Telecom switching systems

The following elements are found in all telecom switching systems:

§  Operating system software that controls the hardware

§  Call processing software that makes connections, provides features, and delivers services

§  Line side interfaces connecting the switch to the end-user telephone set

§  Trunk side interfaces connecting the customer's switch to the network

§  A switching fabric linking the various interfaces

Operating system software

In the telecommunications environment as in the world of computing, hardware is controlled by operating system software. The telecom environment has special needs for multiuser, real-time, fault-tolerant operating systems. The complexities of the features available have resulted in switch products that use proprietary operating systems, a parallel to the proprietary legacy systems of the computing environment.

Call processing applications

The heart of a modern switching system is a set of software applications known collectively as call processing. This software provides all the functionality seen by the user—from the basic call setup to delivering caller ID. This software also provides user features (such as call forwarding), enhanced network services (such as least-cost routing), and specialized call handling for call centers. Call processing is the basis for powerful CTI applications that can make a call center highly effective and productive.

Interfacing hardware

Modern telecom systems operate by converting analog voice signals into a digital format known as pulse code modulation (PCM). The digital format is far superior for clear transmission, storage, compression, and even encryption. This process is accomplished by a silicon chip, a CODEC (for code and decode) designed into the line interface. The CODEC samples the voice signal 8000 times per second and transforms it into the digital signal, ready for transmission. At the other end, another CODEC chip transforms the digital signal into a recognizable voice signal.

The switching device performs the function of connecting the digital signal from the line interface to the destination, which might be another line interface (intercom call), a trunk interface (network call), or a common resource (such as a conference bridge). Once established, the connection stays up for the duration of the call. The trunk interface is a shared pipe into the public network. There are various types of trunk interfaces, defined by their bandwidth capacities—T1, T2, T3. For example, a single T1 trunk provides 24 circuit paths for digitized voice as well as the signaling to access the network services. Typically, these 24 circuits can provide service to about 150 users.