Grade Of Service And Blocking Probability Pdf
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Current wireless networks are characterized by a static spectrum allocation policy, where governmental agencies assign wireless spectrum to license holders on a long-term basis for large geographical regions.
Current wireless networks are characterized by a static spectrum allocation policy, where governmental agencies assign wireless spectrum to license holders on a long-term basis for large geographical regions. The operators claim that the spectrum bands for mobile operation are highly occupied.
Even then, a significant amount of licensed spectrum remains underutilized. Cognitive radio senses the radio environment with a twofold objective: identify those subbands of the radio spectrum that are underutilized by the primary i.
For unlicensed communication, the Quality of Service parameters need to be considered. Quality of Service comprises of channel availability, accessibility, and maintainability.
Assessment of vacant channels of licensed band in a geographical region is termed as availability. An analysis of the collected data lead to arrive at the conclusion that more than one-eighth part of resources of each band are nearly permanently vacant, which is enough to design in-band common control signaling methods for cognitive radio.
Measurement result plot of vacant channels in cities with known population will help to assess availability of vacant channels for any city and hence, measurement complexity can be avoided. The strategy to occupy the vacant channels without disturbing the primary user operation is referred as accessibility or selection. Accessibility of a channel is dependent on blocking probability or Quality of Service measured in duration of minutes instead of hours.
Instantaneous blocking probability has been calculated based on current minute occupancy for all available channels as reference. Validation through actual data establishes that channelized blocking probability estimation model has lower error value compared to estimation through prediction models of other researchers.
It was also observed that hourly basis prediction model has constant blocking probability value during clock hour, whereas minutewise Grade of Service GoS prediction model addresses the local peak demand and hence leads to a stringent GoS estimation. On secondary user request for vacant channel, the cognitive radio network needs to evaluate the expected holding time of the particular Secondary User and to ensure channel maintainability or allocation , and it shall predict that the allotted channel shall be able to provide interruption-free service for holding time duration.
Also, at the instant of SU channel request, the channel allocation processor inputs all PU channel status minutewise, calculates actual mean residual lifetime MRL in minutes for each vacant channel and selects the channel with highest predicted free time.
A simulation program runs on data collected from mobile switch of cellular network, which creates pseudo-live environment for channel allocation. The present work has compared the mean residual lifetime MRL method with the other researchers using probabilistic method of channel allocation and MRL method has been established as more accurate. The selection and allocation process with defined blocking probability model has been verified retrieving big data from data warehouse.
The first decade of the twenty-first century belongs to a new wireless world indeed! The rapid growth of cellphones, Wireless Local Area Networks WLANs , and recently the wireless Internet, in short, wireless communication is driving the whole world toward greater integrity with wireless communications. By , two-thirds i. Licensed bands claim to be heavily congested but different research work shows that the channels in the form of time and frequency are still available.
In future, wireless networks may face the problem to find suitable frequency spectrum to fulfill the demands of future services. To solve the problem of inefficient use of spectrum utilization, a new concept is evolved known as cognitive radio CR [ 2 ]. This new concept of CR which is called as intelligent wireless communications is capable of sensing its environment and dynamically accessing the technology.
It adjusts according to the input variations of statistical data for: a very dependable communication wherever and whenever needed; and b efficiently utilizing the radio spectrum [ 3 , 4 ]. This can be done by sensing the radio environment: i by finding spectrum bands which are unused by the PU i. The underused frequency bands of PUs are called as in-band spectrum holes [ 6 ]. The spectrum holes can be used to allocate the channels to CR user.
However, to ensure efficient communication for such unlicensed communication, the Quality of Service QoS parameters need to be considered. Quality of Service can be defined as a set of specific requirements provided by a network of users, which are necessary in order to achieve the required functionality of a service.
Cognitive radio CR concept is based on vacant spectrum in licensed band which sometimes referred to as combination of channels. In telecommunication, a channel refers either to a physical transmission medium such as wire or to a logical connection over a multiplexed medium such as a radio channel. A large amount of information is transmitted between the MS and the BS, particularly, user information voice or data and control or signaling data.
Depending on the type of information transmitted, different logical channels are used. These logical channels are mapped onto the physical channels time slots. For example, digital speech is carried by the logical channel called the traffic channel which during transmission can be allocated to a certain physical channel.
The CCHs are used to communicate service between network equipment nodes. With CDMA, all users share the same 1. Base stations in the system distinguish themselves from each other by transmitting different portions of the code at a given time. In other words, the base stations transmit time-offset versions of the same pseudo-random code. CR utilizes both licensed and unlicensed bands for communication.
Among these bands, GSM bands have less attenuation; their wavelength is more resilient to phenomenon like diffraction, absorption, scattering, etc.
Cognitive radio users human and machine are low end users users in lower or lowest economic bracket or free public utility users with minimum vocabulary or information and expected mainly to use voice, short message and short data services. These reasons make GSM is a good choice for cognitive radio implementation. The QoS for mobile services which has been defined by ITU-T includes different parameters of QoS like availability, accessibility, maintainability and user perception of service.
These parameters have been defined in context of cognitive radio in Section 2. Availability refers to detection of unused spectrum by way of signal strength measurements. In conventional method, the signal strength of a received radio signal is measured. The measurement setup used for detection of spectrum holes in CR along with the cognitive radio issues for availability has been discussed in detail in Section 4. The new concept of channelized blocking probability has been defined along with the general definitions of blocking probabilities in Section 5.
Allocation of a PU vacant channel on SU call request is done based on prediction that the channel will remain vacant for more than the assessed holding time of SU. The channel allocation model works based on inputs from a the channel call arrival rate prediction model and b SU holding time assessment model and has been discussed in Section 6.
The model accepts collected data as input in time serial manner for running through residual lifetime based prediction model program. The comparison of proposed work has also been done and its results and conclusion has been discussed in Section 6. Quality of Service QoS is the capability of a network to offer better service to selected network traffic over specific underlying technologies [ 7 , 8 ].
The various parameters for QoS are: Availability: The operator maintains a dynamic list of available channels. When the user wants to communicate, operator is liable to assign one or more communication channel to the user as per his demand and within tolerable specified time limit. This function is referred to as availability. Accessibility: When the operator assigns channels to the user, the user equipment UE should be capable to use the allocated spectrum to the extent possible.
This phenomenon is called accessibility. Proper handshaking shall take place between UE and access network AN before establishing communication at acceptable speed by both ends. Maintainability: In mobile communication, as the user is mobile, there is a continuous change in environment and RF condition.
The operator has to take into consideration various parameters like speed of communication, handover, etc. This is known as maintainability.
It is measured by the customer satisfaction using access equipment behavior audit, drive test for mobile as pseudo customer and actual satisfaction through interrogation by customer survey specialists.
Over the last few years, a lot of research has undergone on spectrum sensing SS techniques for the detection of spectrum holes [ 9 ].
Energy detection ED approach, also known as radiometry or periodogram, is a popular technique for spectrum sensing due to low computational and implementation complexities [ 10 ]. WBS is a coherent method that correlates the received signal with the previous patterns available in database [ 11 ]. This technique is susceptible to synchronization errors which can cause false detection of primary users [ 10 ]. MFBS is the best detecting method where the received signal is interrelated with the transmitted signal [ 12 ].
The periodic characteristics of the received signals i. It requires less time to achieve high processing gain due to coherent detection. It indicates that method is not suitable in some bands as some of the communication technologies are not operating with the previous information. CBS has high computational complexity [ 14 ]. Energy-based sensing based on sub-Nyquist sampling shall be beneficial as per as sensing duration is concerned [ 18 ]. A brief comparison various SS techniques is enlisted in Table 1 as follows [ 10 , 20 ]:.
In mobile communication, primary user occupied channels are known to network. So, a new call is eligible to occupy any of the vacant channels. In contrast, in cognitive radio network, a dynamic spectrum management is used which shall include information about the traffic pattern of the channels occupied by primary users at an instant. Basically, a CR should characterize whether the traffic pattern is static or dynamic and based on that it should use different methods for idle time prediction before selecting a channel.
The actual utilization of mobile communication spectrum in licensed band has not yet been taken into consideration. The variation of channel utilization for various types of cities has also not been studied. These studies may be very useful to perfectly recognize the frequency channels with no active or low occupancy so that the CR technology can be successfully deployed.
Few such studies has been mentioned below:. It was observed that the average utilization of frequency band was only 4. The ISM bands and mobile licensed bands are partly utilized and the remaining part of the spectrum band resembles noise.
In India, the RFs are being used for different types of services like mobile communication, broadcasting, radio navigation, satellite communication, defense communication, etc. The wireless equipment are developed and manufactured based on the spectrum utilization in the country as decided by the National Frequency Allocation Plan NFAP. The various frequency spectrums allotted to mobile communication services is shown in Table 2 [ 25 , 26 ].
CR technology has been developed to dynamically access and release channels in licensed bands. There is a scope of getting the unutilized channels in licensed spectrum with or without having a stable infrastructure for CR. Thus it is expected that at zero cost public authorities providing public utility services may be authorized to operate over unutilized spectrum even though licensed.
However, such public utility service providers are very limited. Field test should be essentially conducted for the evaluation of quasi-permanently unused channels for use of in-band common control signaling purposes.
To dynamically measure the occupancy rate of the PUs and to calculate the quantum of vacant channels available for CR use, a measurement setup called drive test equipment is used that collects data on a moving vehicle. A motor vehicle containing mobile radio network air interface measurement equipment is used in the drive test.
The equipment measures different types of virtual and physical parameters of mobile cellular service in a given geographical region.
Grade of service
Grade of service of a telephone exchange. The analytical models used are a the probability linear graph for lost calls and b the traffic flow graph for delayed calls. Both of these models are expressed in terms of the congestion functions of the sub-units of the switching system in the exchange. When these models are used, the engineering procedure of determining the quantity of equipment for the switching system in the exchange to satisfy the objective grade of service is simple. The switching system in the exchange may be divided into three major sub-units: the trunk groups, the switching network, and the common control. The congestion functions for these sub-units are discussed.
The GOS seen by a subscriber (blocking and/or delay in establishing calls) is not At this level, one focuses on the probability that the service is accessible (not.
In telecommunication engineering, and in particular teletraffic engineering , the quality of voice service is specified by two measures: the grade of service GoS and the quality of service QoS. Grade of service is the probability of a call in a circuit group being blocked or delayed for more than a specified interval, expressed as a vulgar fraction or decimal fraction. This is always with reference to the busy hour when the traffic intensity is the greatest. Grade of service may be viewed independently from the perspective of incoming versus outgoing calls, and is not necessarily equal in each direction or between different source-destination pairs.
The telephone system is not dimensioned such that all subscribers can be connected at the same time . Providing sufficient resources to carry all traffic that could be offered in a telecommunications system would be very uneconomical . Subscribers have to share resources because equipment at the exchanges is expensive. As a result of this, telecommunications systems are likely to experience problems at times, for example calls fail to get through. This phenomenon is termed blocking.
The erlang symbol E  is a dimensionless unit that is used in telephony as a measure of offered load or carried load on service-providing elements such as telephone circuits or telephone switching equipment. A single cord circuit has the capacity to be used for 60 minutes in one hour. Full utilization of that capacity, 60 minutes of traffic, constitutes 1 erlang. Carried traffic in erlangs is the average number of concurrent calls measured over a given period often one hour , while offered traffic is the traffic that would be carried if all call-attempts succeeded. How much offered traffic is carried in practice will depend on what happens to unanswered calls when all servers are busy.
Техники и все прочие беспомощно смотрели на ВР. Джабба повернулся к монитору и вскинул руки. - Почему среди нас нет ни одного ядерного физика. Сьюзан, глядя на мультимедийный клип, понимала, что все кончено.
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Джабба повернул голову к экрану ВР. Атакующие линии рвались вперед, они находились уже на волосок от пятой, и последней, стены, Последние минуты существования банка данных истекали. Сьюзан отгородилась от царившего вокруг хаоса, снова и снова перечитывая послание Танкадо.
Прозвучал еще один выстрел. Он принял решение. Под визг покрышек, в снопе искр Беккер резко свернул вправо и съехал с дороги.
Шестьдесят четыре знака… Сьюзан кивнула: - Да, но они… - Она вдруг замерла. - Шестьдесят четыре буквы, - повторил Дэвид. - О мой Бог! - воскликнула Сьюзан. - Дэвид, ты просто гений.
Она в столовой. Консьерж снова покачал головой: - Ресторан закрылся полчаса .