數(shù)據(jù)與計(jì)算機(jī)通信課件10circuit packet

1、William StallingsData and Computer Communications7th EditionChapter 10Circuit Switching and Packet Switching內(nèi)容 10.1 Switching Networks 10.2 Circuit-Switching Networks 10.3 Circuit-Switching Concept 10.4 Control Siganling 10.5 Softswitch Architecture 10.6 Packet-Switching Principles 10.7 X.25 10.8 Fr

2、ame Relay10.1 Switching Networks Long distance transmission is typically done over a network of switched nodes Nodes not concerned with content of data End devices are stationsComputer, terminal, phone, etc. A collection of nodes and connections is a communications network Data routed by being switc

3、hed from node to nodeNodes Nodes may connect to other nodes only, or to stations and other nodes Node to node links usually multiplexed Network is usually partially (部分的) connectedSome redundant (冗余的) connections are desirable for reliability Two different switching technologiesCircuit switchingPack

4、et switchingSimple Switched Network10.2 Circuit Switching Dedicated communication path between two stations Three phasesEstablish (建立)Transfer (傳輸數(shù)據(jù))Disconnect(拆除電路) Must have switching capacity (容量) and channel capacity to establish connection Must have intelligence to work out routingCircuit Switc

5、hing - Applications InefficientChannel capacity dedicated for duration of connectionIf no data, capacity wasted Set up (connection) takes time Once connected, transfer is transparent (透明的) Developed for voice traffic (phone)Public Circuit Switched NetworkTelecomms Components Subscriber (用戶(hù)) Devices

6、attached to network Subscriber line(本地環(huán)路) Local Loop Subscriber loop Connection to network Few km up to few tens of km Exchange(交換) Switching centers(交換中心) End office (端局) - supports subscribers Trunks Branches between exchanges (交換局之間的干線) MultiplexedCircuit EstablishmentCircuit Switch Elements10.3

7、Circuit Switching Concepts Digital Switch (數(shù)字交換的功能) Provide transparent signal path between devices Network Interface(數(shù)字設(shè)備連接到網(wǎng)絡(luò)上所需要的功能和硬件) Control Unit Establish connections Generally on demand Handle and acknowledge requests Determine if destination is free construct path Maintain connection Discon

8、nectBlocking or Non-blocking Blocking(阻塞網(wǎng)絡(luò))A network is unable to connect stations because all paths are in useA blocking network allows thisUsed on voice systems Short duration calls Non-blocking (無(wú)阻塞網(wǎng)絡(luò))Permits all stations to connect (in pairs) at onceUsed for some data connectionsSpace Division (

9、空分) Switching Developed for analog environment Separate physical paths (通路之間物理上是隔開(kāi)的) Crossbar (縱橫) switchNumber of crosspoints grows as square of number of stationsLoss of crosspoint prevents connectionInefficient use of crosspoints All stations connected, only a few crosspoints in useNon-blockingSp

10、ace Division SwitchCrossbar Matrix的缺點(diǎn) 交叉點(diǎn)的數(shù)量已相連站點(diǎn)數(shù)量的平方數(shù)上升 一個(gè)交叉點(diǎn)的損壞使得再該交叉點(diǎn)上相交的線路所屬的設(shè)備之間無(wú)法連接 交叉點(diǎn)的利用率很低Multistage Switch Reduced number of crosspoints More than one path through networkIncreased reliability More complex control May be blockingThree Stage Space Division SwitchTime Division Switching Mod

11、ern digital systems rely on intelligent control of space and time division elements Use digital time division techniques to set up and maintain virtual circuits Partition low speed bit stream into pieces that share higher speed stream(把低速的比特流分割成許多小塊,然后與其他比特流一起共享速率較高的容量) e.g. TDM bus switching(總線交換)

12、based on synchronous time division multiplexing Each station connects through controlled gates to high speed bus Time slot allows small amount of data onto bus Another lines gate is enabled for output at the same time10.4 Control Signaling Functions Audible (可聽(tīng)見(jiàn)的) communication with subscriber Trans

13、mission of dialed number Call can not be completed indication Call ended indication Signal to ring phone Billing (計(jì)費(fèi)) info Equipment and trunk (中繼線) status info Diagnostic (診斷) info Control of specialist (特殊) equipmentControl Signal Sequence Both phones on hook(掛機(jī)) Subscriber lifts receiver (off hoo

14、k) End office switch signaled Switch responds with dial tone Caller dials number If target not busy, send ringer signal to target subscriber Feedback to caller Ringing tone, engaged tone, unobtainable Target accepts call by lifting receiver Switch terminates ringing signal and ringing tone Switch es

15、tablishes connection Connection release when Source subscriber hangs upSwitch to Switch Signaling Subscribers connected to different switches Originating switch seizes interswitch trunk Send off hook signal on trunk, requesting digit register at target switch (for address) Terminating switch sends o

16、ff hook followed by on hook (wink) to show register ready Originating switch sends addressControl SignalsLocation of Signaling Subscriber to network(用戶(hù)與網(wǎng)絡(luò)之間的)Depends on subscriber device and switch Within network (網(wǎng)絡(luò)之間的)Management of subscriber calls and networkore complexIn Channel Signaling (隨路信令)

17、 Use same channel for signaling and call Requires no additional transmission facilities Inband (帶內(nèi)信令) Uses same frequencies as voice signal Can go anywhere a voice signal can Impossible to set up a call on a faulty speech path Out of band(帶外信令) Voice signals do not use full 4kHz bandwidth Narrow sig

18、nal band within 4kHz used for control Can be sent whether or not voice signals are present Need extra electronics Slower signal rate (narrow bandwidth)Drawbacks (缺點(diǎn)) of In Channel Signaling Limited transfer rate Delay between entering address (dialing) and connection Overcome by use of common channe

19、l signalingCommon Channel (共路) Signaling Control signals carried over paths independent of voice channel One control signal channel can carry signals for a number of subscriber channels Common control channel for these subscriber lines Associated Mode(相關(guān)方式) Common channel closely tracks interswitch

20、trunks Disassociated Mode (非相關(guān)方式) Additional nodes (signal transfer points) Effectively two separate networksCommon v. In Channel SignalingCommonChannelSignaling ModesSignaling System Number 7 SS7 Common channel signaling scheme ISDN Optimized for 64k digital channel network Call control, remote con

21、trol, management and maintenance Reliable means of transfer of info in sequence Will operate over analog and below 64k Point to point terrestrial and satellite linksSS7 Signaling Network Elements Signaling point (SP,信令點(diǎn))Any point in the network capable of handling SS7 control message Signal transfer

22、 point (STP ,信號(hào)傳送點(diǎn))A signaling point capable of routing control messages Control planeResponsible for establishing and managing connections Information planeOnce a connection is set up, info is transferred in the information planeTransfer PointsSignaling Network Structures STP capacitiesNumber of si

23、gnaling links that can be handledMessage transfer timeThroughput capacity Network performanceNumber of SPsSignaling delays Availability and reliabilityAbility of network to provide services in the face of STP failures10.5 Softswitch Architecture General purpose computer running software to make it a

24、 smart phone switch Lower costs Greater functionality Packetizing of digitized voice data Allowing voice over IP Most complex part of telephone network switch is software controlling call process Call routing Call processing logic Typically running on proprietary processor Separate call processing f

25、rom hardware function of switch Physical switching done by media gateway Call processing done by media gateway controllerTraditional Circuit SwitchingSoftswitch10.6 Packet Switching Principles Circuit switching designed for voiceResources dedicated to a particular callMuch of the time a data connect

26、ion is idleData rate is fixed(固定的) Both ends must operate at the same rateBasic Operation (基本操作) Data transmitted in small packetsTypically 1000 octetsLonger messages split into series of packetsEach packet contains a portion of user data plus some control info Control infoRouting (addressing) info

27、Packets are received, stored briefly (buffered) and past on to the next nodeStore and forwardUse of PacketsAdvantages Line efficiency Single node to node link can be shared by many packets over time Packets queued and transmitted as fast as possible Data rate conversion Each station connects to the

28、local node at its own speed Nodes buffer data if required to equalize (補(bǔ)償) rates Packets are accepted even when network is busy Delivery may slow down Priorities can be usedSwitching Technique Station breaks long message into packets Packets sent one at a time to the network Packets handled in two w

29、aysDatagram (數(shù)據(jù)報(bào))Virtual circuit (虛電路)Datagram Each packet treated independently(獨(dú)立,即每個(gè)分組是獨(dú)立的,和以前發(fā)送的分組沒(méi)有關(guān)系) Packets can take any practical (可用的) route Packets may arrive out of order Packets may go missing Up to receiver to re-order packets and recover from missing packetsDatagramDiagramVirtual Circ

30、uit Preplanned route established before any packets sent Call request and call accept packets establish connection (handshake) Each packet contains a virtual circuit identifier (標(biāo)識(shí)符) instead of destination address No routing decisions required for each packet Clear request to drop circuit Not a dedi

31、cated (專(zhuān)用的) pathVirtualCircuitDiagramVirtual Circuits v Datagram Virtual circuitsNetwork can provide sequencing and error controlPackets are forwarded more quickly No routing decisions to makeLess reliable Loss of a node looses all circuits through that node DatagramNo call setup phase Better if few

32、 packetsMore flexible(可擴(kuò)充性) Routing can be used to avoid congested parts of the networkPacket SizeCircuit v Packet Switching PerformancePropagation delay傳播時(shí)延)Transmission time (傳輸時(shí)間)Node delay (節(jié)點(diǎn)時(shí)延,即節(jié)點(diǎn)的處理時(shí)間)Event TimingExternal and Internal Operation Packet switching - datagrams or virtual circuits

33、 Interface between station and network node Connection oriented(面向連接) Station requests logical connection (virtual circuit) All packets identified as belonging to that connection & sequentially numbered Network delivers packets in sequence External virtual circuit service e.g. X.25 Different fro

34、m internal virtual circuit operation Connectionless(面向無(wú)連接的) Packets handled independently External datagram service Different from internal datagram operationCombinations (1) External virtual circuit, internal virtual circuitDedicated(專(zhuān)用的) route through network External virtual circuit, internal dat

35、agramNetwork handles(處理) each packet separatelyDifferent packets for the same external virtual circuit may take different internal routesNetwork buffers at destination node for re-orderingCombinations (2) External datagram, internal datagramPackets treated independently by both network and user Exte

36、rnal datagram, internal virtual circuitExternal user does not see any connectionsExternal user sends one packet at a timeNetwork sets up logical connectionsExternal Virtual Circuit andDatagram OperationInternalVirtualCircuit andDatagram Operation10.7 X.25 1976 Interface between host and packet switc

37、hed network(分組交換網(wǎng)絡(luò)) Almost universal on packet switched networks and packet switching in ISDN Defines three layersPhysicalLink (鏈路層)Packet (分組層)X.25 - Physical Interface between attached station and link to node Data terminal equipment DTE (user equipment) Data circuit terminating equipment DCE (nod

38、e) Uses physical layer specification X.21 Reliable (可靠的) transfer across physical link Sequence of frames (順序幀遞交)X.25 - Link Link Access Protocol Balanced (LAPB)Subset of HDLCsee chapter 7X.25 - Packet External virtual circuits (外部虛電路) Logical connections (virtual circuits) between subscribersX.25 U

39、se of Virtual CircuitsVirtual Circuit Service Virtual Call(虛呼叫)Dynamically established Permanent virtual circuit(永久虛電路)Fixed network assigned virtual circuitVirtual Circuit Service Logical connection between two stationsExternal virtual circuit Specific preplanned route through networkInternal virtu

40、al circuit Typically one to one relationship between external and internal virtual circuits Can employ X.25 with datagram style network External virtual circuits require logical channel All data considered part of streamVirtual CallPacket FormatMultiplexing DTE can establish 4095 simultaneous virtua

41、l circuits with other DTEs over a single DTC-DCE link Packets contain 12 bit virtual circuit number 0號(hào)總是為所有虛電路都具有的診斷分組保留；分配給永久虛電路的編號(hào)；Virtual Circuit NumberingFlow and Error Control HDLC(Chapter 7) 使用Flow Control,每個(gè)數(shù)據(jù)分組都含有一個(gè)發(fā)送序號(hào)P(S)和一個(gè)接收序號(hào)P（R） ,默認(rèn)使用的是3位序號(hào)。Packet Sequences(分組序列) Complete packet sequen

42、ces(分組序列) Allows longer blocks of data across network with smaller packet size without loss of block integrity A packets(A分組) M bit 1, D bit 0,分組是完全的,等于最大可允許分組長(zhǎng)度 B packets(B分組) The rest Zero or more A followed by B(一個(gè)完整的分組序列包含了零或者多個(gè)A分組,以及緊跟在后面的一個(gè)B分組.處理分組的方法取決于M和D比特的設(shè)置,若D=1,就是說(shuō)由接收端DTE發(fā)送了一個(gè)端對(duì)端的應(yīng)答到發(fā)送端D

43、TE,若M=1,則說(shuō)明另外還有完整的分組序列跟在后面.)Reset and Restart Reset Reinitialize(重新初始化) virtual circuit Sequence numbers(序號(hào)) set to zero Packets in transit lost Up to higher level protocol to recover lost packets Triggered(觸發(fā)) by loss of packet, sequence number error, congestion, loss of network internal virtual ci

44、rcuit Restart(重啟動(dòng)) Equivalent to(相當(dāng)于) a clear request on all virtual circuits E.g. temporary loss of network accessX.25 Levels User data passes to X.25 level 3 X.25 appends control informationHeaderIdentifies virtual circuitProvides sequence numbers for flow and error control X.25 packet passed down

45、 to LAPB entity LAPB appends further control informationUser Data and X.25 Protocol Control Information10.8 Frame Relay Designed to be more efficient than X.25 Developed before ATM Larger installed base than ATM ATM now of more interest on high speed networksFrame Relay Background - X.25 Call control packets, in band signaling Multiplexing of virtual circuits at layer 3 Layer 2 and 3 include flow and error control Considerable overhead Not appropriate for modern digital systems with high reliabilityFrame Relay - Differences Call control carried in separate logical conn