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 stations Computer, terminal, phone, etc. Computer, terminal, phone, etc. A collection of nodes and connections is a communications network Data routed by being switched from node to node

2. Switching Nodes Nodes may connect to other nodes only (internal), or to stations and other nodes Node-to-node links usually multiplexed FDM or TDM FDM or TDM Network is usually not fully connected Partially connected Partially connected Some redundant connections desirable for reliability Some redundant connections desirable for reliability Two different switching technologies Circuit switching Circuit switching Packet switching Packet switching

3. A Simple Switched Network

4. Circuit Switching Basic Concepts

5. Circuit Switching Dedicated communication path provided between two stations Three phases Circuit establishment Circuit establishment Free channel must be allocated for each leg in the route Data transfer Data transfer Data may be analog or digital, depending upon network type Digital transmission for voice and data becoming dominant Typically full-duplex Circuit disconnect Circuit disconnect As requested by one of two stations involved Action propagated to deallocate the dedicated resources Must have switching capacity and channel capacity between each pair of switching nodes on path to establish connection Switches must have intelligence to make allocations and device route through network

6. Circuit Switching - Applications Inefficient Channel capacity dedicated for duration of connection Channel capacity dedicated for duration of connection When no data, capacity wasted When no data, capacity wasted Setup (connection) takes time But once connected, transfer is transparent! But once connected, transfer is transparent! Developed for voice traffic (phone) Now widely used for data traffic Now widely used for data traffic Best-known example is public telephone network Best-known example is public telephone network Substantial data traffic from modems Gradually being converted to digital network Another example is private branch exchange (PBX) Another example is private branch exchange (PBX) Interconnect phones within a building or office

7. Public Circuit-Switched Network

8. Telecomm Network Components Subscriber Devices attached to network Devices attached to network Local Loop a.k.a. subscriber loop or local loop a.k.a. subscriber loop or local loop Connection to network Connection to networkExchange Switching centers within the network Switching centers within the network End office – switching center supporting subscribers End office – switching center supporting subscribers Intermediate switching nodes in-between Intermediate switching nodes in-betweenTrunks Branches between exchanges Branches between exchanges Carry multiple voice-frequency circuits via FDM or synchronous TDM Carry multiple voice-frequency circuits via FDM or synchronous TDM

9. Circuit Establishment

10. Elements of a Circuit-Switching Node Digital Switch Provide transparent signal path between devices Network interface Functions and hardware to connect digital devices (computers, digital telephones) to network Analog phones can also be attached if interface logic includes converter Control logic Establishes connections on demand of attached device Handle and acknowledge requests Must determine if destination free and construct path through switch Maintains connections Terminates connections at request of attached device or for its own reasons

11. Blocking vs. Non-blocking Blocking A network is unable to connect stations because all paths are in use A network is unable to connect stations because all paths are in use A blocking network allows this situation to occur A blocking network allows this situation to occur Used on voice systems Used on voice systems Tolerance when assuming short-duration calls Non-blocking Permits all stations to connect (in pairs) at once Permits all stations to connect (in pairs) at once Used in some data applications Used in some data applications

12. Space-Division Switching Background Originally developed for analog environment, but carried over into digital realm Originally developed for analog environment, but carried over into digital realm Principles same whether switch carries analog or digital signals Separate physical paths (divided in “space”) Separate physical paths (divided in “space”) Each connection requires establishment of physical path through switch dedicated to data transfer between 2 endpoints Each connection requires establishment of physical path through switch dedicated to data transfer between 2 endpoints Basic building block is crossbar switch Basic building block is crossbar switch Crossbar switch Number of crosspoints grows as square of number of stations Number of crosspoints grows as square of number of stations Loss of crosspoint prevents connection Loss of crosspoint prevents connection Inefficient use of crosspoints Inefficient use of crosspoints All stations connected, yet only a few crosspoints in use Non-blocking Non-blocking

13. Crossbar Switch

14. Multistage Switch Designed to overcome crossbar limitations Scalability Scalability Resilience to failed crosspoints Resilience to failed crosspoints Utilization of crosspoints Utilization of crosspointsCharacteristics Reduced number of crosspoints Reduced number of crosspoints More than one path through network More than one path through network Increased reliability But, more complex control scheme required But, more complex control scheme required May be blocking May be blocking Can be made non-blocking by increasing number and/or size of intermediate switches Can be made non-blocking by increasing number and/or size of intermediate switches At increased cost

15. Three-Stage Switch NOTE: 48 crosspoints instead of 100 in crossbar switch

16. Time Division Switching modern digital systems use intelligent control of space & 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 individual pieces manipulated by control logic to flow from input to output

17. Softswitch Architecture Recent trend in circuit-switching technology is Softswitch General-purpose computer running software to make it a smart phone switch General-purpose computer running software to make it a smart phone switch Lower costs and greater functionality in addition to handling traditional circuit-switching functions Packetizing of digitized voice data Packetizing of digitized voice data Allowing voice over IP Allowing voice over IP Most complex part of telephone network switch is software that controls call processing Call routing Call routing Call processing logic Call processing logic Typically running on proprietary processor Typically running on proprietary processor Difference with Softswitch Separate call processing from h/w switching function of switch Separate call processing from h/w switching function of switch Physical switching performed by media gateway (MG) Physical switching performed by media gateway (MG) Call processing performed by media gateway controller (MGC) Call processing performed by media gateway controller (MGC) MG and MGC perhaps from different vendors; standard protocol MG and MGC perhaps from different vendors; standard protocol

18. Traditional Circuit Switching

19. Softswitch

20. Packet Switching Basic Concepts

21. Principles Circuit switching designed for voice Resources dedicated to a particular call Resources dedicated to a particular call Fairly high utilization with talking Fairly high utilization with talking Shortcomings with data connections Shortcomings with data connections Much of the time the line is idle Inefficient use of resources Inefficient use of resources Data rate is fixed Both ends must operate at same rate Both ends must operate at same rate Limits utility in interconnecting variety of host computers Limits utility in interconnecting variety of host computers Packet switching is far better for data

22. Basic Operation of Packet Switching Data transmitted in small packets e.g. packet length of 1000 octets e.g. packet length of 1000 octets Longer messages split into series of packets Longer messages split into series of packets Each packet contains a portion of user data from message, plus some control info Each packet contains a portion of user data from message, plus some control info Control info Routing (addressing) info Routing (addressing) info At each switching node Each packet received, stored briefly (buffered), and passed on to next node Each packet received, stored briefly (buffered), and passed on to next node Store and forward Store and forward

23. Use of Packets

24. Advantages Line efficiency Single node-to-node link can be shared by many packets over time Single node-to-node link can be shared by many packets over time Packets queued and transmitted as fast as possible Packets queued and transmitted as fast as possible Data rate conversion Each station connects to local node at its own speed Each station connects to local node at its own speed Nodes buffer data if required to equalize rates Nodes buffer data if required to equalize rates Packets accepted even when network is busy Unlike blocking of calls in circuit switching Unlike blocking of calls in circuit switching But, delivery may slow down (i.e. increased delay) But, delivery may slow down (i.e. increased delay) Priorities can be used Useful when packets queued, as can send higher- priority packets first when output link available Useful when packets queued, as can send higher- priority packets first when output link available

25. Switching Technique Station breaks long message into packets Packets sent one at a time to the network Packets handled in one of two approaches Datagram Datagram Virtual circuit Virtual circuit

26. Datagram Basic characteristics Each packet treated independently Each packet treated independently Packets can take any practical route Packets can take any practical route Packets may arrive out of order Packets may arrive out of order Some get through faster than others Packets may go missing Packets may go missing e.g. momentary crash of switching node may cause all queued packets on it to be lost Up to receiver to re-order packets and recover from missing packets Up to receiver to re-order packets and recover from missing packets

27. Example: Datagram

28. Virtual Circuit Basic characteristics Not a dedicated path Not a dedicated path Preplanned route is established before any data packets sent Preplanned route is established before any data packets sent First, call request and call accept packets establish connection (handshake) First, call request and call accept packets establish connection (handshake) Then, each data packet contains a virtual circuit identifier (VCI) instead of destination address Then, each data packet contains a virtual circuit identifier (VCI) instead of destination address No routing decisions required for each packet Clear request packet used to drop circuit Clear request packet used to drop circuit

29. Example: Virtual Circuit

30. Virtual Circuit vs. Datagram Virtual circuit Network can provide sequencing and error control Network can provide sequencing and error control By design, packets arrive in order Retransmission request for missing packets Data packets forwarded more quickly Data packets forwarded more quickly No routing decisions to make Less reliable Less reliable Loss of switching node loses all virtual circuits through that node Datagram No call setup phase No call setup phase Better if few packets More flexible More flexible Routing with each data packet permits it to avoid congested or failed parts of network

31. Packet Size A virtual circuit from station X through nodes a and b to station Y Each packet contains 40 octets and 3 octets of control information Relationship between packet size and transmission time Smaller packets provide more potential for concurrency in network when multiple hops Smaller packets provide more potential for concurrency in network when multiple hops Reduces msg. trans. time Reduces msg. trans. time However, law of diminishing returns applies as usual However, law of diminishing returns applies as usual At some point, too small a packet starts to increase total transmission time again  Of course, other factors also in play with change in packet size e.g. smaller packets exhibit higher ratio of overhead  e.g. smaller packets exhibit higher ratio of overhead 

32. Circuit vs. Packet Switching Delay factors in performance Propagation delay Propagation delay As we’ve seen, speed of signal through medium e.g. 2x10 8 meters/sec through a wire e.g. 2x10 8 meters/sec through a wire A function of distance and medium Transmission time Transmission time As we’ve seen, time for transmitter to emit block of data Clearly a function of packet size and data rate Node delay Node delay Processing delay of node to switch data A function of technology and perhaps packet size

33. Event Timing

34. 34

35. External and Internal Operation Can consider datagrams vs. virtual circuits at two levels Internal vs. external Internal vs. external They need not necessarily be the same at both levels They need not necessarily be the same at both levels Interface between station and network switching node Connection-oriented service Connection-oriented service Station requests logical connection (virtual circuit) All packets identified as belonging to that connection and sequentially numbered Network delivers packets in sequence External VC service e.g. X.25 Different from internal VC operation Connectionless service Connectionless service Packets handled independently External datagram service Different from internal datagram operation Basically, the logical behavior (VC or datagram) provided by service to upper layer (“user”) is the external service (DG vs. VC) Basically, the logical behavior (VC or datagram) provided by service to upper layer (“user”) is the external service (DG vs. VC)

36. External VC and Datagram Operation inside the network fabric (i.e. cloud) is not necessarily the same as external view.

37. Internal VC and Datagram

38. X.25 ITU-T standard for interface between host and packet switched network almost universal on packet switched networks and packet switching in ISDN defines three layers Physical Physical Link Link Packet Packet

39. X.25 - Physical interface between station node link two ends are distinct Data Terminal Equipment DTE (user equipment) Data Terminal Equipment DTE (user equipment) Data Circuit-terminating Equipment DCE (node) Data Circuit-terminating Equipment DCE (node) physical layer specification is X.21 can substitute alternative such as EIA-232

40. X.25 - Link Link Access Protocol Balanced (LAPB) Subset of HDLC Subset of HDLC see chapter 7 see chapter 7 provides reliable transfer of data over link sending as a sequence of frames

41. X.25 - Packet provides a logical connections (virtual circuit) between subscribers all data in this connection form a single stream between the end stations established on demand termed external virtual circuits

42. X.25 Use of Virtual Circuits

43. Virtual-Circuit Service Logical connection between two stations External virtual circuit External virtual circuit Specific preplanned route through network Internal virtual circuit Internal virtual 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 stream All data considered part of stream

44. User Data and X.25 Protocol Control Information

45. Issues with X.25 key features include: call control packets, in band signaling call control packets, in band signaling multiplexing of virtual circuits at layer 3 multiplexing of virtual circuits at layer 3 layers 2 and 3 include flow and error control layers 2 and 3 include flow and error control hence have considerable overhead not appropriate for modern digital systems with high reliability

46. PSN Example: Frame Relay Designed to be more efficient than X.25 Developed before ATM Developed before ATM Larger installed base than ATM Larger installed base than ATM Key features of X.25 Call-control packets, in-band signaling Call-control packets, in-band signaling Multiplexing of virtual circuits at layer 3 Multiplexing of virtual circuits at layer 3 Layer 2 and 3 include flow and error control Layer 2 and 3 include flow and error controlWeaknesses Considerable overhead Considerable overhead Not particularly appropriate for modern digital communications systems with high reliability at high speed Not particularly appropriate for modern digital communications systems with high reliability at high speed

47. Frame Relay – Key Differences Streamlines the communications process Call-control signaling carried on separate logical connection from user data Intermediate nodes in fabric relieved of much work in maintaining state tables and processing Intermediate nodes in fabric relieved of much work in maintaining state tables and processing Multiplexing and switching is at layer 2 Eliminates one layer of processing Eliminates one layer of processing No hop-by-hop error or flow control End-to-end flow and error control (if used) performed by higher layer End-to-end flow and error control (if used) performed by higher layer Single user data frame sent from source to destination and ACK carried back No hop-by-hop exchanges of data and ACK frames No hop-by-hop exchanges of data and ACK frames

48. End-to-End vs Hop-by-hop

49. Advantages and Disadvantages Lost link by link error and flow control Increased reliability makes this less a problem Increased reliability makes this less a problem Streamlined communications process Lower delay Lower delay Higher throughput Higher throughput Better solution for higher data rates, reliable links

50. Protocol Architecture

51. Control Plane Between subscriber and network Separate logical channel used Similar to common channel signaling for circuit- switching services Similar to common channel signaling for circuit- switching services Data link layer LAPD (Q.921) LAPD (Q.921) Reliable data link control Reliable data link control Error and flow control Error and flow control Between user (TE) and network (NT) Between user (TE) and network (NT) Used for exchange of Q.933 control signal messages Used for exchange of Q.933 control signal messages

52. User Plane End-to-end functionality Transfer of info between ends LAPF (Link Access Procedure for Frame Mode Bearer Services) Q.922 Called “LAPF Core” for minimum-function protocol Called “LAPF Core” for minimum-function protocol Frame delimiting, alignment and transparency Frame delimiting, alignment and transparency Frame mux and demux using addressing field Frame mux and demux using addressing field Ensure frame is integral number of octets (zero bit insertion/extraction) Ensure frame is integral number of octets (zero bit insertion/extraction) Ensure frame is neither too long nor short Ensure frame is neither too long nor short Detection of transmission errors Detection of transmission errors Congestion control functions Congestion control functions LAPF Core is similar to LAPB, LAPD, HDLC, etc. LAPF Core is similar to LAPB, LAPD, HDLC, etc. Except no control field! Except no control field!

53. Frame Relay Data Link Connections logical connection between subscribers data transferred over them not protected by flow or error control uses separate connection for call control overall results in significantly less work in network

54. User Data Transfer only have one frame type which carries user data carries user data no control frames means no inband signaling no inband signaling no sequence numbers no sequence numbers flag and FCS function as in HDLC address field carries DLCI DLCI (Data Link Connection Identifier) has local significance only

55. User Data Transfer The information field carries higher-layer data. If the user selects to implement additional data link control functions end to end then a data link frame can be carried in this field. then a data link frame can be carried in this field. A common selection will be to use the full LAPF protocol (known as LAPF control protocol), to perform functions above the LAPF core functions. Note that the protocol implemented in this fashion is strictly between the end subscribers and is transparent to the frame relay network. 55

56. LAPF-Core Formats 56

57. LAPF-Core Address Formats The address field has a default length of 2 octets may be extended to 3 or 4 octets. may be extended to 3 or 4 octets. It carries a data link connection identifier (DLCI) of 10, 16, or 23 bits. It carries a data link connection identifier (DLCI) of 10, 16, or 23 bits. The DLCI serves the same function as the virtual circuit number in X.25 It allows multiple logical frame relay connections to be multiplexed over a single channel. It allows multiple logical frame relay connections to be multiplexed over a single channel. As in X.25, the connection identifier has only local significance: Each end of the logical connection assigns its own DLCI from the pool of locally unused numbers, and the network must map from one to the other. Each end of the logical connection assigns its own DLCI from the pool of locally unused numbers, and the network must map from one to the other. The alternative, using the same DLCI on both ends, would require some sort of global management of DLCI values. The alternative, using the same DLCI on both ends, would require some sort of global management of DLCI values.

58. LAPF-Core Address Formats The length of the Address field, and hence of the DLCI, is determined by the Address field extension (EA) bits. The C/R bit is application specific and not used by the standard frame relay protocol. The remaining bits in the address field have to do with congestion control 58