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PrinciplesCircuit switching designed for voice
Resources dedicated to a particular callMuch of the time a data connection is idleData rate is fixed
Both ends must operate at the same rate
What if we don’t want a dedicated call, or the data rate is bursty? You want packet switching!
Basic OperationData transmitted in small packets
Typically 1000 octetsLonger messages split into series of packetsEach packet contains a portion of user data plus
some control info
Control infoRouting (addressing) info
Packets are received, stored briefly (buffered) and passed on to the next nodeStore and forward
AdvantagesLine efficiency
Single node to node link can be shared by many packets over time
Packets queued and transmitted as fast as possible
Data rate conversionEach station connects to the local node at its own
speedNodes buffer data if required to equalize rates
Packets are accepted even when network is busyDelivery may slow down
Priorities can be used
Switching TechniqueStation breaks long message into packetsPackets sent one at a time to the networkPackets handled in two ways
DatagramVirtual circuit
DatagramEach packet treated independentlyPackets can take any practical routePackets may arrive out of orderPackets may go missingUp to receiver to re-order packets and
recover from missing packets
Virtual CircuitPreplanned route established before any
packets sentCall request and call accept packets
establish connection (handshake)Each packet contains a virtual circuit
identifier instead of destination addressNo routing decisions required for each
packetClear request to drop circuitNot a dedicated path
Virtual Circuits vs DatagramVirtual circuits
Network can provide sequencing and error controlPackets are forwarded more quickly
No routing decisions to make
Less reliableLoss of a node looses all circuits through that node
DatagramNo call setup phase
Better if few packets
More flexibleRouting can be used to avoid congested parts of
the network
External and Internal OperationPacket switching - datagrams or virtual circuitsInterface between station and network node
Connection orientedStation requests logical connection (virtual circuit)All packets identified as belonging to that
connection & sequentially numberedNetwork delivers packets in sequenceExternal virtual circuit servicee.g. X.25Different from internal virtual circuit operation
ConnectionlessPackets handled independentlyExternal datagram serviceDifferent from internal datagram operation
Combinations (1)External virtual circuit (connection-
oriented), internal virtual circuitDedicated route through network
External virtual circuit, internal datagramNetwork handles each packet separatelyDifferent packets for the same external virtual
circuit may take different internal routesNetwork buffers at destination node for re-
ordering
Combinations (2)External datagram (connectionless), internal
datagramPackets treated independently by both network
and user
External datagram, internal virtual circuitExternal user does not see any connectionsExternal user sends one packet at a timeNetwork sets up logical connections
RoutingComplex, crucial aspect of packet switched
networksCharacteristics required
CorrectnessSimplicityRobustnessStabilityFairnessOptimalityEfficiency
Performance CriteriaUsed for selection of routeMinimum hopLeast cost
Dijkstra’s algorithm most commonFinds the least cost path from one starting node
to all other nodesAlgorithm can be repeated for each starting
node
Decision Time and PlaceTime
Packet or virtual circuit basis
PlaceDistributed
Made by each node
CentralizedSource
Network Information Source and Update TimingRouting decisions usually based on knowledge of
network (not always)Distributed routing
Nodes use local knowledgeMay collect info from adjacent nodesMay collect info from all nodes on a potential route
Central routingCollect info from all nodes
Update timingWhen is network info held by nodes updatedFixed - never updatedAdaptive - regular updates
Fixed RoutingSingle permanent route for each source to
destination pairDetermine routes using a least cost
algorithmRoute fixed, at least until a change in
network topology
FloodingNo network info requiredPacket sent by node to every neighborIncoming packets retransmitted on every link
except incoming linkEventually a number of copies will arrive at
destinationEach packet is uniquely numbered so duplicates
can be discardedNodes can remember packets already forwarded to
keep network load in boundsCan include a hop count in packets
Properties of FloodingAll possible routes are tried
Very robust
At least one packet will have taken minimum hop count routeCan be used to set up virtual circuit
All nodes are visitedUseful to distribute information (e.g. routing)
Random RoutingNode selects one outgoing path for
retransmission of incoming packetSelection can be random or round robinCan select outgoing path based on
probability calculationNo network info neededRoute is typically not least cost nor
minimum hop
Adaptive RoutingUsed by almost all packet switching networksRouting decisions change as conditions on the
network changeFailureCongestion
Requires info about networkDecisions more complexTradeoff between quality of network info and
overheadReacting too quickly can cause oscillationToo slowly to be relevant
Adaptive Routing - AdvantagesImproved performanceAid congestion control (See chapter 12)Complex system
May not realize theoretical benefits
ClassificationBased on information sources
Local (isolated)Route to outgoing link with shortest queueCan include bias for each destinationRarely used - do not make use of easily
available info
Adjacent nodesAll nodes
ARPANET Routing Strategies(1)First Generation
1969 – 1979?Distributed adaptiveEstimated delay as performance criterionBellman-Ford algorithm (appendix 10a) (aka
Distance Vector Routing)Node exchanges delay vector with neighborsUpdate routing table based on incoming infoDoesn't consider line speed, just queue lengthQueue length not a good measurement of delayResponds slowly to congestion
ARPANET Routing Strategies(2)
Second Generation1979 – 1987?Known as Link State RoutingUses delay as performance criterionEvery 10s node computes average delay on
each outgoing link; if changes are significant, info is flooded
Delay measured directlyUses Dijkstra’s algorithm (appendix 10a)Good under light and medium loadsUnder heavy loads, little correlation between
reported delays and those experienced
ARPANET Routing Strategies(3)Third Generation
1987 - Known as Open Shortest Path FirstLink cost calculations changedMeasure average delay over last 10 secondsNormalize based on current value and previous
results
More on these in a later course.
X.251976Interface between host and packet switched
networkAlmost universal on packet switched
networks and packet switching in ISDNDefines three layers
PhysicalLinkPacket
X.25 - PhysicalInterface between attached station and link
to nodeData terminal equipment DTE (user
equipment)Data circuit terminating equipment DCE
(node)Uses physical layer specification X.21Reliable transfer across physical linkSequence of frames
Virtual Circuit ServiceVirtual Call
Dynamically established
Permanent virtual circuitFixed network assigned virtual circuit
MultiplexingDTE can establish 4095 simultaneous virtual
circuits with other DTEs over a single DTC-DCE link
Packets contain 12 bit virtual circuit number
Reset and RestartReset
Reinitialize virtual circuitSequence numbers set to zeroPackets in transit lostUp to higher level protocol to recover lost packetsTriggered by loss of packet, sequence number
error, congestion, loss of network internal virtual circuit
RestartEquivalent to a clear request on all virtual circuitsE.g. temporary loss of network access
Review Questions1. List the main differences between circuit
switched and packet switched networks2. What are differences between datagram and
virtual circuit packet switched networks?3. What is the difference between
connectionless and connection-oriented networks?
4. Be able to perform Djkstra’s least cost algorithm
5. Be able to perform a simple flooding algorithm