Computer Networks -LANComputer Networks -LAN

D k J hDeepak JohnDepartment Of MCA,SJCET-Pala

Local Area NetworkLocal Area Network

Attributes Of A LANi. Transmission technology

ii. Signalling methods

iii T i i diiii. Transmission medium

iv. Access Control

v. Topologies

i. Transmission TechnologyTwo Transmission Technologies used by LANs are :Two Transmission Technologies used by LANs are :

a. Broad cast: A single communication channel is shared by all themachines on the networkmachines on the network

b. Point to Point: consists of many connections between individualpairs of machines Packets have to follow multiplepairs of machines.Packets have to follow multipleroutes of different lengths

ii Signalling Methodsii. Signalling Methods Two signaling methods used by LANs are :

B db d Th b d idth f th t i i di i bdi id da. Broadband: The bandwidth of the transmission medium is subdividedby frequency to form two or more sub-channels

b B B d l i l i i d i i ib. Base-Band: only one signal is transmitted at any point in time

iii. Transmission Mediumi i di b l ifi d i f h i l bl Transmission medium can be classified into 4 from physical cable

connection to wireless systems.T i t d P i i t f t i di id l i l t d ia. Twisted Pair: consists of two individual insulated copper wires

physically twisted together to minimize unwantedl i i l f i f i i helectromagnetic signals from interfering with or

radiating from the pairb. Coaxial cables: consists of a central copper core surrounded by a

layer of insulating material.c. Optical fiber cables: contains glass fiber and signals are transmitted

in the form of light pulses .d. Wireless transmission

iv. Access controlh h d i i i i h represents how the devices get permission to communicate on the

network. S f th th d i il l d i l l Some of the access methods primarily employed in local area

networks are : Polling Polling Token-passing Slotted ring Slotted ring CSMA/CD S it hi Switching

v. Topology refers to the way in which the end points, or stations, attached to

the network are interconnected.The common topologies for LANs are:

i. Busii. Treeiii. Ringiv. Star

LAN PROTOCOL ARCHITECTURE The LAN protocol architecture consists of layering of protocols that

contribute to the basic functions of a LANcontribute to the basic functions of a LAN The standardized LAN protocol architecture encompasses 3 layers

Ph i l li. Physical layerii. Medium Access control layer (MAC)

L i l Li k liii.Logical Link control

IEEE 802 Reference Model

The layers of OSI Reference Model can be classified as Networkd lSupport Layers and User support layers

LAN protocols are concerned with the network support layers,i l Ph i l d D t Li k Lmainly Physical and Data Link Layer

IEEE 802 Reference Model is a modified reference model suitablefor LANs built by IEEEfor LANs built by IEEE.

It corresponds to the lower 2 layers of OSI reference model Here the data link la er has been di ided into t o s b la ers: Here the data link layer has been divided into two sub layers:

logical link control (LLC) and media access control (MAC)

Physical layer includes functions such as :i. Encoding/decoding of signalsii Specification of the transmission medium and the topologyii. Specification of the transmission medium and the topologyLogical Link LayerActs as the interface between the Network layer and the MAC sub-Acts as the interface between the Network layer and the MAC sub

layer Functions:

i. Error Controlii. Flow Controliii S i & U Add i F tiiii. Sequencing & User Addressing Functions

LLC standard is common to all LAN’s and offers three types ofservices for controlling the exchange of data between two usersservices for controlling the exchange of data between two users.i. Unacknowledged connectionless service: does not involve any of

the flow- and error control mechanisms and delivery of data is notthe flow and error control mechanisms and delivery of data is notguaranteed.

ii. Connection-mode service:A logical connection is set up betweentwo users exchanging data, and flow control and error control areprovided.

iii Acknowledged connectionless service:datagram are to beiii. Acknowledged connectionless service:datagram are to beacknowledged, but no prior logical connection is set up.

LLC Protocol Data Unit (PDU) which contains 4 fields which contains 4 fields

D ti ti S i A P i t (DSAP) S S i A Destination Service Access Point (DSAP), Source Service AccessPoint (SSAP) : address fields which specify the destination andsource users of LLC These identify the network protocol entitiessource users of LLC.These identify the network protocol entitieswhich use the link layer service

LLC control field: describes type of PDU(U,I and S) and includes LLC control field: describes type of PDU(U,I and S) and includesother information such as sequencing and flow control information

Medium Access Control It i l b l f d t li k l l t th h i l l It is lower sub-layer of data link layer closer to the physical layer Key parameters of MAC technique is where and how

Wh f t h th t l i i d i t li d a. Where: refers to whether control is exercised in a centralized ordistributed fashion.

C l A ll i d i d h h h h i Central: A controller is designated that has the authority togrant access to the network.

Di t ib t d Th t ti ll ti l f MAC f ti t Distributed:The stations collectively perform a MAC function todetermine the order in which stations transmit.

b H i i d b h l d i f lik b. How: is constrained by the topology and competing factors likecost, performance, and complexity.

Basic functions of MAC Sub Layer are:i di li. Media Access Controlii. Error Detectioniii Station Addressingiii. Station Addressing

Defines two medium access control techniques specific for eachLAN.i. Synchronous techniques: a specific capacity is dedicated to a

connection.ii A h ll i i h (d i )ii. Asynchronous: allocate capacity in an asynchronous (dynamic)

fashion, more or less in response to immediate demand.

Asynchronous techniques can be classified into three:i R d R bii. Round Robinii. Reservationiii Contentioniii. Contention

i.Round Robin Each station in turn is given the opportunity to transmit. When it is finished, relinquishes its turn, and the right to transmit

passes to the next station in logical sequence. C t l f b t li d di t ib t d Control of sequence may be centralized or distributed. Efficient when many stations have data to transmit over an

extended period of timeextended period of time

ii. ReservationTime on the medium is divided into slots.A station wishing to transmit reserves future slots for an

extended or even an indefinite period.Reservations may be made in a centralized or distributed

fashion.iii Contentioniii. Contention Useful for bursty type traffic No control is exercised to determine whose turn it is. Stations send data by taking risk of collision (with others’

packets).however they understand collisions by listening to thechannel, so that they can retransmit.

Efficient under light or moderate load,bad under heavy load Round Robin and Contention techniques are the most commonly Round-Robin and Contention techniques are the most commonly

used in LANs.

MAC- Frame Format The MAC layer is responsible for performing functions related to

medium access and for transmitting the data.

MAC Control: contains any protocol control information needed forthe functioning of the MAC protocol

Destination MAC Address: The destination physical attachmentpoint on the LAN for this frame.

Source MAC Address: The source physical attachment point on thep y pLAN for this frame.

LLC PDU: The LLC data from the next higher layer.g y CRC: The Cyclic Redundancy Check field

Standard Title Description

802 1 Hi h l l I f S ifi i f d d f LAN hi802.1 High-level Interface Specification of standards for LAN architecture,interconnection, management

802.2 Logical Link Control Specification of standards for the LLC layer

802.3 Ethernet(CSMA/CD) Specification of standards for CSMA/CD architectures

802.4 Token Bus Specification of standards for token bus architectures

802.5 Token Ring Specification of standards for token ring architectures

802.6 Metropolitan AreaNetworks

Specification of standards for MANsNetworks

802.7 Broadband TechnicalAdvisory Group

Provision of guidance to other groups working onbroadband LANs

802 8 Fib O i T h i l P i i f id h ki802.8 Fiber Optic TechnicalAdvisory Group

Provision of guidance to other groups working onfiber optic-based LANs

802.9 Integrated Data and Specification of standards for interfaces to ISDNVoice Networks

Multiple Access ProtocolsMultiple Access Protocols A MAC (Media Access Control) protocol is a set of rules to control

h d i i di iaccess to a shared communication medium among various users.

RANDOM ACCESSRANDOM ACCESS In random access or contention methods, no station is superior to

another station and none is assigned the control over anotheranother station and none is assigned the control over another. Two features give this method its name: Fi t th i h d l d ti f t ti t t it First, there is no scheduled time for a station to transmit. Second, no rules specify which station should send next. Stations

compete with one another to access the mediumcompete with one another to access the medium

ALOHA

When the user simply transmits a frame, there are chances ofcollision

The user could simply retransmit, but this would not help, other userinvolved in the collision will also retransmit, resulting in anothercollisionO id hi i i d f i b f One way to avoid this is to wait a random amount of time beforeretransmitting which forms the basis of ALOHA

There are t o ersions of ALOHA: p re and slotted There are two versions of ALOHA: pure and slotted.

Pure ALOHAu e O

The system is working as follows:1. let users transmit whenever they have data to be sent.2. collisions will occur.3 using a feedback mechanism to know about the status of frame3. using a feedback mechanism to know about the status of frame.4. the collided frames will be destroyed.5. retransmit the destroyed frame.y

the number of collisions rise rapidly with increased load. After a maximum number of retransmission attempts Kmax

station must give up and try later.

Frames in a pure ALOHA networkFrames in a pure ALOHA network

Suppose L: the average frame length,R: rateR: rate,X=L/R: frame time

1. Transmit a frame at t=t0 (and finish transmission of the frame at t0+X )1. Transmit a frame at t t0 (and finish transmission of the frame at t0 X )2. If ACK does not come after t0+X+2tprop or detect collision, wait for

random time: B

First transmission Retransmission

3. Retransmit the frame at t0+X+2tprop+B

tt0t0-X t0+X t +X+2t t0+X+2tprop +B00 0 t0+X+2tprop

t0 X 2tprop B

Vulnerable Time-out Backoff period: B Retransmissionperiod Retransmissionif necessary

Vulnerable period: t0-X to t0+X, if any other frames are transmittedduring the period the collision will occurduring the period, the collision will occur.Therefore the probability of a successful transmission is the probability

that there is no additional transmissions in the vulnerable period.that there is no additional transmissions in the vulnerable period.Therefore, if a station generates only one frame in this vulnerable time

(and no other stations generate a frame during this time), the framewill reach its destination successfully.

Max channel utilization is 18% - very bad

Slotted ALOHASlotted ALOHA

Slotted ALOHA was invented to improve the efficiency of pureALOHAALOHA.

Here, we divide the time into slots and force the station to send onlyat the beginning of the time slotat the beginning of the time slot

If a station misses this moment, it must wait until the beginning of thenext time slot.

There is still the possibility of collision if two stations try to send atthe beginning of the same time slot

First transmission Retransmission

tt0t0 -X t0+X+2tprop

t0+X+2tprop +B t0+X

Time-out Backoff period: B

Retransmissionif necessaryif necessary

Vulnerable period: t0-X to t0 , i.e., X seconds long

Max channel utilization is 37%,doubles Normal ALOHA, but still low

Carrier Sense Multiple Access(CSMA)Carrier Sense Multiple Access(CSMA) A station wishing to transmit first listens to the medium if another

transmission is in progress (carrier sense)transmission is in progress (carrier sense). If the medium is in use, station waits if the medi m is idle station ma transmit if the medium is idle, station may transmit

Collision probability depends on the propagation delay Longer propagation delay worse the utilization Longer propagation delay, worse the utilization

Collisions can occur only when more than one user beginstransmitting within the period of propagation delay.transmitting within the period of propagation delay.

The vulnerable time for CSMA is the propagation time . If collision occurs If collision occurs Wait random time and retransmit

Suppose tprop is propagation delay from one extreme end to the otherextreme end of the medium. When transmission is going on, a station

Station A begins

g g ,can listen to the medium and detect it. Vulnerable period = tprop

A

gtransmission att=0

sense sense

Station A captures

Achannelat t=tprop CSMA random access scheme

sense sense

After tprop, A’s transmission will arrive the other end; every stationwill hear it and refrain from the transmission, so A captures themedium and can finish its transmission.

Following are some versions of CSMA protocol Based on how todo when medium is busydo when medium is busy• 1-Persistent CSMA• Non-Persistent CSMANon Persistent CSMA• p-Persistent CSMA

1-persistent CSMA if the medium is idle transmit if the medium is idle, transmit. if the medium is busy, continue to

listen until the channel is sensedidle; then transmit immediately.

If more than one station areSense carrier

yessensing, then they will begintransmission the same time whenh l b idl lli i Send the frame

Busy?yes

no

channel becomes idle, so collision.At this time, each station wait for arandom time and then re-senses

with probability 1

random time, and then re sensesthe channel again.

Problem with 1-persistent CSMA isp“high collision rate”.

Nonpersistent If the channel is busy the station does not continually check it for If the channel is busy the station does not continually check it for

detecting the end of ongoing transmission. It waits for a randomtime then checks the channel. If the channel is idle, sends the frame.

Sense carrier Wait random time…

yes

Send the frame

Busy?yes

no

p-persistent CSMAif di i idl i i i h b bili h i if medium is idle, station transmits with a probability p. otherwiseit defers to the next slot with probability 1-p. the process repeatuntil either the frame has been transmitted or another station hasuntil either the frame has been transmitted or another station hasbegun transmission.

Sense carrierSense carrier

Busy?yes

Send the frame with probability p

no

p y p

Behaviour of three persistence methods

CSMA/CD(CSMA with Collision Detection)(CSMA with Collision Detection)

Drawback of CSMA: when two frames collide, the medium remainsunusable for the duration of transmission of both damaged framesunusable for the duration of transmission of both damaged frames.

CSMA/CD: 1 if th di i idl t it th i t t 2 1. if the medium is idle, transmit; otherwise, go to step 2. 2. if the medium is busy, continue to listen until the channel is idle,

then transmitthen transmit. 3. if a collision is detected during transmission, transmit a brief

jamming signaljamming signal 4. after transmitting a jamming signal, wait a random amount of

time, then attempt to transmit., p

CSMA/CD efficiency

tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame Efficiency = 1/(1+5 * tprop / ttrans) For 10 Mbit Ethernet, tprop = 51.2 us, ttrans = 1.2 ms Efficiency is 82.6%!

Much better than ALOHA, i l d h simple, and cheap Efficiency goes to 1 as tprop goes to 0 Goes to 1 as t goes to infinity Goes to 1 as ttrans goes to infinity

Exponential Back off AlgorithmExponential Back off Algorithm used by a transmitting station to determine how long to wait following a

collision before attempting to retransmit the framecollision before attempting to retransmit the frame Each station generate a random number that falls within a specified

range of values. which determines the length of time it must wait beforerange of values. which determines the length of time it must wait beforetesting the carrier. The range of values increases exponentially aftereach failed retransmission.

After c collisions, the range is between 0 and 2c – 1,It then waits thatnumber of slot times before attempting retransmission.

If repeated collisions occur, the range continues to expand ,until after10 attempts when it reaches 1023. After that the range of values staysfi d If t ti i f l i t itti ft 16 tt t thfixed .If a station is unsuccessful in transmitting after 16 attempts, thengives up if cannot transmit

low delay with small amount of waiting stations low delay with small amount of waiting stations large delay with large amount of waiting stations

Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)Avoidance (CSMA/CA)

In CSMA/CA once the channel is clear, it again waits for an additionaltime period before performing the transmissiontime period before performing the transmission.

Before sending a frame, source senses the medium B k ff til th h l i idl Backoff until the channel is idle. After the channel is found idle, the station waits for a period of time

called the Distributed Inter Frame Space (DIFS); then the stationcalled the Distributed Inter Frame Space (DIFS); then the stationsends a control frame called Request To Send (RTS).

After receiving RTS, the destination waits for a period called Short After receiving RTS, the destination waits for a period called ShortInter Frame Space (SIFS), the destination station sends a controlframe, called Clear To Send (CTS) to source. This control frameindicates that the destination station is ready to receive data.

Source sends data after waiting for SIFS Destination sends ACK after waiting for SIFS.

RTS frame indicates theduration of time that theduration of time that thesource needs to occupythe channel.the channel.

Stations that are affectedby this transmissionycreate a timer called aNetwork AllocationV (NAV) h hVector (NAV) that showshow much time mustpass before these stationspass before these stationsare allowed to check thechannel for idleness.

CONTROLLED ACCESS

In controlled access, the stations consult one another to find whichstation has the right to sendstation has the right to send

A station cannot send unless it has been authorized by other stations. Th l t l th d Three popular control access methods are:

PollingR ti Reservation

Token Passing

Polling

Stations take turns accessing the medium Two models: Centralized and distributed polling Two models: Centralized and distributed polling Centralized polling One device is assigned as primary station and the others as

secondary stations All data exchanges are done through the primary. If the primary wants to receive data it asks the secondary's if they If the primary wants to receive data, it asks the secondary s if they

have anything to send; this is called poll function. If the primary wants to send data, it tells the secondary to get ready

t i thi i ll d l t f tito receive; this is called select function Polling can be done in order (Round-Robin) or based on

predetermined orderp

Primary is sending to Secondary Secondary is sending to Primary

ACK i th k l d t f th d ' d t t ACK is the acknowledgment of the secondary's ready status. Secondary responds either with a NAK frame if it has nothing to send

or with data (in the form of a data frame) if it doesor with data (in the form of a data frame) if it does

Distributed pollingNo primary and secondaryp y yStations have a known polling order list which is made based onsome protocol.station with the highest priority will have the access right first,station with the highest priority will have the access right first,then it passes the access right to the next station (it will send apulling message to the next station in the pulling list), which willpasses the access right to the following next stationpasses the access right to the following next station, …

Reservation A station needs to make a reservation before sending data. Transmissions are organized into variable length cycles. Each cycle begins with a reservation interval that consists of (N)

minislots. One minislot for each of the N stations. Wh t ti d t d d t f it k ti i When a station needs to send a data frame, it makes a reservation in

its own minislot. By listening to the reservation interval, every station knows which By listening to the reservation interval, every station knows which

stations will transfer frames, and in which order. The stations that made reservations can send their data frames after

the reservation frame.

Token PassingToken Passing Here the stations in a network are organized in a logical ring.

I thi th d i l k t ll d t k i l t th h In this method, a special packet called a token circulates through the ring.

k i h i h i h h h l d d i token gives the station the right to access the channel and send its dataWh i i h k d h d d i j When a station receives the token and has no data to send, it just passes the data to the next station.

token

LAN SYSTEMS

EthernetEthernet Ethernet is a dominant physical and data link layer technology for

local area networks (LANs)local area networks (LANs). It is a bus based broadcast network using co-axial cable operating at 10

or 100 Mbpsor 100 Mbps. It uses a control method called Carrier Sense Multiple

Access/Collision Detection (CSMA/CD) to transmit dataAccess/Collision Detection (CSMA/CD) to transmit data

Ethernet Evolution

Standard Ethernet

Fast Ethernet

Gigabit Ethernet

Ten-Gigabit Ethernet

100-Gigabit Ethernet

(10 Mbps) (100 Mbps)

IEEE 802.3 MAC Frame

Preamble: Alternating 0s and 1s; used for synchronizing; 7bytes( bi )(56 bits).

Start Frame Delimiter (SFD): 10101011 indicates the start of thef L t t bit l t th t th t fi ld i d ti ti ddframe. Last two bits alerts that the next field is destination address.

Destination Address (DA): 6 bytes (48 bits) physical address ofdestination station(s)destination station(s)

Source Address (SA): 6 bytes (48 bits) physical address of sender Length/T pe: if less than 1500 it indicates the length of data field Length/Type: if less than 1500, it indicates the length of data field.

If greater than 1536, it indicates the type of PDU. Data: 46 to 1500 bytes; Data: 46 to 1500 bytes; CRC: CRC-32 for error detection

Addressing Each station on an Ethernet network has its own network interface

card( NIC) fits inside and provides a 6-byte physical addressEg:06:01:02:01:2C:4B.

First three bytes from left specify the vendor. (Cisco 00-00-0C, 3Com02-60-8C) and the last 24 bit should be created uniquely by thecompany

A source address is always a unicast address where as destinationdd b i l i b daddress can be unicast, multicast, or broadcast.

Unicast: defines one recipient ,second digit from left is even

Multicast: defines a group of recipients ,Second digit from left isodd

Broadcast : defines a group of all stations in the same LAN ,Allonesones

The transmission is left-to-right, byte by byte; however, for eachbyte, the least significant bit is sent first and the most significant bitis sent last.

Physical Layer Implementation The Standard Ethernet defines several physical layer implementations; The Standard Ethernet defines several physical layer implementations;

four of the most common, are

Physical Layer Signaling Uses Manchester encoding. At the sender, data are converted to a digital signal using the

Manchester scheme; at the receiver, the received signal is interpretedas Manchester and decoded into data.

l h i d d Helps synchronize sender and recvr.

10Base5: Thick Ethernet use a bus topology with an external transceiver (transmitter/receiver) use a bus topology with an external transceiver (transmitter/receiver)

connected via a tap to a thick coaxial cable. 10-Mbps transmission speed and 5 represents 500 meters maximum 10 Mbps transmission speed and 5 represents 500 meters maximum

cable segment length. The transceiver is responsible for transmitting, receiving, and detectingp g, g, g

collisions

10Base2: Thin Ethernet uses a bus topology but the cable is much thinner and more flexible uses a bus topology, but the cable is much thinner and more flexible. The transceiver is normally part of the network interface card (NIC),

which is installed inside the station.which is installed inside the station. can transmit 10 Mbps digital signals over coaxial cable. more cost effective than 10Base5 because thin coaxial cable is less more cost effective than 10Base5 because thin coaxial cable is less

expensive than thick coaxial. The length of each segment cannot exceed 185 m (close to 200 m). The length of each segment cannot exceed 185 m (close to 200 m).

10Base-T: Twisted-Pair Ethernet Uses a physical star topology Uses a physical star topology. The stations are connected to a hub via two unshielded twisted pair

cables; One for transmitting data, and the other for receiving datacables; One for transmitting data, and the other for receiving data Maximum length of the cable segment can be 100 meters.

10Base-F: Fiber Ethernet Although there are several types of optical fiber l0-Mbps Ethernet,

the most common is called 10Base-F.It t t l t t t ti t h b It uses a star topology to connect stations to a hub.

The stations are connected to the hub using two fiber-optic cables.

B id d Eth tBridged Ethernet LAN can be divided using bridges Bridges have two effects on an LAN can be divided using bridges. Bridges have two effects on an

Ethernet LAN:• They raise the bandwidthThey raise the bandwidth• They separate collision domains

Raising the Bandwidth Raising the Bandwidth each network is independent. Suppose there are 12 stations And bandwidth is 10 Mbps Suppose there are 12 stations. And bandwidth is 10 Mbps. If we divide the network into 2 networks using bridge, each network

has a capacity of 10 Mbpshas a capacity of 10 Mbps. The 10 Mbps capacity is shared between 7 stations, 6+1(bridge acts

as a station in each segment), not 12 stations.g ),

A network with and without a bridge

Separating collision domains:Collisions domains become m ch smaller and possibilit of collision Collisions domains become much smaller and possibility of collisionis reduced.

With bridging, lesser number of channels compete for access to theWith bridging, lesser number of channels compete for access to themedium.

Switched Ethernet The heart of the system is a switch containing room for typically 4 to

32 plug-in cards each containing one to eight connectors that allows

Switched Ethernet

32 plug-in cards, each containing one to eight connectors that allowsfaster handling of packets.

When a station wants to transmit a frame, it outputs a frame topswitch.

Half duplex

All ports on the same card are wired together to form a local on-cardLANLAN.

Collisions on this on-card LAN are detected and handled usingCSMA/CD protocolCSMA/CD protocol.

One transmission per card is possible at any instant. All the cards cantransmit in paralleltransmit in parallel.

With this design each card forms its own collision domain.

Full-duplex switched Ethernet

Each station is connected to the switch through two links: one totransmit and one to receivetransmit and one to receive.increases the capacity of each domain from 10 to 20 Mbps.no chances of collision, so CSMA/CD is not used,

F E hFast Ethernet IEEE 802.3 u.

f f di d lli i d i l same frame format, media access, and collision detection rules as 10Mbps Ethernet

d t t f t f 100 Mb/ data transfer rate of 100 Mb/s . compatible with Standard Ethernet.

MAC Sub Layer

The only two changes made in the MAC layer are the data rate andthe collision domain

A new feature added called Auto negotiation;allows two devices tonegotiate the mode or data rate of operation.

l d i i h i i f 10 b For example, a device with a maximum capacity of 10 Mbps cancommunicate with a device with a 100 Mbps capacity.

Physical Layer Implementation Implementation

Topologies

Fast Ethernet Topologiesp g

Gigabit Ethernet IEEE 802.3z. All config rations of gigabit Ethernet are point to point

Gigabit Ethernet

All configurations of gigabit Ethernet are point to point. Point-to-point, between two computers or one computer – to –switch. Compatible with 100BASE T and 10BASE T Compatible with 100BASE-T and 10BASE-TMAC Sublayer

It supports two different modes of medium access : full duplexmode and half duplex mode.

Half duplex is used when computers are connected by a hub.C lli i i h b i ibl d CSMA/CD i i dCollision in hub is possible and so CSMA/CD is required.

Full duplex is used when computers are connected by a switch. Nocollision is there and so CSMA/CD is not usedcollision is there and so CSMA/CD is not used.

Carrier Extension tells the hardware to add its own padding bits afterthe normal frame to extend the frame to 512 bytes.y

RRRRRRRRRRRRR

Carrier Extension

Frame

Frame Bursting allows a sender to transmit a concatenated sequence

Carrier Extension512 bytes

Frame Bursting allows a sender to transmit a concatenated sequenceof multiple frames in a single transmission. If the total burst is lessthan 512 bytes, the hardware pads it again.y , p g

ExtensionFrame Frame Frame Frame

512 bytesF b tFrame burst

Physical Layer Topology Topology.

LAN CONNECTING DEVICESLAN CONNECTING DEVICES

Connecting Devices

Networking Devices InternetworkingDevicesDevices

Repeaters Bridges Routers Gateways

Network Connecting DevicesNetwork Connecting Devices

RepeatersRepeaters A repeater (or regenerator) is an electronic device that operates on

only the physical layer of the OSI modelonly the physical layer of the OSI model. A repeater installed on a link receives the signal before it becomes

too weak or corrupted regenerates the original pattern and puts thetoo weak or corrupted, regenerates the original pattern, and puts therefreshed copy back on the link.

A repeater does not actually connect two LANS; it connects twof hsegments of the same LAN.

A repeater forwards every frame; it has no filtering capability

Function of repeater

Hubs

Passive Hubsi h b i j hi h h i i A passive hub is just a connector which connects the wires coming

from different branchesA ti H bActive Hub A Hub is a multiport repeater. used to create connections between

stations in a physical star topologystations in a physical star topology. Connection to the hub consists of two pairs of twisted pair wire one

for transmission and the other for receivingfor transmission and the other for receiving. it copy the received frame onto all other links

B idBridges Bridges operate in both the physical and the data link layers of the

d lOSI model. Bridges can divide a large network into smaller segments. When a frame (or packet) enters a bridge, the bridge not only

regenerates the signal but checks the destination address andforwards the new copy only to the segment the address belongforwards the new copy only to the segment the address belong.

This is done by a bridge table (forwarding table) that contains entriesfor the nodes on the LANfor the nodes on the LAN The bridge table is initially empty and filled automatically by

learning from frames movements in the networklearning from frames movements in the network A bridge runs CSMA/CD before sending a frame onto the channel

Types of Bridges

1. Simple Bridge2. Multiport Bridge3. Transparent BridgeSimple Bridge The address table must be entered manually Whenever a new station is added or removed, the table must

modified. Installation and maintenance of simple bridges are time-consuming

and potentially more.Multiport bridges A multiport bridge can be used to connect more than two LANs.

Transparent Bridges A transparent, or learning, bridge builds its table of station addresses

on its own as it performs its bridge functions.

the stations are completely unaware of the bridge’s existence.

A transparent bridge must meet three criteria: A transparent bridge must meet three criteria:

1. Frames must be forwarded from one station to another.

2. The forwarding table is automatically made by learning framemovements in the networkmovements in the network.

3. Loops in the system must be prevented.

Learning Bridge

multiple paths of bridges and local-Loop Problem multiple paths of bridges and local-

area networks (LANs) exist betweenany two LANs in the internetwork.y

Having more than one transparentbridge between a pair of LANsegments can create loops in thesystem.

Bridging Loops Can Result inInaccurate Forwarding and Learningin Transparent Bridgingin Transparent BridgingEnvironments .

Step 1. Station-A sends a frame to Station-B. Both the bridges forwardthe frame to LAN Y and update the table with the source address of Athe frame to LAN Y and update the table with the source address of A.

Step 2. Now there are two copies of the frame on LAN-Y. The copy sentby Bridge a is received by Bridge b and vice versa As both theby Bridge-a is received by Bridge-b and vice versa. As both thebridges have no information about Station B, both will forward theframes to LAN-X.

Step 3. Again both the bridges will forward the frames to LAN-Ybecause of the lack of information of the Station B in their databaseand again Step-2 will be repeated, and so on.

So, the frame will continue to loop around the two LANsindefinitely.

Looping problem Is avoided by using Blocking ports( f i d t f th t )(no frame is send out of these ports).

Spanning Tree AlgorithmSpanning Tree Algorithm IEEE 802.1d In graph theory a spanning tree is a graph in which there is no loop In graph theory, a spanning tree is a graph in which there is no loop In a bridged LAN, this means creating a topology in which each LAN

can be reached from any other LAN through one path onlycan be reached from any other LAN through one path only A LAN can be depicted as a graph, whose nodes are bridges and

LAN segments (or cables) and whose edges are the interfacesLAN segments (or cables), and whose edges are the interfacesconnecting the bridges to the LAN segments

STEPS Every bridge has a built-in ID and the bridge with smallest ID is

selected as the root bridge The algorithm tries to find the shortest path (a path with the shortest

cost) from the root bridge to every other bridge or LANTh bi i f h h h h h The combination of the shortest paths creates the shortest tree

Based on the spanning tree, we mark the forwarding ports andblocking portsblocking ports

The forwarding ports are shown as solid lines, whereas the blockedports are shown as dotted linesports are shown as dotted lines.

Spanning tree of a network of bridges

Two Layer SwitchesTwo-Layer Switches Two-layer switch performs at the physical and data link layers.

i b id i h d ll b (f ) f is a bridge, with many ports and allows better (faster) performance. A bridge with many ports may be able to allocate a unique port to

h t ti ith h t ti it i d d t titeach station, with each station on its own independent entity. It makes a filtering decision based on the MAC address of the frame

it receivedit received. It can have a buffer to hold the frames for processing. More than one station transmitting at a time More than one station transmitting at a time. It can have a switching factor that forwards the frames faster.

Types

Store-and-forward switch• Accepts frame on input linep p• Buffers it briefly,• Then routes it to appropriate output linepp p p• Delay between sender and receiverCut-through switchCut through switch• Takes advantage of destination address appearing at beginning of

frame• Switch begins repeating frame onto output line as soon as it

recognizes destination address

RoutersRouters Capable of connecting networks of different types Routers separate networks into different broadcast domains. They use the “logical address” of packets and routing tables to

determine the best path for data delivery. RIP(Routing Information Protocol)

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Back Bone NetworksBack Bone Networks A backbone network allows several LANs to be connected. In a backbone network, no station is directly connected to the

backbone; the stations are part of a LAN, and the backbonet th LANconnects the LANs.

The backbone is itself a LAN that uses a LAN protocol such asEthernet and each connection to the backbone is itself anotherEthernet and each connection to the backbone is itself anotherLAN.

The two most common architectures are the bus backbone and the The two most common architectures are the bus backbone and thestar backbone.

Bus Backboneb b kb h l f h b kb i b In a bus backbone, the topology of the backbone is a bus.

Bus backbones are normally used as a distribution backbone tot diff t b ildi i i ticonnect different buildings in an organization.

Star Backbone the topology of the backbone is a star; the backbone is just a switch the topology of the backbone is a star; the backbone is just a switch. mostly used as a distribution backbone inside a building.

Connecting Remote LANsConnecting Remote LANs Consider a situation where 2 LANS are located at some

distance. It is possible to connect these LANS by taking a full duplex leased

ti f t l h t k tconnection from telephone network operator. Use 2 bridges, one at each end of leased connection These bridges are called remote bridges These bridges are called remote bridges Useful when a company has several offices with LANs and needs to

connect them. These bridges have ports that have data rate and signal levels

compatible to telephone network standard. Th b id t bli h d t li k ti th h th l d The bridges establish a data link connection through the leased

circuit and then carry out bridge operation