Chapter 10. Circuits Switching and Packet Switching 10-1

Transcription

1 Chapter 10 Circuits Switching and Packet Switching 10-1

2 Content Switched communication networks Circuit switching networks Circuit-switching concepts Packet-switching principles X.25 (mentioned but not covered) Frame relay (mentioned but not covered) 10-2

3 Switching Networks Long distance transmission is typically done over a network of switched nodes Nodes not concerned with content of data End devices are stations Computer, terminal, phone, etc. A collection of nodes and connections is a communications network Data routed by being switched from node to node 10-3

4 Nodes Nodes may connect to other nodes only, or to stations and other nodes Node to node links usually multiplexed Network is usually partially connected Some redundant connections are desirable for reliability Two different switching technologies Circuit switching Packet switching 10-4

5 Simple Switched Network 10-5

6 Circuit Switching Dedicated communication path between two stations Three phases Establish Transfer Disconnect Must have switching capacity and channel capacity to establish connection Must have intelligence to work out routing 10-6

7 Circuit Switching (II) Inefficient Channel capacity dedicated for duration of connection If no data, capacity wasted Set up (connection) takes time Once connected, transfer is transparent Developed for voice traffic (phone) 10-7

8 Telecommunications Components Subscriber Devices attached to network Subscriber line Local Loop or subscriber loop Connection to network Few km up to few tens of km Exchange Switching centers End office - supports subscribers Trunks Branches between exchanges Multiplexed 10-8

9 Circuit Establishment 10-9

10 Circuit Switching Concepts Digital Switch Provide transparent signal path between devices Network Interface Control Unit Establish connections Generally on demand Handle and acknowledge requests Determine if destination is free Construct path Maintain connection Disconnect 10-10

11 Blocking or Non-blocking Blocking A network is unable to connect stations because all paths are in use A blocking network allows this Used on voice systems Short duration calls Non-blocking Permits all stations to connect (in pairs) at once Used for some data connections 10-11

12 Space Division Switching Developed for analog environment Separate physical paths Crossbar switch Number of crosspoints grows as square of number of stations Loss of crosspoint prevents connection Inefficient use of crosspoints All stations connected, only a few crosspoints in use Non-blocking 10-12

13 Space Division Switch 10-13

14 Multistage Switch Reduced number of crosspoints More than one path through network Increased reliability More complex control May be blocking 10-14

15 Time Division Switching Modern digital systems rely on intelligent control of space and time division elements Use time division multiplexing to achieve switching. Two popular methods Time slot interchange TDM bus 10-15

16 Packet Switching Principles Circuit switching designed for voice Resources dedicated to a particular call Much of the time a data connection is idle Data rate is fixed Both ends must operate at the same rate Data transmitted in small packets Typically 1000 octets Longer messages split into series of packets Each packet contains a portion of user data plus some control information Control information At least, routing (addressing) information Packets are received, stored briefly (buffered) and past on to the next node Store and forward 10-16

17 Use of Packets 10-17

18 Advantages 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 local node at its own speed Nodes buffer data if required to equalize rates Packets are accepted even when network is busy Delivery may slow down Priorities can be used 10-18

19 Switching Technique Station breaks long message into packets Packets sent one at a time to the network Packets handled in two ways Datagram Virtual circuit 10-19

20 Datagram Each packet treated independently Packets can take any practical route Packets may arrive out of order Packets may go missing Up to receiver to re-order packets and recover from missing packets 10-20

21 Virtual Circuit Preplanned route established before any packets sent Call request and call accept packets establish connection (handshake) Each packet contains a virtual circuit identifier instead of destination address No routing decisions required for each packet Clear request to drop circuit Not a dedicated path 10-21

22 What is the difference between the virtual circuit switching we just saw and real circuit switching? 10-22

23 Virtual Circuits vs. Datagram Virtual circuits Network can provide sequencing and error control Packets are forwarded more quickly No routing decisions to make Less reliable Loss of a node loses all circuits through that node Datagram No call setup phase Better if few packets More flexible Routing can be used to avoid congested parts of the network 10-23

24 Circuit vs. Packet Switching Performance Propagation delay Transmission time Node delay 10-24

25 Packet Size 10-25

26 Circuit v Packet Switching performance depends on various delays propagation delay transmission time node delay range of other characteristics, including: transparency amount of overhead 10-26

27 Chapter 11 Asynchronous Transfer Mode (ATM) 10-27

28 ATM Cell ATM divides all data into small, fixed-size cells. Each cell contains exactly 53-octets. 5 octets of header 48 octets of data 10-28

29 An ATM Cell 10-29

30 ATM Logical Connections Connection oriented service VCC (Virtual Channel Connection) Logical connection in ATM A VCC is set up between two end users through the network and a variable rate, full-duplex flow of fixedsize cells is exchanged over the connection. VPC (Virtual Path Connection) A bundle of VCCs that have the same endpoints. VPI (VP Identifier) VCI (VC Identifier) 10-30

31 TPs, VPs, and VCs TP: physical Transmission Path 10-31

32 Architecture of an ATM Network UNI: User Network Interface NNI: Network network Interface 10-32

33 Example of VPs and VCs 10-33

34 Routing with a VPC Switch ATM switch changes the VPI/VCI in each cell it handles

35 A Conceptual View of a VPC Switch 10-35

36 ATM Layers (ATM Adaptation Layer) 10-36

37 ATM Header 10-37

38 Header Fields GFC (General Flow Control) for flow control PT (Payload Type) CLP (Cell Loss Priority) 0: higher priority, should not be discarded 1: lower priority HEC (Header Error Control) CRC: X 8 +X 2 +X

39 Service Classes CBR: Constant Bit Rate VBR: Variable Bit Rate ABR: Available Bit Rate UBR: Unspecified Bit Rate 10-39

40 Service Classes and Capacity of Network 10-40

41 ATM Applications (I) 10-41

42 ATM Applications (II) 10-42

43 ATM Applications (III) 10-43