the packet fulfills the requirements of the security rules, then the firewall retransmits it and may log it. If the packet does not fulfill the security rules, then it is blocked and logged.

There are two required network connections to a firewall and one optional one. The first required connection is to the trusted (protected) network. This consists of the internal systems that one wishes to protect. The second required connection is to the untrusted (unprotected) network, better known as the outside world, or the Internet. The optional network is called the DMZ, or demilitarized zone. Devices that require both protection and public access reside in the DMZ. An example of a device on a DMZ is a web server, which is used to serve web content and needs to be accessible from the outside. See figure 2.18 for an illustration of a firewall.

Hardware Firewall: Hardware firewalls are considered appliance devices that are specifically designed to perform the firewall function. They still need to be patched and updated to ensure that they are kept up to date with the latest patches and updates.

Software Firewall: A software firewall runs on an existing system utilizing the system's operating system. It is an application that runs on it, performing firewall functions. The firewall software needs to be patched and updated to ensure that it is kept up to date with the latest patches and updates, and the host operating system needs to be updated and patched as well.

Firewall limitations: It is important to understand that firewalls have to be configured individually, based on the specific requirements of the environment that needs to be protected. A firewall is not an all-in-one security solution. It needs to be used in conjunction with other technologies and as part of a defense-in-depth approach, utilizing multiple layers of protection.

Multiplexer

A multiplexer (sometimes abbreviated mux) allows for multiple signals to be transferred across a single link, which eliminates the waste of bandwidth and the need for intelligence in the devices. Multiplexers have three significant characteristics:

· They can combine multiple signals on a single communication link, thereby allowing multiple terminals to share a common circuit.

· They are nonintelligent (dumb) devices that do not modify or delay the multiplexed signals in any way, thereby appearing transparent to the end user.

· They are used in pairs connected by a single link. There are an identical number of inputs to and outputs from the pair. The sending multiplexer is called a mux, and the receiving multiplexer is called a demultiplexer or demux. However, the sending and receiving muxes can reverse roles.

There are three types of multiplexing: frequency-division multiplexing (FDM), time-division multiplexing (TDM), and statistical time-division multiplexing (STDM). Two of these types, FDM and TDM, have the three characteristics listed above and therefore involve pure multiplexing. STDM is not pure multiplexing because the multiplexers involved have some intelligence. They differ from the second of the characteristics listed above because they require a terminal identification, require intelligence to identify the receiving terminal, and may delay the signal in a condition of heavy traffic. See figure 2.19 for an illustration of how a multiplexer works.

Figure 2.19 Multiplexer

In module 1, we started to define a network and took a quick look at local area networks and wide area networks. You will recall that a network can be defined as two or more computers that are linked in order to

· share resources

· exchange files

· allow electronic communications

The computers on a network may be linked via different types of media (e.g., cables, telephone lines, radio waves, satellites, or infrared light beams).

The advantages of having a network usually far outweigh the disadvantages, but these should be addressed on a case-by-case basis. Some of the advantages of having a network are:

· Shared resources: This saves time and reduces equipment costs on items such as printers and hard disks.

· Shared data files: This helps eliminate the possibility of having multiple incompatible copies of files and increases productivity by providing access to the same data to several users.

· Shared applications: This allows more than one user to utilize the available applications.

· Electronic communication (e-mail): Users can leave each other electronic mail instead of playing phone tag.

Some of the disadvantages are:

· Installation costs: The initial setup of a network can be expensive. It depends on how many devices will be connected and on the amount of construction that needs to be done. If, for example, your only reason for considering a network is to share resources such as printers, it may be cheaper to buy each user his or her own printer.

· Administration and support costs: Someone skilled in network administration must be consulted when the network is down.

· Security: As you connect multiple computers and networks, you open up the possibility of an unauthorized individual accessing your network and resources.

Networks are generally classified into one of two groups, depending on their size and function. A local area network, or LAN, is the basic building block of any computer network.

Local Area Networks (LANs)

LANs usually consist of two or more devices (i.e., computers), connected directly, or through the use of a hub, switch, or other connectivity device. A LAN can range from the simple (two computers connected by a cable) to the complex (hundreds of interconnected computers and peripherals throughout a large organization). A system of LANs connected in this way is called a wide area network (WAN).

Each computer in the LAN must have a connecting device called a network interface card (NIC). The NIC provides the connections between the computer's motherboard and the network media. See figure 2.20 for an illustration of a LAN.

Figure 2.20 Local Area Network

Most LANs connect workstations and/or personal computers. Each computer (also known as a node) within a LAN not only has its own central processing unit (CPU) with which it executes software programs, but it is also able to access data and devices anywhere on the LAN through an external path or a resource located on a different system. This allows many users to share software applications, information, and expensive devices, such as laser printers, plotters, and storage. Users can also use the LAN to communicate with each other by sending e-mail or instant messages.

LANs typically have the following characteristics:

· Data transfer occurs at high speeds (higher bandwidth than dialup, DSL, and most affordable commercial leased lines).

· They exist in a limited geographic area.

· The organization running the LAN usually manages connectivity and resources, especially the transmission media.

Wide Area Networks (WANs)

As we discussed in module 1, a WAN is a network of networks and other devices that are connected together. As businesses grow, and their locations and the number of LANs begin to multiply, a requirement emerges to connect the multiple LANs together. Routers are used in many cases to join two LANs together, and the Internet, private lines, cable, and/or satellites can be used as the telecommunications path between the LANs. See figure 2.21 for an illustration of a WAN.

Figure 2.21 Wide Area Network

WANs allow for the basic expansion of LANs, linking them and allowing them to communicate with each other. By definition, a LAN becomes a WAN when it crosses a public right of way. At that point, the network is no longer local and requires a public carrier for data transmission. The characteristics of WANs are

· low- to high-speed links

· wide geographic area, ranging in size from regional to coast-to-coast to global

· devices and equipment used to carry signals between networks belong to a commercial carrier (AT&T, Verizon, etc.)

The benefits of LANs and WANs are identical, with the exception that WANs cover larger geographic areas of operation.

The network architecture, which covers all the design aspects of the network, defines the relationship that network devices have with one another. The two main network relationship bases are peer-to-peer and client-server.

Peer-to-Peer

On a peer-to-peer network, all computers are equal. Peers share resources, and there is no server, and thus no centralized management, dedicated to handling requests for those resources. Each workstation acts as both the client, which issues requests, and the server, which receives and processes requests. Smaller networks of 10 or fewer computers may work well under this model. The peer-to-peer network model is an inadequate solution for large networks, because many clients requesting services can put an undue strain on a typical workstation. See figure 2.22 for an illustration of a peer-to-peer network.

Figure 2.22 Peer-to-Peer Network

The advantages of the peer-to-peer network model are

· easy setup

· low installation cost

· best used in small networks of approximately 10 workstations

· amount of hardware necessary to connect all workstations is limited

· low maintenance

The disadvantages of the peer-to-peer network model are:

· The data are scattered across all workstations. If there are a large number of users, the sharing of documents becomes a problem because of the possibility of multiple versions of the same document.

· There is a security risk. Users may be required to remember multiple passwords for each resource on the network, tempting them to write them down, or resources may not have any passwords assigned at all.

· Users must be trained to share their own resources and act as administrators, taking on system management responsibilities such as storage management, sharing resources, and patch and update management for their own machines.

· There is no central management or administration.

Client-Server

A client-server network consists of workstations, or clients, that issue requests to a server. The client workstation is responsible for issuing requests for whatever services are required. The server's function on the network is to service those requests. Servers are generally more powerful than their client counterparts due to the fact that they must service a large number of requests. Some examples of server-based network operating systems (NOSs) are Microsoft Windows Server and UNIX. See figure 2.23 for an illustration of a client-server network.

Figure 2.23 Client-Server Network

The advantages of the client-server network model include

· centralized security

· dedicated servers that take the load of processing client requests off workstations that may not have the capacity to service those requests

· easy accessibility

· synchronized files or a well-organized shared directory structure

· ease of backups

The disadvantages of the client-server network model include

· dependency on an administrator

· expensive hardware (server)

· expensive software (NOS)

· requires highly trained administrators

Network Topology

Network topologies are defined by how the nodes are connected. We will see below how the nodes in mesh, star, tree, bus, ring, and wireless networks are connected and how the different connection topologies affect how messages are sent.

Mesh Topology

In mesh topology, every node is connected by a dedicated point-to-point connection to all other nodes. A message will require only one hop, no matter where the sending and receiving nodes are located in the network. Figure 2.24 shows a mesh network. Note that messages from A to B or from A to C both require one hop.

The number of dedicated point-to-point connections in a mesh network increases at roughly the square of the rate of increase in the number of nodes. The equation for computing the number of connections required is as follows:

number of connections = T x (T – 1)/2 where T = the number of nodes or terminals

Table 2.1 shows that as the size of the network grows, the number of connections required will quickly become unmanageable.

Table 2.1 Connections Required for a Mesh Network

Star Topology

In the star topology, the central node is connected to every other node. Figure 2.25 shows a star network. Messages between A and B, where neither is the central node, always require two hops. Note that the network goes down if the central node goes down.

Figure 2.25 Star Topology

The advantages of the star topology are:

· A break in one cable does not affect all other stations as it does in bus technologies, because there is only one station per segment, generally making the star more reliable.

· Problems are easier to locate.

· It is the easiest to design and install, with the exception of bus topology.

· The star does not require a terminator.

The disadvantages of the star topology are:

· Hubs, which are required for star topology, are more expensive than bus connectors.

· A failure of the hub can affect the entire configuration and all connected stations.

· Star topologies use more cable than bus topologies.

Tree Topology

· The tree topology consists of a root node and a series of attached cascaded nodes. This topology closely resembles a corporate organization chart. Figure 2.26 shows a tree network. Messages from A to B must flow up to reach the lowest common node and then down to B—in our diagram, this consists of three hops from node A up to the root node and then two hops down to node B. A message from B to C simply moves up one node to a common node and then down two nodes.

Figure 2.25 Tree Topology

Bus Topology

· In the bus topology, a cable running the length of the network connects all nodes or workstations with a multipoint connection. Figure 2.27 shows a bus network. The small solid rectangles at each end represent the cable ends. A message from node A to node B is put onto the bus and is received by all nodes on the bus, including node B.

Figure 2.27 Bus Topology

Ring Topology

· In the ring topology, a closed loop connects all of the workstations. Figure 2.28 shows a ring network. Messages from A to B flow through the intervening nodes, in sequence, until they reach B. In our diagram, four hops are required if the traffic flows only in the counterclockwise direction.

Figure 2.25 Ring Topology

The advantages of the ring topology are:

· It prevents network collision because of the media access method.

· Each station functions as a repeater, so the topology does not require additional network hardware.

The disadvantages of the ring topology are:

· A failure at one point can bring down the network.

· Because all stations are wired together, adding a station means that the network must be shut down temporarily.

· Maintenance on a ring is more difficult than on a star topology because an adjustment or reconfiguration affects the entire ring.

Wireless Topologies

Wireless LANs have two basic topologies:

· ad hoc

· infrastructure

In the ad hoc topology, groups of computers are equipped with wireless network interface adapters and are able to communicate freely with each other within a communication range. This topology is useful in a small area, such as a home or a small business, where there are few computers and the installation of cables is difficult.

In the infrastructure topology, wireless-equipped computers communicate with a network using wireless transceivers, called network access points, connected to the LAN by standard cables. This topology is better-suited to a larger network that has only a few wireless computers—for example, laptops belonging to traveling users. See figure 2.29 for an illustration of infrastructure wireless topology.

Figure 2.29 Network Access Points in Infrastructure Wireless Topology

Summary

Here is a recap of the concepts covered in this module.

· Computers use multiple systems to process data electronically, but ultimately, they store data using binary code.

· Computers store and process various types of data, including audio, images, and video.

· Computers require two types of memory in order to function: volatile and nonvolatile.

· There are only 13 root-level domain name servers worldwide, and their purpose is to translate meaningful domain names into numerical decimal identifiers.

· IP addresses are unique numbers used to identify network devices. The IP address is broken down into parts. The first two octets provide the network address, the third octet provides the subnet address, and the fourth octet provides the host address.

· While the OSI networking model is fundamental for understanding data communication, the TCP/IP networking model focuses on computer-specific network connections.

· The seven layers of the OSI model are: physical, data link, network, transport, session, presentation, and application.

· The four layers of the TCP/IP networking model are: network interface, internet, transport, and application.

· A signal is a mechanism that is used to convey data from one place to another, and it can be in either analog or digital form.

· Conducted media are represented by cabling that acts as the transmission medium and carries data signals between devices.

· Wireless media transmit information via frequencies from the radio frequency spectrum. Wireless communications consists of a transmitting device and a receiving device.

· Network components are used to connect to the physical network via the appropriate media. The network components are responsible for translating the data into electronic signals, and sending, forwarding, and routing the data packets until they reach their intended destinations.

· A network can be defined as two computers connected to each other by a medium (wired or wireless) so they can share data. A local area network (LAN) is a group of computers and other devices located in close proximity to each other and connected by a common medium. A wide area network (WAN) is a network of networks and other devices that are connected.

· In a peer-to-peer network, all computers are equal. Peers share resources, and there is no server dedicated to handling requests for those resources.

· A client-server network consists of workstations, or clients, that issue requests to a server. The client workstation is responsible for issuing requests for whatever services are required. The server's function on the network is to service those requests.