Computer Networks UNIT-10

8/7/2019 Computer Networks UNIT-10

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C o n f i d e n t i a l

Course : BCA

Semester : IV

Subject Code : BC 0048

Subject Name : Computer Networks

Unit number : 10

Unit Title : Networking Devices

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Unit-10 Networking Devices

Networking Devices

Objectives

After going through the presentation, you should be able to:

Describe Repeaters and Hubs

Discuss switches, switching methods

Discuss Bridges, types of bridges

Describe Routers, Gateways, modems proxy server, brouter and

WAPs

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Lecture outline

Introduction

Repeaters

Hubs

Switches

Bridges

Routers

Gateways

Other Devices

Summary

Networking Devices

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Introduction

To connects LANs, or segments of LANs, we make use of connecting

devices.

Connecting devices are therefore also referred to as networking

devices. They can operate at different layers of the OSI or TCP/IP

reference model.

Internetworking refers to the equipment and technologies involved in

connecting either LANs to LANs, WANs to WANs, or LANs to WANs.

Typical examples of internetworking devices are repeaters, Hubs,

Switches, bridges, routers, and gateways.

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Repeaters

What is repeater ?

A Repeater is a device that has just two ports. It operates at layer 1 that

is physical layer of the OSI model and TCP/IP model. It simply acts as

transceivers that receives, amplifies, and retransmits information.

A repeater receives a signal and before it becomes too weak or

corrupted, it regenerates the original bit pattern. The repeater then sends

the refreshed signal.

Repeaters are analog devices that connect two cable segments.

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Inter-connecting two Token Ring networks with a Repeater :

Repeaters may introduce side effects where they add cost to the circuit.

Also they introduce distortion as a result of limited bandwidth, additional

noise and also sometimes they may amplify some other undesirable

noise to the signal.

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Repeaters





Uses of Repeaters :

Repeaters are used to either extend a LAN beyond its usual distance

limitation, or to provide electrical isolation, or to accomplish media

conversion.

A Repeater connects segments of a LAN.

A repeater forwards every frame and it has no filtering capability. And

It is a regenerator, and not an amplifier

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Repeaters





Hubs

What is Hub ?It is a thin wire segment with fixed taps. Each tap is provided with

reliable connector (RJ45) as shown in figure. Hubs are the simplest

network devices, and their simplicity is reflected in their low cost.

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What is Hub ? ( continued..)

The basic function of a hub is to take data from one of the connected

devices and forward it to all the other ports on the hub. This method of

operation is very inefficient because, in most cases, the data is intended

for only one of the connected devices.

The salient features of Hubs are listed below:

Frames arriving on any of the lines are transmitted on the others.

All lines coming into a Hub must operate at a same speed.

Hubs do not amplify the incoming signal.

Hubs do not understand frames, packets or headers.

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Hubs





Classification of Hubs :

Passive hubs: These hubs do not need power because they don't

regenerate the signal.

Active hubs: Regenerate a signal before forwarding it to all the ports on

the device.

Stackable Hubs :It improves the network reliability and manageability.

It also gives a scalable and modular approach.

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Hubs





Classification of Hubs : ( continued..)

USB Hub : A USB hub serves as a medium for connecting peripheral

devices to a computer in accordance with a USB standard. A hub typically

includes an upstream port that couples the hub to the host, and several

downstream ports that couple the hub to another hub or peripheral.

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Hubs





Switches

What is a switch ?

A switch is like a hub in that it has multiple ports, but internally it has a

table listing which systems are found on each port, and shuttles the

packets where they need to go. Unlike a hub, signals are transmitted

only to the port where they need to go, rather than to all ports.

Some switches and bridges can be used to connect networks that use

different physical protocols, for instance connecting Ethernet with TokenRing. The name for this is a µtranslating¶ bridge or switch.

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Switches are fast multi-port bridges that provide an economical way to

resolve network congestion.

On the surface, a switch looks much like a hub. Switches can do much

more and offer many more advantages than hubs.

Rather than forwarding data to all the connected ports, a switch forwards

data only to the port on which the destination system is connected.

Switches have two major weaknesses. Under heavy load conditions,

some switches may drop packets, and they lack the ability to filter out or

block unwanted or unauthorized traffic.

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Switches





Switching Methods :

Switches use three methods to deal with data as it arrives:

Cut-through: In a cut-through configuration, the switch begins to

forward the packet as soon as it is received. No error checking is

performed on the packet, so the packet is moved through very quickly.

Store-and-forward: In a store-and-forward configuration, the switch

waits to receive the entire packet before beginning to forward it. It also

performs basic error checking.

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Switches





Switching Methods : ( continued..)

Fragment-free : Building on the speed advantages of cut-through

switching, fragment-free switching works by reading only the part of the

packet that enables it to identify fragments of a transmission.

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Switches





Bridges

Bridges are intelligent devices that operate at layer 2 and are used to

interconnect LANs of the same type.

Packets received on one port may be retransmitted or forwarded on

another port.

A bridge unlike repeaters will not start retransmission until it has

received the complete packet. As a consequence, stations on either side

of a bridge may be transmitting simultaneously without causing

collisions.

Unlike repeaters, bridges may provide filtering and forwarding services

across the link.

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Bridge interconnects two Token Ring LANs :

Bridges are networking devices that divide up networks. A bridge

functions by blocking or forwarding data, based on the destination MAC

address written into each frame of data.

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Bridges





Types of Bridges :

Three types of bridges are used in networks.

1. Transparent bridge:

A transparent bridge is invisible to the other devices on the network.

Transparent bridges only perform the function of blocking or

forwarding data based on the MAC address.

Transparent bridges are the most popular types of bridges.

2. Translational bridge :

A translational bridge can convert from one networking system to

another. Translational bridges are useful for connecting two different

networks, such as Ethernet and Token Ring networks.

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Bridges





Types of Bridges : (continued..)

3. Source-route bridge:

Source-route bridges were designed by IBM for use on Token Ring

networks.

The source-route bridge derives its name from the fact that the entire

route of the frame is embedded within the frame.

This allows the bridge to make specific decisions about how the frame

should be forwarded through the network.

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Bridges





Routers

What is Router ?

A router is an intelligent packet sorter, which can look at the ultimate

destination for a packet and analyze the best way to get it there.

A router is often a full - fledged computer system with multiple

network cards and its own operating system.

Routers operate on Layer 3, the network layer that routes data o

different networks.

Router performance is measured using its packet- forwarding rate,

which is the number of packets transferred per second from input to

output port. This rate depends on packet size, and how many protocols

are being supported.

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Routers

What is Router ? ( continued..)

Unlike bridges and switches, which use the hardware-configured MACaddress to determine the destination of the data, routers use the

software-configured network address to make decisions.

This approach makes routers more functional than bridges or switches,and it also makes them more complex because they have to work harderto determine the information.

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Routers

Advantages of dedicated hardware routers :

Typically faster than server-based routers

Generally more reliable than server-based routers

Easier to harden against attacks than server-based routing solutions

Disadvantages of dedicated hardware routers :

More expensive than server-based router solutions; extra

functionality may have to be purchased

Often require specialized skills and knowledge to manage them

Limited to a small range of possible uses

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Routers

Drawbacks of Routers :

They are complex and difficult to install, configure, and manage.

They have a lower packet ± filtering ± and ±forwarding rate ascompared to switches.

They are protocol dependent, and maybe unable to handle someprotocols with out network reconfiguration.

Furthermore, some protocols are not routable.

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Gateways

Gateways :

A gateway is a network point that acts as an entrance to another

network.

In earlier days a gateway were referred to what we now call a router, and

in fact the gateway and router functions are often combined in the samedevice. Depending on the situation though, a gateway could be a switch

or bridge rather than a router.

Gateways are most commonly used to transfer data between private

networks and the Internet.

A gateway acts as the interface between proximity local area protocols

and wide area protocols. The Internet gateway transfers the packets to

the Internet where Internet routers forward the packets based on the

Internet address.

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Other Devices

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.

Proxy Server :

This is a system that sits in between a workstation and the server that

workstation is accessing. The proxy server can also be used to control

Internet access, which is a major issue at some companies.

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Other Devices

Wireless router :

Wireless routers allow portable electronics to share information and

resources via wireless communications. A router uses a routing protocol

to exchange information with other routers in order to maintain a

consistent view of the network. The routers distinguish data packetsaccording to network protocols and forwards traffic according to network-

level addresses.

Brouter :

Brouter combines the function of a router and a bridge. Local packets can

be routed from one side of the network to the other based on the

destination address, even if they do not conform to any routing protocols.

Packets with the appropriate protocols can be sent on their way to the

outside world.

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Other Devices

Wireless Access Point (WAPs) :

Wireless network devices gain access to the network via WAPs.

WAPs are typically deployed as part of a larger network infrastructure,

but in some environments, such as small businesses or home offices,

they can operate completely independently of a normal network.

When a WAP connects to a wired network, it is often referred to as a

wired access point because it joins the wireless portion of the network

with the wired portion.

WAPs are fairly innocuous, hub-like devices; the only giveaway to their

function is the antennae that protrude from the box.

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Summary

To connects LANs, or segments of LANs, we make use of connecting

devices.

Connecting devices are therefore also referred to as networking

devices.

They can operate at different layers of the OSI or TCP/IP reference

model.

Hubs and switches provide a mechanism to connect devices to a

network that is created with twisted-pair cabling.

Switches offer a speed advantage over hubs because they can use

full-duplex communications. Bridges allow network traffic to be confined to certain network

segments, thereby reducing the amount of network traffic.

Routers are devices that connect networks and thereby create

internetworks.

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A gateway is a device that translates from one data format to

another; it can be a hardware device or a software application.

A modem translates a signal from digital to analog so that it can be

transmitted across a conventional phone line.

WAPs are a relative newcomer to the networking equipment field.

Wireless network clients use WAPs to connect to the network.

Summary

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Computer Networks UNIT-10