09-05-2011, 09:29 AM
Submitted By
ACHARYA MIRAJ SHAILESH BHAI
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OVERVIEW OF TELECOMMUNICATION NETWORKS
Introduction
The telephone is a telecommunication device that is used to transmit and receive electronically or digitally encoded speech between two or more people conversing. It is one of the most common household appliances in the world today. Most telephones operate through transmission of electric signals over a complex telephone network which allows almost any phone user to communicate with almost any other user.
Telecommunication networks carry information signals among entities, which are geographically far apart. An entity may be a computer or human being, a facsimile machine, a teleprinter, a data terminal and so on. The entities are involved in the process of information transfer that may be in the form of a telephone conversation (telephony) or a file transfer between two computers or message transfer between two terminals etc.
With the rapidly growing traffic and untargeted growth of cyberspace, telecommunication becomes a fabric of our life. The future challenges are enormous as we anticipate rapid growth items of new services and number of users. What comes with the challenge is a genuine need for more advanced methodology supporting analysis and design of telecommunication architectures. Telecommunication has evaluated and growth at an explosive rate in recent years and will undoubtedly continue to do so.
The communication switching system enables the universal connectivity. The universal connectivity is realized when any entity in one part of the world can communicate with any other entity in another part of the world. In many ways telecommunication will acts as a substitute for the increasingly expensive physical transportation.
The telecommunication links and switching were mainly designed for voice communication. With the appropriate attachments/equipments, they can be used to transmit data. A modern society, therefore needs new facilities including very high bandwidth switched data networks, and large communication satellites with small, cheap earth antennas.
Voice Signal Characteristics
Telecommunication is mainly concerned with the transmission of messages between two distant points. The signal that contains the messages is usually converted into electrical waves before transmission. Our voice is an analog signal, which has amplitude and frequency characteristics.
Voice frequencies: - The range of frequencies used by a communication device determines the communication channel, communicating devices, and bandwidth or information carrying capacity. The most commonly used parameter that characterizes an electrical signal is its bandwidth of analog signal or bit rate if it is a digital signal. In telephone system, the frequencies it passes are restricted to between 300 to 3400 Hz.
In the field of telecommunications, a Telephone exchange or a Telephone switch is a system of electronic components that connects telephone calls. A central office is the physical building used to house inside plant equipment including telephone switches, which make telephone calls "work" in the sense of making connections and relaying the speech information.
Switching system fundamentals
Telecommunications switching systems generally perform three basic functions: they transmit signals over the connection or over separate channels to convey the identity of the called (and sometimes the calling) address (for example, the telephone number), and alert (ring) the called station; they establish connections through a switching network for conversational use during the entire call; and they process the signal information to control and supervise the establishment and disconnection of the switching network connection.
In some data or message switching when real-time communication is not needed, the switching network is replaced by a temporary memory for the storage of messages. This type of switching is known as store-and-forward switching.
Signaling and control
The control of circuit switching systems is accomplished remotely by a specific form of data communication known as signaling. Switching systems are connected with one another by telecommunication channels known as trunks. They are connected with the served stations or terminals by lines.
In some switching systems the signals for a call directly control the switching devices over the same path for which transmission is established. For most modern switching systems the signals for identifying or addressing the called station are received by a central control that processes calls on a time-shared basis. Central controls receive and interpret signals, select and establish communication paths, and prepare signals for transmission. These signals include addresses for use at succeeding nodes or for alerting (ringing) the called station.
Most electronic controls are designed to process calls not only by complex logic but also by logic tables or a program of instructions stored in bulk electronic memory. The tabular technique is known as translator. The electronic memory is now the most accepted technique and is known as stored program control (SPC). Either type of control may be distributed among the switching devices rather than residing centrally.
Microprocessors on integrated circuit chips are a popular form of distributed stored program control.
Network Architecture.
When electronic devices were introduced in the switching systems, a new concept of switching evolved as a consequence of their extremely high operating speed compared to their former counter-parts, i.e., the Electro-mechanical systems, where relays, the logic elements in the electromechanical systems, have to operate and release several times which is roughly equal to the duration of telephone signals to maintain required accuracy.
Research on electronic switching started soon after the Second World War, but commercial fully electronic exchange began to emerge only about 30 years later.
However, electronic techniques proved economic for common control systems much earlier. In electromechanical exchanges, common control systems mainly used switches and relays, which were originally designed for use in switching networks. In common controls, they are operated frequently and so wear out earlier. In contrast, the life of an electronic device is almost independent of its frequency of operation. This gave a motivation for developing electronic common controls and resulted in electronic replacements for registers, markers, translators etc. having much greater reliability than their electromechanical predecessors.
In electromechanical switching, the various functions of the exchange are achieved by the operation and release of relays and switch (rotary or crossbar) contacts, under the direction of a Control Sub-System. These contracts are hard - wired in a predetermined way. The exchange dependent data, such as subscriber’s class of service, translation and routing, combination signaling characteristics are achieved by hard-ware and logic, by a of relay sets, grouping of same type of lines, strapping on Main or Intermediate Distribution Frame or translation fields, etc. When the data is to be modified, for introduction of a new service, or change in services already available to a subscriber, the hardware change ranging from inconvenient to near impossible, are involved.
Implementation of Switching Network.
In an electronic exchange, the switching network is one of the largest sub-systems in terms of size of the equipment. Its main functions are Switching (setting up temporary connection between two or more exchange terminations), Transmission of speech and signals between these terminations, with reliable accuracy.
There are two types of electronic switching system. viz. Space division and Time Division.
Space Division
In this type of switching System In a space Division Switching system, a continuous physical path is set up between input and output terminations. This path is separate for each connection and is held for the entire duration of the call. Path for different connections is independent of each other. Once a continuous path has been established, Signals are interchanged between the two terminations. Such a switching network can employ either metallic or electronic cross points. Previously, usage of metallic cross-points using reed relays and all were favored. They have the advantage of compatibility with the existing line and trunk signaling conditions in the network.
Time Division Switching System
In Time Division Switching, a number of calls share the same path on time division sharing basis. The path is not separate for each connection, rather, is shared sequentially for a fraction of a time by different calls. This process is repeated periodically at a suitable high rate. The repetition rate is 8 KHz, i.e. once every 125 microseconds for transmitting speech on telephone network, without any appreciable distortion. These samples are time multiplexed with staggered samples of other speech channels, to enable sharing of one path by many calls. The Time Division Switching was initially accomplished by Pulse Amplitude Modulation (PAM) Switching. However, it still could not overcome the
performance limitations of signal distortion noise, cross-talk etc. With the advent of Pulse Code Modulation (PCM), the PAM signals were converted into a digital format overcoming the limitations of analog and PAM signals. PCM signals are suitable for both transmission and switching. The PCM switching is popularly called Digital Switching.
Control of switching systems
Switching systems have evolved from being manually controlled to being controlled by relays and then electronically. The change from the manual system to the Strowger step-by-step system brought about a change from centralized to distributed control. However, as systems developed and offered more services to customers, it became economic to perform particular functions in specialized equipments that were associated with connections only when required, thus, common control was introduced.
Later, the development of digital computer technology enabled different functions to be performed by the same hardware by using different programs; thus switching system entered the era of stored-program control (SPC).
There are basically two approaches to organizing stored program control: centralized and distributed. Early electronic switching systems (ESS) developed during the period 1970-75 almost invariably used centralized control. Although many present day exchange designs continue to use centralized SPC, with the advent of low cost powerful microprocessors and very large scale integration (VLSI) chips such as programmable logic arrays (PLA) and programmable logic controllers (PLC), distributed SPC is gaining popularity.
The figure below shows the evolution of electronic switching systems from the manual switching systems. The figure also depicts the changing scenario from digital switching to Broadband where the focus will be for high bit rate data transmissions
