Data and Computer Communications Part 3 Wide area

Data and Computer Communications Part 3 - Wide area Networks Concepts Chapter 9 Circuit Switching

Switching Networks z Long distance transmission is typically done over a network of switched nodes z Nodes are not concerned with content of data z End devices are referred to as stations y. Computer, terminal, phone, etc. z A collection of nodes and connections is a communications network z Data is routed by being switched from node to node 2

Switching Networks z Communication Network y. Switching Network (used in WAN) y. Broadcast Network (used in LAN) z Switching Networks y. Circuit-switching networks y. Packet-switching networks 3

Nodes z Nodes may connect to other nodes only, or to stations and other nodes z Node to node links are usually multiplexed z A network is usually partially connected y. But some redundant connections are desirable for reliability z Two different switching technologies exist: y. Circuit switching y. Packet switching 4

Simple Switched Network 5

Simple switching network Communication Network Node l provides switching facility (routing) Network Station l end node (source & destination) Communication is achieved by transmitting data from source to destination through a network of switching nodes Communication Network 6

Switching technology l Circuit switching n n l need a connection established between end nodes connection is maintained until one of end nodes terminates Connection is dedicated to the communication between two nodes Example : Public Switch Telephone Network (PSTN) Packet switching n n data are transmitted in short messages called packets a connection between the two end-nodes is not maintained a node-to-node link can be dynamically shared by many packets Example : Public Data Network (PDN) like X. 25, Frame Relay 7

Key differences Keys Circuit switching Packet switching data node status connection utilization data rate Prioritization dedicated single route different routes both must be ready sender ready to send is enough dedicated shared poor good fixed varies not supported 8

Public Switching Telephone Network subscriber loop end office Long-distance office subscriber loop (local loop) Intercity trunk link between subscriber and network connecting trunk (exchanges) switching center; localized support for subscriber Intercity trunk (trunks) connecting trunk branches between exchanges; carry multiple voice circuits 9

PSDN • PSDN is designed specifically for the transmission of data rather than voice • Communication is shared LAN PSDN= Public Switched Data Network LAN PSDN LAN 10

Packets data header data packet header data header packet l Messages are broken up into a series of packets l Header is used to route the packet through the network data packet 11

PSDN switching approach l Virtual Circuit n n l network protocols establish a logical route called virtual circuit packets use the same route; data arrive in order similar to circuit switching ! but the route is not dedicated need call setup Datagram n n each packet is transmitted independently network protocols route each packet as though it were a separate message packet may not arrive in order; need protocol to ensure ordering call setup is not required 12

Virtual circuit 1. 3 1. 2 1. 1 2. 3 2. 2 2. 1 1. 3 1. 2 1. 1 B l A logical connection known as virtual circuit (VC) is set up between two stations. A 2. 3 2. 2 C 2. 1 l Packets are labeled with a virtual circuit number and a sequence number virtual circuit #1 B A 2 4 3 5 1 virtual circuit #2 C 13

Datagram 1 2 3 B. 2 B. 1 C. 3 C. 2 C. 1 B. B. B l Each packet is transmitted independently. A C. 3 B. 3 C C. 1 B B. 2 4 2 A C. 2 l Packets are labeled with a destination address and may arrive out of sequence C. 1 1 B. 1 3 C. 2 5 C 14

Routing methods in packet switching l Fixed routing l l Flooding l l l network protocol establishes a logical route called virtual circuit packets use the same route; data arrive in order Random routing l l each packet is transmitted independently the route taken is random Adaptive routing l the route reacts to changing conditions within the network 15

Circuit-Switching Networks z During communication, a dedicated communication path exists between sender and receiver; e. g. , telephone system. z Communication involves 3 phases: y Circuit establishment: “Call request” propagates, hop by hop through the network, to establish a dedicated link (channels in each component link in some path, from sender to receiver) y Data transfer: continuous transfer; either analog or digital signal y Circuit disconnect: “circuit disconnect” signal to deallocate the links 16

Circuit-Switching z Definition: Communication in which a dedicated communications path is established between two devices through one or more intermediate switching nodes z Dominant in both voice and data communications today ye. g. PSTN is a circuit-switched network z Relatively inefficient (100% dedication even without 100% utilization) 17

Circuit Switching z Dedicated communication path between two stations z Three phases y. Establish y. Transfer y. Disconnect z Must have switching capacity and channel capacity to establish connection z Must have intelligence to work out routing 18

Circuit-Switching Stages z Circuit establishment z Transfer of information ypoint-to-point from endpoints to node yinternal switching/multiplexing among nodes z Circuit disconnect 19

Circuit-Switching Networks z Disadvantages: y. Both stations need to be simultaneously ready y. Inefficient use of link capacities, especially with bursty intermittent traffic z Advantages y. Low delay once circuit is established 20

Circuit-Switching Networks z Public telephone network y Best-known example of a circuit-switching network y Four generic architectural components x. Subscribers - Devices that attach to the network x. Local loop - The link between the subscriber and the network (end office). Also called subscriber loop x. Exchanges - Switching centers in the network. End offices, long-distance offices, … x. Trunks - Links between exchanges. Carry multiple voice channels by using FDM or synchronous TDM 21

Circuit Switching - Applications z Inefficient y. Channel capacity dedicated for duration of connection y. If there is no data, capacity wasted z Set up (connection) takes time z Once connected, transfer is transparent z Developed for voice traffic (phone) 22

Public Circuit Switched Network Connecting Trunk 23

Circuit-Switching Networks 24

AT&T telephone hierarchy 25

Switching Concepts z Single circuit-switched node network y. A collection of stations attached to a central switching unit y. The central switch establishes a dedicated path between any two devices that wish to communicate 26

Switching Concepts z Digital switch y Establishes a dedicated path between any two devices z Control unit y Establishes, maintains, and tears down the connection z Network interface z (circuit) switch y Blocking y Nonblocking 27

Telecommunication Components z Subscriber y. Devices attached to network z Local Loop y. Subscriber loop y. Connection to network z Exchange y. Switching centers y. End office - supports subscribers z Trunks y. Branches between exchanges y. Multiplexed 28

Circuit Switching Node's Elements 29

Circuit Switching Node's Elements z Digital Switch y. Provides transparent signal path between devices z Network Interface (functions & hardware needed to connect devices to the network) z Control Unit; its function is to: y. Establish connections x. Generally on demand x. Handle and acknowledge requests x. Determine if destination is free xconstruct path y. Maintain connection y. Disconnect 30

Circuit Switching Node: Digital Switch z Provides transparent signal path between any pair of attached devices z Typically full-duplex 31

Circuit-Switching Node: Network Interface z Provides hardware and functions to connect digital devices to switch z Analog devices can be connected if interface includes CODEC functions z Typically full-duplex 32

Circuit-Switching Node: Control Unit z Establishes on-demand connections z Maintains connection while needed z Breaks down connection on completion 33

Blocking or Non-blocking z An important characteristic of a circuit-switching device is whether it is blocking or non-blocking. z Blocking y. A network is unable to connect stations because all paths are in use y. Used on voice systems xexample: Short duration calls z Non-blocking y. Permits all stations to connect (in pairs) at once y. Used for some data connections 34

Blocking/Nonblocking Networks z Blocking: network is unable to connect two stations because all possible paths are already in use z Nonblocking: permits all possible connection requests because any two stations can be connected 35

There are 2 Types of Switching Techniques Internal to a Single Circuit-Switching Node: 1. Space-Division Switching 2. Time-Division Switching 36

Switching Techniques z Space-Division Switching y Developed for analog environment, but has been carried over into digital communication y Requires separate physical paths for each signal connection y Uses metallic or semiconductor “gates” z Time-Division Switching y Used in digital transmission y Utilizes multiplexing to place all signals onto a common transmission path y Bus must have higher data rate than individual I/O lines 37

Space Division Switch z Developed for the analog environment (but is now used for both analog & digital signals) z Separate physical paths for every channel (signal path) z Its basic device is the Crossbar switch y. Number of crosspoints grows as square of number of stations y. Loss of crosspoint prevents connection y. Inefficient use of crosspoints x. All stations connected, only a few crosspoints in use y. Non-blocking 38

Crossbar Matrix (switch) 39

Multistage Switch z Reduced number of crosspoints z More than one path through network y. Results in increased reliability z More complex control z May be blocking 40

Three Stage Switch 41

Time Division Switching z Partition low speed bit stream into pieces that share higher speed stream z e. g. TDM bus switching ybased on synchronous time division multiplexing y. Each station connects through controlled gates to high speed bus y. Time slot allows small amount of data onto bus y. Another line’s gate is enabled for output at the same time 42

Routing in Circuit-Switched Networks z Traditional circuit-switched model is hierarchical, sometimes supplemented with peer-to-peer trunks z Newer circuit-switched networks are dynamically routed: all nodes are peer-to-peer, making routing more complex 43

Routing in Circuit-Switching Networks z Many connections will need paths through more than one switch z Therefore, the network needs to find an appropriate route. Important factors are: y. Efficiency y. Resilience z Public telephone switches are a tree structure y. Static routing uses the same approach all the time y. Dynamic routing allows for changes in routing depending on traffic 44

Alternate Routing z Possible routes between two end offices are predefined z Originating switch selects the best route for each call z Routing paths can be fixed (1 route) or dynamic (multiple routes, selected based on current and historical traffic) 45

Alternate Routing Diagram 46

Control Signaling z Control unit manages the establishment, maintenance, and termination of signal paths z Includes signaling from subscriber to network, and signals within network z In-channel signaling uses the same channel for control signals and calls z Common-channel signaling uses independent channels for controls (e. g. SS 7) 47

Control Signaling Functions z The means by which the network is managed, calls are established, maintained, and terminated z Audible communication with subscriber z Transmission of dialed number z Call cannot be completed indication z Call ended indication z Signal to ring phone z Billing info z Equipment and trunk status info z Diagnostic info z Control of special equipment 48

Control Signal Sequence z Both phones on hook z Subscriber lifts receiver (off hook) z End office switch signaled z Switch responds with dial tone z Caller dials number z If target is not busy, ringer signal is sent to target subscriber z Feedback to caller y Ringing tone, engaged tone, unobtainable (disconnected line, etc. ) z Target accepts call by lifting receiver z Switch terminates ringing signal and ringing tone z Switch establishes connection z Connection is released when Source subscriber hangs up 49

Switch to Switch Signaling z Subscribers connected to different switches z Originating switch seizes inter-switch trunk z Off hook signal is sent on trunk, requesting a digit register at target switch (so that address may be communicated) z Terminating switch sends off hook followed by on hook (known as wink) to show register-ready status z Originating switch sends address 50

Control Signals through a Circuit-Switching Telephone Network 51

Location of Signaling z Subscriber to network y. Depends on subscriber device and switch z Within network yis concerned with management of subscriber calls and network y. More complex z Two types of control signaling are used in circuit switching networks: y. In-channel signaling y. Common channel signaling 52

In-Channel Signaling z Use same channel for signaling and call y Requires no additional transmission facilities z Inband y Uses same frequencies as voice signal y Can go anywhere a voice signal can y Impossible to set up a call on a faulty speech path (because the control signals used to set up the path have to follow same path) z Out of band y Voice signals do not use full 4 k. Hz bandwidth y Narrow signal band within 4 k. Hz used for control y Can be sent whether or not voice signals are present y Requires extra electronics y Slower signal rate (narrow bandwidth) 53

Drawbacks of In Channel Signaling z Limited transfer rate z Delay between entering address (dialing) and connection z This is overcome by use of common channel signaling 54

Common Channel Signaling z Control signals carried over paths independent of voice channel z One control signal channel (path) can carry signals for a number of subscriber channels and is therefore the common control channel for these subscriber lines (channels) z Two modes of operation are used in Common channel signaling: z Associated Mode y Common channel closely tracks inter-switch trunks z Disassociated Mode y Additional nodes (signal transfer points) y Effectively two separate networks 55

Common v. In Channel Signaling 56

Common Channel Signaling Modes 57

Signaling System Number 7 z SS 7 z It is the most widely used common channel signaling scheme z Used in ISDN z Optimized for 64 kbps digital channel network z Call control, remote control, management and maintenance z Reliable means of transfer of info in sequence z Will operate over analog and below 64 k z Point to point terrestrial and satellite links z Although the network being controlled is a circuit switched network, the control signaling is implemented 58 using packet switching technology

SS 7 Signaling Network Elements z Signaling point (SP) y. Any point in the network capable of handling SS 7 control messages z Signal transfer point (STP) y. A signaling point capable of routing control messages z Control plane y. Responsible for establishing and managing connections z Information plane y. Once a connection is set up, info is transferred in the information plane 59

Transfer Points 60

Signaling Network Structures z STP capacities y. Number of signaling links that can be handled y. Message transfer time y. Throughput capacity z Network performance y. Number of SPs y. Signaling delays z Availability and reliability y. Ability of network to provide services in the face of STP failures 61