NETWORK ENGINEERING PDF

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PDF | According to need of communication in societies as well as an to a vast network and they can build a global network engineering with. “No man but a blockhead ever wrote, except for money.” - Samuel Johnson. The textbook world is changing. On the one hand, open source. Computer Networking: Principles, Protocols, and Practice was written by Dr. . oped within the Internet Engineering Task Force (IETF) using an open process.


Network Engineering Pdf

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all part of, computer networks let us share information and resources. In business , In this chapter, you'll begin by relating networks to situations and concepts. Describe the functionality of LAN, MAN, and WAN networks A network refers to two or more connected computers that Facilitates modular engineering. As a student on one of our Computer and Network. Engineering courses you will benefit from fantastic facilities and resources. These include numerous.

A small local exchange is some- times known as a remote switching unit RSU and it performs the switching and concentration functions just as all local exchanges do. A local exchange reduces the required transmission capacity number of speech channels typi- cally by a factor of 10 or more; that is, the number of subscribers of the local exchange is 10 times higher than the number of trunk channels from the exchange for external calls.

The number of required trunk circuits is analyzed in Section 2. It is a large con- struction with huge number of connectors. Subscriber pairs are connected to one side and pairs from the local exchange to the other. Between these con- nector fields there is enough space for free cross-connections. Cables and connectors are usually arranged in a logical way considering the subscriber cable network structure and switching arrangements.

This fixed cabling stays the same over long periods of time, but connections between sides change daily, for example, because a subscriber moves to another house in the same switching area. A cross-connection in the MDF is usually done with twisted open pairs that are able to carry data rates up to 2 Mbps.

ADSL and ordinary ana- log telephone circuits use the same 2W subscriber loop. A digital exchange may include both analog and digital subscriber interfaces. If the local switch has ISDN capability, basic rate and primary rate interfaces are avail- able. Ordinary subscriber pairs are used for ISDN basic rate connections Kbps bidirectional and a network terminal NT is required on cus- tomer premises. The primary rate interface of ISDN 1. It requires two pairs, one for each transmission direc- tion, and supports many simultaneous external calls.

In addition to MDF, network operators may use other distribution frames for transmission network management and maintenance. An optical distribution frame ODF contains two fields of optical fiber connectors. The optical cables of the network are connected to one connector field and the other one is connected to optical line terminal equipment. Cross- connections between two connector fields are created with optical fibers. This allows maintenance personnel, for example, to replace a faulty optical cable connection with a spare one.

A digital distribution frame DDF is a cross-connection system to which digital interfaces from line systems and the exchange or other network equipment are connected. With the help of a 1. DXC is managed via its network management interface and an operator may change its cross- connection configuration from a network management system NMS site.

From a remote NMS he may, for example, define to which of the 1. 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: 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. 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.

Network engineering

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.

An Overview 53 and ciphering and then the Internet can be used instead of a more expensive leased or circuit-switched data connection. 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.

Examples of supplementary services include the following: 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.

A company with several offices in different parts of a country may have the same number throughout the coun- try. The cost of the call is the same no matter to which office the call is connected. Information provision over the phone, for instance, doctor and layer services. The service provider charges sub- scribers via the telephone bill. The charge is dependent on the called service number.

Companies that want to provide free customer service use this service in which the receiver pays for the call. A service user can pay with his or her credit card by dialing his or her account number and identity code. 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.

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The follow- ing section gives an overview of the importance of network management and of the standardized structure of network management. An Overview 59 this area is not as advanced as the standardization of telecommunications sys- tems that carry the actual traffic and provide the services. 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: 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. The main concerns of network managers of a company include these: 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.

An Overview 61 the case of packet-switched service, the amount of transferred data is recorded to generate bills to customers. 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.

The complete standardization of TMN is designed to cover the follow- ing specifications: 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 management system performs or is used to perform following actions: 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.

An Overview 65 network element responds. 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. 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.

Consider these examples: 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. The Poisson distribution is used as a probability model for these calcu- lations and it gives a probability for occurrence of x events when the average number of events is A according to this formula: 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. This probability is given by: An Overview 69 Now we have the Poisson formula, which is also known as the Molina lost calls held trunking formula, for blocking probability and it is as follows: 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.

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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. For example, VPNs allow you to establish a secure dial-up connection to a remote server.

This is a protocol that provides a way for multiple computers on a common network to share single connection to the Internet. The Network layer is responsible for data routing, packet switching and control of network congestion.

Routers operate under this layer. Network topology dictates what media you must use to interconnect devices. It also serves as basis on what materials, connector and terminations that is applicable for the setup. RIP, short for Routing Information Protocol is used by routers to send data from one network to another.

It efficiently manages routing data by broadcasting its routing table to all other routers within the network. It determines the network distance in units of hops. There are several ways to do this. Install reliable and updated anti-virus program on all computers.

Make sure firewalls are setup and configured properly.

User authentication will also help a lot. All of these combined would make a highly secured network. This is a peripheral card that is attached to a PC in order to connect to a network. It is an interconnection of computers and devices that are geographically dispersed. It connects networks that are located in different regions and countries.

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The physical layer does the conversion from data bits to electrical signal, and vice versa. This is where network devices and cable types are considered and setup. There are four layers: Proxy servers primarily prevent external users who identifying the IP addresses of an internal network.

Without knowledge of the correct IP address, even the physical location of the network cannot be identified. Proxy servers can make a network virtually invisible to external users. This layer provides the protocols and means for two devices on the network to communicate with each other by holding a session. This includes setting up the session, managing information exchange during the session, and tear-down process upon termination of the session.

Are there limitations? A fault tolerance system ensures continuous data availability. This is done by eliminating a single point of failure.

However, this type of system would not be able to protect data in some cases, such as in accidental deletions. The 10 refers to the data transfer rate, in this case is 10Mbps. The word Base refers to base band, as oppose to broad band.

T means twisted pair, which is the cable used for that network. Private IP addresses are assigned for use on intranets. These addresses are used for internal networks and are not routable on external public networks.

These ensures that no conflicts are present among internal networks while at the same time the same range of private IP addresses are reusable for multiple intranets since they do not "see" each other. NOS, or Network Operating System, is specialized software whose main task is to provide network connectivity to a computer in order for it to be able to communicate with other computers and connected devices. DoS, or Denial-of-Service attack, is an attempt to prevent users from being able to access the internet or any other network services.

Such attacks may come in different forms and are done by a group of perpetuators. One common method of doing this is to overload the system server so it cannot anymore process legitimate traffic and will be forced to reset.

It is made up of 7 layers, with each layer defining a particular aspect on how network devices connect and communicate with one another. One layer may deal with the physical media used, while another layer dictates how data is actually transmitted across the network.

The main purpose of this is to prevent crosstalk. Crosstalks are electromagnetic interferences or noise that can affect data being transmitted across cables. By using address translation instead of routing, address sharing provides an inherent security benefit. That's because host PCs on the Internet can only see the public IP address of the external interface on the computer that provides address translation and not the private IP addresses on the internal network.

It is also known as physical address or Ethernet address. A MAC address is made up of 6-byte parts. If the first octet begins with a 0 bit, that address is Class A. If it begins with bits 10 then that address is a Class B address.

If it begins with , then it's a Class C network. OSPF, or Open Shortest Path First, is a link-state routing protocol that uses routing tables to determine the best possible path for data exchange. Firewalls serve to protect an internal network from external attacks. These external threats can be hackers who want to steal data or computer viruses that can wipe out data in an instant.

It also prevents other users from external networks from gaining access to the private network. This is one of the easiest to setup and maintain. Gateways provide connectivity between two or more network segments. It is usually a computer that runs the gateway software and provides translation services. This translation is a key in allowing different systems to communicate on the network. One major disadvantage of star topology is that once the central hub or switch get damaged, the entire network becomes unusable.

This is one of the protocols that are used for remote access. Tracert is a Windows utility program that can used to trace the route taken by data from the router to the destination network. It also shows the number of hops taken during the entire transmission route. A network administrator has many responsibilities that can be summarize into 3 key functions: When you are accessing the resources that are shared by one of the workstations on the network, that workstation takes a performance hit.

A hybrid network is a network setup that makes use of both client-server and peer-to-peer architecture. Free Access. Summary PDF Request permissions. Part One: Part Two: Part Three: Part Four: Part Five: Tools Get online access For authors.

Email or Customer ID. Forgot password? Old Password. New Password.Software related problems can be any or a combination of the following: client server problems. OSPF, or Open Shortest Path First, is a link-state routing protocol that uses routing tables to determine the best possible path for data exchange. This book can be used as a first year graduate course in computer, network, and software engineering; as an on-the-job reference for computer, network, and software engineers; and as a reference for these disciplines.

This is usually one Kbps time slot of a primary 2- or 1. 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. This coding and struc- ture make national and international digital connections between networks possible.

All digital local exchanges have a capability to use either pulse or tone dialing on a subscriber loop.

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