data communications tdc 362 / tdc 460 circuit switching and packet switching 1

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  • Slide 1
  • Data Communications TDC 362 / TDC 460 Circuit Switching and Packet Switching 1
  • Slide 2
  • 8.1 Circuit Switching Space-Division Switch Time-Division Switch TDM Bus Combinations 2
  • Slide 3
  • Figure 8.1 Circuit-switched network 3
  • Slide 4
  • Figure 8.2 A circuit switch 4
  • Slide 5
  • 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 5
  • Slide 6
  • Figure 8.4 Crossbar switch 6
  • Slide 7
  • Figure 8.5 Multistage switch 7
  • Slide 8
  • Figure 8.6 Switching path 8
  • Slide 9
  • Three Stage Switch 9
  • Slide 10
  • Figure 8.7 Time-division multiplexing, without and with a Time-slot interchange 10
  • Slide 11
  • Figure 8.8 Time-slot interchange 11
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  • Figure 8.9 TDM bus 12
  • Slide 13
  • Figure 8.10 TST (Time-space-time) switch 13
  • Slide 14
  • Circuit-Switched Routing Many connections will need paths through more than one switch Need to find a route Efficiency Resilience Public telephone switches are a tree structure Static routing uses the same approach all the time Dynamic routing allows for changes in routing depending on traffic Uses a peer structure for nodes 14
  • Slide 15
  • Alternate Routing 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 15
  • Slide 16
  • Alternate Routing Diagram 16
  • Slide 17
  • 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 Control of specialist equipment 17
  • Slide 18
  • Control Signals 18
  • Slide 19
  • Location of Signaling Subscriber to network Depends on subscriber device and switch Within network Management of subscriber calls and network ore complex 19
  • Slide 20
  • In Channel Signaling Use same channel for signaling and call Requires no additional transmission facilities Inband Uses same frequencies as voice signal Can go anywhere a voice signal can Impossible to set up a call on a faulty speech path Out-of-band Voice signals do not use full 4kHz bandwidth Narrow signal band within 4kHz used for control Can be sent whether or not voice signals are present Need extra electronics Slower signal rate (narrow bandwidth) 20
  • Slide 21
  • Drawbacks of In Channel Signaling Limited transfer rate Delay between entering address (dialing) and connection Overcome by use of common channel signaling 21
  • Slide 22
  • Common Channel Signaling Control signals carried over paths independent of voice channel One control signal channel can carry signals for a number of subscriber channels Common control channel for these subscriber lines Associated Mode Common channel closely tracks interswitch trunks Disassociated Mode Additional nodes (signal transfer points) Effectively two separate networks 22
  • Slide 23
  • Common vs. In Channel Signaling 23
  • Slide 24
  • Signaling Modes 24
  • Slide 25
  • Signaling System Number 7 SS7 Most widely used common channel signaling scheme Internationally standardized and general purpose 25
  • Slide 26
  • SS7 SS7 network and protocol used for: Basic call setup, management, tear down Wireless services such as PCS, roaming, authentication Toll free and toll (900) wireline services Enhanced features such as call forwarding, caller ID, 3-way calling Efficient and secure worldwide telecommunications 26
  • Slide 27
  • SS7 SS7 messages are exchanged between central offices and specialized databases via signal transfer points (packet switches). Control plane Responsible for establishing and managing connections Information plane Once a connection is set up, info is transferred in the information plane 27
  • Slide 28
  • SS7 Signaling Network Elements Service switching point (SSP) SSPs enable central offices to communicate with SS7 databases (the user entry point into SS7) Signal transfer point (STP) A signaling point (packet switch) capable of routing control messages Service control point (SCP) SCPs contain databases with call routing instructions 28
  • Slide 29
  • SS7 SSP STP SCP Central Office Central Office Central Office 29
  • Slide 30
  • SS7 Characteristics SSPs are telephone switches that send signaling messages to other SSPs to setup, manage, and release voice circuits An SSP may also send a query message to a centralized database (an SCP) to determine how to route a call (e.g. a toll-free number) Because the SS7 network is critical to call processing, SCPs and STPs are deployed in mated pair configurations in separate physical locations Links between signaling points are also in pairs 30
  • Slide 31
  • 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 What if we dont want a dedicated call, or the data rate is bursty? You want packet switching! 31
  • Slide 32
  • Basic Operation Data transmitted in small packets Typically 1000 bytes Longer messages split into series of packets Each packet contains a portion of user data plus some control info (such as addressing info or packet type) Packets are received, stored briefly (buffered) and passed on to the next node Store and forward (only ATM does not do this) 32
  • Slide 33
  • 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 33
  • Slide 34
  • Two Basic Forms of Packet Switching Packets handled in two ways Datagram Virtual circuit 34
  • Slide 35
  • Datagram Each packet treated independently Packets can take any practical route Packets may arrive out of order Packets may get lost or delayed Up to receiver to re-order packets and recover from missing packets 35
  • Slide 36
  • 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 36
  • Slide 37
  • Figure 18.2 Virtual Circuit Identifier (VCI) VCI is known only between two switches. (It is not a global address.) 37
  • Slide 38
  • Figure 18.4 Switch and table 38
  • Slide 39
  • Figure 18.5 Source-to-destination data transfer 39
  • Slide 40
  • S(witched)VC vs. P(ermanent)VC setup A virtual circuit can be either switched or permanent. If permanent, an outgoing VCI is given to the source, and an incoming VCI is given to the destination. The source always uses this VCI to send frames to this particular destination. The destination knows that the frame is coming from that particular source if the frame carries the corresponding incoming VCI. If a duplex connection is needed, two virtual circuits are established. 40
  • Slide 41
  • S(witched)VC vs. P(ermanent)VC setup A PVC has several drawbacks: 1. Always connected, so always paying 2. Connection is between two parties only. If you need a connection to another point, you need another PVC. Dont like these disadvantages? Use an SVC. 41
  • Slide 42
  • Figure 18.6 SVC setup request 1 - Setup frame sent from A to Switch I. Note how the Outgoing VCI is not yet known. 42
  • Slide 43
  • Figure 18.7 SVC setup acknowledgment As the acknowledgment frame goes back, the VCI number is placed into the Outgoing VCI entry in each table. 43
  • Slide 44
  • 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 looses 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 44
  • Slide 45
  • Packet Size 45
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  • Event Timing 46
  • Slide 47
  • Routing Complex, crucial aspect of packet switched networks Characteristics required Correctness Simplicity Robustness Stability Fairness Optimality Efficiency 47
  • Slide 48
  • Performance Criteria Used for selection of route