Wireless Communication

Wireless Communication

Wireless communication uses radio frequencies (RF) or infrared (IR) waves to transmit data between devices on a LAN. For wireless LANs, a key component is the wireless hub, or access point, used for signal distribution.

Figure 1.1:Wireless Network

To receive the signals from the access point, a PC or laptop must install a wireless adapter card (wireless NIC). Wireless signals are electromagnetic waves that can travel through the vacuum of outer space and through a medium such as air. Therefore, no physical medium is necessary for wireless signals, making them a very versatile way to build a network. Wireless signals use portions of the RF spectrum to transmit voice, video, and data. Wireless frequencies range from 3 kilohertz (kHz) to 300 gigahertz (GHz). The data-transmission rates range from 9 kilobits per second (kbps) to as high as 54 Mbps. The primary difference between electromagnetic waves is their frequency. Low-frequency electromagnetic waves have a long wavelength (the distance from one peak to the next on the sine wave), while high-frequency electromagnetic waves have a short wavelength.

Some common applications of wireless data communication include the following:

• Accessing the Internet using a cellular phone

• Establishing a home or business Internet connection over satellite

• Beaming data between two hand-held computing devices

• Using a wireless keyboard and mouse for the PC

Another common application of wireless data communication is the wireless LAN (WLAN), which is built in accordance with Institute of Electrical and Electronics Engineers (IEEE) 802.11 standards. WLANs typically use radio waves (for example, 902 megahertz [MHz]), microwaves (for example, 2.4 GHz), and IR waves (for example, 820 nanometers [nm]) for communication. Wireless technologies are a crucial part of the today’s networking. See Chapter 28, “Wireless LANs,” for a more detailed discuss on wireless networking.

Comparing Media Types

Presented in Table 1.1 are comparisons of the features of the common network media. This chart provides an overview of various media that you can use as a reference. The medium is possibly the single most important long-term investment made in a network. The choice of media type will affect the type of NICs installed, the speed of the network, and the capability of the network to meet future needs.

Table 1.1 Media Type Comparison

Media Type	Maximum Segment Length	Speed	Cost	Advantages	Disadvantages
UTP	100 m	10 Mbps to 1000 Mbps	Least expensive	Easy to install; widely available and widely used	Susceptible to interference; can cover only a limited distance
STP			More expensive than UTP	Reduced crosstalk; more resistant to EMI than Thinnet or UTP	Difficult to work with; can cover only a limited distance
Coaxial	500 m (Thicknet) 185 m (Thinnet)	10 Mbps to 100 Mbps	Relatively inexpensive, but more costly than UTP	Less susceptible to EMI interference than other types of copper media	Difficult to work with (Thicknet); limited bandwidth; limited application (Thinnet); damage to cable can bring down entire network
Fiber-Optic	10 km and farther (single mode) 2 km and farther (multimode)	100 Mbps to 100 Gbps (single mode) 100 Mbps to 9.92 Gbps (multimode)	Expensive	Cannot be tapped, so security is better; can be used over great distances; is not susceptible to EMI; has a higher data rate than coaxial and twisted-pair cable	Difficult to terminate

Summary:

• Coaxial cable consists of a hollow outer cylindrical conductor that surrounds a single inner wire conductor.

• UTP cable is a four-pair wire medium used in a variety of networks.

• STP cable combines the techniques of shielding, cancellation, and wire twisting.

• Fiber-optic cable is a networking medium capable of conducting modulated light transmission.

• Wireless signals are electromagnetic waves that can travel through the vacuum of outer space and through a medium such as air.

Coaxial Cable

Coaxial Cable

Coaxial cable consists of a hollow outer cylindrical conductor that surrounds a single inner wire made of two conducting elements. One of these elements, located in the center of the cable, is a copper conductor. Surrounding the copper conductor is a layer of flexible insulation. Over this insulating material is a woven copper braid or metallic foil that acts both as the second wire in the circuit and as a shield for the inner conductor. This second layer, or shield, can help reduce the amount of outside interference. Covering this shield is the cable jacket (See Figure 1.1)

Figure 1.1 Coaxial Cable (

Coaxial cable supports 10 to 100 Mbps and is relatively inexpensive, although it is more costly than UTP on a per-unit length. However, coaxial cable can be cheaper for a physical bus topology because less cable will be needed. Coaxial cable can be cabled over longer distances than twisted-pair cable. For example, Ethernet can run approximately 100 meters (328 feet) using twisted-pair cabling. Using coaxial cable increases this distance to 500m (1640.4 feet). For LANs, coaxial cable offers several advantages. It can be run with fewer boosts from repeaters for longer distances between network nodes than either STP or UTP cable. Repeaters regenerate the signals in a network so that they can cover greater distances. Coaxial cable is less expensive than fiber-optic cable, and the technology is well known; it has been used for many years for all types of data communication.

When working with cable, you need to consider its size. As the thickness, or diameter, of the cable increases, so does the difficulty in working with it. Many times cable must be pulled through existing conduits and troughs that are limited in size. Coaxial cable comes in a variety of sizes. The largest diameter (1 centimeter [cm]) was specified for use as Ethernet backbone cable because historically it had greater transmission length and noise-rejection characteristics. This type of coaxial cable is frequently referred to as Thicknet. As its nickname suggests, Thicknet cable can be too rigid to install easily in some situations because of its thickness. The general rule is that the more difficult the network medium is to install, the more expensive it is to install. Coaxial cable is more expensive to install than twisted-pair cable. Thicknet cable is almost never used except for special-purpose installations. A connection device known as a vampire tap was used to connect network devices to Thicknet. The vampire tap then was connected to the computers via a more flexible cable called the attachment unit interface (AUI). Although this 15-pin cable was still thick and tricky to terminate, it was much easier to work with than Thicknet. In the past, coaxial cable with an outside diameter of only 0.35 cm (sometimes referred to as Thinnet) was used in Ethernet networks. Thinnet was especially useful for cable installations that required the cable to make many twists and turns. Because it was easier to install, it was also cheaper to install. Thus, it was sometimes referred to as Cheapernet. However, because the outer copper or metallic braid in coaxial cable comprises half the electrical circuit, special care had to be taken to ensure that it was properly grounded. Grounding was done by ensuring that a solid electrical connection existed at both ends of the cable. Frequently, however, installers failed to properly ground the cable. As a result, poor shield connection was one of the biggest sources of connection problems in the installation of coaxial cable. Connection problems resulted in electrical noise, which interfered with signal transmittal on the networking medium. For this reason, despite its small diameter, Thinnet no longer is commonly used in Ethernet networks.

The most common connectors used with Thinnet are BNC, short for British Naval Connector or Bayonet Neill Concelman, connectors (see Figure 8-5). The basic BNC connector is a male type mounted at each end of a cable. This connector has a center pin connected to the center cable conductor and a metal tube connected to the outer cable shield. A rotating ring outside the tube locks the cable to any female connector. BNC T-connectors are female devices for connecting two cables to a network interface card (NIC). A BNC barrel connector facilitates connecting two cables together.

Figure 1.2 Thinnet and BNC Connector

The following summarizes the features of coaxial cables:

• Speed and throughput—10 to 100 Mbps

• Average cost per node—Inexpensive

• Media and connector size—Medium

• Maximum cable length—500 m (medium)

Plenum Cable

Plenum cable is the cable that runs in plenum spaces of a building. In building construction, a plenum (pronounced PLEH-nuhm, from Latin meaning “full”) is a separate space provided for air circulation for heating, ventilation, and air-conditioning (sometimes referred to as HVAC), typically in the space between the structural ceiling and a drop-down ceiling. In buildings with computer installations, the plenum space often is used to house connecting communication cables. Because ordinary cable introduces a toxic hazard in the event of fire, special plenum cabling is required in plenum areas. In the United States, typical plenum cable sizes are AWG sizes 22 and 24. Plenum cabling often is made of Teflon and is more expensive than ordinary cabling. Its outer material is more resistant to flames and, when burning, produces less smoke than ordinary cabling. Both twisted pair and coaxial cable are made in plenum cable versions.

Fiber-Optic Cable

Fiber-optic cable used for networking consists of two fibers encased in separate sheaths. If you were viewing it in a cross-section, you would see that each optical fiber is surrounded by layers of protective buffer material, usually a plastic shield, then a plastic such as Kevlar, and finally an outer jacket. The outer jacket provides protection for the entire cable, while the plastic conforms to appropriate fire and building codes. The Kevlar furnishes additional cushioning and protection for the fragile, hair-thin glass fibers (see Figure 8-6). Wherever buried fiber-optic cables are required by codes, a stainless-steel wire sometimes is included for added strength.

Figure 1.3 Fiber-Optic Cable

Figure 1.4 Fiber-Optic Cable

The light-guiding parts of an optical fiber are called the core and the cladding. The core is usually very pure glass with a high index of refraction. When a cladding layer of glass or plastic with a low index of refraction surrounds the core glass, light can be trapped in the fiber core. This process is called total internal reflection. It allows the optical fiber to act like a light pipe, guiding light for tremendous distances, even around bends. Fiber-optic cable is the most expensive but it supports line speeds of more than 1 Gbps.

Two types of fiber-optic cable exist:

• Single-mode—Single-mode fiber cable allows only one mode (or wavelength) of light to propagate through the fiber. It is capable of higher bandwidth and greater distances than multimode, and it is often used for campus backbones. This type of fiber uses lasers as the light-generating method. Single-mode cable is much more expensive than multimode cable. Its maximum cable length is more than 10 km (32808.4 feet).

Figure 1.4 Fiber-Optic Cable

• Multimode—Multimode fiber cable allows multiple modes of light to propagate through the fiber. It is often used for workgroup applications and intrabuilding applications such as risers. It uses light-emitting diodes (LEDs) as a light-generating device. The maximum cable length is 2 km (6561.7 feet). The characteristics of the different transport media have a significant impact on the speed of data transfer. Fiber-optic cable is a networking medium capable of conducting modulated light transmissions. Compared to other networking media, it is more expensive. However, it is not susceptible to EMI, and it is capable of higher data rates than any of the other types of networking media discussed in this chapter. Fiber-optic cable does not carry electrical impulses as other forms of networking media that use copper wire do. Instead, signals that represent bits are converted into beams of light.

NOTE- Even though light is an electromagnetic wave, light in fibers is not considered wireless because the electromagnetic waves are guided in the optical fiber. The term wireless is reserved for radiated, or unguided, electromagnetic waves.

Fiber-optic connectors come in single-mode and multimode varieties. The greatest difference between single-mode connectors and multimode connectors is the precision in the manufacturing process. The hole in the single-mode connector is slightly smaller than in the multimode connector. This ensures tighter tolerances in the assembly of the connector. The tighter tolerances make field assembly slightly more difficult. A number of different types of fiber-optic connectors are used in the communications industry. The following list briefly describes two of the commonly used connectors:

• SC—SC type connectors feature a push-pull connect and disconnect method. To make a connection, the connector is simply pushed into the receptacle. To disconnect, the connector is simply pulled out.

• ST—ST fiber-optic connector is a bayonet type of connector. The connector is fully inserted into the receptacle and is then twisted in a clockwise direction to lock it into place.

Figure 8-7 ST Fiber-Optic Connector

The following summarizes the features of fiber-optic cables:

• Speed and throughput   —More than 1 Gbps

• Average cost per node  —Expensive

• Media and connector size—Small

• Maximum cable length   —More than 10 km for single 

                                        mode; up to 2 km for multimode

Network Media Types

Network Media Types

Network media:  In the networked world, networking media is some sort of physical cable, and electromagnetic radiation also with wireless networking. 

Network media is the actual path over which an electrical signal travels as it moves from one component to another. This chapter describes the common types of network media, including twisted-pair cable, coaxial cable, fiber-optic cable, and wireless.

Network Media Types

1. Twisted-Pair Cable
2. Coaxial Cable
3. Wireless Communication
4. Comparing Media Types

    1. Twisted-Pair Cable: Twisted-pair cable is a type of cabling that is used for telephone communications and most modern Ethernet networks. A pair of wires forms a circuit that can transmit data. The pairs are twisted to provide protection against crosstalk, the noise generated by adjacent pairs. When electrical current flows through a wire, it creates a small, circular magnetic field around the wire. When two wires in an electrical circuit are placed close together, their magnetic fields are the exact opposite of each other. Thus, the two magnetic fields cancel each other out. They also cancel out any outside magnetic fields. Twisting the wires can enhance this cancellation effect.   Using cancellation together with twisting the wires, cable designers can effectively provide self-shielding for wire pairs within the network media. Two basic types of twisted-pair cable exist: unshielded twisted pair (UTP) and shielded twisted pair (STP). The following sections discuss UTP and STP cable in more detail.

UTP Cable

UTP cable is a medium that is composed of pairs of wires. UTP cable is used in a variety of networks. Each of the eight individual copper wires in UTP cable \is covered by an insulating material. In addition, the wires in each pair are twisted around each other.

UTP cable relies solely on the cancellation effect produced by the twisted wire pairs to limit signal degradation caused by electromagnetic interference (EMI) and radio frequency interference (RFI). To further reduce crosstalk between the pairs in UTP cable, the number of twists in the wire pairs varies. UTP cable must follow precise specifications governing how many twists or braids are permitted per meter (3.28 feet) of cable.

Figure 1.1  Unshielded Twisted-Pair Cable

UTP cable often is installed using a Registered Jack 45 (RJ-45) connector. The RJ-45 is an eight-wire connector used commonly to connect computers onto a local-area network (LAN), especially Ethernets.

When used as a networking medium, UTP cable has four pairs of either 22- or 24-gauge copper wire. UTP used as a networking medium has an impedance of 100 ohms; this differentiates it from other types of twisted-pair wiring such as that used for telephone wiring, which has impedance of 600 ohms.

UTP cable offers many advantages. Because UTP has an external diameter of approximately 0.43 cm (0.17 inches), its small size can be advantageous during installation. 

Figure 1.2  RJ-45 Connectors

Because it has such a small external diameter, UTP does not fill up wiring ducts as rapidly as other types of cable. This can be an extremely important factor to consider, particularly when installing a network in an older building. UTP cable is easy to install and is less expensive than other types of networking media. In fact, UTP costs less per meter than any other type of LAN cabling. And because UTP can be used with most of the major networking architectures, it continues to grow in popularity.

Disadvantages also are involved in using twisted-pair cabling, however. UTP cable is more prone to electrical noise and interference than other types of networking media, and the distance between signal boosts is shorter for UTP than it is for coaxial and fiber-optic cables.

Although UTP was once considered to be slower at transmitting data than other types of cable, this is no longer true. In fact, UTP is considered the fastest copper-based medium today. The following summarizes the features of UTP cable:

• Speed and throughput—10 to 1000 Mbps
• Average cost per node—Least expensive
• Media and connector size—Small
• Maximum cable length—100 m (short)

Commonly used types of UTP cabling are as follows:

• Category 1—Used for telephone communications. Not suitable for transmitting data.
• Category 2—Capable of transmitting data at speeds up to 4 megabits per second (Mbps).
• Category 3—Used in 10BASE-T networks. Can transmit data at speeds up to 10 Mbps.
• Category 4—Used in Token Ring networks. Can transmit data at speeds up to 16 Mbps.
• Category 5—Can transmit data at speeds up to 100 Mbps.
• Category 5e —Used in networks running at speeds up to 1000 Mbps (1 gigabit per second [Gbps]).
• Category 6—Typically, Category 6 cable consists of four pairs of 24 American Wire Gauge (AWG) copper wires. Category 6 cable is currently the fastest standard for UTP.

Shielded Twisted-Pair Cable

Shielded twisted-pair (STP) cable combines the techniques of shielding, cancellation, and wire twisting. Each pair of wires is wrapped in a metallic foil. The four pairs of wires then are wrapped in an overall metallic braid or foil, usually 150-ohm cable. As specified for use in Ethernet network installations, STP reduces electrical noise both within the cable (pair-to-pair coupling, or crosstalk) and from outside the cable (EMI and RFI). STP usually is installed with STP data connector, which is created especially for the STP cable. However, STP cabling also can use the same RJ connectors that UTP uses.

Although STP prevents interference better than UTP, it is more expensive and difficult to install. In addition, the metallic shielding must be grounded at both ends. If it is improperly grounded, the shield acts like an antenna and picks up unwanted signals. Because of its cost and difficulty with termination, STP is rarely used in Ethernet networks. STP is primarily used in Europe.

Figure 1.3 Shielded Twisted-Pair Cable

The following summarizes the features of STP cable:

• Speed and throughput—10 to 100 Mbps
• Average cost per node—Moderately expensive
• Media and connector size—Medium to large
• Maximum cable length—100 m (short)

When comparing UTP and STP, keep the following points in mind:

• The speed of both types of cable is usually satisfactory for local-area distances.
• These are the least-expensive media for data communication. UTP is less expensive than STP.
• Because most buildings are already wired with UTP, many transmission standards are adapted to use it, to avoid costly rewiring with an alternative cable type.

Network Devices

I'm posting another discussion of network devices. This will may make sense all about network devices

Network Devices

Computer network devices also known as communication devices and they constitute a data communication network. These devices are routers, switches, hubs, LAN cards, gateway, modems, hardware firewall, CSU/DSU, ISDN terminals and transceivers.  In an Ethernet or WAN network, the data communication cannot be performed without these devices.  We must have the good understanding of these devices. Much of the information will serve as a review if we have studied the CompTIA Network+.

Video: Networking Basics Introduction

1.1 Network Interface:

Network interfaces connect clients, servers, and peripherals to the network. Most network interfaces consist of a small circuit board that you insert into one of your computer's internal slots. Alternatively, modern computers sometimes include the network interface as part of their main circuit boards (motherboards). Each network interface is associated with a unique address called its media access control (MAC) address. The MAC address helps route information within your local area network and is used by switches and bridges. The MAC address is just one of several network addresses assigned to each networked client, server, or peripheral. Another network address is the device’s Internet, or TCP/IP, address. This address helps route information between networks. Every networked device maintains multiple, simultaneous network addresses which are used for different purposes.

Practical advice

• Make sure that the network interfaces on all computers are compatible with the physical and data link protocol you have chosen. For example, if you are running a 10BaseT Ethernet network, then all network interfaces must also use this protocol.

• Make sure that the network interface is compatible with the slot into which it will be inserted. Slots provide places on your computer's main circuit board (motherboard) where you can insert daughter circuit boards that add functionality to your computer (for example, network interfaces, modems, and so forth). Common slot types include PCI (Peripheral Component Interconnect), ISA (Industry Standard Architecture), EISA (Extended Industry Standard Architecture), among others. Each slot type specifies the speed, number of data bits used in the signal, and the number and position of wires on the motherboard used for communication inside the computer. PCI is the newest and fastest of the slots, although EISA and ISA slots are sufficient for most common network interface cards such as those for 10BaseT Ethernet. Most computers include slots of several different types. Before you order a network interface, check your computer to determine which slots are available, and then check your motherboard manual to ascertain the slot type. Order a card appropriate for your slot.

• Purchase network interfaces from a known manufacturer whose support you trust. Make sure the manufacturer provides a competitive warranty.

• Macintosh computers usually come with network interfaces as part of their main circuit boards. Some Windows PCs, however, still require that you purchase a network interface (for new PCs, your vendor may install the interface for you).

Common Network Devices

1.2 Hubs

A hub connects individual devices on an Ethernet network so that they can communicate with one another. The hub operates by gathering the signals from individual network devices, optionally amplifying the signals, and then sending them onto all other connected devices. You should use a hub or a switch on your Ethernet network if the network includes more than two clients, servers, or peripherals. You can see a diagram of a network containing hub.

Figures #1.2:  HUB diagram network

Video: Understanding HUB

While connect dozens of clients, peripherals, and servers via hubs, network performance may degrade if too many devices try to communicate within one area of the network. To improve performance by adding switches, bridges, or routers to the network. Each switch port, bridge port, or router port regulates traffic so that devices on the port are protected from the interfering signals of devices on other ports. Most hubs operate by examining incoming or outgoing signals for information at OSI level 1, the physical level.

Practical advice

• Like network interfaces, your hubs must be compatible with your physical and data link level protocols. If you are running a 10BaseT Ethernet network, then you must purchase 10BaseT hubs. Some hubs, called multiprotocol hubs, can accommodate more than one physical and data link level protocol. For example, modern hubs may accommodate both 10BaseT and 100BaseTX protocols.

• If you purchase a multiprotocol hub, then make sure that it automatically senses which protocol is being used on each port. Autosensing hubs ensure that you can connect any part of the network to any hub port. 

• Make sure that your hub includes an AUI port (connector). (AUI is an abbreviation for attachment unit interface.) AUI ports are intended to connect with a kind of cabling called thick coaxial cable (like that used for cable TV). While this cable is no longer used frequently for Ethernet networks, AUI ports are versatile in the sense that they can be fitted with adapters to connect to many different kinds of cable (for example, thin coaxial cable or fiber).

• Make sure that your hub includes a crossover port. Unlike regular hub ports, which connect hubs to clients, servers, or peripherals, a crossover port connects one hub to another. In order to understand this distinction, you must consider how network devices use the Ethernet cable to send and receive information. All devices on 10BaseT or 100BaseTX Ethernet networks send their information over one particular pair of wires within the cable. This particular pair of wires is called the transmit pair. Similarly, all devices receive information from a different pair of wires, called the receive pair. The location of each pair of wires within the cable is specified by the wiring standard—for example, T568B—that was selected when your network was installed. All devices on your network conform to the same standard. When regular ports on hubs receive incoming information, they transfer it from the transmit pair of the sending device to the receive pair of the destination device. Crossover ports work in a different manner than regular ports. When crossover ports on hubs receive information, they simply pass it on without transferring it between transmit and receive pairs. By refraining from any change of pairs, crossover ports ensure that the next hub on the connection receives the original information intact.

• Some hubs can be stacked. Stackable hubs look like one, giant hub to the network. That is to say, the Ethernet restriction on the number of hubs that can be traversed in a single network does not apply to stacked hubs.

• Purchase hubs from a known manufacturer whose support you trust. Make sure the manufacturer provides a competitive warranty. 

• Install your hubs in a room that is cool and free of dust, if possible. Additionally, plug your hubs into an uninterrupted power supply (UPS) to ensure that they receive clean power. 

1.3 Switches

Like a hub, a switch is a device that connects individual devices on an Ethernet network so that they can communicate with one another. But a switch also has an additional capability; it momentarily connects the sending and receiving devices so that they can use the entire bandwidth of the network without interference. If you use switches properly, they can improve the performance of your network by reducing network interference.

Switches have two benefits: 

• Switches provide each pair of communicating devices with a fast connection; and 

• Switches segregate the communication so that it does not enter other portions of the network. (Hubs, in contrast, broadcast all data on the network to every other device on the network.) 

These benefits are particularly useful if your network is congested and traffic pools in particular areas. However, if your network is not congested or if your traffic patterns do not create pools of local traffic, then switches may cause your network performance to deteriorate. This performance degradation occurs because switches examine the information inside each signal on your network (to determine the addresses of the sender and receiver) and therefore process network information more slowly than hubs (which do not examine the signal contents). Most switches operate by examining incoming or outgoing signals for information at OSI level 2, the data link level.

Figure #1.3: Switch diagram network

Figure #1.3.1: Cisco 2960 Switch

Figure #1.3.2: Switch Interface Types

Switches, however, are more powerful than hubs and can substantially increase network performance. In order to understand how they perform this magic, it is necessary to understand first how they work. Most common switches operate by learning the MAC addresses of all connected clients, servers, and peripherals, and associating each address with one of its ports. When a switch receives an incoming signal, it creates a temporary circuit between the sender and receiver. The temporary circuit provides two important benefits.

• First, the circuit allows the sender and receiver momentarily to exchange information without intrusion from other devices on the network. That is, each pair of communicating devices utilizes the full bandwidth (data carrying capacity) of the network instead of sharing that bandwidth, as they do in unswitched Ethernet networks. To say this another way, each switch port defines a collision domain containing only a small number of devices and thereby helps provide maximum performance for Ethernet networks.

• Second, the circuit ensures that information travels directly between the communicating computers. This behavior differs markedly from un-switched Ethernet networks. In unswitched networks, data from a transmitting computer is sent by the nearest hub to all connected devices (not just to the recipient) and therefore congests parts of the network needlessly.

Like all network equipment, switches benefit your network only if they are deployed in the proper manner. If your network is congested and if traffic pools in certain areas, then you can improve network performance by replacing hubs with switches, or by connecting hubs to switches in a hierarchical manner. 

Practical advice

• Your switches must be compatible with your physical and data link level protocols. If you are running a 10BaseT Ethernet network, then you must purchase a 10BaseT switch.

• Some switches can accommodate more than one physical or data link level protocol. For example, modern switches accommodate both 10BaseT and 100BaseTX protocols. It is wise to purchase a switch with at least one 100BaseTX port, since you can interconnect your switches via their high speed ports to improve network performance even if the remainder of your network uses 10BaseT.

• If you purchase a switch that accommodates more than one protocol, then make sure that it automatically senses which protocol is being used on each port. Autosensing switches ensure that you can connect any part of the network to any switch port. Older switches required that you attach each segment of the network to a port compatible with its physical and data link level protocol. Keeping the segments and ports straight presents a management headache.

• Purchase switches from a known manufacturer whose support you trust. Make sure the manufacturer provides a competitive warranty.

• Install your switches in a room that is cool and free of dust, if possible. Additionally, plug your switches into an uninterrupted power supply (UPS) to ensure that they receive clean power.

1.4 Bridges

A bridge is a device that connects two or more local area networks, or two or more segments of the same network. For example, suppose that your network includes both 10BaseT Ethernet and LocalTalk connections. You can use a bridge to connect these two networks so that they can share information with each other. In addition to connecting networks, bridges perform an additional, important function. They filter information so that network traffic intended for one portion of the network does not congest the rest of the network. (You may remember from the previous section that switches also perform

Like switches, bridges learn the MAC addresses of all connected clients, servers, and peripherals, and associate each address with a bridge port (network connection). When a bridge (or switch) receives an incoming frame, it opens and reads its destination MAC address. If the port that will receive the frame is different from the port connected to the sender, then the bridge forwards the frame to the destination port. If the port that will receive the frame is the same as the port connected to the sender, the bridge drops the frame. (Since the bridge is by definition at the end of the network segment, the receiving computer presumably intercepted a copy of the frame on its way to the bridge.) If the bridge cannot determine which port is associated with a destination address, it passes the frame along to all ports.

Bridges are relatively simple and efficient traffic regulators. However, in some networks they have been replaced by their more powerful cousins—hubs, switches, and routers. Each of these traffic regulators brings a unique set of strengths and weaknesses to its work:

• Hubs, switches, bridges, and routers can interconnect two different kinds of networks such as 10BaseT Ethernet and 100BaseTX.

• Hubs (unlike switches, bridges, and routers) do not filter traffic between the two networks.

• Switches have the unique capability to enable communicating devices momentarily to utilize the full bandwidth (data carrying capacity) of the network.

• However, switches (and hubs) cannot accommodate the variety of protocols and cabling types that bridges can.

• Routers are much more expensive and much more difficult to install and manage than hubs, switches, or bridges, but they can filter and route information much more precisely.

Practical advice

• Before you decide on your purchase, take a moment to clarify what you wish to achieve . Then work with your technical staff, or with manufacturers and consultants, to determine your options. You can often use a hub, switch, or router in the same places that you can use a bridge. Each device brings its unique set of strengths and weaknesses to the job.

• Make sure that the bridge is compatible with your physical and data link protocols.

• Purchase bridges from a known manufacturer whose support you trust. Make sure the manufacturer provides a competitive warranty.

• Install your bridges in a room that is cool and free of dust, if possible. Additionally, plug your bridges into an uninterrupted power supply (UPS) to ensure that they receive clean power.

1.5 Routers

Like bridges, routers connect two or more networks. However, routers are much more powerful than bridges. Routers can filter traffic so that only authorized personnel can enter restricted areas. They can permit or deny network communications with a particular Web site. They can recommend the best route for information to travel. As network traffic changes during the day, routers can redirect information to take less congested routes.

If your school is connected to the Internet, then you will most likely use a router to make that connection. Routers ensure that your local area network traffic remains local, while passing onto the Internet all your electronic mail, Web surfing connections, and other requests for Internet resources.

Figure #1.5: Modular Cisco Router with a Blank Slot to the Right

Routers are generally expensive to purchase and difficult to configure and maintain. Be sure that your staff have the resources necessary to manage them well. Routers quickly become critical components of your network. If they fail, your network services will be significantly impaired. As part of your network plan, you should consider how you might

deal with the failure of key routers on your network. Many sites include redundant connections—additional routers and network cable connections—configured to take over if one router or connection fails. Most routers operate by examining incoming or outgoing signals for information at OSI level 3, the network addressing level.

Routers operate primarily by examining incoming data for its network routing and transport information—for example, information carried within the TCP/IP, IPX/SPX, or AppleTalk portions of the network signal. This information includes the source and destination network routing addresses. (Remember that every client, server, and peripheral on the network maintains multiple addresses, including both a data link and network routing addresses. Among other things, the network routing address provides information on which routers base traffic management decisions.) However, most routers also include the same functionality as bridges. That is, they can inspect the data link level portions of the network signals for such information as the Ethernet or LocalTalk destination address. 

Based on complex, internal tables of network information that it compiles, a router then determines whether or not it knows how to forward the data packet towards its destination. If the router has been configured with sufficient information to know which of its ports is en route to the destination, it transmits the packet. If the router has not been so configured, it typically drops the packet. Dropping unknown packets provides an important service to your network by eliminating restricted, wayward, or damaged information from your network. Bridges lack this capability they forward unknown packets to all ports and the misinformation they forward often creates extra network traffic. 

Routers can be programmed to prevent information from being sent to or received from certain networks or computers based on all or part of their network routing addresses. If you have sensitive student records on a server, for example, you can use a router to filter packets headed for the server so that only authorized personnel—for example, personnel whose network addresses match a specified list—can connect to it.

Since routers play a key role in connecting networks, they can cause significant problems if they malfunction. As part of your network plan, you should consider how you might deal with the failure of key routers on your network. Many sites include redundant connections—additional routers and network cable connections—configured to take over if one router or connection fails.  Because routers depend upon network routing addresses, we say in the parlance of the OSI model that they are level 3 devices level 3 manages routing information between different networks.

Practical advice

• It is best to purchase all routers from a single manufacturer. Purchasing routers from a single manufacturer ensures that the software you use to configure and manage the routers via the network will be compatible across devices. Additionally, your staff will find it easier to learn about and operate devices that are relatively uniform because they come from a single source. Make sure that your router manufacturer offers a wide variety of routers, including models for local area networks, dial-up connections, and wide area networks so that you can continue to purchase from the same manufacturer as your network grows. Consult with other educators to see which router manufacturers they have used and liked.

• Before you purchase a router, you should draw a picture of your network, including the place where intend to put your router. Then label the segments on either side of the router with the kind of cable used as well as with the protocols that will travel across the router to/from each segment. Your router must accommodate the cable types on all adjacent segments. In addition, the router must be compatible with protocols that appear on both sides of the router.

• Work with your router manufacturer or network integrator to choose the router model(s) that you need. Be sure that you can describe to your manufacturer/integrator not only the protocols in use, but also the kind of information that will be exchanged on the attached network, the kinds of information that may be restricted, the number of users, and their patterns of usage. You must match your router's capabilities to your particular network needs.

• Routers are often expensive. Your router should be easily upgraded so that you need not replace the entire device as your network incorporates additional kinds of cable or protocols. Ask your manufacturer about the particular expansion modules they offer, and what is involved in purchasing, installing, and maintaining them.

• Some managers plan to deliver multimedia applications over the Internet. These applications require a fast, steady stream of data to function properly. To deliver this increased performance, Internet standards organizations have defined options that allow routers and other network devices to reserve the bandwidth they need on the Internet. Such equipment is assures quality of service, or QoS, for specified purposes. Not all routers are capable of providing QoS services. If you are planning for multimedia delivery over the Internet, you may wish to make sure that your router does so.

• Price should not be the determining factor in purchasing a router. Routers, like servers, are key components of your network. It is far better to purchase durable equipment from premium manufacturers than to suffer equipment breakdowns or malfunctions.

1.6 Firewalls and proxy servers

A firewall is a device that prevents unauthorized electronic access to your entire network. The term firewall is generic, and includes many different kinds of protective hardware and software devices. Routers, discussed in the previous section, comprise one kind of firewall. Most firewalls operate by examining incoming or outgoing packets for information at OSI level 3, the network addressing level.

Firewalls can be divided into 3 general categories: packet-screening firewalls, proxy servers (or application-level gateways), and stateful inspection proxies.

Packet-screening firewalls examine incoming and outgoing packets for their network address information. You can use packet-screening firewalls to restrict access to specific Web sites, or to permit access to your network only from specific Internet sites.

Figure #1.6: Firewall

Proxy servers (also called application-level gateways) operate by examining incoming or

outgoing packets not only for their source or destination addresses but also for information carried within the data area of each network packet. The data area contains information written by the application program that created the packet—for example, your Web browser, FTP, or TELNET program. Because the proxy server knows how to examine this application-specific portion of the packet, you can permit or restrict the behavior of individual programs.

Stateful inspection proxies monitor network signals to ensure that they are part of a legitimate ongoing conversation (rather than malicious insertions).

Besides firewalls, other types of security software may also be useful. For example, intrusion detection software monitors your network for particular kinds of malicious activity (attempts to steal passwords, for example). Filtering software maintains lists of Web sites that are permitted or restricted for students, and enforces those restrictions.

Many schools combine one or more of these solutions to create their network security system. Each solution has strengths and weaknesses. In order to choose a solution, you should begin by defining your security policy (the resources you wish to share or restrict, and the personnel who will have access to each resource). Then work with your manufacturer to ensure that your security

solution meets your needs.

Practical advice

Firewalls (packet-screening, proxy, and stateful inspection) provide logs of traffic which you should monitor frequently. Logs indicate the people and resources that are active on your network. Also, the firewall should contain two network interfaces—one connected to the outside world and the other connected to your private network; the firewall operates by controlling the flow of information between the two.

Firewalls provide logs of traffic which you should monitor frequently. Logs indicate the people and resources that are active on your network. Also, the firewall should contain two network interfaces—one connected to the outside world and the other connected to your private network; the firewall operates by controlling the flow of information between the two.

When you add a firewall to your network, you must situate the firewall equipment so that it is the single point of access to all those resources on your network that you consider private. To visualize such a configuration, you can examine a picture of typical firewall and network at

When the firewall forms a single point of access, it can review all inbound network traffic to determine whether it should reach private data, and it can review all outbound traffic to determine whether it is bound for an acceptable destination.

Computers that contain public information, such as Web servers, are not usually protected by firewalls. While it is relatively straightforward to define which outsiders should access your private information and to configure your firewall appropriately, it is very hard to define which outsiders should be excluded from your public information. In order to protect Web servers, you generally approach the problem from an entirely different point of view. You try to ensure that no one can add information to your server unless they have specific privilege to do so, and that malicious users cannot disrupt its activities. To enforce these restrictions, you configure the Web server operating system,

Web server software, and associated software. These topics are beyond the scope of this Primer, but you can find an excellent discussion at the World Wide Web Consortium,

You should avoid mixing security products from different manufacturers. Incompatibilities among equipment can cause unnecessary work and security risks. Security is a very complex topic, and you must understand the possible solutions in order to make a good selection of hardware and software. There are many possible types of equipment, network configurations, and manufacturers. Take your time and research the area thoroughly before you purchase. 

Video: Common Network Components