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.
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.
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#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
Figure#8: Wireless Network Card
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.
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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