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Data Communications TDC 362 / TDC 460. Circuit Switching and Packet Switching. 8.1 Circuit Switching. Space-Division Switch Time-Division Switch TDM Bus Combinations. Figure 8.1 Circuit-switched network. Figure 8.2 A circuit switch. Blocking or Non-blocking. Blocking
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Data CommunicationsTDC 362 / TDC 460 Circuit Switching and Packet Switching
8.1 Circuit Switching Space-Division Switch Time-Division Switch TDM Bus Combinations
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
Figure 8.7Time-division multiplexing, without and with a Time-slot interchange
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
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
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
Location of Signaling Subscriber to network Depends on subscriber device and switch Within network Management of subscriber calls and network ore complex
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)
Drawbacks of In Channel Signaling Limited transfer rate Delay between entering address (dialing) and connection Overcome by use of common channel signaling
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
Signaling System Number 7 SS7 Most widely used common channel signaling scheme Internationally standardized and general purpose
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
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
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
SS7 SCP SCP STP SSP Central Office STP SSP Central Office SSP Central Office
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
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 don’t want a dedicated call, or the data rate is bursty? You want packet switching!
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)
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
Two Basic Forms of Packet Switching Packets handled in two ways Datagram Virtual circuit
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
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
Figure 18.2Virtual Circuit Identifier (VCI) VCI is known only between two switches. (It is not a global address.)
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.
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. Don’t like these disadvantages? Use an SVC.
Figure 18.6SVC setup request 1 - Setup frame sent from A to Switch I. Note how the Outgoing VCI is not yet known.
Figure 18.7SVC setup acknowledgment As the acknowledgment frame goes back, the VCI number is placed into the Outgoing VCI entry in each table.
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
Routing Complex, crucial aspect of packet switched networks Characteristics required Correctness Simplicity Robustness Stability Fairness Optimality Efficiency
Performance Criteria Used for selection of route Minimum hop Least cost Dijkstra’s algorithm most common Finds the least cost path from one starting node to all other nodes Algorithm can be repeated for each starting node
