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Switching Networks

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

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Switching Networks

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  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. • 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 • Network is usually not fully connected • Partially connected • Some redundant connections desirable for reliability • Two different switching technologies • Circuit 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 • Free channel must be allocated for each leg in the route • 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 • 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 • When no data, capacity wasted • Setup (connection) takes time • But once connected, transfer is transparent! • Developed for voice traffic (phone) • Now widely used for data traffic • 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) • Interconnect phones within a building or office

  7. Public Circuit-Switched Network

  8. Telecomm Network Components • Subscriber • Devices attached to network • Local Loop • a.k.a. subscriber loop or local loop • Connection to network • Exchange • Switching centers within the network • End office – switching center supporting subscribers • Intermediate switching nodes in-between • Trunks • Branches between exchanges • 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 blocking network allows this situation to occur • Used on voice systems • Tolerance when assuming short-duration calls • Non-blocking • Permits all stations to connect (in pairs) at once • Used in some data applications

  12. Space-Division Switching • Background • 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”) • Each connection requires establishment of physical path through switch dedicated to data transfer between 2 endpoints • Basic building block is crossbar switch • Crossbar switch • Number of crosspoints grows as square of number of stations • Loss of crosspoint prevents connection • Inefficient use of crosspoints • All stations connected, yet only a few crosspoints in use • Non-blocking

  13. Crossbar Switch

  14. Multistage Switch • Designed to overcome crossbar limitations • Scalability • Resilience to failed crosspoints • Utilization of crosspoints • Characteristics • Reduced number of crosspoints • More than one path through network • Increased reliability • But, more complex control scheme required • May be blocking • 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 • Lower costs and greater functionality in addition to handling traditional circuit-switching functions • Packetizing of digitized voice data • Allowing voice over IP • Most complex part of telephone network switch is software that controls call processing • Call routing • Call processing logic • Typically running on proprietary processor • Difference with Softswitch • Separate call processing from h/w switching function of switch • Physical switching performed by media gateway (MG) • Call processing performed by media gateway controller (MGC) • 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 • Fairly high utilization with talking • Shortcomings with data connections • Much of the time the line is idle • Inefficient use of resources • Data rate is fixed • Both ends must operate at same rate • 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 • Longer messages split into series of packets • Each packet contains a portion of user data from message, plus some control info • Control info • Routing (addressing) info • At each switching node • Each packet received, stored briefly (buffered), and passed on to next node • Store and forward

  23. Use of Packets

  24. 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 local node at its own speed • Nodes buffer data if required to equalize rates • Packets accepted even when network is busy • Unlike blocking of calls in circuit switching • 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

  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 • Virtual circuit

  26. Datagram • Basic characteristics • Each packet treated independently • Packets can take any practical route • Packets may arrive out of order • Some get through faster than others • 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

  27. Example:Datagram

  28. Virtual Circuit • Basic characteristics • Not a dedicated path • Preplanned route is established before any data packets sent • First, call request and call accept packets establish connection (handshake) • 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

  29. Example:Virtual Circuit

  30. Virtual Circuit vs. Datagram • Virtual circuit • Network can provide sequencing and error control • By design, packets arrive in order • Retransmission request for missing packets • Data packets forwarded more quickly • No routing decisions to make • Less reliable • Loss of switching node loses all virtual circuits through that node • Datagram • No call setup phase • Better if few packets • 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 • Reduces msg. trans. time  • 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 

  32. Circuit vs. Packet Switching • Delay factors in performance • Propagation delay • As we’ve seen, speed of signal through medium • e.g. 2x108 meters/sec through a wire • A function of distance and medium • 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 • Processing delay of node to switch data • A function of technology and perhaps packet size

  33. Event Timing

  34. External and Internal Operation • Can consider datagrams vs. virtual circuits at two levels • Internal vs. external • They need not necessarily be the same at both levels • Interface between station and network switching node • 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 • 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)

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

  36. Internal VC and Datagram

  37. 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 • Link • Packet

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

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

  40. 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

  41. X.25 Use of Virtual Circuits

  42. Virtual-Circuit Service • Logical connection between two stations • External virtual circuit • Specific preplanned route through network • 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

  43. User Data and X.25 Protocol Control Information

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

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

  46. 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 • Multiplexing and switching is at layer 2 • 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 • Single user data frame sent from source to destination and ACK carried back • No hop-by-hop exchanges of data and ACK frames

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

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

  49. Protocol Architecture

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