6.2. - LANs

CablingTopologiesSegments and bridgesSwitches and routers

At the end of this session you will be able to:Describe different types of cabling and other communication methodsDescribe the term topologyDescribe the operation of different types of networks such as Bus, Ring, Star, Switched Ethernet and hubsCompare advantages and disadvantages of eachDefine and explain the terms Segment and Bridge and why they are used in LANs.

Give another name for Network Adapter

A. ModemB. ASDLC. Network Interface CardD.Network Information Centre

Network adapter card

Why are NICs used?

A. Converts analogue signal into digitalB. Allows the PC to send and receive data

with other devices on the network.C. To connect to the Internet.D.To remotely access parts of an

internal network.

Identify the advantage of using a bus network

A. It is easy to locate a fault if a cable failsB. Easy to add more stations without disrupting the

networkC. If the main cable fails only one station is affectedD. The performance is not affected under a heavy

load

Give an advantage of star network

A. If one cable fails, only one station is affected

B. Is easy and inexpensive to install as it requires the least amount of cabling

C. The whole network goes down if a cable fails at any point

D.There is no central computer

Identify a common feature of Bus topology and Star with hub.

A. Easy to expand to a larger network.B. Cheap to install and maintain.C. Every node competes for a fraction of the

total bandwidth by broadcasting the packets to all nodes.

D.Uses intelligent routing of packets.

Describe the function of the hub at the centre of a star network.

A. Hub stores information temporarily then send it out to another destination on the network

B. Hardware device which will enable any printer to be connected to the network

C. It regenerates the incoming signals and sends them out to all nodes connected to hub.

Cables

Twisted pairBaseband Coaxial cable (digital transmission e.g .Bus Ethernet)Broadband coaxial Cable (analog transmission e.g .cable TV – is more expensive, larger geo area)Fibre Optics

Other Wireless Communication methods used in LANs

Radio Waves – WLAN Bluetooth (uses time division full duplex)

Microwaves –max 30 miles apart because of earth’s curvature. Mobile phones

Satellite – travel in Geosynchronous orbit about 22,000 miles above earth.

Cable versus Wireless LANs

Task – 5 minutes:Group 1:

Identify advantages/disadvantages of cabling over wireless

Group 2Identify advantages/disadvantages of wireless over cabling

Group 1 -disadvantages of cabling over wireless

Group 2 -disadvantages of wireless over cabling

Advantages/disadvantages of Wireless

Wireless +Convenience - locationMobilityProductivity due to increased mobility. Deployment – initial set up costs – less complexity because less cablingExpandability – easy to add devicesCost – initial cost but maintenance / expansion cost is low

Wireless -Security – more complex to secure than wired LAN, wireless packets can be intercepted, encryption neededRange is limited- repeaters are required – costlyReliability – radio frequency – interference from other devices such as printersSpeed –can be slow compared to slowest common wired LANs – low bandwidth

Advantages/disadvantages of Cabling

Cable +Reliable technologyHigher speed than wireless (100 Mbps Fast Ethernet )Security – more difficult to intercept packets

Cable –Static location – cost and expertise to move devicesCables and connection need to be set up -more complex and costlyExpandability – more costly as extra cables are required

Evolution of LAN Devices

NICs, Repeaters, & Hubs

Bridges

Switches

Routers

NIC Specifics

NICs provide hosts with access to media by using a MAC address.

MAC stands for Media Access Control

NICs operate at Layer 1 – Network Access layer of TCP/IP model

NICs, Repeaters, & Hubs

To connect two computers, you must...Install a NIC card in each.

The First LAN

NICNIC

NIC NIC

Attach computers using a crossover cable

NICs, Repeaters, & HubsRepeaters can be

used to increase the distance

What’s the maximum distance for Cat 5 cable?

100 meters or approx. 300 feetSo what can we use if this

distance is greater than 100 meters?

NIC NIC

Repeaters amplify and retime signals

NICs, Repeaters, & Hubs

NIC NIC

NIC NIC

Using repeaters was fine as long as a business only needed two computers

networked.What if a business

wanted a third computer attached?

Or a fourth? What device would they

need?

NICs, Repeaters, & Hubs

NIC NIC

NIC NIC

A multi-port repeater! Also called a...

Hub

A Dilemma!

NIC NIC

NIC NIC

As businesses expanded their networks, they began to cascade hubs.

What’s The Problem?

1) Hubs share bandwidth between all attached devices.

2) Hubs are stupid, Layer 1 devices. They cannot filter traffic.

3) Most LANs use a “broadcast topology,” so every device sees every packet sent down the media.

BroadcastsIn this picture, all hubs forward all traffic to all devices.

So, if Host 1 wants contact Host 2, all hosts see the packets. This is what we mean by a broadcast topology

Broadcasts

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The red arrows show that all hosts receive the contact request. Only Host 2 will respond.

What’s The Solution?

We need a smarter hub!

What’s a “smarter hub” called?

A Bridge!

Bridges filter network traffic based on MAC addresses.

Let’s take a look at how this works.

SegmentDefinition: A run of cable to which are attached a

number of workstations

The portion of a computer network in which computers can access each other using a Network Access layer protocol (e.g., in Ethernet, this would be the ability to send an Ethernet packet to others using their MAC addresses). In this case, it is synonymous with broadcast domain. In Ethernet a segment is also known as the collision domain

This area should be kept small or heavy traffic can slow down network considerable

Bridge

Definition: Connects two segments at MAC (Media Access Control) address level.

Amplifies signal that passes throughBridge ‘learns’ address connected to ports.Examines data frame and extracts source and destination (MAC) addresses.If not on same segment as source – bridge will broadcast the frame on the other segment.

BridgePort 1

Port 2

segment

segment

BridgeTo lessen the amount of LAN traffic, businesses began to uses bridges to filter frames based on MAC addresses.

BridgeNow, if Host 1 contacts Host 2, only the hosts on that LAN segment see the packets. The bridges stop the packets.

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SwitchA switch (also know as a multi-port bridge), can effectively replace these four bridges.

SwitchAnother benefit of a switch is that each LAN segment gets dedicated bandwidth.

The Cloud

10 Mbps10 Mbps

10 Mbps

10 Mbps

10 Mbps

Ethernet Switch

Each node can transmit to switch at same time. No collisions occur as switches can receive and send data at same time to all connections.Two wire pairs providing full duplex operation.

Since a switch is a multi-port bridge, we know it will stop local pings from traveling to other network segments.

Switch

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But a switch cannot stop a ping destined for a different LAN segment from traveling to all other LAN segments as it uses Mac Addresses.

Switch

16

1

For example, Host 1 pings Host 16. Since Host 16 is on another LAN segment, the switch will flood the ping request out all ports.

What device will solve this problem?

Router

1

16

1

Routers filter traffic based on IP addresses. The IP address tells the router which LAN segment the ping belongs to.

Devices Function At Layers

Know These!

Devices Function At Layers

Also know that each device not only works at its layer, but all layers below it.

Devices Function At Layers

For example, a router is a layer 2 device but also uses MAC addresses (layer 1) and repeats the signal (layer 1)

Why are LANs based on a bus topology segmented?

A. Is cheaper as less wiring is requiredB. It reduces traffic in the LAN as the bridge

will block frames by looking at destination IP address and operates at internet layer or network layer.

C. It reduces traffic in the LAN as the bridge will block frames destined for an address on the same segment from being passed to the other segment and operates at Network Access layer or data link layer (physical layer).

Give an advantage of a Switched Star network (Ethernet)

A. Collisions cannot occurB. Is easy and inexpensive to install as it

requires the least amount of cablingC. Supports portable devicesD.Difficult to find cable failure between

node and switch.

What is a MAC address

A. A flat 48 bit number on the network card representing the manufacturer and vendor of card.

B. A dynamically assigned address representing the PC in a LAN

C. A flat 32 bit number that can be used to identify a computer in WAN

Which of the following elements does a peer-to-peer network require?

A. WorkstationsB. FileserverC. Network Interface Cards