Komunikasi Data dan Jaringan Komputer (Bagian 2)

Magister Teknik ElektroMagister Teknik Elektro 11

Komunikasi Data dan Jaringan Komunikasi Data dan Jaringan KomputerKomputer

(Bagian 2)(Bagian 2)

Dr. Tb. Maulana KusumaDr. Tb. Maulana [email protected]@staff.gunadarma.ac.id

http://staffsite.gunadarma.ac.id/mkusumahttp://staffsite.gunadarma.ac.id/mkusuma

22Magister Teknik ElektroMagister Teknik Elektro

Review of OSI Networking ModelReview of OSI Networking Model

Application ApplicationDataAH

Presentation PresentationData unitPH

Session SessionData unitSH

Transport TransportData unitTH

Network NetworkData unitNH

Data link Data linkData unitLH LT

Physical PhysicalBits

Physical transmission medium

DataProgram X Program Y


Data Link LayerData Link Layer

Means of activating, maintaining and Means of activating, maintaining and deactivating a reliable linkdeactivating a reliable link

Error detection and controlError detection and control

Higher layers may assume error free Higher layers may assume error free transmissiontransmission


IntroductionIntroduction

The PDU at the Data Link Layer (DL-PDU) is typically called a Frame. A Frame has a header, a data field, and a trailer

Example:


FramingFraming

Problem: Identify the beginning and the end of a frame in a bit streamSolution (bit-oriented Framing): A special bit pattern (flag) signals the beginning and the end of a frame (e.g., "01111110")

Problem: The sequence '01111110' must not appear in the data

of the frame


Bit-oriented framing and bit stuffingBit-oriented framing and bit stuffing

'Bit stuffing': If the sender detects five consecutive '1‘ it adds a '0' bit into the bit stream. The receiver removes the '0' from each occurrence of the sequence '111110'

Note: The flags itself are not bit-stuffed.


Flow controlFlow control

Flow Control is a technique for speed-matching of transmitter and receiver. Flow control ensures that a transmitting station does not overflow a receiving station with data

We will discuss two protocols for flow control: Stop-and-Wait Protocol Sliding Window Protocol

For the time being, we assume that we have a perfect channel between sender and receiver (no errors)


Stop-and-wait flow controlStop-and-wait flow control

Simplest form of flow control

In Stop-and-Wait flow control, the receiver indicates its readiness to receive data for each frame

Operations:1. Sender: Transmit a single frame2. Receiver: Transmit acknowledgment (ACK)3. Go to 1.


Analysis of stop-and-waitAnalysis of stop-and-wait



Transmission delay is the time that the sender needs to transmit a frame

Transmission delay is dependent on the size of a frame and the maximum data rate

Example:

Frame Size = 1000 bit

Data rate of network = 1 Mbps

Transmission delay = 1000 bit / 1 Mbps = 1 ms



Propagation delay is the time that a transmitted bit needs to travel from sender to the receiver

Propagation delay is only dependent on the speed of the transmission medium and the distance between sender and receiver.

Speed of light: 300000 km/sec,Speed in guided media (approx.): 200000 km/sec

Example:Distance = 1000 kmPropagation delay = 1000 km / (200000 km/sec)

= 5 ms


Sliding window flow controlSliding window flow control

Major Drawback of Stop-and-Wait Flow Control:

Only one frame can be in transmission at a time

Sliding Window Flow Control Allows transmission of multiple frames Assigns each frame a k-bit sequence number Range of sequence number is [0..2k-1], i.e., frames

are counted modulo 2k


Operation of sliding windowOperation of sliding window

Sending Window: At any instant, the sender is permitted to send

frames with sequence numbers in a certain range

The range of sequence numbers is called the sending window



Receiving Window: The receiver maintains a receiving window

corresponding to the sequence numbers of frames that are accepted



Operations at the sender:Operations at the sender:



Operations at the sender:Operations at the sender:



Operations at the receiverOperations at the receiver



Operations at the receiverOperations at the receiver



How is “flow control” achieved? Receiver can control the size of the sending

window By limiting the size of the sending window

data flow from sender to receiver can be limited

Interpretation of ACK N message: Receiver acknowledges all packets until (but

not including) sequence number N


Analysis of sliding windowAnalysis of sliding window


Error controlError control

Two basic approaches to handle bit errors:

Error-detecting codes plus retransmission (Automatic Repeat reQuest / ARQ)

Used if retransmission of corrupted data is feasible Receiver detects error and requests retransmission of a frame.

Error-correcting codes Used if retransmission of the data is not possible Data are encoded with sufficient redundancy to correct bit errors Examples: Hamming Codes, Reed Solomon Codes, etc.


Error detection techniquesError detection techniques

Error Detection Techniques: Parity Checks Cyclic Redundancy Check (CRC)


Parity checksParity checks

General Method: Append a parity bit to the end of each character in a

frame such that the total number of '1' in a character is:

even (even parity) orodd (odd parity)

Example: With ASCII code, a parity bit can be attached to an 7-bit character

ASCII "G" = 1 1 1 0 0 0 1 with even parity = with odd parity =


Cyclic-Redundancy CodesCyclic-Redundancy Codes

General Method: The transmitter generates an n-bit check sequence

number from a given k-bit frame such that the resulting (k+n)-bit frame is divisible by some number

The receiver divides the incoming frame by the same number

If the result of the division does not leave a remainder, the receiver assumes that there was no error


Cyclic-Redundancy CodesCyclic-Redundancy Codes

CRC is used by all advanced data link protocols, for the following reasons: Powerful error detection capability CRC can be efficiently implemented in

hardware


Additional facts on CRCAdditional facts on CRC

CRC can be efficiently implemented in hardware by a set of XOR gates and a shift registerThe following generator polynomials are widely used:

CRC-12: P(x) = x12 + x11 + x3 + x2 + x + 1

CRC-16: P(x) = x16 + x15 + x2 + 1CRC-CCITT: P(x) = x16 + x12 + x5 + 1CRC-32: P(x) = x32 + x26 + x23 + x22 + x16

+ x12 + x11 + x10 + x8 + x7 + x5 + x4 + x2 + x + 1


ARQ error controlARQ error control

Two types of errors: Lost frames Damaged Frames

Most Error Control techniques are based on (1) Error Detection Scheme (e.g., Parity checks, CRC), and (2) Retransmission Scheme

Error control schemes that involve error detection and retransmission of lost or corrupted frames are referred to as Automatic Repeat ReQuest (ARQ) error control



All retransmission schemes use all or a subset of the following procedures: Receiver sends an acknowledgment (ACK)

if a frame is correctly received Receiver sends a negative acknowledgment

(NAK) if a frame is not correctly received The sender retransmits a packet if an ACK is

not received within a timeout interval All retransmission schemes (using ACK, NAK

or both) rely on the use of timers



Note: Once retransmission is used, a sequence number is required for every data packet to prevent duplication of packets

Both ACKs and NAKs can be sent as special frames, or be attached to data frames going in the opposite direction (Piggybacking)


ARQ schemesARQ schemes

The most common ARQ retransmission schemes: Stop-and-Wait ARQ Go-Back-N ARQ Selective Repeat ARQ

The protocol for sending ACKs in all ARQ protocols are based on the sliding window flow control scheme


Stop-and-wait ARQStop-and-wait ARQ

Stop-and-Wait ARQ is an addition to the Stop-and-Wait flow control protocol:

Frames have 1-bit sequence numbers (SN = 0 or 1) Receiver sends an ACK (1-SN) if frame SN is

correctly received Sender waits for an ACK (1-SN) before transmitting

the next frame with sequence number 1-SN If sender does not receive anything before a timeout

value expires, it retransmits frame SN



Lost frameLost frame



Lost ACKLost ACK


Go-back-N ARQGo-back-N ARQ

Go-Back-N uses the sliding window flow control protocol. If no errors occur the operations are identical to Sliding Window

Operations: A station may send multiple frames as allowed by the

window size Receiver sends a NAKi if frame i is in error. After that,

the receiver discards all incoming frames until the frame in error was correctly retransmitted

If sender receives a NAKi it will retransmit frame i and all packets i+1, i+2,... which have been sent, but not been acknowledged






Lost ACKLost ACK


Details Go-back-N ARQDetails Go-back-N ARQ

Scenario 1:A transmits frame i, and B detects error in frame i, buthas received frames i-1, i-2,... correctly

➨ B sends NAKi

Scenario 2:Frame i is lost or B does not recognize frame iAssume that A sends frame i+1 and B receives it

➨ B sends NAKi, or A will timeout and retransmit frame i and all subsequent frames


Details Go-back-N ARQDetails Go-back-N ARQ

Scenario 3: B receives frame i and sends ACK(i+1) which is lost

➨ B may send an ACK(i+k) later which also acknowledges all frames < i+k (ACKs are “cumulative”)

orA retransmits frame i and all subsequent frames

Scenario 4: NAKi is lost ➨ A will eventually time out


Example of Go-back-N ARQExample of Go-back-N ARQ


Selective-repeat ARQSelective-repeat ARQ

Similar to Go-Back-N ARQ. However, the sender only retransmits frames for which a NAK is received

Advantage over Go-Back-N: Fewer Retransmissions.

Disadvantages: More complexity at sender and receiver Each frame must be acknowledged individually (no

cumulative acknowledgements) Receiver may receive frames out of sequence


Selective-repeat ARQSelective-repeat ARQ



Example of Selective-repeat ARQExample of Selective-repeat ARQ


Analysis of ARQ protocolsAnalysis of ARQ protocols

What is the efficiency of the discussed ARQ protocols?

A number of assumptions: ACKs and NAKs are never lost, and frames

are not dropped. Sizes of ACKs, NAKs, and frame headers are

negligible.


Error correction techniquesError correction techniques

Forward error correction (FEC)Forward error correction (FEC)

Hybrid-ARQ (H-ARQ)Hybrid-ARQ (H-ARQ) Type-I H-ARQType-I H-ARQ Type-II H-ARQType-II H-ARQ Type-III H-ARQType-III H-ARQ


NetworkingNetworking

Point to point communication not usually Point to point communication not usually practicalpractical Devices are too far apartDevices are too far apart Large set of devices would need impractical Large set of devices would need impractical

number of connectionsnumber of connections

Solution is a communications networkSolution is a communications network


Simplified Network ModelSimplified Network Model


Two types of networks at the Two types of networks at the data link layerdata link layer

Broadcast NetworksBroadcast Networks: All stations share a single : All stations share a single communication channelcommunication channel

Point-to-Point NetworksPoint-to-Point Networks: Pairs of hosts (or routers) : Pairs of hosts (or routers) are directly connectedare directly connected

Typically, local area networks (LANs) are broadcast and Typically, local area networks (LANs) are broadcast and wide area networks (WANs) are point-to-pointwide area networks (WANs) are point-to-point

Broadcast Network Point-to-Point Network



CComputer networkomputer network A collection of A collection of computing devices that are connected in computing devices that are connected in various ways in order to communicate and various ways in order to communicate and share resourcesshare resources

Usually, the connections between Usually, the connections between computers in a network are made using computers in a network are made using physical wires or cablesphysical wires or cables

However, some connections are However, some connections are wirelesswireless, , using radio waves or infrared signalsusing radio waves or infrared signals



The generic term The generic term nodenode or or hosthost refers to refers to any device on a networkany device on a network

Data transfer rateData transfer rate The speed with which The speed with which data is moved from one place on a data is moved from one place on a network to anothernetwork to another

Data transfer rate is a Data transfer rate is a key issuekey issue in in computer networkscomputer networks


Switching NetworksSwitching Networks

Long distance transmission is typically done Long distance transmission is typically done over a network of switched nodesover a network of switched nodes

Nodes not concerned with content of dataNodes not concerned with content of data

End devices are stationsEnd devices are stations Computer, terminal, phone, etc.Computer, terminal, phone, etc.

A collection of nodes and connections is a A collection of nodes and connections is a communications networkcommunications network

Data routed by being switched from node to Data routed by being switched from node to nodenode


NodesNodes

Nodes may connect to other nodes only, or Nodes may connect to other nodes only, or to stations and other nodesto stations and other nodes

Node to node links usually multiplexedNode to node links usually multiplexed

Network is usually partially connectedNetwork is usually partially connected Some redundant connections are desirable for Some redundant connections are desirable for

reliabilityreliability

Two different switching technologiesTwo different switching technologies Circuit switchingCircuit switching Packet switchingPacket switching


Simple Switched NetworkSimple Switched Network


Circuit SwitchingCircuit Switching

Dedicated communication path between two Dedicated communication path between two stationsstations

Three phasesThree phases EstablishEstablish TransferTransfer DisconnectDisconnect

Must have switching capacity and channel Must have switching capacity and channel capacity to establish connectioncapacity to establish connection

Must have intelligence to work out routingMust have intelligence to work out routing


Circuit Switching - ApplicationsCircuit Switching - Applications

InefficientInefficient Channel capacity dedicated for duration of Channel capacity dedicated for duration of

connectionconnection If no data, capacity wastedIf no data, capacity wasted

Set up (connection) takes timeSet up (connection) takes time

Once connected, transfer is transparentOnce connected, transfer is transparent

Developed for voice traffic (phone)Developed for voice traffic (phone)


Public Circuit Switched NetworkPublic Circuit Switched Network


Telecomm ComponentsTelecomm Components

SubscriberSubscriber Devices attached to networkDevices attached to network

Local LoopLocal Loop Subscriber loopSubscriber loop Connection to networkConnection to network

ExchangeExchange Switching centersSwitching centers End office - supports subscribersEnd office - supports subscribers

TrunksTrunks Branches between exchangesBranches between exchanges MultiplexedMultiplexed


Circuit Switch ElementsCircuit Switch Elements


Circuit Switching ConceptsCircuit Switching Concepts

Digital SwitchDigital Switch Provide transparent signal path between devicesProvide transparent signal path between devices

Network InterfaceNetwork Interface

Control UnitControl Unit Establish connectionsEstablish connections

Generally on demandGenerally on demand

Handle and acknowledge requestsHandle and acknowledge requests

Determine if destination is freeDetermine if destination is free

construct pathconstruct path Maintain connectionMaintain connection DisconnectDisconnect


Blocking or Non-blockingBlocking or Non-blocking

BlockingBlocking A network is unable to connect stations A network is unable to connect stations

because all paths are in usebecause all paths are in use A blocking network allows thisA blocking network allows this Used on voice systemsUsed on voice systems

Short duration callsShort duration calls

Non-blockingNon-blocking Permits all stations to connect (in pairs) at oncePermits all stations to connect (in pairs) at once Used for some data connectionsUsed for some data connections


Space Division SwitchingSpace Division Switching

Developed for analog environmentDeveloped for analog environment

Separate physical pathsSeparate physical paths

Crossbar switchCrossbar switch Number of crosspoints grows as square of number of Number of crosspoints grows as square of number of

stationsstations Loss of crosspoint prevents connectionLoss of crosspoint prevents connection Inefficient use of crosspointsInefficient use of crosspoints

All stations connected, only a few crosspoints in All stations connected, only a few crosspoints in useuse

Non-blockingNon-blocking


Crossbar MatrixCrossbar Matrix


Time Division SwitchingTime Division Switching

Partition low speed bit stream into pieces that share Partition low speed bit stream into pieces that share higher speed streamhigher speed stream

e.g. TDM bus switchinge.g. TDM bus switching based on synchronous time division multiplexingbased on synchronous time division multiplexing Each station connects through controlled gates to Each station connects through controlled gates to

high speed bushigh speed bus Time slot allows small amount of data onto busTime slot allows small amount of data onto bus Another line’s gate is enabled for output at the same Another line’s gate is enabled for output at the same

timetime


Control Signaling FunctionsControl Signaling Functions

Audible communication with subscriberAudible communication with subscriber

Transmission of dialed numberTransmission of dialed number

Call can not be completed indicationCall can not be completed indication

Call ended indicationCall ended indication

Signal to ring phoneSignal to ring phone

Billing infoBilling info

Equipment and trunk status infoEquipment and trunk status info

Diagnostic infoDiagnostic info

Control of specialist equipmentControl of specialist equipment


Control Signal SequenceControl Signal Sequence

Both phones on hookBoth phones on hook

Subscriber lifts receiver (off hook)Subscriber lifts receiver (off hook)

End office switch signaledEnd office switch signaled

Switch responds with dial toneSwitch responds with dial tone

Caller dials numberCaller dials number

If target not busy, send ringer signal to target subscriberIf target not busy, send ringer signal to target subscriber

Feedback to callerFeedback to caller Ringing tone, engaged tone, unobtainableRinging tone, engaged tone, unobtainable

Target accepts call by lifting receiverTarget accepts call by lifting receiver

Switch terminates ringing signal and ringing toneSwitch terminates ringing signal and ringing tone

Switch establishes connectionSwitch establishes connection

Connection release when Source subscriber hangs upConnection release when Source subscriber hangs up


Packet SwitchingPacket Switching

Data transmitted in small packetsData transmitted in small packets Typically 1000 octetsTypically 1000 octets Longer messages split into series of packetsLonger messages split into series of packets Each packet contains a portion of user data plus Each packet contains a portion of user data plus

some control infosome control info

Control infoControl info Routing (addressing) infoRouting (addressing) info

Packets are received, stored briefly (buffered) and past Packets are received, stored briefly (buffered) and past on to the next nodeon to the next node Store and forwardStore and forward


Use of PacketsUse of Packets


AdvantagesAdvantages

Line efficiencyLine efficiency Single node to node link can be shared by many Single node to node link can be shared by many

packets over timepackets over time Packets queued and transmitted as fast as possiblePackets queued and transmitted as fast as possible

Data rate conversionData rate conversion Each station connects to the local node at its own Each station connects to the local node at its own

speedspeed Nodes buffer data if required to equalize ratesNodes buffer data if required to equalize rates

Packets are accepted even when network is busyPackets are accepted even when network is busy Delivery may slow downDelivery may slow down

Priorities can be usedPriorities can be used


Switching TechniqueSwitching Technique

Station breaks long message into packetsStation breaks long message into packets

Packets sent one at a time to the networkPackets sent one at a time to the network

Packets handled in two waysPackets handled in two ways DatagramDatagram Virtual circuitVirtual circuit


DatagramDatagram

Each packet treated independentlyEach packet treated independently

Packets can take any practical routePackets can take any practical route

Packets may arrive out of orderPackets may arrive out of order

Packets may go missingPackets may go missing

Up to receiver to re-order packets and Up to receiver to re-order packets and recover from missing packetsrecover from missing packets


Virtual CircuitVirtual Circuit

Preplanned route established before any Preplanned route established before any packets sentpackets sent

Call request and call accept packets establish Call request and call accept packets establish connection (handshake)connection (handshake)

Each packet contains a virtual circuit identifier Each packet contains a virtual circuit identifier instead of destination addressinstead of destination address

No routing decisions required for each packetNo routing decisions required for each packet

Clear request to drop circuitClear request to drop circuit

Not a dedicated pathNot a dedicated path


Virtual Circuits v DatagramVirtual Circuits v Datagram

Virtual circuitsVirtual circuits Network can provide sequencing and error controlNetwork can provide sequencing and error control Packets are forwarded more quicklyPackets are forwarded more quickly

No routing decisions to makeNo routing decisions to make Less reliableLess reliable

Loss of a node looses all circuits through that nodeLoss of a node looses all circuits through that node

DatagramDatagram No call setup phaseNo call setup phase

Better if few packetsBetter if few packets More flexibleMore flexible

Routing can be used to avoid congested parts of the networkRouting can be used to avoid congested parts of the network


Circuit v Packet SwitchingCircuit v Packet Switching

PerformancePerformance Propagation delayPropagation delay Transmission timeTransmission time Node delayNode delay


External Virtual Circuit and Datagram External Virtual Circuit and Datagram OperationOperation


Internal Virtual Circuit and Datagram Internal Virtual Circuit and Datagram OperationOperation

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Komunikasi Data dan Jaringan Komputer (Bagian 2)