Chapter 9 Circuit Switching Switching Networks CircuitSwitching Concept

Chapter 9: Circuit Switching • Switching Networks • Circuit-Switching Concept Ø Space-Division Switching Ø Time-Division Switching • Routing in Circuit-Switching Networks • Control Signaling • SS 7 3/2/2021 Spring, 2003 EE 4272

Switching Networks • Long distance transmission is typically done over a network of switched nodes End node device • Nodes not concerned with content of data • End devices are stations: Computer, terminal, phone, etc. 3/2/2021 Spring, 2003 EE 4272

Switching Networks (Con’t) • A collection of nodes (switches) and connections is a communications network • Data routed by being switched from node to node • Nodes may connect to other nodes (switches) only, or to stations (hosts)and other nodes • Node to node links usually multiplexed (TDM &FDM) • Network is usually partially connected Ø Some redundant connections are desirable for reliability • Two different switching technologies Circuit switching Ø Packet switching Ø 3/2/2021 Spring, 2003 EE 4272

Circuit Switching & features • 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 • Inefficient Channel capacity dedicated for duration of connection Ø If no data, capacity wasted Ø • Set up (connection) takes time • Once connected, transfer is transparent, no delay -> • Developed for voice traffic (phone) 3/2/2021 Spring, 2003 EE 4272

Telecomm. Networks & Components • • Subscriber: Devices attached to network Local Loop: Subscriber loop -> Connection to network Exchange: Switching centers; end office supports subscribers Trunks (Multiplexed): Branches between exchanges 3/2/2021 Spring, 2003 EE 4272

Circuit Switch Concepts & Elements • Digital Switch Ø Provide transparent signal path between devices • Network Interface • Control Unit Ø Establish connections Ø Generally on demand Ø Handle and acknowledge requests Ø Determine if destination is free Ø construct path Maintain connection Ø Disconnect Ø 3/2/2021 Spring, 2003 EE 4272

Digital Switch: Blocking vs. Non-blocking • Blocking A network is unable to connect end stations because all paths are in use Ø In blocking network this is possible Ø Used on voice systems Ø Ø Short duration calls • Non-blocking Permits all stations to connect (in pairs) at once Ø Used for some data connections Ø • Switching Techniques Space-Division Switching Ø Time-Division Switching Ø 3/2/2021 Spring, 2003 EE 4272

Flat Crossbar Switch: Example of space-division Switch • Separate physical paths for signal for one another • E. g. , Crossbar switch Ø When a request comes in from an incoming line for an outgoing line, the corresponding crosspoint is closed to enable information to flow from the input to the output Ø The connection requests are never blocked due to the lack of connectivity resource (crosspoint), but a busy outgoing line rejects a connection request Ø The complexity of the crossbar switch is measured by the number of crosspoints (N 2) 3/2/2021 Spring, 2003 EE 4272

Multistage Space-Division Switch nxk 1 N inputs N/n x N/n 1 nxk N/n x N/n 2 3 2 kxn N outputs 3 kxn nxk N/n 1 kxn nxk 2 kxn N/n x N/n k • • Reduced number of crosspoints: 2(N/n)nk + k (N/n)2 crosspoints More than one path through network->Increased reliability More complex control May be blocking: can be solved by increasing # of intermediate switch 3/2/2021 Spring, 2003 EE 4272

Three Stage Switch 3/2/2021 Spring, 2003 EE 4272

Time Division Switching • All digital switching technology is base on the use of STDM • During a time slot, data are switched from the enabled input line to the enabled output line, # of time slot = # of devices • The data rate on the controller must be high enough: e. g. 100 device @ 19. 2 kbps, controller date rate at 1. 92 Mbps • Partition low speed bit stream into pieces that share higher speed stream • e. g. TDM bus switching based on synchronous time division multiplexing Ø Each station connects through controlled gates to high speed bus Ø 3/2/2021 Spring, 2003 EE 4272

Routing • Many connections will need paths through more than one switch • Need to find a route Efficiency Ø Resilience Ø • Static routing uses the same approach all the time • Dynamic routing allows for changes in routing depending on traffic 3/2/2021 Spring, 2003 EE 4272

Alternate Routing: an example routing approach • • • Possible routes between end offices predefined Originating switch selects appropriate route Routes listed in preference order Different sets of routes may be used at different times Can be implemented statically or dynamically 3/2/2021 Spring, 2003 EE 4272

Alternate Routing Diagram 3/2/2021 Spring, 2003 EE 4272

Control Signaling Functions • • Audible communication with subscriber Transmission of dialed number Call can not be completed indication Call ended indication Signal to ring phone Billing info Equipment and trunk status info Diagnostic info 3/2/2021 Spring, 2003 EE 4272

Control Signal Sequences 3/2/2021 Spring, 2003 EE 4272

Location of Signaling • Subscriber to network Ø Depends on subscriber device and switch • Within network Management of subscriber calls and network Ø more complex Ø • Mapping is needed between the two signaling techniques @ the local switching office 3/2/2021 Spring, 2003 EE 4272

In. Channel Signaling: an traditional signaling approach • Use same channel for signaling and call Ø Requires no additional transmission facilities • Inband Uses same frequencies as voice signal Ø Can go anywhere a voice signal can Ø Impossible to set up a call on a faulty speech path Ø • Out of band Voice signals do not use full 4 k. Hz bandwidth Ø Narrow signal band within 4 k. Hz used for control Ø Can be sent whether or not voice signals are present Ø Need extra electronics Ø Slower signal rate (narrow bandwidth) Ø 3/2/2021 Spring, 2003 EE 4272

Drawbacks of In. Channel Signaling • Limited control signaling transfer rate Inband signaling: when there is no voice data Ø Out-band signaling: a very narrow bandwidth is for signaling Ø • Delay between entering address (dialing) and connection • Overcome by use of common channel signaling 3/2/2021 Spring, 2003 EE 4272

Common vs. In. Channel Signaling 3/2/2021 Spring, 2003 EE 4272

Common Channel Signaling 1. Control signals carried over paths independent of voice channel 2. One control signal channel can carry signals for a number of subscriber channels • Associated Mode Ø Common channel closely tracks interswitch trunks 1. Disassociated Mode Additional nodes (signal transfer points) 2. Effectively two separate networks 3/2/2021 Spring, 2003 1. EE 4272

Signaling System Number 7 1. SS 7: a real world Common channel signaling scheme 2. Used in ISDN (Integrated Service Digital Network) 3. Optimized for 64 k digital channel network 4. Functionalities: Call control, remote control, management and maintenance 5. Reliable means of transfer of info in sequence 6. Suitable to operate over analog and below 64 k 7. Suitable to point-to-point terrestrial and satellite links 3/2/2021 Spring, 2003 EE 4272

SS 7 Signaling Network Elements • Signaling point (SP) Ø Any point in the network capable of handling SS 7 control message • Signal transfer point (STP) Ø A signaling point capable of routing control messages • Control plane Ø Responsible for establishing and managing connections • Information plane Ø Once a connection is set up, info is transferred in the information plane 3/2/2021 Spring, 2003 EE 4272