Komunikasi Data dan Jaringan Komputer (Bagian 2)
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Transcript of Komunikasi Data dan Jaringan Komputer (Bagian 2)
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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
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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
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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
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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:
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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
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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.
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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)
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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.
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Analysis of stop-and-waitAnalysis of stop-and-wait
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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
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Analysis of stop-and-waitAnalysis of stop-and-wait
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
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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
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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
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Operation of sliding windowOperation of sliding window
Receiving Window: The receiver maintains a receiving window
corresponding to the sequence numbers of frames that are accepted
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Operation of sliding windowOperation of sliding window
Operations at the sender:Operations at the sender:
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Operation of sliding windowOperation of sliding window
Operations at the sender:Operations at the sender:
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Operation of sliding windowOperation of sliding window
Operations at the receiverOperations at the receiver
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Operation of sliding windowOperation of sliding window
Operations at the receiverOperations at the receiver
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Operation of sliding windowOperation of sliding window
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
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Analysis of sliding windowAnalysis of sliding window
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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.
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Error detection techniquesError detection techniques
Error Detection Techniques: Parity Checks Cyclic Redundancy Check (CRC)
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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 =
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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
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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
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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
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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
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ARQ error controlARQ 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
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ARQ error controlARQ error control
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)
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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
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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
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Stop-and-wait ARQStop-and-wait ARQ
Lost frameLost frame
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Stop-and-wait ARQStop-and-wait ARQ
Lost ACKLost ACK
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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
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Go-back-N ARQGo-back-N ARQ
Lost frameLost frame
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Go-back-N ARQGo-back-N ARQ
Lost ACKLost ACK
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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
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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
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Example of Go-back-N ARQExample of Go-back-N ARQ
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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
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Selective-repeat ARQSelective-repeat ARQ
Lost frameLost frame
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Example of Selective-repeat ARQExample of Selective-repeat ARQ
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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.
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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
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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
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Simplified Network ModelSimplified Network Model
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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
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NetworkingNetworking
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
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NetworkingNetworking
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
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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
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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
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Simple Switched NetworkSimple Switched Network
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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
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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)
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Public Circuit Switched NetworkPublic Circuit Switched Network
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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
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Circuit Switch ElementsCircuit Switch Elements
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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
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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
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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
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Crossbar MatrixCrossbar Matrix
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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
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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
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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
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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
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Use of PacketsUse of Packets
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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
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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
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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
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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
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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
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Circuit v Packet SwitchingCircuit v Packet Switching
PerformancePerformance Propagation delayPropagation delay Transmission timeTransmission time Node delayNode delay
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External Virtual Circuit and Datagram External Virtual Circuit and Datagram OperationOperation
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Internal Virtual Circuit and Datagram Internal Virtual Circuit and Datagram OperationOperation