Common Network

Common Network Devices

 Introduction

Computer networking devices are units that mediate data in a computer network and are also called network equipment. Units which are the last receiver or generate data are called hosts or data terminal equipment. All but the most basic of networks require devices to provide connectivity and functionality. Understanding how these networking devices operate and identifying the functions they perform are essential skills for any network administrator and requirements for a Network. This chapter introduces commonly used networking devices, and, although it is true that you are not likely to encounter all of the devices mentioned.

Common Network Devices

ü  Hubs

ü  Switches

ü  Bridges

ü  Routers

ü  Gateways

ü  CSU/DSU (Channel Service Unit/Data Service Unit)

ü  NICs (Network Interface Card)

ü  ISDN (Integrated Services Digital Network) adapters

ü  WAPs (Wireless Access Point)

ü  Modems

ü  Transceivers (media converters)

ü  Firewalls

HUBS:

At the bottom of the networking food chain, so to speak, are hubs. Hubs are used in networks that use twisted-pair cabling to connect devices. Hubs can also be joined together to create larger networks. Hubs are simple devices that direct data packets to all devices connected to the hub, regardless of whether the data package is destined for the device. This makes them inefficient devices and can create a performance bottleneck on busy networks. In its most basic form, a hub does nothing except provide a pathway for the electrical signals to travel along. Such a device is called a passive hub.

Far more common nowadays is an active hub, which, as well as providing a path for the data signals, regenerates the signal before it forwards it to all of the connected devices. A hub does not perform any processing on the data that it forwards, nor does it perform any error checking. Hubs come in a variety of shapes and sizes.Hubs don’t perform any processing, they do little except enable communication between connected devices. For today’s high-demand network applications switches come in.

SWITCHES:

Like hubs, switches are the connectivity points of an Ethernet network. Devices connect to switches via twisted-pair cabling, one cable for each device. The difference between hubs and switches is in how the devices deal with the data that they receive. Whereas a hub forwards the data it receives to all of the ports on the device, a switch forwards it only to the port that connects to the destination device. It does this by learning the MAC address of the devices attached to it, and then by matching the destination MAC address in the data it receives. 

Figure#1 – Cisco 2960 Switch

By forwarding data only to the connection that should receive it, the switch can improve network performance in two ways. 

First by creating a direct path between two devices and controlling their communication, it can greatly reduce the number of collisions on the network. Collisions occur on Ethernet networks when two devices attempt to transmit at exactly the same time. In addition, the lack of collisions enables switches to communicate with devices in full-duplex mode. In a full-duplex configuration, devices can send and receive data from the switch at the same time. Contrast this with half-duplex communication, in which communication can occur in only one direction at a time. The net result of these measures is that switches can offer significant performance improvements over hub-based networks, particularly when network use is high.

HUB & SWITCH Cabling

In addition to acting as a connection point for network devices, hubs and switches can also be connected to create larger networks. This connection can be achieved through standard ports with a special cable or by using special ports with a standard cable. The ports on a hub to which computer systems are attached are called Medium Dependent Interface-Crossed (MDI-X). The crossed designation is derived from the fact that two of the wires within the connection are crossed so that the send signal wire on one device becomes the receive signal of the other. Because the ports are crossed internally, a standard or straight-through cable can be used to connect devices. Another type of port, called a Medium Dependent Interface (MDI) port, is often included on a hub or switch to facilitate the connection of two switches or hubs. Because the hubs or switches are designed to see each other as simply an extension of the network, there is no need for the signal to be crossed. If a hub or switch does not have an MDI port, hubs or switches can be connected by using a crossover cable between two MDI-X ports. The crossover cable serves to uncross the internal crossing. You can see diagrams of the cable pinouts for both a straight-through and crossover cable.

Figure#2: Straight Through Cable

Figure#3: Crossover Cable

Figure#4: Straight Through & Crossover Cable

BRIDGES:

Bridges are used to divide larger networks into smaller sections. They do this by sitting between two physical network segments and managing the flow of data between the two. By looking at the MAC address of the devices connected to each segment, bridges can elect to forward the data (if they believe that the destination address is on another interface), or block it from crossing (if they can verify that it is on the interface from which it came). Figure-5 shows how a bridge can be used to segregate a network.

 Figure#5: Bridge Network

When bridges were introduced, the MAC addresses of the devices on the connected networks had to be entered manually, a time-consuming process that had plenty of opportunity for error. Today, almost all bridges can build a list of the MAC addresses on an interface by watching the traffic on the network. Such devices are called learning bridges because of this functionality.

Routers

In a common configuration, routers are used to create larger networks by joining two network segments. CISCO router used to connect user to the Internet. A router can be a dedicated hardware device or a computer system with more than one network interface and the appropriate routing software. All modern network operating systems include the functionality to act as a router.

Figure#6: CISCO Router

A router derives its name from the fact that it can route data it receives from one network onto another. When a router receives a packet of data, it reads the header of the packet to determine the destination address. Once it has determined the address, it looks in its routing table to determine whether it knows how to reach the destination and, if it does, it forwards the packet to the next hop on the route. The next hop might be the final destination, or it might be another router.

Gateways

Any device that translates one data format to another is called a gateway. Some examples of gateways include a router that translates data from one network protocol to another, a bridge that converts between two networking systems, and a software application that converts between two dissimilar formats. The key point about a gateway is that only the data format is translated, not the data itself. In many cases, the gateway functionality is incorporated into another device.

Network Cards (NIC)

Network cards, also called Network Interface Cards, are devices that enable computers to connect to the network. When specifying or installing a NIC, you must consider the following issues:

Figure#7: Network Card

➤ System bus compatibility—If the network interface you are installing is an internal device, bus compatibility must be verified. The most common bus system in use is the Peripheral Component Interconnect (PCI) bus, but some older systems might still use Industry Standard Architecture (ISA) expansion cards.

➤ System resources—Network cards, like other devices, need IRQ and memory I/O addresses. If the network card does not operate correctly after installation, there might be a device conflict.

➤ Media compatibility—Today, the assumption is that networks use twisted-pair cabling, so if you need a card for coaxial or fiber-optic connections, you must specify this. Wireless network cards are also available. Even more than the assumption you are using twisted-pair cabling is that the networking system being used is Ethernet. If you require a card for another networking system such as Token Ring, this must be specified when you order.

Figure#8: Wireless Network Card

To install or configure a network interface, you will need drivers of the device, and might need to configure it, although many devices are now plug and play. Most network cards are now software configured. Many of these software configuration utilities also include testing capabilities. The drivers and software configuration utilities supplied with the cards are often not the latest available, so it is best practice to log on to the Internet and download the latest drivers and associated software.

Wireless Access Points

Wireless access points (APs) are a transmitter and receiver (transceiver) device used to create a wirelessLAN (WLAN). APs are typically a separate network device with a built-in antenna, transmitter, and adapter. APs use the wireless infrastructure network mode to provide a connection point between WLANs and a wired Ethernet LAN. APs also typically have several ports allowing a way to expand the network to support additional clients.

Figure#9: CISCO Wireless Access Points

Depending on the size of the network, one or more APs might be required. Additional APs are used to allow access to more wireless clients and to expand the range of the wireless network. Each AP is limited by a transmissions range—the distance a client can be from an AP and still get a usable signal. The actual distance depends on the wireless standard being used and the obstructions and environmental conditions between the client and the AP.

Saying that an AP is used to extend a wired LAN to wireless clients doesn’t give you the complete picture. A wireless AP today can provide different services in addition to just an access point. Today, the APs might provide many ports that can be used to easily increase the size of the network. Systems can be added and removed from the network with no affect on other systems on the network. Also, many APs provide firewall capabilities and DHCP service. When they are hooked up, they will provide client systems with a private IP address and then prevent Internet traffic from accessing client systems. So in effect, the AP is a switch, a DHCP Server, router, and a firewall. APs come in all different shapes and sizes. Many are cheaper and designed strictly for home or small office use. Such APs have low powered antennas and limited expansion ports. Higher end APs used for commercial purposes have very high powered antennas enabling them to extend the range that the wireless signal can travel.

Modems

A modem, short for modulator/demodulator, is a device that converts the digital signals generated by a computer into analog signals that can travel over conventional phone lines. The modem at the receiving end converts the signal back into a format the computer can understand. Modems can be used as a means to connect to an ISP or as a mechanism for dialing up to a LAN. Modems can be internal add-in expansion cards, external devices that connect to the serial or USB port of a system, PCMCIA cards designed for use in laptops, or proprietary devices designed for use on other devices such as portables and handhelds. The configuration of a modem depends on whether it is an internal or external device. For internal devices, the modem must be configured with an interrupt request (IRQ) and a memory I/O address. It is common practice, when installing an internal modem, to disable the built-in serial interfaces and assign the modem the resources of one of those (typically COM2). Table-1 shows the resources associated with serial (COM) port assignments.

Table#1: Common Serial (COM) Port Resource Assignments

Port ID	IRQ	I/O	Address Associated Serial I/F Number
COM1	4	03F8	1
COM2	3	02F8	2
COM3	4	03E8	1
COM4	3	02E8	2

Table#1

For external modems, you need not concern yourself directly with these port assignments, as the modem connects to the serial port and uses the resources assigned to it. This is a much more straightforward approach and one favored by those who work with modems on a regular basis. For PCMCIA and USB modems, the plug-and-play nature of these devices makes them simple to configure, and no manual resource assignment is required. Once the modem is installed and recognized by the system, drivers must be configured to enable use of the device. Two factors directly affect the speed of the modem connection—the speed of the modem itself and the speed of the Universal Asynchronous Receiver/Transmitter (UART) chip in the computer that is connected to the modem. The UART chip controls the serial communication of a computer, and although modern systems have UART chips that can accommodate far greater speeds than the modem is capable of, older systems should be checked to make sure that the UART chip is of sufficient speed to support the modem speed. The UART chip installed in the system can normally be determined by looking at the documentation that comes with the system. Table 2 shows the maximum speed of the commonly used UART chip types.

Table#2: UART Chip Speeds
UART Chip	Speed (Kbps)
8250	9600
16450	9600
16550	115,200
16650	430,800
16750	921,600
16950	921,600

Table#2: UART Chip Speeds

Transceivers (Media Converters)

The term transceiver does describe a separate network device, but it can also be technology built and embedded in devices such as network cards and modems. In a network environment, a transceiver gets its name from being both a transmitter and a receiver of signals—thus the name transceivers. Technically, on a LAN, the transceiver is responsible for placing signals onto the network media and also detecting incoming signals traveling through the same wire. Given the description of the function of a transceiver, it makes sense that that technology would be found with network cards. Although transceivers are found in network cards, they can be external devices as well. As far as networking is concerned, transceivers can ship as a module or chip type. Chip transceivers are small and are inserted into a system board or wired directly on a circuit board. Module transceivers are external to the network and are installed and function similarly to other computer peripherals, or they can function as standalone devices. There are many types of transceivers—RF transceivers, fiber optic transceivers, Ethernet transceivers, wireless (WAP) transceivers, and more. Though each of these media types is different, the function of the transceiver remains the same. Each type of the transceiver used has different characteristics, such as the number of ports available to connect to the network and whether full-duplex communication is supported.

Listed with transceivers in the CompTIA objectives are media converters. Media converters are a technology that allows administrators to interconnect different media types—for example, twisted pair, fiber, and Thin or thick coax—within an existing network. Using a media converter, it is possible to connect newer 100Mbps, Gigabit Ethernet, or ATM equipment to existing networks such as 10BASE-T or 100BASE-T. They can also be used in pairs to insert a fiber segment into copper networks to increase cabling distances and enhance immunity to electromagnetic interference (EMI).

Firewalls

A firewall is a networking device, either hardware or software based, that controls access to your organization’s network. This controlled access is designed to protect data and resources from an outside threat. To do this, firewalls are typically placed at entry/exit points of a network—for example, placing a firewall between an internal network and the Internet. Once there, it can control access in and out of that point.

Figure#8: Firewall

Although firewalls typically protect internal networks from public networks, they are also used to control access between specific network segments within a network—for example, placing a firewall between the Accounts and the Sales departments. As mentioned, firewalls can be implemented through software or through a dedicated hardware device. Organizations implement software firewalls through network operating systems (NOS) such as Linux/UNIX, Windows servers, and Mac OS servers. The firewall is configured on the server to allow or permit certain types of network traffic. In small offices and for regular home use, a firewall is commonly installed on the local system and configured

to control traffic. Many third-party firewalls are available. Hardware firewalls are used in networks of all sizes today. Hardware firewalls are often dedicated network devices that can be implemented with very little configuration and protect all systems behind the firewall from outside sources. Hardware firewalls are readily available and often combined with other devices today. For example, many broadband routers and wireless access points have firewall functionality built in. In such case, the router or WAP might have a number of ports available to plug systems in to.

MAC Addresses

A MAC address is a unique 6-byte address that is burned into each network interface or more specifically, directly into the PROM chip on the NIC. The number must be unique, as the MAC address is the basis by which almost all network communication takes place. No matter which networking protocol is being used, the MAC address is still the means by which the network interface is identified on the network. Notice that I say network interface. That’s very important, as a system that has more than one network card in it will have more than one MAC address. MAC addresses are expressed in six hexadecimal values. In some instances, the six values are separated by colons (:); in others, hyphens (-) are used; and in still others, a space is simply inserted between the values. In any case, because the six values are hexadecimal, they can only be numbers 0–9 and the letters A–F. So, a valid MAC address might be 00-D0-56-F2-B5-12 or 00-26-DD-14-C4-EE. 

There is a way of finding out whether a MAC address exists through the IEEE, which is responsible for managing MAC address assignment. The IEEE has a system in place that lets you identify the manufacturer of the network interface by looking at the MAC address. For example, in the MAC address 00-80-C8-E3-4C-BD, the 00-80-C8 portion identifies the manufacturer and the E3-4C-BD portion is assigned by the manufacturer to make the address unique. The IEEE is the body that assigns manufacturers their IDs, called Organizationally Unique Identifiers, and the manufacturer then assigns the second half, called the Universal LAN MAC address. From the IEEE’s perspective, leaving the actual assignment of The method by which you can discover the MAC address of the network interfaces in your equipment depends on which operating system is being used. Table 3.5 shows you how to obtain the MAC address on some of the more common platforms.

Table 3.4 Network Devices Summary (continued)

Table#3 Commands to Obtain MAC Addresses

Platform	Method
Windows 95/98/Me	Run the winipcfg utility.
Windows NT/2000	Run ipconfig /all from a command prompt.
Linux/Some UNIX	Run the ifconfig -a command.
Novell NetWare	Run the config command.
Cisco Router	Run the sh int <interface name> command.

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