Chapter 9. Circuit Switching Switching Networks Circuit-Switching Networks Switching Concepts Routing in Circuit-Switched Networks Control Signaling

Switching Networks

Circuit-Switching Networks Dedicated communication path btw two stations Three phases of communications Circuit establishment Data transfer Circuit disconnect Application of circuit switching Private Branch Exchange (PBX)

Telecommunications Network Subscribers The devices that attach to the network e.g. telephones Local loop The link between the subscriber and the network also referred to as the subscriber loop Exchange The switching centers in the network End office Trunks The branches between exchanges

Public Circuit-Switching Network

Circuit-Switching Technology Driven by applications that handle voice no transmission delay no variation in delay i.e. constant signal transmission rate Transparency once a circuit is established, it appears as a direct connection to the two attached stations; no special networking logic is needed at either point

Switching Concepts

Space-division Switching

Space-division Switching (cont)

Space-division Switching (cont) Limitations of Crossbar Costly for a large switch loss of a crosspoint prevents connection between two devices inefficiently utilized Multiple-stage switches The number of crosspoints is reduced more than one path through the network to connection two endpoints it may be blocking

E.g. Banyan Switch (ATM) 1 1 100 2 2 3 3 4 4 011 5 5 6 6 7 7 1 1

Time-division Switching

Time-division Switching (cont) Control of a TDM bus switch 1 X 2 X 3 X 4 X 5 X 6 X 1  3 2  5 Control logic Control memory 4  6 3  1 5  2 6  4

Routing in Circuit-Switched Networks Traditionally The switches of a network were organized into a tree structure, or hierarchy Dynamic routing routing decisions are influenced by current traffic conditions Two broad classes of dynamic routing alternate routing adaptive routing

Alternate Routing Possible routes to be used between two end offices are predefined The routing sequences reflects an analysis based on historical traffic patterns Fixed alternate-routing scheme only one routing sequence Dynamic alternate-routing scheme a different set of preplanned routes is used for different time periods

Alternate Routing (e.g.)

Alternate Routing (e.g.) Alternate routes from end office X to end office Y If only one route sequence  Fixed alternate-routing

Adaptive Routing Enable switches to react to changing traffic patterns Require greater management overhead Switches must exchange information to learn of network conditions More effectively optimizing the use of network resources E.g. Dynamic traffic management (DTM) by Northern Telecom DTM uses a central controller to find the best alternate route choices depending on congestion in the network

Control Signaling Control Signals Management Common Channel Signaling the means by which the network is managed and by which calls are established, maintained, and terminated Management requires that information be exchanged between subscriber and switch, among switch, and between switch and network management center Common Channel Signaling

Signaling Functions Audible communication with the subscriber Dial tone, ringing tone, busy signal, etc. Transmission of the number dialed to s/w offices Information between switches Indicating that a call cannot be completed Indicating that a call has ended and the path can be disconnected A signal to make a telephone ring Information for billing purpose Information for routing and maintenance purpose Information for diagnosing and isolating failures Control of special equipment

Control Signaling (e.g.) Circuit-switched telephone network

In-Channel Signaling The same channel is used to carry control signals as is used to carry the call Two forms of in-channel signaling in-band out-of-band Drawbacks The information transfer is quite limited amount of delay from the time a subscriber enter an address to when the connection is established

Common Channel Signaling Signal path is physically separate from the path for calls (voice) Two modes of operation associated mode non-associated mode used in ISDN

Common Channel Signaling Modes

Common Channel Signaling Modes

Discussion

Discussion (cont) Common Channel Signaling In-Channel Signaling control signals are transfer directly from one processor to another, w/o being tied to a voice signal call setup time is reduced complexity of the technique In-Channel Signaling is still needed from some of the communication with the subscriber e.g. dial tone, ringback, and busy signals