categories of computer networks

8/8/2019 Categories of Computer Networks

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Categories of computer networks


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LAN

LAN is generally a privately owned network

with in a single office, building or campuscovering a distance of a few kms.

The main reason for designing a LAN is to

share resources such as disks, printers,

programs and data


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MAN

A network spanning a physical area larger

than a LAN but smaller than a WAN, such asa city. A MAN might be owned and operated

by a single organization, but it usually will be

used by many individuals and organizations.


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WAN

WAN spans a large geographic area, such as

a state, or country. WANs often connectmultiple smaller networks, such as local area

networks (LANs) or metro area networks

(MANs).


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Error detection

CRC (cyclic redundancy check)

A mathematical algorithm is used on the data blockto be sent to arrive at the CRC (a small block of bitswhich are appended to the data block and sent bythe sender).

At the destination, the receiver separates the data

block, recomputes the CRC using the samealgorithm and matches the received CRC with thecomputed CRC.

A mismatch indicates an error.


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LAN can use star, bus or ring topology. However,

bus topology is popular in the Ethernet LANs andToken Bus LANS and Ring topology is popular in

the Token Ring LANs.

A modified version of Token Ring is FDDI

Ethernet and Token Ring are the most popular LANs


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LAN broad cast their message to all host on that

LAN. This is because, all the host share a singletransmission wire.

The address in the packet or frame enables the

destination host to receive that packet, while all

other hosts ignore it.

Broadcast network can be

1. Static

2. Dynamic


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In static method each host is given a fixed

time slice to send the information. If a hostdoes not have anything to send, that timeslice is wasted. That is why this method isnot very popular.

In dynamic method, a host can send aframe any time. Thus, if two host send aframe at the same time, the two frames couldcollide with each other.


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We need some arbitration to solve this problem. A protocolcalled MAC (Media Access Control) performs this job and

decides which node can access the medium and when.Dynamic method can be further subdivided into:

1. Centralized

2. Decentralized

In Centralized method, there is a single entity which decides

who would send the data next. This is typically a master-slavemethod. A host wanting to send data can explicitly request thisunit for the permission.

In star topology, the hub can play the role of the master.However if the arbitration unit goes down, the entire networkgoes down.


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In decentralized method, we do not require

any external arbitrator for this.So, more efficient and more popular.

This can be implemented in two major ways

1. Ethernet

2. Token Passing


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Ethernet

Ethernet is the name of a popular packet

switching LAN technology. Ethernet uses a single coaxial cable as the

transport medium. All hosts in the Ethernet

LAN connect to this cable.

A device called transceiveris used to

establish the connection between a computer

and Ethernet.


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At any point of time, the Ethernet bus can be in any ofthe three states:

Bus is idle

Carries a legitimate signal i.e. it is busy

Carries a erratic signal generated by a collision

The transceiver constantly listens to the bus i.e. itmeasures the signal level to determine the state ofthe bus.


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The transceiver does not connects to the hostdirectly. Instead, it connects to NIC, a small cardplugged on the motherboard of the host.

It has a small CPU, memory and a limited instructionset. It performs all the network related functions onbehalf of the host.

Each NIC bears a unique hardware address thatidentifies a host uniquely.


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Ethernet properties

Broadcast network

Every message travels across the entire cable and all NICs

receive it, but only the NIC of the correct destination accepts it.

Best effort delivery

Ethernet hardware or software do not take care of missingframe.

Decentralized access controlThere is no single centralized authority that dictates if a hostcan transmit data. Instead, the Ethernet uses the approach ofcarrier sense multiple access with collision detection(CSMA/CD).


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Multiple host can access the Ethernet bus simultaneouslythrough their transceivers and can determine if it is idle by

looking for the presence/absence of a carrier wave on the bus.For this reason it is called CSMA

When a host determines that no other host is using theEthernet, it transmits its own data on to the Ethernet.

When the transceiver of a host begins transmission, the signaldoes not immediately reach all parts of the network. It takensome finite time. Therefore, until the signal reaches anotherhost continues to believe that cable is idle. Thus, twotransceiver believe that Ethernet is free for transmission andcan transmit data almost exactly at the same time.


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When this happens, the electrical signals of the twotransmissions intermingle and neither remains a meaningful

transmission. Such incidents termed as collisions, whichproduce erratic signals, which the transceiver of both thesending hosts can detect.

To resolve collisions, while a host transmits the data, thetransceiver of the host continues to listen to the Ethernet to seeis a collision has occurred. If it has, the transceiver informs itsNIC about it.

NIC stops wait for some time to retransmit the data. However, as soon as it detects a collision, it generates a

specific jamming signal across the bus, informing all thenodes of the collision.


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The NIC of the other node trying to send some dataalso detects the jamming signal and backs off.

now, both the nodes wanting to send data wait for awhile. The question is : How long should they wait?

The Ethernet standard specifies a binaryexponential back off policy where a sender waits

for a random time after a first collision, twice as longif a retransmission also results in a collision, fourtimes as long if the retransmission also results in acollision and so on.


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Ethernet Frame Format

Preamble

(8 bytes)

Destination

Address (6

bytes)

Source

Address (6

bytes)

Frame

Type

(2 bytes)

Frame data

(64-1500

bytes)

CRC (4

bytes)


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Token ring

Token ring network is based on ring topology

A token ring network employs a mechanism called token

passing. All host on a token ring share the same physical medium, as all

the hosts are arranged to form a circular ring.

The sending computer transmits a frame, which travels acrossthe ring. Each host on the ring check the destination addressand if it is not meant for it, forward it along. Only the actualdestination makes a copy of it i.e. accepts it. In last framecomes back to the sender.

In this, only the host which have a token will be able to sendframe.


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However, at the destination node, before it forwards a frame,that host checks the CRC to ensure that there are no errors and

then not only accepts the frame, but also changes a flag bit inthe frame to indicate the receipt of a correct frame.

The frame comes back to the sender, the sender can check theflag bit to verify if the frame was received by the destinationsuccessfully, or if there were any errors during transmission.

Such an acknowledgement scheme is implemented only in the

token ring, unlike Ethernet, where there is no such theme.


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Medium Access Control (MAC)

Token ring does not employ CSMA/CD.

The token ring hardware ensures that the permissionfor data transmission is granted to each host on the

token ring in turn.

The hardware facilitates this coordination by using a

special three byte frame called token

A token contains a bit pattern that is completely

different from any other data frame, so that both can

be easily distinguished from each other.


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Token ring frame format

Token ring describes three frame formats:

Data Token

Abort


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Data Frame

Destination

Address

(6 bytes)

Source

Address

(6 bytes)

Frame Data

(up to

4,500 bytes)

CRC

(4 bytes)

ED

(1 byte)

FS

1byte

Preamble

(3bytes)


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Token Frame

SD

(1 byte)

AC

(1 byte)

ED

(1 byte)


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Abort Frame

SD

(1 byte)

ED

(1 byte)


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FDDI

FDDI developed using optical fiber because

of optical fiber could support high data rates(100 mbps)

These days copper wires support such rates.So, the copper version of FDDI is calledCDDI.

FDDI uses glass fibers for data transmissiontherefore encodes data bits in the form ofpulses of light.


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FDDI properties

1. Token passing for media access control

2. Self Healing MechanismThis is possible by second ring when a

network error occurs, or a host is down. In

such a case, the NIC uses the second ring,

which is used as backup for such failuresfor data transmission. This is called

loopback.


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FDDIFrame

Same as token data frame

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