network devices notes

8/10/2019 Network Devices Notes

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NETWORK DEVICES

Introduction:

A network device possesses special communications hardware to make the necessary physical

connections to other devices. Most modern network devices have communication electronics

integrated onto their circuit boards.

Devices on computer networks function in different roles. The two most common roles are

clientsand servers. Examples of network clients include PCs, phones and tablets, and network

printers. Clients generally make request and consume data stored in network servers , devices

generally designed with large amounts of memory and/or disk storage and high performance

processors to better support clients. Examples of network servers include Web serversand game

servers. Networks naturally tend to support many more clients than servers. Both clients and

servers are sometimes called network nodes.

Network devices may also be capable of functioning as both clients and servers. In apeer to peer

networking,for example, pairs of devices share files or other data with each other, one acting as

a server hosting some data while simultaneously working as a client to request different data

from other peer devices.

Repeater:

A repeater connects two segments of your network cable. It regenerates the signals to proper

amplitudes and sends them to the other segments.In the ethernet topology, we are probably

using a hub as a repeater. Repeaters require a small amount of time to regenerate the signal. This

can cause a propagation delay which can affect network communication when there are several

repeaters in a row. Many network architectures limit the number of repeaters that can be used in

a row. Repeaters work only at the physical layer of the OSI network model.

Network repeaters regenerate incoming electrical, wireless or optical signals. With physical

media like Ethernet or Wi-Fi, data transmissions can only span a limited distance before the

quality of the signal degrades. Repeaters attempt to preserve signal integrity and extend the

distance over which data can safely travel. Actual network devices that serve as repeaters usually

have some other name as Active hubs which arealso called "multiport repeaters.
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Bridge:

A bridge is adevice that filters data traffic at a network boundary. Bridges reduce the amount

of traffic on alocal area network (LAN)by dividing it into two segments.

Bridges operate at the data link layer (Layer 2) of the OSI model. Bridges inspect incoming

traffic and decide whether to forward or discard it. An Ethernet bridge, for example, inspects

each incoming Ethernet frame - including the source and destination MAC addresses, and

sometimes the frame size - in making individual forwarding decisions.

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 con-nected 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 cross-

ing (if they can verify that it is on the interface from which it came).Bridges can also be used toconnect two physical LANs into a larger logical LAN.

Types of Bridges

Three types of bridges are used in networks:

Transparent bridgeDerives its name from the fact that the deviceson the network

are unaware of its existence. A transparent bridge does nothing except block or forward

data based on the MAC address.

Source route bridgeUsed in Token Ring networks. The source routebridge derives

its name from the fact that the entire path that the packet is to take through the network is

embedded within the packet.

Translational bridgeUsed to convert one networking data format to another; for

example, from Token Ring to Ethernet and vice versa.
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Router:

Routersare small physical devices that join multiple networks together. Technically, a router is

a Layer 3 gatewaydevice, meaning that it connects two or more networks and that the router

operates at the network layer of theOSImodel.Home networks typically use a wireless or wired

Internet Protocol (IP) router, IP being the most common OSI network layer protocol. An IP

router such as a DSL or cable modem broadband routerjoins the home's local area network

(LAN)to thewide-area network (WAN)of the Internet.

By maintaining configuration information in a piece of storage called therouting table,wired or

wireless routers also have the ability to filter traffic, either incoming or outgoing, based on the IP

addresses of senders and receivers. Some routers allow a network administrator to update the

routing table from a Web browser interface. Broadband routers combine the functions of a router

with those of a network switch and a firewall in a single unit.

In a common configuration, routers are used to create larger networks by joining two network

segments. A router can be a dedicated hardware device or a com-puter system with more than

one network interface and the appropriate routing software. All modern network operating

systems include the func-tionality to act as a router.Routers will normally create, add, or divide

on the Network Layer as they are normal-ly IP-based devices.

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.
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Multiprotocol Routers:

Static Routing

In environments that usestatic routing, routes and route information are entered into the routingtables manually. Not only can this be a time-con-suming task, but also errors are more common.

Additionally, when there is a change in the layout, or topology, of the network, statically

configured routers must be manually updated with the changes. Again, this is a time-consuming

and potentially error-laden task. For these reasons, static routing is suited to only the smallest

environments with perhaps just one or two routers.

Dynamic Routing

In a dynamic routing environment, routers use special routing protocols to communicate. The

purpose of these protocols is simple; they enable routers to pass on information about themselves

to other routers so that other routers can build routing tables. There are two types of routing

protocols usedthe older distance vector protocols and the newer link state protocols.

Distance-Vector Protocols

With distance-vector routing protocols, each router communicates all the routes it knows about

to all other routers to which it is directly attached (that is, its neighbors). Because each router in

the network knows only about the routers to which it is attached, it doesnt know how to

complete the entire journey; instead, it only knows how to make the next hop. Hops are the

means by which distance-vector routing protocols determine the shortest way to reach a given

destination. Each router constitutes one hop; so if a router is four hops away from anoth-errouter, there are three routers, or hops, between itself and the destination.

Link-State Protocols

A router that uses a link-state protocol differs from a router that uses a distance-vector pro-tocol

because it builds a map of the entire network and then holds that map in memory. On a network

that uses a link-state protocol, routers send out link-state advertisements (LSAs) that contain

information about what networks they are connected to. The LSAs are sent to every router on the

network, thus enabling the routers to build their network maps.


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Brouter

There is a device called a brouter which will function similar to a bridge for network transportprotocols that are not routable, and will function as a router for routable protocols. It functions at

the network and data link layers of the OSI network model.

Hubs

Hubs are used in networks that use twisted-pair cabling to connect devices. Hubs can also be

joined together to create larger networks. Hubsare 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 ineffi-cient 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 passivehub. An activehub, 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.


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Hubs come in a variety of shapes and sizes. Small hubs with five or eight con-nection ports are

commonly referred to as workgroup hubs. Others can accommodate larger numbers of devices

(normally up to 32). These are referred to as high-density devices. Hubs dont perform any

processing, except enable communication between connected devices.

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 learningthe MAC address of the devices attached to it,

and then by matching the destination MAC address in the data it receives.

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 great-ly 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 configura-tion, 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 onlyone direction at a time. Full-duplex transmission speeds are double that of a standard, half-

duplex, connection. So, a 10Mbps connection becomes 20Mbps, and a 100Mbps connection

becomes 200Mbps.


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Modem:

Modem is a contraction of the terms modulator and demodulator. Modems perform a simple function:

They translate digital signals from a computer into analog signals that can travel across conventional

phone lines. The modem modulates the signal at the sending end and demodulates at the receiving end. Modems provide a relatively slow method of communication. In fact, the fastest modem avail-

able on the market today has a maximum speed of 56Kbps.

Modems are available as internal devices that plug into expansion slots in a system; external

devices that plug into serial or USB ports; PCMCIA cards designed for use in laptops; and spe-

cialized devices designed for use in systems such as handheld computers.

Modem speeds can be expressed in either baud rate or bits per second (bps). The baud raterefers

to the number of times a signal changes in each second, and the bps rateis the number of bits of

data that can be sent or received in a second.


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CSUs/DSUs(CHANNEL SERVICE/DATA SERVICE UNIT)

A Channel Service Unit/Data Service Unit (CSU/DSU) acts as a translator between the LAN

data format and the WAN data format. Such a conversion is necessary because the technologies

used on WAN links are different from those used on LANs. Some consider a CSU/DSU as atype of digital modem; but unlike a normal modem, which changes the signal from digital to

analog, a CSU/DSU changes the signal from one digital format to another.

A CSU/DSU has physical connections for the LAN equipment, normally via a serial interface,

and another connection for a WAN. Traditionally, the CSU/DSU has been in a separate box from

other networking equipment; however, the increasing use of WAN links means that some routerManufacturers are now including the CSU/DSU functionality in routers or are providing the

expansion capability to do so.


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CSU/DSUs essentially function as the digital counterpart to analog modems. They are typically

external units that look similar to an external modem, but they can also come in sizes that can be

mounted in a rack. Unlike analog modems, CSU/DSUs do not perform signal conversion

because the signal at both ends is already digital. CSU/DSUs package digital data into a format

suitable for the particular digital transmission line they are servicing, and buffer and rate-adapt

digital signals going to and from the telephone company network. CSU/DSUs ensure that data

frames are properly formed and timed for the telephone company network and provide a

protective barrier to electrical disturbances that can harm customer premises equipment (CPE).

Network Interface Cards (NICs)

In computer networking, a NIC provides the hardware interface between a computer and a

network. A NIC technically is network adapter hardware in the form factor of an add-in card

such as aPCIorPCMCIAcard.Some NIC cards work with wired connections while others are

wireless. Most NICs support either wiredEthernetorWiFiwireless standards.

Ethernet NICs plug into the system bus of the PC and include jacks for network cables, while

Wi-Fi NICs contain built-in transmitters / receivers (transceivers).

NICssometimes called network cardsare the mechanisms by which computers connect to a

network. NICs come in all shapes and sizes, and they come in prices to suit all budgets. Many

different terms are used to refer to NICs, such as network card, network adapter, and LAN

adapter.Newly purchased NICs will almost certainly use the PeripheralComponent Interconnect

(PCI) bus Generally a NIC has only one port, for twisted-pair cabling.

Before installing a network card into a system, we must verify compatibility between the

network card and the operating system on the PC in which we are installing the NIC.

In new computers, many NICs are now pre-installed by the manufacturer. All NICs feature a

speed rating such as 11 Mbps, 54 Mbps or 100 Mbps that suggest the general performance of the

unit.
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Transceivers

The term transceiver does not necessarily describe a separate network device but rather an

integrated technology embedded in devices such as network cards. In a network environment, a

transceiver gets its name from being both a transmitter and a receiver of signals, such as analogor digital. Technically, on a LAN the transceiver is responsible to place signals onto the network

media and also detecting incoming signals traveling through the same cable. 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 transceiversare 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 computerperipherals, or they may

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 isdifferent, 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.

Media converters are a technology that allows administrators to interconnect different media

typesfor example, twisted pair, fiber, and thin or thick coaxwithin 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 electro-magnetic interference (EMI).


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