Rider , The primary purpose of this volume is to give such pertinent information as will furnish a general outline of frequency modulation: the manner in which it differs from amplitude modulation, and details associated with f-m receiver servicing problems. The practical applications that are described follow closely good engineering practice. Schultz, R.
Tinnell - Delmar Publishers , This manual is a result of experience gained with students in technical schools and community colleges throughout the country.
We shall cover a number of basic areas including: receivers, transmitters, and introductory communications systems. The main purpose is to understand how models could be constructed and how to analyze them. These study areas are often called teletraffic. Schelkunoff - J. Wiley , This book is about antennas -- about the physical principles underlying their behavior, the theory needed in sound antenna design and in planning meaningful experiments, the applications of theory to antennas in various frequency ranges.
It promises high data rate communication over short distances as well as innovative radar sensing and localization applications. Book topics include discussion of arrays, spectral domain, optimization, multiband, dual and circular polarization, etc. Optical components and other enhanced signal processing functions are also considered in depth. We will look at both broadcast and bi-directional data networks. The book will focus on existing technology, not too much on theory.
It will model and study the effects of channel impairments on the performance of communication systems and introduce signal processing, modulation, and coding. Theory, rather than industry standards, motivates the approaches, and key results are stated with all the required assumptions.
It identifies and examines the most pressing research issues in Telecommunications. This book focuses on the fundamental techniques, concepts, and mechanisms used in the design, development, and operation of telecommunication networks. Topics c. Multidimensional Queueing Models in Telecommunication Networks.
The increasing complexity of telecommunication networks requires us to develop adequate mathematical models. We must find their characteristics, optimize them s. This book contains the best papers of the First International Conference on e-Business and Telecommunication Networks held in The book presents recent res.
Optical and Microwave Technologies for Telecommunication Networks. This is a self-contained book on the foundations and applications of optical and microwave technologies to telecommunication networks application, with an empha. Loss networks ensure that sufficient resources are available when a call arrives. Operation of DXC is discussed in Chapter 4. The local exchanges are connected to these trunk exchanges, which are linked to provide a network of connections from any customer to any other subscriber in the country.
High-capacity transmission paths, usually optical line systems, with capacities up to 10 Gbps, interconnect trunk exchanges. Note that a trans- port network has alternative routes. If one of these transmission systems fails, switches are able to route new calls via other transmission systems and trunk exchanges to bypass the failed system Figure 2. Connections between local and trunk exchanges are usually not fault protected because their faults affect on a smaller number of subscribers.
Digital Connections trunk Digital to other regions Digital exchange trunk local exchange exchange Transmission network Common channel Digital signaling trunk exchange Digital Digital local trunk exchange High capacity exchange optical transmission systems Figure 2.
Its basic purpose is simply to provide a required number of channels or data transmission capacity from one exchange site to another. Exchanges use these channels of the transport net- work for calls that they route from one exchange to another on subscriber demand.
The trunk exchanges are usually located in major cities. They are digital and use the international common channel signaling standard SS7 to exchange routing and other signaling information between exchanges. The transmission lines between exchanges have conventionally carried TDM tele- phone channels, as explained in Chapter 4.
Currently the use of IP networks for connections among exchanges is increasing and it requires media gateways MGWs between the exchange and IP network to take care of signaling and real-time transmission through the IP network. Via this highest switching hierarchy level, international calls are connected from one country to another and any subscriber is able to access any of the other more than 2 billion sub- scribers around the world.
High-capacity optical systems interconnect international exchanges or switching centers of national networks. Submarine cables coaxial cable or optical cable systems , microwave radio systems, and satellites connect conti- nental networks to make up the worldwide telecommunications network.
The first submarine cable telephone system across the north Atlantic Ocean was installed in , and it had the capacity of 36 speech channels. Modern optical submarine systems have a capacity of several hundred thou- sand speech channels and new high capacity submarine systems are put into use every year. In addition to speech, submarine systems carry intercontinen- tal Internet traffic, which is estimated to take most of the capacity of the new systems under installation.
Submarine systems are the main paths for inter- continental telephone calls and Internet communication. Satellite systems are sometimes used as backup systems in the case of congestion. We described the common structure of the global telecommunications network without separating the different network technologies.
We need dif- ferent network technologies to provide different types of services, and the telecommunications network is actually a set of networks, each of them hav- ing characteristics suitable for the service it provides. However, the public network contains many other networks that are optimized to provide services with different characteristics. We review these different network technologies in this section. We can divide telecommunications networks into categories in any of many different ways.
If we consider the customers of networks and the avail- ability of services, there are two broad categories: public networks and private or dedicated networks. These network operators have a license to provide telecommunica- tions services and that is usually their core business. Sometimes we refer its service to as POTS if we want to distinguish ordinary fixed telephone service from other services provided by telecommunications networks today. In addition to voice communications between fixed telephones, data can be substituted for speech with the help of a voice-band modem.
They are regional or national access net- works and connected to the PSTN for long-distance and international con- nections. We introduce mobile networks in Chapter 5. The bit rate of telex is very slow, 50 or 75 bps, which makes it robust. It was once widely used but its importance has been reduced as other messaging systems such as electronic mail and facsimile have reduced its market share. Pagers are low-cost, lightweight wireless communication systems for contacting customers without the use of voice.
The impor- tance of paging systems has been reduced in countries where penetration on cellular systems, providing text-messaging service, is high. Leased point-to-point lines are often an economical solution for connections between the LANs of corporate offices in a region.
Circuit-switched networks dedicated to data transmission are not widely used today. Packet-switched data service is provided by the X. These networks were developed to provide commercial data com- munication service and they provide charging functionality so that the cus- tomer bill may be based on the amount of transferred data. The importance of these networks has been reduced because of expansion of the Internet.
Internet e-mail has replaced X. Public wireless data networks, such as general packet radio service GPRS , have been implemented to provide data services for mobile users. Department of Defense. The ARPANET grew until it became a wide-area computer network called the Internet, which was used in the s and s mainly by academic institutes such as universities.
Because of its his- tory the Internet does not provide charging functions, and customer billing is usually based on the access data rate and fixed monthly fee. In the first half of the s the user-friendly graphical user interface WWW was intro- duced; since then the use of the Internet has expanded very rapidly.
Cur- rently, the Internet is the major information network in the world, and many Internet service providers ISPs have sprung up to provide Internet services for both businesses and residential customers. The expansion of the Internet continues, and the evolving commercial services e. With the help of some hardware and software updating, modern digital telephone exchanges are able to provide ISDN service.
The main hardware modifica- tion required is the replacement of analog subscriber interface units with digital ones, as shown in Figure 2. The ordinary two-wire subscriber loop of the telephone network is upgraded to the basic rate access of ISDN by an NT on the subscriber premises and by a basic rate interface unit and ISDN software in the local exchange. The bidirectional data rate in the subscriber loop is Kbps, which carries Kbps of user data and additional framing information.
D-channel, 16 Kbps, is used for signaling. Total information rate is Kbps, which makes Kbps when framing information is added. User data contain two independent Kbps circuit- switched user channels, B channels, and a Kbps signaling channel, the D channel.
Subscribers may use user channels, B channels at 64 Kbps, for ordi- nary speech transmission, data, facsimile, or videoconferencing connections. Subscribers may use both B channels independently at the same time and dial them up independently, for example, using one of these channels for a telephone call and another for an Internet connection.
For Internet surfing B channels can be combined to provide a single Kbps data rate connec- tion. Users may connect up to eight terminals to a network terminal and two of them may be in use at the same time. The advantages of ISDN over the analog telephone service are a higher data rate and the availability of two con- nections at the same time. ISDN technology has been available for some time but its usage has been low because of high tariffs in the past.
On the other hand, higher rate access technologies, such as xDSL and cable modems, provide better performance and they have cut the growth of ISDN. The Telecommunications Network: An Overview 51 However, the existing low-cost ISDN technology makes it feasible for net- work operators to provide ISDN connections sometimes at a lower cost than two conventional analog telephone connections.
Traditionally, the operators of these net- works have not provided dial-up bidirectional telecommunications services. Access to these networks is currently available in urban areas via cable TV networks built by cable TV operators.
These operators have not been allowed to provide other telecommunications services and their wideband cable net- work to homes has not supported bidirectional communication. As the deregulation of the telecommunications business has proceeded, these opera- tors have become active in providing other telecommunications services as well, especially fixed telephone service and high-data-rate Internet access. To provide interactive services, the cable TV networks need to be upgraded with the technologies that allow subscribers not only to receive TV and radio signals, but to transmit data to the network.
Most of the invest- ment was already made when wideband cables were installed. This existing medium is especially attractive for providing Internet service to every home connected to a cable TV network. Typically, a data connection made via a cable TV network is shared between many home users; that is, there is no physically separate connection to every home as we have in the case of ISDN or xDSL.
This service is has often attractive tariffs because of shared investments, but it may suffer from temporary congestion when many users happen to be active at the same time. They usually own and maintain the networks themselves. Services provided are a tailored mix of voice, data, and, for example, special control information. They are called private or professional mobile radio PMR. Railway companies also have private tele- phone networks that use cables that run alongside the tracks.
They can incorporate LANs with mainframe computers feeding information to the branch offices. Banks, hotel chains, and travel agencies, for example, have their own separate data networks to update and distribute credit and reserva- tion information. Another choice is to lease resources, which are also shared with other users, from a public network operator. This virtual private network VPN provides a service similar to an ordinary private network, but the sys- tems in the network are the property of the network operator.
In effect, a VPN provides a dedicated network for the customer with the help of public network equipment. As companies concentrate more and more on their core businesses, they are willing to outsource the provision, management, and maintenance of their telecommunications services to a public network operator that has skilled professionals dedicated to telecommunications.
An important application of VPN is intranet use. An intranet is a pri- vate data network that uses open Internet technology. Physically, an intranet may be made up of many LANs at different sites. Note that the Internet uses the packet- switching principle and there are no physically separate channels for each VPN as in the previously explained voice VPN.
Because the packets are not separated into dedicated point-to-point channels, security risks arise when the public Internet is used for interconnections instead of leased lines or a circuit-switched network such as ISDN.
To overcome this problem, firewalls are used in an intranet at the interface between each LAN and the public Internet. The firewalls perform the authentication duties for the communi- cating parties and they encrypt and encapsulate data for transmission through the public Internet from one office to another.
Another network related to an intranet is an extranet. An extranet is connected between selected users of the Internet and an intranet. These external users of a private intranet may be, for example, customers or mate- rial suppliers. Like an intranet, an extranet uses Internet technology, and for security reasons firewalls or other security gateway arrangements are used for user authentication purposes and data encryption.
Connection setup is always done in the same way, whether the intended B subscriber is available or not. In the old days, a human operator performed the switching process manually on a switchboard. In a modern telecommunications network this intelli- gence is implemented with help of IN technology.
The IN is an ordinary digital telephone network with some additional capabilities like flexible routing of calls and voice notifications. Traditionally, a telephone number has been the identifier of a certain physical subscriber line and a socket.
In an IN the physical number and service number have no fixed relation and may change with time. For example, emergency service may be available at daytime in multiple locations but at nighttime only in one location of the area. We can implement these services by updating corresponding functions to each local exchange. Forwarded calls are regarded as being made from your home telephone and will therefore be charged to the telephone bill of the subscriber who has forwarded the call.
You hear the message as a faint tone in the receiver, while the caller simultaneously hears a normal ringing tone. You can alternate between these two calls. A subscriber notifies the system that you want to have a call established when the called party becomes free and she will be informed when this happens. When the subscriber then lifts the receiver, the number will be automatically dialed again. These short numbers can be used by all home telephones that are connected to the same subscriber loop.
This service is implemented by the telephone service provider according to a customer request. A subscriber may, with the help of this service, avoid charges that may be very high when expen- sive service numbers are called from his telephone. Implementation of supplementary services in local exchanges is reason- able because these services are related to only one subscriber connected to one exchange.
A subscriber is also able to modify the service and there is no need to transfer service information to other exchanges. However, some services should be available in all exchanges. Examples of this include use of the same emer- gency number all over the country and establishment of nationwide service numbers. Calls to these numbers are to be routed to one physical telephone number depending on where the call is initiated or time of day.
As more and more of these kinds of services have been introduced, the updating of new serv- ices to many exchanges has become a great burden to the network operator. The IN structure was developed to help network operators and service provid- ers introduce, update, and develop new services in a more efficient way. With central intelligence, control information is stored in a central place and the same information is available for all exchanges in the network. Exchanges request information when they need it for call handling.
Otherwise, service information would need to be updated to all exchanges when a change is made. The service management system SMS provides tools for introduction of new services and service updates. The database DB contains control information, such as emergency numbers and corresponding physical num- bers, for the service control point SCP , which controls service switching point SSP exchanges. The intelligent peripheral IP is a system that provides voice notifications when required, and the service transfer point STP is an intermediate exchange, which routes signaling messages between the SSP and STP.
A certain range of telephone numbers is reserved for IN services only. The SCP then provides information about how that call should be handled. In principle, we could implement all intelligence in the SCP and its database could store all the routing information. This would require heavy signaling between the switching points and the SCP.
In practice, the services that do not require a centralized database are implemented in switching points to reduce the load on the SCP and the signaling connections between SCP and SSPs. The cost of the call is the same no matter to which office the call is connected. The service provider charges sub- scribers via the telephone bill.
The charge is dependent on the called service number. The modern telecommunications networks using IN technology pro- vide many other services and a few new ones appear annually. An example of these is inexpensive home-to-mobile and mobile-to-home calls for which you dial a specific number given by an operator. Another example is a card service for which a serviceperson dials a specific service number and security code and the network operator charges his or her employer instead of the tele- phone from which he or she is calling.
One category of services implemented with the help of IN technology is value-added services. This term refers to the services that give additional value, not just point-to-point telephone conversation. Separate service pro- viders, not the telecommunications service provider, often provide these serv- ices. Examples of value-added services are telebanking, telephone doctor or lawyer services, and participation to TV games.
IN technology provides flexi- ble routing and service-specific charging for these services. In previous sections we described the structure and operation of the telephone network and we have also looked at different network technologies that we need to provide different services.
In the following section we look at how all of this fits together. Internet users are connected to the global Internet via the hosts of their ISPs. Networks of national ISPs are connected and this interconnection is extended to the networks of ISPs of neighboring countries, and these networks together make up the global Internet.
Some different means of accessing telecommunications networks are also shown in Figure 2. This connection is called the primary rate interface in the case of ISDN. Each analog line twisted pair carries one telephone call with signaling.
This analog signaling is close to the ordinary analog subscriber loop signaling that we described previously. For data communication via an analog network or digital network with analog subscriber interfaces, a modem is required.
If a subscriber has ISDN service, which is fully digi- tal, no modem is needed and an end-to-end bidirectional or Kbps digital circuit is available with the help of a network terminal that takes care of the digital bidirectional transmission over the subscriber loop.
For active Internet users who require continuous connection or higher data rates, circuit-switched services are expensive because the cost is based on the dura- tion of the call and they do not provide high enough performance. An attrac- tive access method for these types of users is ADSL, which provides data rates up to a few megabits per second with a fixed monthly fee. Leased lines, which interconnect two offices in Figure 2.
Different options for data connections are discussed in Chapter 6. As we have seen, telecommunications networks contain a huge number of different complex systems that are located in multiple sites.
In the old days, when the structure of the network was simple, most of the equipment sites had personnel to keep systems operational and they carried out fault location and performed needed maintenance operations.
Nowadays systems are so numerous and so complicated that this way of network operations and maintenance is not possible anymore and implementation of automated net- work management tools is mandatory for all network operators.
The follow- ing section gives an overview of the importance of network management and of the standardized structure of network management. Efficient network management is a key tool in helping a network operator improve services and make them more competitive.
Operation functions cover subscriber management functions and enable the network operator, for example, to collect charging data and move and terminate subscriptions. Operation also includes traffic monitoring and controlling the network in such a way that the risk of overload is minimized, for example, by switching traffic from overloaded connections to other systems. Maintenance includes monitoring of the network and, when a fault occurs, corrective actions are performed.
Bit error rates and other parameters are continuously measured for the early detection of faults. This used to be quite a difficult task because it was done manually and many systems may detect a fault even when the actual fault may be in only one of them or even somewhere else. Corporate networks are private networks containing LANs interconnected by circuits provided by a public telecommunications network operator. We can divide corporate networks into two main areas of network management responsibility: local networks in corporate sites and interconnections between sites implemented in a public network that provides interconnec- tions as shown in Figure 2.
Network management responsibility is often divided hierarchically. Local or site managers only take care of LAN networks at each office. A cen- tralized organization of the company manages the usage and availability of wide-area network WAN connections between sites.
A centralized organiza- tion offers service to business units at various sites and optimizes the utiliza- tion of expensive long-distance or even international WAN connections. Most network elements of LANs provide network management func- tions via a standardized management interface.
Software packages for centralized management workstations for LANs are commercially available. The public network operator manages the public network in order to be able to provide reliable service to customers. Network optimization to avoid unnecessary investments as well as quick repairs in the case of faults is important. Short delivery times of leased-line circuits are an important com- petitive advantage today, and a network operator can make delivery time shorter with the help of sophisticated network management tools.
In addition to private network management needs, accounting func- tions are needed in a public network for switched circuits. Accounting functions of the Internet are very limited but in packet-switched cellular networks, such as in the gen- eral packet data service GPRS of the GSM, accounting based on the amount of transferred data is implemented.
Today these organizations usually have their own dedicated and incompatible network management systems, probably with some kind of geographical hierarchy, and the integration of these is an important issue for the future. At least some level of integration is needed because, for example, all services usually use the same transmission network. In the following section we describe the data communications network DCN , which belongs to the TMN concept and is responsible for the transmission of management data.
ITU-T has worked a long time to define a vendor-independent network manage- ment concept. It is called TMN. The transportation network of management data is called the DCN. Even though DCN is supposed to be a logically separate network from the actual telecommunications network, the management messages often use the same network as the actual telecommunications services. Most transmis- sion systems, for example, synchronous digital hierarchy SDH as described in Chapter 4, provide data channels for network management purposes.
This requires careful planning of the DCN because a fault on a transmission link may disturb management messages that are necessary for fault localization. Therefore, the DCN should be designed to be as independent as possible from the network that transmits user data.
Sometimes a network operator can physically separate management data from user data by using another independent network for management links. For example, the packet-switched X. The use of another network may also be feasi- ble to implement redundant routes to DCN, that is, the management data are sent via another connection when the one in use fails.
TMN is understood to be separate from the actual telecommunications network, though network systems have to provide the management interfaces and management functions that they are able to perform. The physical architec- ture of TMN Figure 2. The most important and most difficult standardization issue has been the specification of the highest layer of the management interface, Q3. Lower-level protocols, like the physical network that carries actual data and formats messages, are already standardized, but detailed information models are not.
The specification work of information models is an endless task, because new systems require their own models and an update to a system often requires a revision of the information model.
The information model defines the managed objects manageable resources of a system and their relationships. The specification of an information model is mandatory before we can talk about vendor-independent network management. The information model is specified by the management information tree MIT or management information base MIB , which defines all managed objects in a system.
The managed objects contain all resources that the man- agement system can access. Each managed object has a unique identification that consists of a sequence of names or numbers starting from the root and having multiple options at each level.
The highest levels of the MIT are standardized, but the compatibility of the systems from dif- ferent vendors requires detailed standardization down to the managed object and its behavior. For example, if we want to get information about whether subscriber 1 of an exchange is busy, we must have a complete specification of what kind of message, transmitted to the exchange, will produce the wanted response regardless of the manufacturer of that exchange. For example, all exchanges should respond with exactly the same message if subscriber 1 is busy.
Much work remains to standardize the network management functions of the present systems in the public telecommunications network and new systems require their own standards for network management. In this chapter we have looked at telecommunications networks, their structure, and functionality; we also introduced network management, which network operators use to improve the performance of their networks and to maintain their network in an effective way.
Telecommunications net- work operators who build up and maintain their network have to provide good performance service at as low an investment level as possible if they are to be competitive. Their problem is how to minimize investment but still keep customers happy. To find out where they should invest and what the bottlenecks of the network are, they continuously perform traffic engineer- ing, which is introduced in the next section.
Nowadays, network operators have to pay more and more attention to these aspects because of increasing competition in the telecom- munications services market. The capacity of the network e. Therefore, the utilization of the network is continuously meas- ured and traffic demand in the future is estimated. Then, based on these esti- mates, the capacity of the network can be increased before severe problems occur. An important capacity planning method is based on theoretical analy- ses of capacity demand and introduction to these calculations is given next.
The GoS depends on the network capacity that should meet the service demand of the customers. System faults, error rates, and other quality measures are not considered here. We instead con- centrate only on the evaluation of the blocking probability. For the probability of unsuccessful calls, operators define the target value, the highest probability of an unsuccessful call that they assume to be acceptable for their customers.
The smaller this probability is, the more capacity they have to build into the network. Another factor we could use to define GoS is how long the subscriber has to wait until the service becomes available. We could design the network to keep customers in a queue until, for example, a transmission channel becomes free.
This factor is also essential to those who plan the telephone service where a person answers incoming calls e. Busy hour is an hour in the year when the average traffic intensity gets the highest value. To be accurate, the busy hour is determined by first selecting the 10 working days in a year with the highest traffic intensity; four consecutive minute peri- ods of those 10 days with the highest traffic intensity make up the busy hour.
The basic goal is to find a minimum capacity that gives the defined grade of service. If more than n subscribers make an external call at a time, some of them are Subscriber Trunk lines lines, n channels Local telephone exchange Figure 2.
The Telecommunications Network: An Overview 67 blocked and they have to try again. The number of external calls varies in a random manner and to be sure that blocking never occurs n should be equal to the number of subscribers.
This is a far too expensive solution because the number of subscribers connected to a local exchange is usually very large and on average only a small portion of them place external calls at the same time. Erlang, the founder of traffic theory. The erlang unit is defined as 1 a unit of telephone traffic specifying the percentage of average use of a line or circuit one channel or 2 the ratio of time during which a circuit is occupied and the time for which the circuit is available to be occupied.
Traffic that occupies a circuit for 1 hour during a busy hour is equal to 1 erlang. The typical average busy-hour traffic volume generated by one sub- scriber is in the range of 10 to mErl. Low values are typical for residential use and high values for business subscribers. The term offered traffic refers to the average generated total traffic including the traffic that is blocked in the system.
Clearly the capacity should at least usually be higher than offered traffic; otherwise, many users would not be able to get service because all lines would be occupied all the time on average.
The essential question is this: How much higher should the capac- ity be for the subscribers to feel that the grade of service is acceptable? The starting point is how often subscribers are allowed to be blocked and receive a busy tone. This probability of blockage for an acceptable GoS is usually set to be in the range of 0. When the average traffic load is estimated to increase to a certain volume, the network operator should increase the network capacity to keep the blocking probability below the defined GoS level.
Now the average number of occupied channels is A in erlangs and 2. Blocking occurs if all n channels are occupied or there may even be a need for a larger number of channels. For this we take a value of the average total offered traffic as A and calculate the probability that traffic occupies all n channels or is even higher at that point in time. The offered traffic load may be higher than n even though actual traffic can never exceed n.
We get this by subtracting from 1 the probability that traffic is smaller than n according to 2. The probability density function P x in the fig- ure tells the probability for each value of x, that is, the number of occupied channels. We get the result, when there are three channels available and average offered traffic is 1 Erl i.
This means that every twelfth call the user makes is blocked and a busy signal is received. Note that this blocking rate allows that only one channel is in use, two channels are free, and only one-third of the capacity can be utilized on average. Equation 2. Then the probability of blockage is 5. This means that, on average, during the busy hour every ninteenth call is blocked, a busy tone is heard, and the subscriber has to redial.
When the number of channels or servers n is high, precalculated tables like Table 2. When we look at Table 2. Only one 9-minute call during an hour is allowed and both lines may be occupied on average only 4. When the number of servers is high, allowed average traffic intensity is close to the maximum or even higher. Even when most of the channels are occupied, some channels are still free for a new call because the number of channels is large.
A part of offered traffic is blocked and actual traffic, that part which is not blocked, naturally never gets a higher value than the number of channels in erlangs.
Blockage probability can be calculated in many different ways. Table 2. It gives slightly more optimistic results than the Poisson for- mula in 2. Erlang B formula is used in Europe and the Poisson formula in used in the United States for network planning. Hint: Draw the current coming from the micro- phone in Figure 2.
Problem 2. Each subscriber is connected directly to all other subscribers. List a few examples of both public and private networks. How does the service and structure of the subscriber inter- face differ from the conventional analog telephone service?
List some examples of IN services. During the busy hour, on average, 3. What is the average traffic intensity of her subscriber line during a —, b —, c —, and d — of that day? Use traffic engineering Table 2.
What can you say about net- work utilization when the number of circuits n is small? How does the utili- zation of the circuits depend on the allowed probability of blocking? What is the blocking probability when each user generates a mErl offered traffic? Each of them uses the phone for an external call of 15 minutes in a busy hour. How many lines are reserved on average during an hour? What is the blocking probability?
What do you think about the capacity of this system? What should the number of trunk channels be if the number of subscribers in the area is a 10, b , c 1,, and d 4,? Use Table 2. Required characteristics depend on the applications we use. To meet these different requirements, many different network technologies that are optimized for each type of service are in use.
To understand the present structure of the telecommunications network, we have to understand what types of signals are transmitted through the telecommunications network and their requirements.
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