local area networks 2

8/14/2019 Local Area Networks 2

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Local Area

Networks

Types of cable Twisted pair, baseband coaxial cable,broadband coaxial cable, optical fibre.

Topology

Bus

Ring

Star

Switched

Ethernet and

Hubs

Define the term topology.

Describe in general terms the operation of

these networks.

Compare the advantages and disadvantages of

each.

Segment Define the term and explain why local-areanetworks based on a bus topology are

segmented.

Bridge Define the term and explain why it is used.

Local Area Networks

Local area networks emerged in the early 1970s, as a substitute for large

mainframe computers. It had become apparent that for many companies it

was more economical to have a number of small computers, each with theability to run applications, rather than a single large system. Since each

small computer needed access to peripherals such as magnetic hard disks,

printers, et cetera as well as needing to share data it became necessary to

inter-connect these small computers and the peripherals they shared. The

inter-connections became the local area network.

Local area networks cover a small geographical area such as a single

building. The close proximity of computers to each other in a local area

network enables communication links to be used that have higher speeds

and lower error rates than their counterparts in wide area networks.

The links most commonly used in local area networks are twisted pair,

baseband coaxial cable, broadband coaxial cable and optical fibre.


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Topology

Linking computers to form a network requires careful planning. Consider

the problem of adding another computer to either of the layouts shown in

Figure NL.5.

Question NL.1

For the network shown in Figure NL.5a, connecting more computers is a problem,

why?

Question NL.2

Why might the network shown in Figure NL.5b be considered to have a better

structure?

The way in which computers are cabled together or linked to form a

network is very important. The term topology is used to describe the form

of a network.

The term topology, in the context of networking, refers to the shape,

configuration or structure of the inter-connections connecting devicesto the network.

Computer Link

(a)(b)Figure NL.5


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The most common network topologies are star, ring and multi-access bus.

These are shown in outline in Figure NL.6.

Bus

In this topology, all computers are attached to through a network

interface card to a linear transmission medium, or bus as shown in Figure

NL.7

Each connected computer has its own unique hardware address provided by

the network interface card. In this way each computer attached to thebus can be uniquely identified. In baseband bus systems a transmission

Bus

Ring

Star

Figure NL.6 Local Area Network topologies

Linear transmission medium or bus

Computer Computer

Computer

Figure NL.7 Bus topology


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from any computer consists of pulses of voltage that propagate the length

of the transmission medium in both directions and can be received by all

other computers connected to the bus. This is shown in Figure NL.8. The

pulses of voltage cease to exist when they reach the end of the bus.

However, a problem arises when two computers transmit onto the bus at

the same time.

The pulses of voltages from each will eventually collide resulting in higher

voltage swings. A computer attempting to read these pulses will fail to do

so correctly. When this happens, a collision is said to have occurred and

the bus becomes unusable for the duration of the transmissions from bothcomputers. To reduce the effect of this, transmissions are limited in

duration to a frame of pulses. A frame consists of a number of pulses up

to some maximum. The frame must also have a minimum number of pulses,

as well. This is so that a transmitting computer can detect a collision by a

rise in pulse voltage. Frames transmitted from two computers situated at

opposite ends of a bus must overlap before each stops transmitting. In

Figure NL9 (a) the frames do not overlap. In Figure NL.9(b) they do.

Networked

computer

Voltage pulses0 volts

5

volts

Figure NL.8 Data transmission from a computer connected to

the bus


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For collision detection to work, successfully, there must be a minimum

length of data frame and a maximum separation of computers for a given

speed of transmission.

In a bus system based upon an Ethernet standard the details are as

follows:

Data transmission rate: 10Mbits/sec

Maximum station separation: 2.5km

Frame size: variable length 72 1526 bytes

Even though collisions do occur from time to time, it is possible to operate

this bus system successfully if each connected computer follows a

protocol when transmitting. A commonly used bus protocol is CarrierSense Multiple Access with Collision Detection or CSMD/CD.

Computer

A

Computer

B

Frame from A

Frame from B

(a)

Computer

A

Computer

B

Frame from A

Frame from B

(b)Figure NL.9 Two computers transmitting a frame each at the same time


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The rules for CSMA/CD are as follows:

1. If bus is quiet : transmit frame

2. If bus is busy, continue to listen until bus is idle, then transmit immediately3. Whilst transmitting monitor bus for a collision, if one is detected transmit a

brief jamming signal to let all computers know that there has been a collision

and then stop transmitting.

4. After transmitting the jamming signal, wait a random amount of time, then

attempt to transmit again, starting from step 1 again.

One very popular bus system, Ethernet, mentioned already, uses the

CSMD/CD protocol. The term Ethernet generally refers to a standardpublished in 1982 by Digital Equipment Corporation, Intel Corporation and

Xerox Corporation. It is the predominant form of local area technology

used with TCP/IP today. It operates at three speeds: 10 Mbits/sec

(standard Ethernet), 100 Mbits/sec (fast Ethernet) and 1000 Mbits/sec

(gigabit Ethernet). It uses 48 bit addresses. Data to be transmitted is

broken into variable size packets called frames as shown in Figure NL10.

Data

Data bits

Frame

Control

bits

Control

bits

Data bits Data bits

Figure NL.10


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Network Adapter

A computer communicates on the network through a network interface card or

network adapter. A network adapter plugs into the motherboard of a computer

and into a network cable. Network adapters perform all the functions requiredto communicate on a network. They convert data from the form stored in the

computer to the form transmitted or received on the cable. Figure 1 illustrates

this.

A network adapter receives data to be transmitted from the motherboard of a

computer into an area of memory called a buffer. The data in the buffer is then

passed through some electronics that calculates a checksum value for the block

of data and adds address information, which indicates the address of the

destination card and its own address, which indicates where the data is from.

Each network adapter card is assigned a permanent unique address at the time

of manufacture. The block is now referred to as a frame. The network adapter

then transmits the frame one bit at a time onto the network cable. The address

information is sent first, followed by the data and then the checksum.

A network adapter card must match both the bus of the computer it is placed in,

the type of network to which it is connected and the media type to which it is

attached.

The bus could be one of ISA, EISA, Micro Channel, VESA Local Bus, PCI, NuBus,

PC Card (PCMCIA) or a proprietary local bus. Some computers have more than

one bus, e.g. both an ISA and a PCI bus.

Serial data flows from the network

adapter card onto the network

Parallel data flows from the

computer to the network

adapter card

Network Adapter

card

Computer Motherboard

Figure 1 Network Adapter


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Each system only needs to contend for bandwidth with systems on its own

segment. These segments are referred to as collision segments.

Ethernet Segments

Ethernet bus networks are often split into smaller sections in order toimprove their performance.

These sections or parts are referred to as network segments. A segment refers

to one physical string of computers, whether connected in a star, a bus or a ring

topology.

It is not possible to add more and more stations to the same physical network

because physical networks have a maximum station limit (the node limit). For

example, in a 10Base2 network the node limit is 30. However, before the limitfor a particular Ethernet bus network is reached the addition of more and more

stations will cause network traffic to slow. This happens because each

additional computer increases the amount of broadcast traffic on the segment

and in turn the number of packet collisions (two or more computers trying to

talk simultaneously) and delays in packet sending.

One solution to this problem is to split the network into two or more separate

networks each with its own server. Traffic is reduced because each network will

have fewer computers. However, communication between a computer in onenetwork and a computer in another will not be possible as the networks are

isolated from each other.

An inexpensive alternative to separate physical networks is to assign the

computers to different groups (segments) and to connect each segment to a

different network card in the same server. This solution reduces the traffic on

each segment but doesnt reduce the servers workload. Data packets between

senders and recipients on the same segment do not enter the other segments.

Data packets between senders and recipients on different segments are routed

through the server so that they may pass from one segment to another. Theserver performs the task of a router.

In Ethernet terminology packets are referred to as frames.

Ethernet Frames

Data moved from one computer to another in a network is packaged inside a

delivery envelope called a frame.

Frames are topology specific. An Ethernet frame conveys differentinformation from a Token ring or an ATM frame.


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An Ethernet frame consists of a set of digital pulses transmitted by the

transmission medium. An Ethernet frame can be between 64 and 1518 bytes in

size and is organised into four sections:

Preamble

Header

Data

Frame check sequence

Preamble

The preamble alerts receiving stations to the presence of a frame on the

transmission medium. The standard preamble is eight bytes long.

Header

A header contains information about who sent the frame and where it is going.

It may also contain other information such as how many bytes the frame

contains. The latter information is used for error correction. The header may

also contain information which specifies what type of Ethernet frame it is. The

header size is always fourteen bytes.

Data

The data section contain the actual data the computer is transmitting as well asany protocol information such as source and destination IP addresses.The data

field may be from 46 to 1500 bytes in size. A computer that needs to send more

than 1500 bytes of data will send the data in multiple frames. Each frame is

assigned a sequence number which is used by the destination computer to

reassemble the data in the correct order.

Frame Check Sequence (FCS)

This is used to check that the data received is actually the data that has been

sent. An algorithm called a cyclic redundancy check (CRC) is applied to the

received bytes from the other fields. The result is a four-byte value. This value

is compared with the four-byte value sent in the Frame Check Sequence. The

latter was caculated in a similar way at the sending computer. If there is a

difference then an error has occured while the frame has been moving across

the transmission medium. The receiving station can request that the frame in

error be sent again by the transmitting system.

Another possible solution is to use a bridge. A bridge is a small box with twonetwork connectors that attach to two separate portions of the network. A


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bridge will amplify the signals that pass through it (in this respect it functions

as a repeater). In addition, a bridge will examine the data frames and extract

the frame header information. The bridge uses the source and destination MAC1

addresses it extracts from the frame header to learn where all the network

nodes are located. It stores this information in a table that it uses when itneeds to lookup which nodes are connected to each of its ports. Figure 1 shows a

bridge with segment one connected to port A and segment two connected to

port B.

A packet/frame sent by computer 1will not get into segment two if it is

intended for a computer in segment 1. Similarly a packet/segment sent by

computer three will not get into segment one if it is intended for a

computer in segment two. However, when a computer in segment onesends

a packet for a computer located in segment two the bridge responds by

recognising the destination MAC address as belonging to a computer in

segment two. The bridge then broadcasts the packet onto segment two.

Figure 2 shows the contents of the table stored in the bridge for the

example network shown in figure 1. The table rows contain the MAC

addresses of each connected computer.

1 Media Access Control address, a hardware address that uniquely identifies each node of a network.

Bridge

Port

A

Port

B

Desktop 1

Desktop 2

Desktop 3

Desktop 4

Segment 1 Segment 2

Figure 1

12-36-56-78-9A-BC

12-34-56-78-9A-BC 12-38-56-78-9A-BC

12-3A-56-78-9A-BC


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Port A Port B

12-34-56-78-9A-BC 12-38-56-78-9A-BC

12-36-56-78-9A-BC 12-3A-56-78-9A-BC

...... ......

...... ......

...... ......

Figure 2

The bridge ensures that communications in each segment stay isolated.

Each system only needs to contend for bandwidth on its own segment.Therefore, it is impossible for a computer to have a collision outside of its

segment. For this reason segments are referred to as collision domains.

This means that in effect by splitting a physical network into two

segments connected by a bridge the potential bandwidth of it can be

doubled.

When a bridge is first connected its table of MAC addresses will be

empty. Therefore, until it gains enough information about which computers

are connected to its ports, a bridge will relay every packet from a

connected segment to all other connected segments.A bridge will work with any network protocol because it operates at the

MAC level.

Ethernet (10BaseT)

Standard Ethernet has a maximum data throughput of 10 Mbps (10 Megabit per

second).

Fast Ethernet (100BaseT)

Fast Ethernet is capable of sending and receiving data at 100 Mbps.

Cat5 Cabling

In both standard Ethernet and fast Ethernet data packets are transmitted over

two pairs of UTP Category 5 cabling.


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Hubs

Figure 3 shows the physical layout of a small local area network consisting

of three desktop computers and one server interconnected via a hub. The

network is Ethernet-based. The three desktop computers and the file

server are known as nodes of the network.

A hub works with one computer at a time and only sends or receives data

since it cannot handle simultaneous two-way communication. The result is

that the bandwidth of the transmission system is shared amongst all the

connected computers. Although in a hub-based network the computers arephysically wired in a star configuration the network behaves as a bus-wired

system. For example, a data packet sent to the hub from desktop

computer 1 is broadcast to all the other connected computers until it finds

its correct destination. During this time no other computer can use the

system.

In a hub-based network every node on the network competes for a

fraction of the total bandwidth.

Figure 4 shows the bus equivalent of the hub-based system shown in figure

3.

Figure 3

Desktop Computer

1

Hub

Port

A

Port

B

Port

C

Port

D

Desktop Computer

2

Desktop Computer

3 Server

Desktop

Computer 1

Desktop

Computer 3

DesktopComputer 2

Server

Port APort B

Port CPort D

Figure 4


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More on Hubs

When bus-based local area networks were first built stations were attached to

a line of cable using taps or t-pieces as shown in Figure NL.13. The cable used

was typically coaxial cable.

Cabling a bus in this linear fashion is fine if the stations are arranged linearly

but often this is not physically possible. People working in a building often do so

from small offices. To cable every office in a building using one long length of

coaxial cable is not often feasible. Furthermore, any break in the cable will stop

the network functioning. Hubs solve the problems associated with a linearly

organised physical bus. A separate cable is run from the site of each station to

a central office. Typically, the cable would sheath two twisted pairs of wires,

one for transmitting and one for receiving. Each cable entering the central

office would be connected to a port of a hub as shown in Figure NL.14.

In order for the cabling layout to function as a bus local area network the hub

must first detect the presence of a packet on a cable. It must then broadcast

this packet onto all the other cables connected to the hub. Although thephysical layout resembles a star network, logically a hub system functions as a

Station Station

Bus

Tap

Figure NL.13 Tap connections to a bus

Station Station Station Station

Hub

Two twisted

pairs

Transmit

Receive

Hub sited in central

office

Cable

Figure NL.14 Hub

connected bus


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bus network. The hub-based network may use Ethernet technology. Therefore

packet collisions may occur.

Switches

Figure 5 shows a system in which the hub has been replaced by a switch.

Each wire run involves only the switch and the computer attached to it.

Therefore each collision domain is limited to two devices, the computer and the

switch it is connected to. Computers attached to the switch only see the trafficintended for them.

A switch looks at the destination address of each packet it receives and delivers

each packet directly to the correct destination.

In a switched-based network all connected computers can talk to the switch at

the same time. Switches can receive and send data simultaneously to all

connections. This means that each node is allocated a dedicated bandwidth of 10

Mbps in a 10BaseT network and 100 Mbps in a 100BaseT network with full-

duplex tranfer (simultaneous both way communication). For example in a

100BaseT network, if four computers are connected to an eight-port switch,

each computer has a connection bandwidth of 100 Mbps. If a further four

computers are connected to the switch, the original four still have a connection

bandwidth of 100 Mbps. The four new computers will also have a connection

bandwidth of 100 Mbps each. Compare this with the hub-based network in figure

3. If four more computers are connected to the hub the connection bandwidth

will be reduced for the original four and the four new computers will each have a

connection bandwidth equal to the new reduced bandwidth.

In a bus-based system a transmitting computer can listen to its owntransmission. In a switch-based system a transmitting computer can listen for

Figure 5

Desktop Computer 1

Switch

Port A Port B

Port C Port D

Desktop Computer 2

Desktop Computer 3Server


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transmissions at the same time as it is sending. However, the difference is that

in a switch-based system a computer can be transmitting and at the same time

receiving a completely different transmission. This is the meaning of full-duplex

operation. There are two wire pairs connecting a computer to a switch. One is

used for transmitting while the other is used for receiving.

Switched Ethernet

Central switches are available with line cards that accept one or more Ethernet

connections and line cards with operating speeds from standard Ethernet to

gigabit Ethernet so that standard, fast and gigabit Ethernet can be connected

together.

When each cable is operated as an Ethernet cable, each station must have an

Ethernet card. If each line card has only one Ethernet connected to it and this

Ethernet has only one station connected to it, collisions no longer occur.

However, packets can still be lost due to buffer overflow on a line card. This

version of Ethernet is known as Switched Ethernet.

Backbone

A backbone is a network segment used to connect other segments. It usually

runs a faster topology than the segments it is interconnecting in order to cater

for the larger bandwidth demands that it may experience. Figure 6 shows how a

backbone segment could be used to inter-connect three other segments via

switches.

B

ac

k

b

o

n

e

Switch

Computer 1

Computer 2

Server 1

Switch

Computer 3

Computer 4

Server 2

Switch

Computer 5

Computer 6

Server 3

Figure 6

local area networks 2

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