Circuit Switching Systems J.TCO7D045 М.Цэлмэг J.TC07D707 Ж.Энхболд J.TC07D719 Б.Эрдэнэсувд
Circuit switches serve the PSTN as end offices and tandems, and the enterprise network as PBXs and key systems. A circuit switching system for interconnecting end point systems, such as packet switches, compressed voice concentrators, or data multiplexers, by allowing multiple logical links on each logical channel. The circuit switching system is connected to the end point system by a number of communication facilities with each facility having a plurality of logical channels. For packet switches, the circuit switching system is responsive to a first request to establish a first logical link on a logical channel to a packet switch and is responsive to a second request for another logical link to that packet switch to establish a second logical link on the same logical channel. The circuit switching system is responsive to a third request for establishing a logical link to another packet switch on the same logical channel for denying this request and for negotiating another logical channel. Circuit Switching Systems
CIRCUIT SWITCHING ARCHITECTURE All digital circuit-switching systems include the elements shown in Figure 14-1.Our focus in this chapter is on central office switches, but PBX architectures are similar. The principal elements are
Switching System Control Circuit switches may distribute control and some call-processing functions to peripheral units, but ultimate control is centralized. The central controller monitors the health of peripherals, collects traffic data, provides access to the database, and other functions common to an operating system as well as providing basic call processing.
Call Processing Most digital switches use similar techniques for processing calls. The following is a short description of a typical call process, although different manufacturers have different terminology for the elements. The discussion assumes that the call originatesand terminates in the same central office and does not consider number portability. The principal processing elements are
PCM Switching Networks The capacity of a switching network is a function of the number of switching stages it has.
The switch modules are of two types:
Time division switching is implemented in a time slot interchange element as shown in Figure 14-3.
Central Processor The central processor in digital switches is similar to that used in commercial computers, but with some important differences.
First, the SPC processor is not only fault tolerant; it is almost fail safe.
A second difference is in the nature of the processing task.
Line Circuit Functions In a digital central office, line circuits have seven basic functions that can be remembered with the acronym B O R S C H T . Analog central office line circuits require five of the seven functions because they have two-wire switching networks, the hybrid and coding functions are omitted. The B O R S C H T functions are B= Battery O= Over R= Ringing S= Supervision C=Coding H=Hybrids T=Testing
Remote Switch Unit (RSU) An RSU essentially extends the line circuits and sometimes trunk circuits closer to the subscriber. An RSU supports a full complement of subscriber line options. Some RSUs also support local trunking, which is important if the RSU serves a distant community because much calling is local and running the connection back to the host is not economical.
Digital Loop Carrier (DLC) DLC is an alternate form of remote. DLCs come in both single and double-ended configurations. A double-ended DLC has matching units in the field and the central office. The central office end connects to analog line ports in the switch and effectively extends the ports to the field over T1/E1 lines. Small DLCs may serve 24 or 30 channels over a single T1/E1 line.
TANDEM SWITCHING Tandem switching capability is an optional feature of most digital switches. Most class 5 switching products can function as a combination end office and tandem switch, but where the application is pure trunk switching, a switch designed for the purpose is often used. The industry has three classes of tandems: local, LATA access, and toll.
Virtual Networks Some IXCs offer virtual private networks to their customers, a service that requires support from the tandem switch. A virtual private voice network is one that operates as if it is composed of switched private lines, but which, in reality, is derived by shared use of the carrier’s switched facilities. A virtual private network handles calls in three manners:
Dedicated access line to dedicated access line
Dedicated access line to switched access line
Switched access line to switched access line
IP INTEGRATION The digital central office is undergoing a transformation into a combination digital switch and softswitch. The new architecture retains the conventional line and trunk ports, but adds line and trunk gateways to interface the VoIP world. The system so configured can therefore serve existing customers with no change, but can add VoIP when the demand arises.