William Stallings Data and Computer Communications Chapter 10 Packet Switching

Principles zCircuit switching designed for voice yResources dedicated to a particular call yMuch of the time a data connection is idle yData rate is fixed xBoth ends must operate at the same rate

Basic Operation zData transmitted in small packets yTypically 1000 octets yLonger messages split into series of packets yEach packet contains a portion of user data plus some control info zControl info yRouting (addressing) info zPackets are received, stored briefly (buffered) and past on to the next node yStore and forward

Use of Packets

Advantages zLine efficiency ySingle node to node link can be shared by many packets over time yPackets queued and transmitted as fast as possible zData rate conversion yEach station connects to the local node at its own speed yNodes buffer data if required to equalize rates zPackets are accepted even when network is busy yDelivery may slow down zPriorities can be used

Switching Technique zStation breaks long message into packets zPackets sent one at a time to the network zPackets handled in two ways yDatagram yVirtual circuit

Datagram zEach packet treated independently zPackets can take any practical route zPackets may arrive out of order zPackets may go missing zUp to receiver to re-order packets and recover from missing packets

Virtual Circuit zPreplanned route established before any packets sent zCall request and call accept packets establish connection (handshake) zEach packet contains a virtual circuit identifier instead of destination address zNo routing decisions required for each packet zClear request to drop circuit zNot a dedicated path

Virtual Circuits v Datagram zVirtual circuits yNetwork can provide sequencing and error control yPackets are forwarded more quickly xNo routing decisions to make yLess reliable xLoss of a node looses all circuits through that node zDatagram yNo call setup phase xBetter if few packets yMore flexible xRouting can be used to avoid congested parts of the network

Packet Size

Circuit v Packet Switching zPerformance yPropagation delay yTransmission time yNode delay

Event Timing

External and Internal Operation zPacket switching - datagrams or virtual circuits zInterface between station and network node yConnection oriented xStation requests logical connection (virtual circuit) xAll packets identified as belonging to that connection & sequentially numbered xNetwork delivers packets in sequence xExternal virtual circuit service xe.g. X.25 xDifferent from internal virtual circuit operation yConnectionless xPackets handled independently xExternal datagram service xDifferent from internal datagram operation

Combinations (1) zExternal virtual circuit, internal virtual circuit yDedicated route through network zExternal virtual circuit, internal datagram yNetwork handles each packet separately yDifferent packets for the same external virtual circuit may take different internal routes yNetwork buffers at destination node for re-ordering

Combinations (2) zExternal datagram, internal datagram yPackets treated independently by both network and user zExternal datagram, internal virtual circuit yExternal user does not see any connections yExternal user sends one packet at a time yNetwork sets up logical connections

External Virtual Circuit and Datagram Operation

Internal Virtual Circuit and Datagram Operation

Routing zComplex, crucial aspect of packet switched networks zCharacteristics required yCorrectness ySimplicity yRobustness yStability yFairness yOptimality yEfficiency

Performance Criteria zUsed for selection of route zMinimum hop zLeast cost ySee Stallings appendix 10A for routing algorithms

Costing of Routes

Decision Time and Place zTime yPacket or virtual circuit basis zPlace yDistributed xMade by each node yCentralized ySource

Network Information Source and Update Timing zRouting decisions usually based on knowledge of network (not always) zDistributed routing yNodes use local knowledge yMay collect info from adjacent nodes yMay collect info from all nodes on a potential route zCentral routing yCollect info from all nodes zUpdate timing yWhen is network info held by nodes updated yFixed - never updated yAdaptive - regular updates

Routing Strategies zFixed zFlooding zRandom zAdaptive

Fixed Routing zSingle permanent route for each source to destination pair zDetermine routes using a least cost algorithm (appendix 10A) zRoute fixed, at least until a change in network topology

Fixed Routing Tables

Flooding zNo network info required zPacket sent by node to every neighbor zIncoming packets retransmitted on every link except incoming link zEventually a number of copies will arrive at destination zEach packet is uniquely numbered so duplicates can be discarded zNodes can remember packets already forwarded to keep network load in bounds zCan include a hop count in packets

Flooding Example

Properties of Flooding zAll possible routes are tried yVery robust zAt least one packet will have taken minimum hop count route yCan be used to set up virtual circuit zAll nodes are visited yUseful to distribute information (e.g. routing)

Random Routing zNode selects one outgoing path for retransmission of incoming packet zSelection can be random or round robin zCan select outgoing path based on probability calculation zNo network info needed zRoute is typically not least cost nor minimum hop

Adaptive Routing zUsed by almost all packet switching networks zRouting decisions change as conditions on the network change yFailure yCongestion zRequires info about network zDecisions more complex zTradeoff between quality of network info and overhead zReacting too quickly can cause oscillation zToo slowly to be relevant

Adaptive Routing - Advantages zImproved performance zAid congestion control (See chapter 12) zComplex system yMay not realize theoretical benefits

Classification zBased on information sources yLocal (isolated) xRoute to outgoing link with shortest queue xCan include bias for each destination xRarely used - do not make use of easily available info yAdjacent nodes yAll nodes

Isolated Adaptive Routing

ARPANET Routing Strategies(1) zFirst Generation y1969 yDistributed adaptive yEstimated delay as performance criterion yBellman-Ford algorithm (appendix 10a) yNode exchanges delay vector with neighbors yUpdate routing table based on incoming info yDoesn't consider line speed, just queue length yQueue length not a good measurement of delay yResponds slowly to congestion

ARPANET Routing Strategies(2) zSecond Generation y1979 yUses delay as performance criterion yDelay measured directly yUses Dijkstra’s algorithm (appendix 10a) yGood under light and medium loads yUnder heavy loads, little correlation between reported delays and those experienced

ARPANET Routing Strategies(3) zThird Generation y1987 yLink cost calculations changed yMeasure average delay over last 10 seconds yNormalize based on current value and previous results

X.25 z1976 zInterface between host and packet switched network zAlmost universal on packet switched networks and packet switching in ISDN zDefines three layers yPhysical yLink yPacket

X.25 - Physical zInterface between attached station and link to node zData terminal equipment DTE (user equipment) zData circuit terminating equipment DCE (node) zUses physical layer specification X.21 zReliable transfer across physical link zSequence of frames

X.25 - Link zLink Access Protocol Balanced (LAPB) ySubset of HDLC ysee chapter 7

X.25 - Packet zExternal virtual circuits zLogical connections (virtual circuits) between subscribers

X.25 Use of Virtual Circuits

Virtual Circuit Service zVirtual Call yDynamically established zPermanent virtual circuit yFixed network assigned virtual circuit

Virtual Call

Packet Format

Multiplexing zDTE can establish 4095 simultaneous virtual circuits with other DTEs over a single DTC-DCE link zPackets contain 12 bit virtual circuit number

Virtual Circuit Numbering

Flow and Error Control zHDLS (Chapter 7)

Packet Sequences zComplete packet sequences zAllows longer blocks of data across network with smaller packet size without loss of block integrity zA packets yM bit 1, D bit 0 zB packets yThe rest zZero or more A followed by B

Reset and Restart zReset yReinitialize virtual circuit ySequence numbers set to zero yPackets in transit lost yUp to higher level protocol to recover lost packets yTriggered by loss of packet, sequence number error, congestion, loss of network internal virtual circuit zRestart yEquivalent to a clear request on all virtual circuits yE.g. temporary loss of network access

Required Reading zStalling Chapter 10 zX.25 info from ITU-T web site zRouting information from Comer