Data and Computer Communications Chapter 10 Packet Switching

Basic Operation zData transmitted in small packets 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 operation

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 yBut delivery 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 missing packets

Virtual Circuit zPreplanned route established before any packets sent zCall request and call accept packets establish connection (handshaking) [Similar to circuit switching except that it is done with packets rather than signals] zEach packet contains a virtual circuit identifier instead of destination address zNo routing decisions required for individual packets zClear request packet is used to drop circuit zNot a dedicated path (unlike circuit switching, the path may be shared)

Virtual Circuits versus 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

Effect of Packet Size on Transmission Time Small packet size decreases total transmission time; but, there is a limit to this approach because of the header. Here, d is larger than c. So, transmission time starts to increase again as a function of header to data ratio.

Circuit v Packet Switching zPerformance; 3 types of delay may affect performance: yPropagation delay: Time for a signal to propagate from one node to another (mostly negligible) yTransmission time: Time for a transmitter to transmit a block of data yNode delay: Node's data processing time (delay)

Event Timing

External and Internal Operation zOne of the most important characteristics of a packet switching network is whether it uses datagrams or virtual circuits. There are 2 dimensions of this characteristic; one for the interface between station and it corresponding network node (external), and the other for the network itself (internal). zInterface between station and network node yConnection oriented [External Virtual Circuit Service] 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 [External Datagram Service] xPackets handled independently xExternal datagram service xDifferent from internal datagram operation

Possible 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 and based on: zMinimum hop or: zLeast cost ySee Stallings appendix 10A for routing algorithms

Decision Time and Place z2 key characteristics concerned with routing decisions are: zDecision Time yPacket or virtual circuit basis meaning decision is made when packet is sent (datagram approach), or when virtual circuit is set up zDecision Place yDistributed xDecision made by each node yCentralized xDecision made by a central network node ySource xDecision made by source

Network Information Source and Update Timing zRouting decisions are usually (not not always) based on knowledge of network 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 happens

Fixed Routing Tables For each source- destination pair, the routing table shows the next node on the route.

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 zHop count is set to a maximum value. When packet passes a node, it decrements the count. Packet is discarded if the count reaches zero before reaching its destination

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 neither 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 are more complex zThe tradeoff here is between quality of network info and overhead associated with the time involved in gathering the information, etc.

Adaptive Routing - Advantages zImproved performance zAids in congestion control (covered in chapter 12)

X.25 zOriginally approved in 1976 zSpecifies interface 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 Layer zSpecifies interface 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, but often other standards such as RS232 are used instead zProvides for a reliable transfer across physical link by transmitting the data as a sequence of frames

X.25 - Link Layer zUses Link Access Protocol Balanced (LAPB) ySubset of HDLC ysee LAPB frame structure (chapter 7 page 222) and User Data and X.25 Control Information (chapter 10 page 331)

X.25 - Packet zProvides external virtual circuit service which enables logical connections (virtual circuits) between subscribers

X.25 Use of Virtual Circuits

Virtual Circuit Service zProvides for 2 types of virtual circuits: zVirtual Call yDynamically established virtual circuits using a call setup and call clearing procedure zPermanent virtual circuit yFixed network assigned virtual circuit; data transfer happens same way as virtual calls, but no call setup and clearing is required.

Virtual Call · See figure and explanations on pages · Left side shows the packets exchanged between user machine A and the packet switching node to which it is attached

Packet Format (see Page 333)

Multiplexing zMost important service provided by X.25 zPackets contain a 12 bit virtual circuit number zDTE is allowed to establish up to 4095 ( ) simultaneous virtual circuits with other DTEs over a single DTC-DCE link zNumber zero is reserved for diagnostic packets common to all virtual circuits zThe rest of the numbers are assigned by the DCE or DTE depending on which one is initiating the virtual circuit call

Virtual Circuit Numbering Used when address overflow from top or bottom happens

Flow and Error Control zLike HDLC (Chapter 7) using sliding window protocol [3 bit, 7 bit, or 15 bit sequence numbers] with: zP(S)=Send sequence number and zP(R)=Receive sequence number [number of next packet expected from the other side] zAcknowledgement has either local or end-to-end significance. zWhen D bit=0, acknowledgement is exercises between DTE and the network. zWhen D bit=1, acknowledgement is exercises between DTE and the remote DTE zThe error control scheme is Go-Back-N ARQ

Packet Sequences zX.25 provides the capacity to identify an adjacent sequence of data packets, which is called a complete packet sequence zThis allows the network to form longer blocks of data sent across network with smaller packet size without loss of block integrity zTo specify this mechanism, X.25 defines 2 types of packets: zA packets yM bit 1 (means there are additional complete packets to follow), D bit 0 zB packets yThe rest (all other packets) zIn a complete packet sequence, there are zero or more A packets followed by a B packet. The network may combine or break down this sequence to make larger or smaller packets for transmission. zSee figure 10.19

Reset and Restart zTwo methods that X.25 uses to recover from errors are: 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