Maintenance involves detection and correction of trouble in the switching system to insure reliable and efficient operation. Maintenance functions include automatic detection and isolation of a problem and, in some cases, location of the trouble while customer service involves the additional features that expand a customer’s telephone service, and are provided by most modern switching system. These features include call-forwarding, conference calls, speed dialing, directory assistance, and special business services, such as Centrex.

Signaling is the common language by which switching machines communicate. In addition to transmitting information, telephone networks also transmit control signals. These signals are transmitted between network control devices, which enable them to set up, disconnect and control the transmissions. Many such supervisory signals are needed to set up a call through a complex network.

Signaling is the common language by which switching machines communicate. In addition to transmitting information, telephone networks also transmit control signals. These signals are transmitted between network control devices, which enable them to set up, disconnect and control the transmissions. Many such supervisory signals are needed to set up a call through a complex network. There are three classes of signaling that are frequently used - Altering Indicators which inform the customer of call-processing conditions, Inter-office Trunk Signals which are used to exchange information between switching machines and special Services Signals which enable custom telephone features to be implemented by pressing pre defined keys on the telephone set.

The control unit is the brain of the switching system. It can switch a call in a fraction of a second, and therefore can service many stations and lines. The control unit receives and stores digital pulses. It interprets the digital pulses and selects an available path through the switching hardware to complete a connection. Efficient high speed control equipment can complete many calling connections during the time of conversation.

An important advantage of an ESS is the ability of the computer to analyze the traffic situation and reroute the call. It can also perform telephone-related accounting functions. There are two types of Electronic Switching Systems called the Analog ESS and the Digital ESS. More on this in the next post.

To route a call to a specific region or a specific country, a numbering plan is used. Depending on the digits dialed, the switching system identifies whether the call is a local call, an inter-regional call or an inter-country call, and routes the call accordingly. The numbering plan is as follows. Prefix-comprises one or more digits, that allow the selection of different numbering formats; for example, local, national or international; transit networks or a service.

International prefix-allows a calling subscriber to obtain access to outgoing international equipment if the called person is in another country. National (trunk) prefix-allows a subscriber to make a call to a subscriber within the same country, but outside the local area. Escape code-is used to indicate that the digits following are from a different numbering plan. Escape codes are currently being used in the numbering plan for Public Data Networks to interconnect with ISDN and telex number plans.

Ringing during this stage, a path is established between the originating and terminating exchanges. A ringing signal is sent to the called line and an audible ring is sent to the calling party. Talking begins when the called line is answered. A talking connection is established between two telephone lines so that conversation can take place. Disconnection happens when one of the lines changes to on-hook. The talking path between the two lines is released.

A switching system controls the flow of information being transmitted over telephone lines. The switching centers are of two types-those that form part of a public network and those that are private and installed for use within an organization (called private board exchanges). A switching system controls the way it connects calls by performing three functions

Monitoring function determines when an action is needed, that is, when a service is requested by a subscriber and when it is no longer required. Logic function determines what action is needed, that is, what lines must be connected and what is the best route to complete the call. Commands function initiates the needed action. They operate those parts of the system which complete the connection and release them when the connection is no longer required.

Delay is caused in some cases due to lower frequencies. Lower frequencies have less resistance in a metallic medium. Therefore, they arrive sooner at the receiver. This causes signals with different frequencies to arrive at the receiver at different times. The difference between the signal that reaches first and subsequent signals is called delay. Delay has no major effect on speech transmission if it is kept at 600 ms or less. Amplitude Frequency Distortion is present when a waveform is altered in amplitude, at a certain frequency, during transmission. This may happen in an unconditioned transmission line where, at a certain frequency, the amplitude is reduced.

To correct echo for a short-haul carrier facility, a compensation circuit is built into the hybrid device. This circuit is adjusted to balance the echo. Echo encountered on long-haul carrier facilities is countered by two types of devices namely, Echo suppressers and Echo cancellers. Echo suppressers are voice-activated devices that do not affect the speaker’s transmission. However, they reduce the level of transmission moving towards the speaker by about 35 dB. This prevents people from hearing their own voice as echo. Echo cancellers are used at the receiving end of a long-haul carrier facility (usually satellite). The echo canceller constantly monitors the incoming signal and produces a negative image signal of amplitude equal to the echo, thus canceling the echo.