Network Devices

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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