Introduction to Telecoms

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(1)Introduction to Telecoms Course Code: TY2600. Duration: 3 days. Technical Level: 2. Other areas of expertise include: n. IP Networks and Protocols. n. Service Enablers. n. WiMAX. n. LTE. n. UMTS. n. GSM and GPRS. n. TETRA. n. CDMA. n. Transport and Signalling. n. Telecoms Industry Dynamics. www.wraycastle.com.

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(3) INTRODUCTION TO TELECOMS. First published 2008 Last updated February 2011 WRAY CASTLE LIMITED BRIDGE MILLS STRAMONGATE KENDAL LA9 4UB UK. Yours to have and to hold but not to copy The manual you are reading is protected by copyright law. This means that Wray Castle Limited could take you and your employer to court and claim heavy legal damages. Apart from fair dealing for the purposes of research or private study, as permitted under the Copyright, Designs and Patents Act 1988, this manual may only be reproduced or transmitted in any form or by any means with the prior permission in writing of Wray Castle Limited.. © Wray Castle Limited.

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(5) INTRODUCTION TO TELECOMS. CONTENTS Section 1. Telecommunication Services in the Modern World. Section 2. The PSTN and ISDN. Section 3. Transmission Networks. Section 4. Mobile Cellular Networks. Section 5. IP Packet Networks. Section 6. Fixed and Wireless Broadband Access Technologies. Section 7. VoIP, NGNs and the IMS. Appendix. Telecoms Evolution to Packet-Switched Data Networks. © Wray Castle Limited. III.

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(7) Introduction to Telecoms. SECTION 1. TELECOMMUNICATION SERVICES IN THE MODERN WORLD. © Wray Castle Limited. V.

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(9) Introduction to Telecoms. CONTENTS Telecommunication Network Evolution The Range of Telecoms Services. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.2. Requirements of a Telecommunication Network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.3 Transmission Networks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.4. Transmission Media . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.5 Traditional Legacy Networks and Transmission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.6 The Importance of Data Rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.7 Circuit Switching. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.8. Packet Switching. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.9. Traditional Telecommunication Signalling Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.10 Factors Affecting System Choice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.11 Bearer Services . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.13 Types of Bearer Service . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.14 Teleservices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.15 Supplementary Services (SSs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.16 Value Added Services (VASs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.17 Telecommunication Services Summary Telecommunications Regulation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.18. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.19. Telecommunciations Standards Bodies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.20 Standards Bodies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.21. © Wray Castle Limited. VII.

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(11) Telecommunication Services in the Modern World. OBJECTIVES At the end of this section you will be able to:. . list the basic services that are provided by today’s telecommunication networks. . identify the main components of a telecommunication network. . outline the operating principles of circuit-switched and packet-switched networks. . describe the functions of signalling. . identify the factors that affect the choice of transmission system. . differentiate between bearer services, teleservices and supplementary services. . discuss the similarities and differences between services offered by fixed-line operators and those offered by mobile networks. . describe the features of supplementary services. . explain the benefits to subscribers and operators of Value Added Services (VASs) and give examples of VASs. . explain the functions of the various telecommunications standards bodies. © Wray Castle Limited. IX.

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(13) Telecommunication Services in the Modern World. Telecommunication Network Evolution The evolution of telecommunication technologies is a continuous process. The original telephone network was designed to provide voice communication and, in the very early days, a manually operated switchboard was used to interconnect the calling and the called parties. An electro-mechanical system was developed to allow the call to be connected by the calling party using a dial. This system became known as the Strowger switch (named after its inventor) and was the first automatic telephone switching system. The Public Switched Telephone Network (PSTN) was based on analogue technologies and was voice centric. Customers connected to the network over analogue copper wires. In the 1980s, the network evolved to a digital infrastructure, which paved the way to the Integrated Services Digital Network (ISDN). This allowed customers to have digital access to the network, and in addition to voice, computer data could be transmitted over the network at faster data rates. Until the 1990s, a limited number of services were available to customers. These were mainly provided through fixed networks. Since the emergence of digital mobile networks, and the rapid increase in mobile-phone usage, many more services have become available and the dividing lines between fixed and mobile services have become blurred. Fixed and mobile networks have some services in common, such as telephony and data, while other services, such as the Short Message Service (SMS) and location-based services, are particular to mobile networks. With the introduction of fixed and mobile broadband technologies, many services are now being provided on both fixed and mobile networks. The services themselves are carried by the Internet Protocol (IP), which provides a common protocol and technology, thereby reducing costs. The data rates available in mobile broadband networks are approaching those provided by the fixed broadband architecture. This trend is enabling a convergence of services between fixed and mobile networks. Access to the World Wide Web (WWW), video streaming, file downloads, online shopping, music downloads, social networking and general information exchange can now be carried over fixed access networks utilizing copper wire infrastructure and mobile networks utilizing radio access networks. IP and other emerging technologies are changing telecommunications dramatically. These packet-based systems allow a wide range of services to be provided over both fixed and mobile broadband networks.. TY2600/v4.1. © Wray Castle Limited. 1.1.

(14) Introduction to Telecoms. Mobile Phones. Broadcast (TV and Radio). Blogs Telephony. WWW Internet. Facsimile. £. File Transfer. Telebanking. Fixed Networks Mobile Networks IP Based Networks. E-mail. e-commerce. Video Streaming. File sharing Music Downloads. Social Networking. Instant Messaging. Telemetry Short Messages LANs. MANs. WANs. Alarm Calls. The Range of Telecoms Services The world has become accustomed to a sophisticated telecommunication system that delivers a wide range of services to its users. Although the most widespread service is still telephony, there is an increasing need to move information in the form of data for a variety of applications. The applications that a particular system or network supports and offers to its users are termed ‘services’. Each service has different information related to it, and each requires the use of different techniques. The provision of equipment within a telecoms system must therefore take this into account. The ideal system has the flexibility to use the same equipment for all services rather than providing separate, specialist equipment. By employing technologies such as IP, new, advanced services can be combined with traditional services into a converged network. With this network, much of the equipment can be simultaneously utilized for real-time and non-real-time traffic, for data rates which are variable or constant or for high-speed or low-speed data transfer. This makes for a far more efficient use of network resources. The way in which traditional services have been provided is changing. For example, in addition to receiving broadcast television over a radio link from the main transmitter, television programmes can be carried over the Internet using IPTV and Video on Demand (VoD) technologies. Social networking web sites and web-logs (blogs) are popular services. Blogs are mainly text based, but some focus on particular interests such as art, photography, videos, music and audio (podcasting). Services such as micro-blogging (e.g. Twitter) are expanding rapidly. The traditional PSTN voice network is evolving towards Voice over IP (VoIP), which can be combined with other services to create new multimedia services and content.. 1.2. © Wray Castle Limited. TY2600/v4.1.

(15) Telecommunication Services in the Modern World. User (Application). Transmission System. The general requirements of the network are to convey information reliably and faithfully between any two points, over any distance, and at a reasonable cost.. Switching System. Signalling System. Requirements of a Telecommunication Network As the services offered to customers become more sophisticated, so the equipment and network infrastructure needed to support them becomes more complex. Although some services may be supported within the standard telephony network, others require a dedicated network in order to satisfy these requirements. A number of network types have evolved to support a variety of applications. However, certain general requirements are common to most networks. These are to convey information reliably and faithfully between any two points, over any distance, and at a reasonable cost. To meet these requirements, there are four major components of a communications network: transmission, switching, signalling (or control), and user equipment with applications. The type of information conveyed depends upon the user application and its associated equipment. Reliable and faithful transport of information over a distance requires the use of transmission techniques, while switching and signalling ensures that any two points can be interconnected automatically, wherever they are in the network.. TY2600/v4.1. © Wray Castle Limited. 1.3.

(16) Introduction to Telecoms. Equipment A. Input Data. Customer Premises Equipment (CPE) Switches etc.. Output Data. Information Pipe. Equipment B Customer Premises Equipment (CPE) Switches etc.. Transmission Network high availability low Error Rate resilient easy circuit provisioning. Digital Transmission Networks Tributary Interfaces. Multiplexing Equipment. Copper Optical Radio Systems. Add/Drop Facility. Cross Connect. PDH SDH WDM OTN. Transmission Networks Transmission networks are designed to transfer information over any distance from one point to another. To the user, the ideal transmission system would be error free and available at all times. From an operator’s perspective a transmission network should enable ease of circuit provisioning, be resilient to failures and reduce the cost per bit transmitted. Transmission systems can be said to provide a simple information pipe between two devices. The size or capacity of the pipe will vary depending on the amount of data to be transferred between the input and the output. In general terms, transmission systems consist of tributary interfaces, multiplexers, add/drop facilities, cross-connects and line systems. Four main transmission technologies are in use in modern systems. They are Plesiochronous Digital Hierarchy (PDH) and Synchronous Digital Hierarchy (SDH), which are digital systems; Wavelength Division Multiplexing (WDM), which provides optical multiplexing, and Optical Transport Networks (OTNs), which may be considered as hybrid networks providing capabilities similar to SDH and WDM systems. PDH systems were originally introduced into trunk systems in the USA in the early 1960s. SDH systems reflect the modern approach to digital transmission systems, having been in use since 1985; again originating in North America under the title of Synchronous Optical Network (SONET). The physical components of a transmission system consist of termination equipment and the transmission link.. 1.4. © Wray Castle Limited. TY2600/v4.1.

(17) Telecommunication Services in the Modern World twisted pair in the access network twisted pair on main distribution frames  coaxial cable between equipment in a transmission building  computer Local Area Networks (LANs)  USB cables  etc.  . Metallic Wire. between towns and cities international links  submarine cables  between equipment in secure data centres  etc. . Fibre Optic Cable. . radio access for mobile phones microwave links between towns and cities  outside broadcasts  satellite communication  etc. . Radio. . Transmission Media There are three main types of transmission media: copper wire, optical fibre and radio. Copper wire is now mainly to be found in the fixed access network between the user and the network. By employing Digital Subscriber Line (DSL) techniques, copper twisted pairs are capable of bit rates up to 100 Mbit/s, although this figure will depend on the length of the wire and its thickness (au). Optical fibre has a much greater capacity than copper wire and is used on major routes between towns, cities and countries. By employing WDM techniques using up to 160 optical channels, with each channel carrying 10 Gbit/s, the aggregate bit rate of an optical fibre can be very large. Total data rates can be several terabits per second, sufficient for transmitting many millions of telephone channels simultaneously. The OTN enables the multiplexing and switching of optical wavelengths between fibres. In addition, an optical fibre cable can contain multiple fibres. Radio has less capacity than fibre but it has the advantage of being cheaper and more flexible to install when compared with cables. Another advantage of radio is that it can enable mobility, as in a cellular network. Point-to-point radio links can carry data rates in excess of 600 Mbit/s.. TY2600/v4.1. © Wray Castle Limited. 1.5.

(18) Introduction to Telecoms. Traditional Legacy Networks and Transmission Traditional transmission networks concentrated on supporting circuit-switched services and were required to support access connections and, in the communication provider’s core network, inter-switch site connections. The term ‘access network’ refers to the part of the network used to connect the customer to the network. Traditionally this has been implemented with a pair of twisted metallic wires (copper or aluminium) between the customer’s premises and the local switch (exchange). The average length of this wire in the UK is about 4 km. When the customer picks up the telephone handset, the two copper wires are connected together forming a loop. Consequently, the access network is often known as the ‘local loop’. Other names have been given to the copper fixed access network, such as the ‘subscriber line’ and the ‘last mile’. The copper wires terminate in the local exchange on a Main Distribution Frame (MDF), which allows flexible connections inside the exchange building. A concentrator is often provided to maximize the use of revenue-generating switch ports; this also provides a range of customer interface types, a basic telephony line or a line termination from a PBX, for example, while allowing the switch to have a single interface type. The former function is achieved by dynamically connecting customer lines to switch ports when the customer picks up the phone to use the line. By doing this, the concentrator can support more customer-facing lines than switch-facing ‘lines’; ratios of between 4 and 10:1 are typical. Using a 10:1 concentration ratio, for example, there may be 100,000 customer-facing terminations and only 10,000 switch terminations. In this case it is more likely that 10,000 out of 100,000 customers will make simultaneous calls than would be the case if only 10,000 customers were directly connected to the switch, hence making best use of the revenue-generating capabilities of the switch. Calls set up between customers via the switch network are referred to as ‘switched services’. As well as connection to the local exchange some corporate customers require secure dedicated inter-site connections for their data. Typically this is provided as a ‘leased line’ service via the transmission network. Leased lines can be used for voice, data or Internet services. Typically, leased lines are available at data rates from 64 kbit/s increasing in increments to 2 Mbit/s (E1). Higher-bit-rate services in the access loop to the customer are often provided using optical fibre connections or microwave radio links as an alternative to metallic pairs. The transmission links between switches are normally implemented as multiple 2 Mbit/s (E1) connections. PDH and SDH transmissions systems provide an effective mechanism for combining multiple E1 connections onto a single higher-capacity signal. This signal may then be transmitted between towns and cities over optical fibre or microwave radio systems.. 1.6. © Wray Castle Limited. TY2600/v4.1.

(19) Telecommunication Services in the Modern World GPRS 40 k e-mail. Web Page. (no attachments). Broadband (DSL) 8 M. UMTS 384 k. 50 kbytes. 200 kbytes HSDPA 3.6 M. GPRS 40 k Photograph Broadband (DSL) 8 M. Video Clip. UMTS 384 k. 700 kbytes. GPRS – General Packet Radio Service UMTS – Universal Mobile Telecommunications System DSL – Digital Subscriber Line. HSDPA 3.6 M. 10 Mbytes. The Importance of Data Rates The time taken to transmit and receive data is dependent upon the capacity of a network and, if a radio system, on the spectrum bandwidth allocated. The higher the data rate of a system, the shorter the time taken to transmit and receive the data. Where services are offered using radio systems, sufficient spectrum bandwidth must be allocated to provide a data rate which satisfies the speed requirements of the service. For example, waiting 33 minutes to download a short video clip would be unacceptable. The diagram shows the data rates of various telecommunication systems and the length of time it takes to receive different types of information, assuming no system delays and no inclusion of additional data. It is important to recognize that the higher the data rate, the greater the range of multimedia services that can be provided, such as voice, video or IPTV.. TY2600/v4.1. © Wray Castle Limited. 1.7.

(20) Introduction to Telecoms C. A. Sorry, all the lines are busy. Circuit Switching Initial set-up delay Short delay. B. Constant delay Single connection. D. Circuit Switching In any given network, switching points are usually interconnected to form a mesh through which calls are routed from one terminal to another, based on the terminals’ unique addressing (numbering) scheme. The network is provided for every terminal to use, but the way in which the resources are allocated may vary depending on the switching mode implemented. Different modes of switching may be present in the same transmission network. There are two main switching methods: circuit switching and packet switching. Circuit switching involves a physical circuit being established between two terminals for a period of time. The circuit is allocated exclusively to the users for the duration of the connection. These resources only become available to other terminals upon release of the connection. Circuit switching has several advantages. Firstly, the end terminals/devices/users are allocated the full data carrying capability of the connection for the duration of the call irrespective of whether they have data to send. In addition, although the initial routing process (or set-up) takes time, further data exchange via the switches is relatively short because it does not involve the analysis of addresses; the data simply flows through the physical connection. Finally, as all data for the connection takes the same physical route, the time taken for data to be transferred between terminals is kept constant throughout the call. There are also disadvantages with circuit switching. Although the parties may cease to talk, network resources are still allocated to them. This is a disadvantage to both the user and the network operator. From the user’s point of view, a connection is being paid for even though no data is being sent; from the network operator’s view, the network may have no resources available for waiting users. Also, should the network fail, the connection is lost completely and a new connection will need to be created.. 1.8. © Wray Castle Limited. TY2600/v4.1.

(21) Telecommunication Services in the Modern World. A. C. B. D. Packet Switching Long delay Variable delay Shared resources. Packet Switching Packet switching involves the segmentation of users’ information into smaller blocks of data known as ‘packets’. Routing is carried out on a packet-by-packet basis; each packet is fed into the network and passed from one switching point to the next until it reaches its destination. Each packet must therefore contain addressing information to allow the routing process to take place at every switch on the route through the network. Within this basic type of packet network the packets are referred to as datagrams. An advantage of packet switching is that terminals share the network, each terminal being allocated network resources only when they have packets (or datagrams) to send. From the users’ perspective this is advantageous as they pay only for the data sent as opposed to the length time they are connected, i.e. users can be charged per packet as opposed to per second. From the network operator’s point of view, packet switching allows all users to be given access to the network. It also allows for more efficient network dimensioning, which can lead to financial savings that can ultimately be extended to the user. In addition, as each packet is handled separately, any failure within the network need not affect the transfer of packets. The network can simply route the packets via an alternative path so bypassing any failed elements. One disadvantage of packet switching is that the delays can be relatively long within the switches themselves. Although packet switching does not incur the set-up and release times associated with circuit switching, routing must be performed on every packet arriving at every switch and this will incur a delay. In addition, the delay between a packet arriving at a switch and it being routed onwards may vary. This is because a packet switch may need to queue packets for sending onwards, the length of the queue varying with the number of packets involved with the onward leg.. TY2600/v4.1. © Wray Castle Limited. 1.9.

(22) Introduction to Telecoms Intelligent Network (IN) Value Added Services (VAS). Access signalling system. Network signalling system. Access signalling system. Network signalling e.g. SS7 Access signalling e.g. loop disconnect, DTMF. Access signalling DSS1. Private network signalling e.g. DPNSS, Q.SIG PBX. PBX. DPNSS – Digital Private Network Signalling System. Traditional Telecommunication Signalling Systems Signalling is used within a network to set up, maintain and terminate a call. Signalling comprises the access signalling system, for customer access, and the network signalling system, for use in the network itself. The access signalling system is extended out to the customer and works in conjunction with recognizable tones to keep the customer informed about the progress of the call. In general, this does not have to be as sophisticated as the network signalling system, but again, the greater the level of sophistication, the more flexible and complex services can be. Access signalling systems include Loop Disconnect with pulse or, more efficiently, Dual Tone Multi-Frequency (DTMF) dialling and Digital Subscriber Signalling System No 1 (DSS1). The main network signalling is based on Signalling System No. 7 (SS7), which is used between switches in the network. The introduction of SS7 allowed the implementation of new services such as Calling Line Identification (CLI). SS7 is the basis of Intelligent Networking (IN). An IN is a service-independent telecommunications network where intelligence is taken out of the switch and placed in computer nodes that are known as Service Control Points (SCPs), which are distributed throughout the network. This provides the network operator with the means to develop and control services more efficiently. New capabilities can be introduced rapidly into the network and then the services are easily customized to meet individual customers’ needs. With the IN infrastructure, service providers are equipped with mechanisms allowing them to create, test, deploy, and support value-added voice services. Examples are credit card calling, malicious call identification and selective call forward on busy or no answer. In addition to access and network signalling, it is possible to provide the means for end users to communicate with each other using a specified signalling system. This is usually achieved via the access and network signalling systems. An example of a private network signalling system is the Digital Private Network Signalling System (DPNSS). This is a signalling protocol used to connect two PBXs (Private Branch Exchanges).. 1.10. © Wray Castle Limited. TY2600/v4.1.

(23) Telecommunication Services in the Modern World User (Application). Factors information type bandwidth/data rate application analogue or digital real time/non-real time cost. Transmission System information type distance and terrain cost bandwidth/data rate security and reliability analogue or digital choice of transmission medium, copper/coaxial cable, optical fibre, radio. Switching System. network size network capacity information type analogue or digital dedicated circuit or only when data to send switching speed position – private/public switch. Factors Affecting System Choice Any potential network operator, whether fixed or mobile, needs to take several considerations into account in order to maximize their network’s potential and satisfy the needs of the customers they are targeting. These considerations include the application (also known as the ‘user’, such as voice); the transmission system; the switching system; the signalling system, and the cost. Although telecommunications usually deals with electromagnetic signals, the form, rate and use of the information varies greatly from application to application. The type of information to be transmitted has a great impact on how the system is engineered. For example, basic voice information needs to be sent from one telephone to another at the instant the user speaks, but a high degradation of the signal may be allowed because of the tolerance of the human ear. On the other hand, computers need to exchange accurate information, but often the time at which it is sent is not critical. The transmission techniques employed must take into account the type of information, the distances which need to be covered, the terrain and the cost of provision. If the information requires a high bandwidth (for example, video pictures) a medium capable of supporting these requirements must be chosen: fibre optics or satellites, for example. Long distances may be best served by providing satellite links or radio links, although cable systems could ultimately provide better quality. It is usual to have many users connected to a telecommunication system. Users need to be able to contact other users. Each terminal connected to the system must therefore have a unique identifier, for example a telephone number or an Internet address. This number or address is used to route the information to the correct end-point. Routing employs dynamic switches, which use this identifier to provide a path from the originating terminal to the destination. A hierarchical approach is usually used to provide switching (and transmission) throughout a network at reasonable cost. The switching technique varies depending on whether a dedicated circuit is needed, or whether the application allows the switched path to be provided only when there is information to send.. TY2600/v4.1. © Wray Castle Limited. 1.11.

(24) Introduction to Telecoms. Signalling System A. Factors B. information to be transferred complexity of the services offered position within the network (access or core network) cost security. Cost acceptable service at acceptable cost a balance between: – quality – reliability – features – services. Factors Affecting System Choice (continued) Signalling informs the switches of the paths that must be provided through the network. In addition, signalling is generally used to set up, supervise and release calls and to provide control of supplementary services and advanced features. The bottom line is to provide an acceptable service at a cost which the customer is willing to pay. A balance has to be struck between quality, reliability, features and services.. 1.12. © Wray Castle Limited. TY2600/v4.1.

(25) Telecommunication Services in the Modern World. Bearer. Bearer. The network provides the means to transfer information. Audio or data e.g. 3.1 kHz audio 64 kbit/s Datagram (real time/non-real time). Circuit Switch or Packet Switch. Bearer Services Every telecommunication network offers services to its users. These can be identified as bearer services, teleservices and supplementary services. Networks may also choose to offer a range of value-added services to their customers. The bearer service is the most basic service offered by the network. It is simply a means of getting the user data (analogue or digital) from source to destination. The bearer can vary depending on the user’s requirement. For example, the network may offer a bearer service of a 3.1 kHz audio circuit; this could be via a pair of wires across the network. Or it could offer a bearer service of 64 kbit/s, down which the user could put different types of information (speech, data, etc.). Another type of bearer service is a packet-switched service, where the information is broken down into packets at one end of the circuit and reassembled at the other end. The information contained in the packet may either be real time (sensitive to delay), or non-real time (not sensitive to delay).. TY2600/v4.1. © Wray Castle Limited. 1.13.

(26) Introduction to Telecoms. Data Communication Services. Telephony. Telephone Network (PSTN/ISDN). PSPDN. VoIP. IP Network. Bearer Networks. Analogue GP3 Fax (ITU-T T.30). Facsimile. Fax over IP (ITU-T T.38). PSPDN – Packet-Switched Public Data Network. Types of Bearer Service Examples of bearer services are telephone networks, i.e. the Public Switch Telephone Network (PSTN) or the Integrated Services Digital Network (ISDN) and Packet-Switched Public Data Networks (PSPDNs). Customers may wish to subscribe to the bearer service (information transfer mechanism) and provide the means to complete the communications package themselves. In this case, the customer will have to ensure that any hardware, such as the terminal, is compatible with the bearer service. A bearer can be used for several teleservices at the same time. Thus, the telephone network may be used as a bearer for teleservices including telepnony and fax and other forms of data. A number of bearer services may exist within the same transmission network, perhaps occupying different lines within the same cable. These lines could belong to such bearers as the telephone network or a data network.. 1.14. © Wray Castle Limited. TY2600/v4.1.

(27) Telecommunication Services in the Modern World. Telephony. Telephony. Bearer. Bearer SMS. SMS. Fax. Fax. End-to-end communication using the network’s bearer services. Switching. Teleservices Teleservices use a network’s bearer services for end-to-end communication. An example of a teleservice is speech, the speech being carried over the bearer service from source to destination. Networks offer a range of teleservices to their subscribers. Teleservices may include Telephony, Fax, Short Message Service (SMS) and Emergency Calls. Teleservices define the compatibility of terminals as well as a suitable bearer service type.. TY2600/v4.1. © Wray Castle Limited. 1.15.

(28) Introduction to Telecoms. SOFTWARE Ring Back Call Waiting Advice of Charge Calling Line Identification Call Forwarding Ring Hold Call Barring Multiparty. Supplementary Services (SSs) Although basic teleservices include speech, fax and data, a network operator may want to make its overall service appear more attractive to subscribers. Supplementary Services (SSs) which modify or provide a basic telecommunications service, include:     . calling line ID (presentation or restriction) call divert advice of charge barring of outgoing calls barring of incoming calls. It is common for networks to offer a wide range of such services. Some of these are shown in the diagram.. 1.16. © Wray Castle Limited. TY2600/v4.1.

(29) Telecommunication Services in the Modern World Problem: Call charges are going down. How can I generate new revenue streams to increase profitability?. Call Charges. Time. Solution: Value Added Services (VAS) INBOX Fax, SMS, Voicemail, e-mail. Multimedia Messaging. Unified Messaging. Intelligent Network (IN). Location-based Services. Service Enabler. … and many others.. Value Added Services (VASs) Call costs are constantly being driven down due to competition. This is ideal from the customer’s perspective, but from the service provider or network operator’s view, it poses a question: ‘How can I generate new revenue streams to compensate for the drop in call charges?’ One solution is the provision of VASs in addition to teleservices and SSs. Examples of VASs are found in both fixed and mobile networks, and include such services as location-based services, multimedia messaging and Unified Messaging (UM). Typically, VASs are implemented using INs. INs do not define the services to be delivered; they provide an overlay infrastructure on existing circuit-switched networks. An IN acts as a service enabler allowing communication providers to develop, test and deploy attractive operator-specific services which may act as a differentiator between themselves and other providers. This helps the provider to retain their existing customers and generate new revenue streams from them, and also to attract new customers and further increase revenue.. TY2600/v4.1. © Wray Castle Limited. 1.17.

(30) Introduction to Telecoms Telecommunication Services. PSTN PSPDN etc.. Teleservices. Bearer Service. Basic Service. Supplementary Service. telephony. call forwarding. data. multiparty. fax. call waiting. 64 kbit/s data. call hold. SMS. calling line ID. 9.6 kbit/s data. Basic Service. Supplementary Service. 4.8 kbit/s data 3.1 kHz audio. Plus VASs to increase revenue. Telecommunication Services Summary In telecommunications, any service that is offered to a customer is known as a telecommunication service. This collective term can be divided into two main categories, bearer services and teleservices. The basic services for both teleservices and bearer services can be modified by a supplementary service. For example, the basic bearer service of 64 kbit/s (for voice) can be modified by the network building a bridging circuit to link more than two subscribers together, a service known as ‘multiparty’. A network operator may attempt to increase revenue by offering SSs and VASs to their customers.. 1.18. © Wray Castle Limited. TY2600/v4.1.

(31) Telecommunication Services in the Modern World. Government. Appoints/ runs. Office of Telecommunications (Oftel) Independent Broadcasting Authority (IBA) Independent Television Commission (ITC) Broadcasting Standards Commission (BSC). Telecoms. Radiocommunications Agency (RA) Radio Authority. Broadcast Radio Convergence of technologies and market leads to. Single (‘super’) Regulator. Telecommunications Regulation The telecommunications industry is highly regulated. Operators have critical obligations over and above providing a range of services. These include the provision of emergency call services (999, 112, 911, etc.) and, in some cases, ‘universality’ – the right for customers to be connected to networks at a fair and level cost for all (even the connection cost may vary dramatically). A telecommunications regulator is usually an agency or a department of central government. A regulator’s role encompasses regulation and dealing with competition issues; it often acts as a mediator in disputes between network operators. With the increasing convergence between services, networks and technologies the regulators in many countries are being restructured. Traditionally, different bodies or regulators were responsible for telecommunications, radio (spectrum) and broadcast, but these are sometimes now combined in a single agency. An example of this regulation convergence is the creation of the Office of Communications (Ofcom) within the United Kingdom. Ofcom assumed its authority at the end of 2003. It was created by bringing together the following roles:      . Radiocommunications Agency (RA) Independent Broadcasting Authority (IBA) Independent Television Commission (ITC) Broadcasting Standards Commission (BSC) Office of Telecommunications (Oftel) Radio Authority. The combined role of Ofcom covers all aspects of telecommunications and broadcasting including the regulation of content and the handling of complaints from customers.. TY2600/v4.1. © Wray Castle Limited. 1.19.

(32) Introduction to Telecoms. International Telecommunication Union (ITU). International Organization for Standardization (ISO). Third Generation Partnership Projects (3GPP/3GPP2). Telecoms Industry Association (TIA). Regional Telecoms Institutes World Wide Web Consortium (W3C). European Telecommunications Standards Institute (ETSI). Internet Engineering Task Force (IETF) Regional Standards Institutes (ANSI/BSI). Electronic Industries Alliance (EIA) Broadband Forum (BBF). Telecommunciations Standards Bodies Standards are critical in an industry such as telecoms because they are key to providing a competitive environment. Standards create so-called ‘open systems’, which by means of standard interfaces allow a network operator to build a multi-vendor network and allow customers to take their terminal from one network to another network. However, the legacy of telecommunication development in different countries and regions means that there are variations in technical standards around the globe. For example, a telephone terminal purchased in the UK will not be capable of connection in other European countries or North America as the line jack is different; and even if this was replaced, line signal differences would still lead to incompatibility. The same is true for mobile telephony, where different standards are used in different global regions. A diverse range of national, regional and international bodies are responsible for standards that apply to telecommunications networks, terminals and services. There is often a complex relationship between these bodies and large manufacturers and network operators will ensure that they have representatives in the important groups within the standards committees. The subject of standards can be very ‘political’, particularly when a company wants its technology to become, or be included in, a standard. This way the company may gain a lead in the market by virtue of already having the technology working; secondly, they may earn revenue from licensing their technology to others (so-called Intellectual Property Rights – IPR). IPR has caused major legal issues in telecoms in recent years and most standards bodies insist that for a technology to be adopted within a standard it must be available for license on a fair and reasonable basis.. 1.20. © Wray Castle Limited. TY2600/v4.1.

(33) Telecommunication Services in the Modern World. International Telecommunication Union (ITU). International Organization for Standardization (ISO). Third Generation Partnership Projects (3GPP/3GPP2). Telecoms Industry Association (TIA). Regional Telecoms Institutes World Wide Web Consortium (W3C). European Telecommunications Standards Institute (ETSI). Internet Engineering Task Force (IETF) Regional Standards Institutes (ANSI/BSI). Electronic Industries Alliance (EIA) Broadband Forum (BBF). Standards Bodies The diagram shows some the major standardization bodies responsible for producing standards and recommendations used by the telecommunications industry. The principal organizations are described below. International Telecommunication Union (ITU) The ITU has its headquarters in Geneva and is part of the UN. It is responsible for the international coordination of telecommunications and networks. To achieve this aim it is divided into three entities: the Telecommunication Standardization Sector (ITU-T), the Radiocommunication Sector (ITU-R) and the Development Sector (ITU-D). European Telecommunications Standards Institute (ETSI) ETSI, based in France, is an independent, non-profit organization that is officially responsible for standardization of Information and Communication Technologies (ICT) within Europe. ETSI comprises 766 members from 63 countries including manufacturers, network operators, research bodies and users. Third Generation Partnership Project (3GPP) 3GPP, formed in December 1998, unites the telecommunications standards bodies ARIB, CCSA, ETSI, ATIS, TTA, and TTC, which are known as ‘Organizational Partners’. 3GPP was formed to produce globally applicable specifications and reports for a third-generation (3G) network, which evolved into the Universal Mobile Telecommunications System (UMTS). 3GPP also has responsibility for the specifications for the Global System for Mobile Communications (GSM), the General Packet Radio Service (GPRS) and Enhanced Data rates for Global Evolution (EDGE) technologies. Internet Engineering Task Force (IETF) The IETF is an international body of network designers, operators, vendors and researchers concerned with the evolution of the Internet architecture and effective Internet operation. The IETF’s work is done in Working Groups (WGs), which are organized by topic (e.g. routing, transport, security). International Organization for Standardization (ISO) The ISO identifies requirements for international standards and goes on to develop those standards in collaboration with the industry sectors that will put them into use. It is a non-governmental organization comprising the standards organizations of 162 counties (one per country) from every world region. In 2010, the ISO’s portfolio contained over 18,500 standards.. TY2600/v4.1. © Wray Castle Limited. 1.21.

(34) Introduction to Telecoms. International Telecommunication Union (ITU). International Organization for Standardization (ISO). Third Generation Partnership Projects (3GPP/3GPP2). Telecoms Industry Association (TIA). Regional Telecoms Institutes World Wide Web Consortium (W3C). European Telecommunications Standards Institute (ETSI). Internet Engineering Task Force (IETF) Regional Standards Institutes (ANSI/BSI). Electronic Industries Alliance (EIA) Broadband Forum (BBF). Standards Bodies (continued) Telecommunications Industry Association (TIA) The TIA represents those who provide information and communications technology products and services to the global marketplace. It does this through standards development, domestic and international policy advocacy, and business opportunities for members. The Association enables convergence between communications networks as well as promoting a competitive and innovative market environment. Electronics Industries Alliance (EIA) The EIA is a trade organization comprising an alliance of trade associations for electronics manufacturers in the USA. Those associations in turn govern sectors of EIA standards activity. The associations include the TIA and the Electronic Components Association. The EIA is accredited by ANSI to help develop standards on electronic components, consumer electronics, electronic information, telecommunications and Cyber security. For more information see http://www.ecaus.org/eia/site/index.html The Broadband Forum The Broadband Forum is the central organization driving broadband wireline solutions and empowering converged packet networks worldwide to better meet the needs of vendors, service providers and their customers. Their aim is to develop multi-service broadband packet networking specifications addressing interoperability, architecture and management. For more information see http://www.broadband-forum.org/about/mission.php The World Wide Web Consortium (W3C) Established in 1994 and with over 400 member organizations from around the world, the World Wide Web Consortium (W3C) promotes the WWW application of the Internet by developing common protocols and ensuring interoperability. The W3C’s overall goal is to ensure that the Web is accessible to all and has a standardized design. The W3C plays an active role in setting standards for the Internet and fostering co-operation to make the standards freely available. W3C standards include: the language of the Web, Hypertext Markup Language (HTML), the ‘next generation’ language, eXtensible Markup Language (XML) and MathML for marking up mathematics for the Web.. 1.22. © Wray Castle Limited. TY2600/v4.1.

(35) Introduction to Telecoms. SECTION 2. THE PSTN AND ISDN. © Wray Castle Limited. I.

(36) Introduction to Telecoms. CONTENTS Basic Structure of the Public Switched Telephone Network (PSTN) . . . . . . . . . . . . . . . . . . . .2.1 Analogue and Digital Telecommunication Networks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.2. The Analogue World . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.3 Signal Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.4 Speech Signals. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.5. Commercial Speech Channels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.6 Two-Wire Transmission in the Local Loop. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.7. Switching in Telephony Networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.8 Public Switches Hierarchy and Functions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.9. Private Switches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.10 Numbering Plans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.11 Call Routing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.12 Analogue and Digital Signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.13 Digital Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.14 Analogue Signal Quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.15 Digital Signal Quality. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.16. Bit Rate and Baud Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.17 Analogue to Digital Conversion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.18. Interworking between A-Law and µ-Law. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.19. Other Types of Voice Coders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.20 The Need for Multiplexing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.21 Time Division Multiplexing (TDM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.22 Primary Level Multiplexers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.23 Timeslot Interchange . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.24. II. © Wray Castle Limited.

(37) The PSTN and ISDN. CONTENTS PSTN and ISDN Digital Switches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.25 Evolution from PSTN to ISDN. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.26. The ISDN Digital Subscriber Line – Basic Rate Access . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.27 BRA Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.28 The ISDN Subscriber Line – Primary Rate Access (PRA) . . . . . . . . . . . . . . . . . . . . . . . . . . .2.29 Signalling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.30 Channel Associated Signalling (CAS). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.31. Common Channel Signalling (CCS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.32 Access/Network Signalling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.33 Basic Call Process and the ISDN User Part (ISUP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.34 End-to-End Call Setup – Basic Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.35 Services for Revenue Generation The IN Concept. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.37. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.38. IN Implementation Simplified . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.39 An Example of an IN Service . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.40 IN-based Services. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.41. © Wray Castle Limited. III.

(38) Introduction to Telecoms. IV. © Wray Castle Limited.

(39) The PSTN and ISDN. OBJECTIVES At the end of this section you will be able to:. . state the general requirements of a telephony network. . explain the terms ‘local loop’ and ‘final mile’. . outline the architecture of the Public Switched Telephone Network (PSTN). . identify the respective roles of access and core networks. . describe the international numbering plan for telephony services. . describe how numbers are used by switches to route calls. . list the advantages of digital signals over analogue signals. . describe the principles of Pulse Code Modulation (PCM). . define the Integrated Services Digital Network (ISDN) concept. . describe the concept of an Intelligent Network (IN). © Wray Castle Limited. V.

(40) Introduction to Telecoms. VI. © Wray Castle Limited.

(41) The PSTN and ISDN Access Signalling e.g. DTMF. Telephone Local Exchange. PBX. Computer Copper Wire Local Loop Access Network. Core Transmission Networks (Fibre or Radio). Core Exchanges. Core Signalling System No. 7 (SS7). (National and International). PBX Telephone Local Exchange DTMF – Dual Tone Multi Frequency. Computer Copper Wire Local Loop Access Network. Basic Structure of the Public Switched Telephone Network (PSTN) The Public Switched Telephone Network (PSTN), also known as the Plain Old Telephone Service (POTS), is the fixed telephone network over which landline telephone calls are made. The PSTN relies on the principle of circuit switching, where the connection is set up through the network by signalling protocols before speech can commence. To connect one phone to another, the call is routed through numerous switches or exchanges operating on a local, regional, national or international level. The connection established between the two phones is called a circuit. In the early days, phone calls were routed as analogue signals all the way to the receiving end. During the 1970s, analogue voice calls began to be converted into digital format and the switching was carried out by microprocessor-controlled digital switches. This provided the basis for the Integrated Services Digital Network (ISDN). Digital techniques allowed a more efficient way of combining different types of traffic such as speech and data, and enabled a better quality of service for the customer. A variety of terminal equipment can be used by the customer including telephones, fax machines, computers and Private Branch Exchanges (PBXs). Within a company or larger organization, each employee or department can have their own extension telephone. Extensions from the main phone number are routed through a PBX that is located at the company’s premises. The local loop has for the most part remained as a twisted copper pair and is dedicated to one customer. However, in some situations radio is used in the access network, which is known as Fixed Wireless Access (FWA). The development of optical fibre transmission techniques now allows thousands of calls to be transferred between switches. However, these high-capacity fibre and radio transmission links have not changed the basic nature of the circuit-switched network, which still requires a connection or circuit to remain open for the duration of the phone call. Various signalling systems have been developed to control the switches in the network. The access signalling protocols are used between the customer and the local exchange – some for basic telephone connections such as DTMF and others for private signalling from a PBX. In the core network between switches, Signalling System No. 7 (SS7) is used.. TY2600/v4.1. © Wray Castle Limited. 2.1.

(42) Introduction to Telecoms (Integrated modem). (Integrated modem). Fax. Fax. Original Analogue PSTN. PBX. PBX. (voice centric) Telephone. Telephone Modem Computer. Analogue Access Network. Modem Computer. Digital Access Network Digital fax. Digital fax. Integrated Digital. Digital PBX. Digital PBX Internet Service Provider (ISP). Network Computer A/D Digital Telephone. Analogue Telephone. D/A. Analogue Access Network. Telephone. Analogue and Digital Telecommunication Networks The original PSTN was analogue in operation and was voice centric. An electrical circuit was connected from one end to the other and analogue telephones and PBXs were connected directly to the access network. If a digital device such as a computer or fax machine required connection across this network, then the digital signal had to be converted to analogue using a modem (modulator/demodulator) to enable it to be carried over the network. The network itself was unaware it was carrying computer data and treated it just like any other analogue telephone signal. Another modem was required at the far end to recover the digital signal. With the development of the digital switching infrastructure, the customer could connect digital devices directly to the network without a modem. However, if an analogue telephone required connection then an analogue/digital converter was necessary. This is normally placed in the local telephone exchange. This way of connecting telephones to the network is still in common use. A wider variety of services could now be provided over this integrated digital network such as fax, computer data, images, compressed video and voice.. 2.2. © Wray Castle Limited. TY2600/v4.1.

(43) The PSTN and ISDN. Sound. Velocity Light. Amplitude Pressure Optical Fibres. Time. The Analogue World The world is analogue in nature since all aspects of the real world, such as temperature, humidity, pressure, velocity, light and sound, vary continuously with time. It is possible to sense these variables electrically and represent them as a continuously varying signal.. TY2600/v4.1. © Wray Castle Limited. 2.3.

(44) Introduction to Telecoms a) Sinusoidal Wave in the Time Domain. Time. Amplitude. 1 millisecond. f =. 1 t. b) Sinusoidal Wave in the Frequency Domain. Amplitude Frequency 0. 1 kHz. 5 kHz. f =. 1 t. Signal Concepts In a modern telecommunication network, electrical signals, light and radio waves (sinusiodal waves) are used to transfer information between network nodes or points. Another important fact is the awareness of the relationship between the time-domain and frequencydomain representation of signals. The figure illustrates a 1 kHz sine wave in the time domain. Such is the display expected on an oscilloscope. The diagram illustrates the same sine wave in the frequency domain. Such is the display expected on a spectrum analyzer.. 2.4. © Wray Castle Limited. TY2600/v4.1.

(45) The PSTN and ISDN (a). Amplitude. Time. (b). Amplitude. Frequency. 0 Bandwidth of Speech Signal 100 Hz. 5 kHz. Speech Signals The previous example was of a single audio tone of 1 kHz. Human speech is a much more complex signal. It consists of a range of frequencies from approximately 100 Hz to anywhere up to 10 kHz. When represented in the time domain it appears as a constantly varying complex signal. As speech consists of a number of frequencies constantly varying in amplitude and frequency, then the representation in the frequency domain at any instant may resemble that shown in the diagram. The time domain representation shows the instantaneous amalgam of all the frequency components present. The concept is that every waveform that is not sinusoidal in the time domain is a complex wave and consists of a number of sinusoidal frequency components to be seen in the frequency domain. The examples of a single tone and speech have introduced the concept of bandwidth. It follows that to hear (receive) the single tone all that needs to be received is that same single tone. To hear (receive) the speech signal exactly as transmitted all frequency components need to be received. All of these frequencies are constituent parts of the information. The speech signal occupies a wider bandwidth than the single tone.. TY2600/v4.1. © Wray Castle Limited. 2.5.

(46) Introduction to Telecoms. Speech Analogue Signal. Speech energy Emotion. 15 Hz 100 Hz 300 Hz 3.4 kHz. 5 kHz. 10 kHz. 15 kHz. 3.1 kHz Commercial Speech Human Speech Human Hearing. Commercial Speech Channels In basic telephony, speech is converted into an electrical signal and carried over a transmission medium as an electrical representation. The electrical information is analogue in nature, as the voltage level varies with the analogue speech itself. Certain constraints need to be applied to the electrical signal in order to control its transmission. The range between the lowest and highest frequencies used for a particular purpose may be defined as bandwidth. The human ear can detect frequencies in a range between 15 and 15,000 Hz. However, it would be too costly to design a telephone system for this bandwidth and therefore more moderate standards are aimed for. Measurements show that if certain frequency components are removed from the speech signal, it is still possible to retain its intelligibility. The diagram shows that the majority of voice energy is transferred in the low frequencies of 600–700 Hz. Although basic speech continues at a much lower level beyond 10,000 Hz, this adds little to the intelligibility of the signal to the human ear, as shown by the solid line. The dashed line shows the portion of the frequency band that carries emotion. From this it can be seen that for economical transfer of intelligible speech, it is possible to use a much narrower band than 15–15,000 Hz. Speech is therefore limited to the range of 300–3,400 Hz, which gives a bandwidth of 3.1 kHz, as specified in the ITU-T Recommendations G.132 and G.151 and often referred to as the nominal 4 kHz voice channel. It is considered a good compromise between quality and cost.. 2.6. © Wray Castle Limited. TY2600/v4.1.

(47) The PSTN and ISDN Single two-wire copper pair connected together for continuous circuit. Local Telephone Exchange. Analogue representation of human speech or modem tones. Cross Connection Point. Average UK distance 3–4 km Distribution Point. Known as:. :Local Loop Final Mile Access Loop. Two-Wire Transmission in the Local Loop A telephone conversation requires transmission in both directions. This can be achieved on a two-wire cable and is known as two-wire transmission. However, as with any cable, attenuation occurs, giving the cable a finite effective length before the signal being carried requires amplification in the form of a repeater. For a two-wire cable used to connect a typical home telephone to the local exchange, the average UK distance is 3–4 km and is known as the local loop, final mile or access loop. The diagram shows a simplified connection to the local exchange.. TY2600/v4.1. © Wray Castle Limited. 2.7.

(48) Introduction to Telecoms a). UK. USA. The PSTN. Australia. France. Exchanges. b). Local. TXMN Section. National International National. TXMN Section. TXMN TXMN Section Section. Local. TXMN Section. TXMN Section. Routing info. Incoming call indication. Switched connection Routing info. Routing info. Routing info. Routing info. End-to-end switched circuit. Switching in Telephony Networks Telephony networks provide global telephony services to customers connected to the network. The network can be viewed as a single switch which provides circuit-switched connections on demand between any two end users located in any country. In reality, however, the network is made up of many systems constituting thousands of interconnected switches throughout the world. The establishment of a complete end-to-end switched circuit between a calling and a called party may involve many switches on many levels, i.e. local exchanges, national exchanges and international exchanges. Each switch assigns and provides a connection between the different transmission channels connected to the switch. Thus, for the duration of any call, an end-to-end circuit-switched connection consists of a number of Transmission (TXMN) sections joined together by one or more switches. This connection is cleared down at the end of a call and the resources become available for other users.. 2.8. © Wray Castle Limited. TY2600/v4.1.

(49) The PSTN and ISDN. Typical Functions ISC: Switching Trunk Termination Signalling Operations and Maintenance Charging TE: Switching Trunk Termination Signalling Operations and Maintenance. International Switching Centre (ISC). Regulatory Interconnect. Transit Exchange (TE) CP. Local Exchange (LE). LE: Switching Trunk Termination Signalling Operations and Maintenance Subscriber Termination End User Subscriber Services Charging Circuit-related signalling Traffic signalling and traffic. Public Switches Hierarchy and Functions Public switches (or telephone exchanges) can be categorized as one of three general types of exchange: Local Exchange (LE), Transit Exchange (TE) (trunk switching centre) and International Exchange (IE), also known as an International Switching Centre (ISC). All of these switch types need to be able to carry out a range of functions. Specific functions will vary from switch to switch depending on their place in the switching hierarchy. Local Exchange (LE) An LE must perform all the necessary actions to allow subscribers to access the public network. This includes signalling, routing, analogue-to-digital conversion, services such as call back, and auxiliary functions such as power supplies, charging, and operations and maintenance. Routing may be within the LE, or externally via a transit switch. Transit Exchange (TE) A TE transfers traffic between exchanges which are subordinate in the hierarchy, or between a subordinate exchange and one further up in the network. Some exchanges may be designated for both local and trunk traffic, combining the functionality of both the LE and TE. International Exchange (IE) An IE provides the interface between national networks and the international network. Charging and accounting are major features of an IE. It will also have an international operator service.. TY2600/v4.1. © Wray Castle Limited. 2.9.

(50) Introduction to Telecoms PBX Functions: to connect telephones within same organization to provide connection to rented lines towards the network concentration (lower number of lines for larger number of extensions) additional services, such as – Call Logging – Call Diversion – Call Waiting – Abbreviated Dialling – Call Barring – Multi-Party. To Secondary Trunk Switching Centre. Primary Trunk Switching Centre Other LEs. Effectively an extra ‘branch’ in the switching hierarchy. Local Exchange. PBX. Other subscribers or PBXs Note: PBXs may be interconnected to form a network, with single or multiple access points to the PSTN.. Private Switches In large organizations there is a need for internal communications between staff in different locations. A public network could be used to provide this service, with telephones interconnected via transmission lines to the LE, and public switches providing the interconnection. However, this would be wasteful, especially when the communicating parties are located within the same building or even the same room. Therefore, private switches are normally used to connect telephones within the same organization. Not only does this eliminate the cost of internal calls, but the number of rented lines from the private switch to the public network is normally far fewer than the number of internal extensions. Usually, a private switch takes the form of a Private Branch Exchange (PBX), although it is also possible to use a manual version, the Private Manual Branch Exchange (PMBX). Many private switches offer supplementary services like those available in public networks such as call logging, call diversion, call waiting, abbreviated dialling, call barring and multi-party calls. If a suitable signalling system is used, these services may also be extended across a network of PBXs (perhaps supporting a larger business with more than one site in a local area).. 2.10. © Wray Castle Limited. TY2600/v4.1.

(51) The PSTN and ISDN a). Country Code. National Destination Code. Subscriber Number. National Significant Number International ISDN number Notes: 1 The national and international prefixes are not shown – they are not considered to be part of the international ISDN number. 2 Maximum length – 15 digits.. b) Example of E.164 Number Structure to Identify a Subscriber (Prefix International Call). (Prefix) National Call. 0044 (0)1539 Identifies a country. 742742. Kendal Transit Switch. Wray Castle ‘Customer’. Numbering Plans Within a telephony network, each customer is given a personal number. This is an identifying reference for that customer on their local switch. It also provides routing information so that incoming calls can locate the correct local switch within a network. Each network is allocated its own group of numbers which it can distribute to its customers on a switchby-switch basis. The function of a line does not affect the number a customer is given, i.e. it does not matter if it is used for speech or data calls. The ITU has issued Recommendations on the use of numbering plans. It allocates codes to identify subscribers uniquely in different countries. It also recommends that numbering plans for new networks/services adhere to the ITU-T Recommendation E.164. The national significant number allocation is a matter for national authorities; the country code is only used when dialling between countries. The maximum length of the dialled number is 15 digits.. TY2600/v4.1. © Wray Castle Limited. 2.11.

(52) Introduction to Telecoms ‘A’ Dials Number: 00 44 1234 567890 Note: The ‘00’ International prefix is not part of the international ISDN number and may vary from country to country. Route towards subscriber based on ‘7890’ (Number of subscriber on the LE). Route towards ‘E’ based on 56 (Network’s own allocation for LE ‘E’). Party ‘B’ Local Exchange. E Network Gateway. Route towards ‘D’ based on ‘1234’ (National destination code). D. International Gateway. International Gateway. C. Route towards ‘B’ based on international prefix ‘00’. A B Route towards ‘C’ based on ‘44’ (UK code). Usually via transit network. Local Exchange. Party ‘A’. Call Routing Routing analysis based on the E.164 numbering plan simply involves selecting routes towards the correct country as specified by the country code, and then towards the correct network within that country based on the national destination code. It then uses the subscriber number to identify the correct line at the local exchange.. 2.12. © Wray Castle Limited. TY2600/v4.1.

(53) The PSTN and ISDN. Continuous time signal. (Analogue). Discrete in both time and amplitude. (Digital). Analogue and Digital Signals Most of the signals considered so far have been analogue signals. However, most telecommunications systems are now based on digital technology. An analogue signal has an infinite range of amplitude values in respect of time, which is continuous, whereas a digital signal has a discrete limited range of amplitude values in respect of time, which is discontinuous. The term ‘data’ is often used when people talk about digital technology; but what is data? The literal meaning of the word is simply ‘information’. However, in this context it means information which is conveyed in a digital form. It should be noted that information itself may be analogue. Modern telephony, music and television are examples of analogue information (speech, music and moving pictures) that is converted to a digital form.. TY2600/v4.1. © Wray Castle Limited. 2.13.

(54) Introduction to Telecoms. Analogue Signal. Digital Signal Analogue to Digital Converter. 0 11 0 111 00 1 0. Analogue Signal Digital to Analogue Converter. Digital Systems All modern telecommunication networks are digital. The benefits of digital networks include quality improvements and the integration of different services onto one network. Information, such as speech, does not originate as a digital signal – it is analogue. The process of digitization – conversion from analogue to digital – will be performed as close to the point of origin as possible. In a fixed telephone network, this would be at the local exchange. For a digital mobile network, it takes place in the handset.. 2.14. © Wray Castle Limited. TY2600/v4.1.

(55) The PSTN and ISDN. Pure Signal. Signal and Noise. Signal and Noise Amplifier. Amplifier. Transmission Media. Transmission Media. Attenuation. Attenuation. Analogue Signal Quality In any electronic environment, whether it is a piece of equipment or a link between equipment (cable or radio), there will be random signals present referred to collectively as ‘noise’. This noise may originate from several sources but is always present to a greater or lesser degree. Once noise becomes mixed with an analogue signal, the two components (noise and signal) can never be fully separated. This means that any attempt to amplify an attenuated (diminished) analogue signal will amplify not only the wanted signal, but noise as well. Hence over any transmission path the quality of an analogue signal will continuously degrade.. TY2600/v4.1. © Wray Castle Limited. 2.15.

(56) Introduction to Telecoms. Bit Stream. 10110. Regenerator. Regenerator. 10110. 10110 Original Bit Stream. Transmission Media. Transmission Media. Attenuation. Attenuation. Digital Signal Quality Digital signals also become corrupted by noise. However, it is a relatively simple process to remove the noise by regenerating the digital waveform. Although a clean digital bit stream is transmitted, it becomes corrupted by noise along the transmission link to its destination. The receiver examines each bit as it arrives and interprets its value in relation to a threshold value. For example, if the value is above the threshold, the value will be interpreted as a logical ‘1’, and if it is below the threshold, it will be interpreted as a logical ‘0’. Assuming the correct decision is made, the digital bit stream may then be regenerated to its original noise-free form. If the signal is pushed too far along the transmission medium without regeneration, the received waveform will be so corrupt that the system will be unable to recognize it. During transmission, therefore, the signal needs to pass through a regenerator before it becomes too corrupt.. 2.16. © Wray Castle Limited. TY2600/v4.1.

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