Chapter 14

Circuit Switching Systems
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 ARCHITECTURE
 DISTRIBUTED SWITCHING
 TANDEM SWITCHING
 ADVANCED INTELLIGENT NETWORK (AIN)
 IP INTEGRATION
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
 A TDM switching fabric that connects paths between input and output line and trunk ports.
 A central controller that directs the connection of paths through the switching network. The central control often offloads part of its load to peripheral processors.
 Databases that store the system configuration and trunk and subscriber line features.
 Line ports that interface outside plant for connection to users. All local and PBX switching systems include line ports; tandem switches may have only a few specialized line ports.
 Trunk ports that interface interoffice trunks, service circuits, and testing equipment.
 Service circuits that provide call progress signals such as dial tone, ringing, and busy tones.
 Common equipment such as battery plants, power supplies, testing equipment, and distributing frames.
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. The central controller in a digital switch is always redundant to provide high reliability. Switching systems are designed as central control, multiprocessing, or distributed control. In a central control system, all the call processing is concentrated in a single location. A multiprocessing system has two or more processors that share call-processing functions.



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
 Scanners to detect changes in states of lines and trunks.
 Signal distributors to transmit signals from scanners to call-processing programs.
 Registers that furnish dial tone and accept and register dialed digits. Registers are normally dial pulse (rotary dial) or DTMF, and they reside on the trunk side of the switch.
 Generic program, which is the call-processing instructions contained in the program store.
 Call store to provide temporary scratch-pad memory that is used to store the details of calls in progress.
 Data store, which stores the line translations.
 Time slots, which, in a digital switch, are units of time reserved for the parties to share during a session.
 Switching fabric to make connections between lines and trunks.

PCM Switching Networks
The capacity of a switching network is a function of the number of switching stages it has. The switching matrix is physically limited by the number of terminations it can support. To avoid blocking, the controller must have multiple choices of paths through the network. These paths are obtained through multiple switching stages so that each stage has enough choices that the probability of blocking is reduced to a level consistent with the grade-of-service objectives.
The switch modules are of two types:
• time switches
• space switches.
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. The processor is also involved in collecting statistical information for administering trunk groups and common equipment. The information is usually dumped to an external processor that assists by finding the busy hour and calculating trunk requirements.
• 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 BORSCHT. 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 BORSCHT functions are

• Battery feeds from the office to the line to operate station transmitters and DTMF dials.
• Over voltage protection is provided to protect the line circuit from damaging external voltages that can occur during the time that it takes the protector to operate.
• Ringing connects from a central ringing supply to operate the telephone bell.
• Supervision refers to monitoring the on-hook/off-hook status of the line.
• Coding converts the analog signal to a PCM bit stream in digital line circuits.
• Hybrids in digital line circuits convert between the four-wire switching fabric and the two-wire cable pair.
• Testing access is provided so an external test system can access the cable pair for trouble isolation.

DISTRIBUTED SWITCHING
• Remote Switch Unit (RSU)
• Digital Loop Carrier (DLC)

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. Most switches have a maximum range over which they can host RSUs, but it is generally in the range of hundreds of miles. Most switches have a maximum range over which they can host RSUs, but it is generally in the range of hundreds of miles.

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. A typical configuration might serve 96 subscribers over two T1 lines, which provides two-to-one concentration.

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
Adedicated-to-dedicated call bypasses the LEC and avoids access charges in both the originating and the terminating direction. The dedicated-to-switched calleliminates the access charge in the originating, but not the terminating direction.

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. The major advantage this configuration brings, aside from avoiding obsolescence, is the ability to serve a wide area from one central office. IP modules can be placed in large customer concentrations to provide Centrex service or to relieve outside plant congestion.