The steadily increasing number of computers being installed adds greater and greater emphasis to the use of facts in decision making. Information as a management tool has already acquired high status. Going forward more importance is likely to be given for this.
To meet the needs of telephone customers, modem switching systems are designed to perform four major functions - Call Processing, Administration, customer service and Maintenance. Call Processing comprises all necessary tasks to connect one telephone to another. These tasks include Supervision while Administration involves monitoring and recording of the system and customer-related conditions. System administration functions include rerouting of network traffic to avoid congestion and reporting traffic data to measure telephone patterns at certain times or areas of network. Customer-related features include recording information about toll calls for billing purposes.
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.
Analog ESS perform line and/or trunk connections that are in analog form when they pass through the switch while the Digital ESS connects lines and/or trunk circuits that convert the analog signals into digital form when they pass through the switch.The digital ESS is more efficient than the analog ESS because it has electronic components that are more reliable, less expensive, and faster than the electro-mechanical devices used in the analog ESS. It requires conversion of analog lines to digital lines only at end offices. It handles more complex signaling data, which increases the processing speed and provides more customer services. It directly accommodates signals for many services, such as fax, video and data transfer. It is designed in a flexible, modular manner that can be easily expanded or modified.
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.
Electronic Switching Systems offer a great potential for voice and data communications. An ESS consists of Computer Memory and Rapid Switching Component. Switching systems are programmed to handle various situations and perform specialized functions. Additional features can be added by modifying the program that controls the switching system. All switching equipments have two basic units such as the Control Unit and the Switching Unit.
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.
There are two main types of public network offices through which a call is routed. These are the local offices and the toll offices. Subscribers are connected to local offices. The switching offices used to interconnect the local offices are called toll offices. The toll offices in tum can be at different levels.
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.
An inter-exchange call is switched between two switching systems. There are five main stages of processing an inter-exchange call. Call Origination begins when the caller first picks up the telephone handset and completes a circuit to the local exchange. Digit Analysis is done during this stage where dialed digits are received and the originating exchange determines how to route the call to the local exchange of the called line.
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.
In 1876, Alexander Graham Bell invented the first telephone. The early telephone system was simply a wired connection between two points. Today there are over 650 million telephones all over the world. The world’s telephones must be capable of being interconnected. These telephones cannot be interlinked with each other as laying a separate line is not feasible for so many connections. A switching system is needed to switch calls between various points.
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.
Digital signals are susceptible to the same impairments as analog signals when they are transmitted over analog facilities. However, they are less susceptible to most impairments. Digital transmission can be designed to correct errors automatically. For example, digital information signals between computers may be transmitted along with error-detecting codes. When an error is detected, the information is retransmitted. The impairments over a digital transmission facility are digital attenuation and echo. Digital signals are subject to attenuation or signal loss. With digital transmission, regenerators are used to reconstruct, or regenerate, the digital signal.
Message circuit noise comes from several sources and includes local exchange battery/ground noise. Two other types of message circuit noise are Thermal noise and Impulse noise. Thermal noise, which is random noise that is generated when electrical circuit elements vary in temperature. Impulse noise, which includes noise generated because of power fluctuations and induced noise
spikes. Intelligible crosstalk is an understandable speech signal from one channel affecting adjacent channels. If channels are a separate pair of wires, EMR from a high amplitude wire affects adjacent low amplitude wire pairs. It also occurs when signals using Frequency Division Multiplexing vary beyond the bandwidth limits of their channels.
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.
Echo occurs within transmission channels. There are two types of echo. Talker echo, which occurs when the speaker’s electrical speech signal is reflected back along the transmission line to the speaker, and listener echo, which is caused by a current circulating in a transmission path. Echo Return Loss (ERL) is a type of measurement used to determine the amount of echo. The decibel level is taken along two or more points in the transmission line. Signal Return Loss occurs when there is an imbalance on both hybrid circuits and echo cycles, over and over until the amplifiers are over-driven.
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.