grade of service
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telecommunication Telecommunication is the transmission of information by various types of technologies over wire, radio, optical, or other electromagnetic systems. It has its origin in the desire of humans for communication over a distance greater than that fe ...
engineering, and in particular
teletraffic engineering Teletraffic engineering, telecommunications traffic engineering, or just traffic engineering when in context, is the application of transportation traffic engineering theory to telecommunications. Teletraffic engineers use their knowledge of stat ...
, the quality of voice service is specified by two measures: the grade of service (GoS) and the
quality of service Quality of service (QoS) is the description or measurement of the overall performance of a service, such as a telephony or computer network, or a cloud computing service, particularly the performance seen by the users of the network. To quantitat ...
(QoS). Grade of service is the
probability Probability is the branch of mathematics concerning numerical descriptions of how likely an Event (probability theory), event is to occur, or how likely it is that a proposition is true. The probability of an event is a number between 0 and ...
of a
call Call or Calls may refer to: Arts, entertainment, and media Games * Call, a type of betting in poker * Call, in the game of contract bridge, a bid, pass, double, or redouble in the bidding stage Music and dance * Call (band), from Lahore, Paki ...
in a circuit ''group'' being blocked or delayed for more than a specified interval, expressed as a
vulgar fraction A fraction (from la, fractus, "broken") represents a part of a whole or, more generally, any number of equal parts. When spoken in everyday English, a fraction describes how many parts of a certain size there are, for example, one-half, eight ...
or
decimal fraction The decimal numeral system (also called the base-ten positional numeral system and denary or decanary) is the standard system for denoting integer and non-integer numbers. It is the extension to non-integer numbers of the Hindu–Arabic num ...
. This is always with reference to the busy hour when the
traffic Traffic comprises pedestrians, vehicles, ridden or herded animals, trains, and other conveyances that use public ways (roads) for travel and transportation. Traffic laws govern and regulate traffic, while rules of the road include 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. "Grade of Service" sometimes means a measure of inbound
call center A call centre ( Commonwealth spelling) or call center (American spelling; see spelling differences) is a managed capability that can be centralised or remote that is used for receiving or transmitting a large volume of enquiries by telephone. ...
traffic to verify adherence to conditions to measure the success of customers served. On the other hand, the quality of service which a ''single'' circuit is designed or conditioned to provide, e.g. voice grade or program grade is called the quality of service. Quality criteria for such circuits may include equalization for amplitude over a specified
band Band or BAND may refer to: Places *Bánd, a village in Hungary *Band, Iran, a village in Urmia County, West Azerbaijan Province, Iran * Band, Mureș, a commune in Romania *Band-e Majid Khan, a village in Bukan County, West Azerbaijan Province, I ...
of frequencies, or in the case of
digital data Digital data, in information theory and information systems, is information represented as a string of discrete symbols each of which can take on one of only a finite number of values from some alphabet, such as letters or digits. An example i ...
transported via analogue circuits, may include equalization for
phase Phase or phases may refer to: Science *State of matter, or phase, one of the distinct forms in which matter can exist *Phase (matter), a region of space throughout which all physical properties are essentially uniform * Phase space, a mathematic ...
. Criteria for mobile quality of service in cellular telephone circuits include the probability of abnormal termination of the call.


What is Grade of Service and how is it measured?

When a user attempts to make a telephone call, the routing equipment handling the call has to determine whether to accept the call, reroute the call to alternative equipment, or reject the call entirely. Rejected calls occur as a result of heavy traffic loads (congestion) on the system and can result in the call either being delayed or lost. If a call is delayed, the user simply has to wait for the traffic to decrease, however if a call is lost then it is removed from the system.Kennedy I., Lost Call Theory, Lecture Notes, ELEN5007 – Teletraffic Engineering, School of Electrical and Information Engineering, University of the Witwatersrand, 2005 The Grade of Service is one aspect of the
quality Quality may refer to: Concepts *Quality (business), the ''non-inferiority'' or ''superiority'' of something *Quality (philosophy), an attribute or a property *Quality (physics), in response theory *Energy quality, used in various science discipli ...
a customer can expect to experience when making a telephone call.Peuhkuri M., IP Quality of Service, Helsinki University of Technology, Laboratory of Telecommunications Technology, 1999. In a Loss System, the Grade of Service is described as that proportion of calls that are lost due to congestion in the busy hour.Farr R.E., Telecommunications Traffic, Tariffs and Costs – An Introduction For Managers, Peter Peregrinus, 1988. For a Lost Call system, the Grade of Service can be measured using ''Equation 1''.Flood, J.E., Telecommunications Switching, Traffic and Networks, Chapter 4: Telecommunications Traffic, New York: Prentice-Hall, 1998. :\mbox=\frac\qquad(1) For a delayed call system, the Grade of Service is measured using three separate terms: *The mean delay t_d – Describes the average time a user spends waiting for a connection if their call is delayed. *The mean delay t_o – Describes the average time a user spends waiting for a connection whether or not their call is delayed. *The probability that a user may be delayed longer than time ''t'' while waiting for a connection. Time ''t'' is chosen by the telecommunications service provider so that they can measure whether their services conform to a set Grade of Service. * Where and when is Grade of Service measured? The Grade of Service can be measured using different sections of a network. When a call is routed from one end to another, it will pass through several exchanges. If the Grade of Service is calculated based on the number of calls rejected by the final circuit group, then the Grade of Service is determined by the final circuit group blocking criteria. If the Grade of Service is calculated based on the number of rejected calls between exchanges, then the Grade of Service is determined by the exchange-to-exchange blocking criteria. The Grade of Service should be calculated using both the access networks and the core networks as it is these networks that allow a user to complete an end-to-end connection. Furthermore, the Grade of Service should be calculated from the average of the busy hour traffic intensities of the 30 busiest traffic days of the year. This will cater for most scenarios as the traffic intensity will seldom exceed the reference level. The grade of service is a measure of the ability of a user to access a trunk system during the busiest hour. The busy is based upon customer demand at the busiest hour during a week month or year.


Class of Service

Different telecommunications applications require different Qualities of Service. For example, if a telecommunications service provider decides to offer different qualities of voice connection, then a premium voice connection will require a better connection quality compared to an ordinary voice connection. Thus different Qualities of Service are appropriate, depending on the intended use. To help telecommunications service providers to market their different services, each service is placed into a specific class. Each Class of Service determines the level of service required. To identify the Class of Service for a specific service, the network’s switches and routers examine the call based on several factors. Such factors can include: *The type of service and priority due to precedence *The identity of the initiating party *The identity of the recipient party


Quality of Service in broadband networks

In broadband networks, the Quality of Service is measured using two criteria. The first criterion is the probability of packet losses or delays in already accepted calls. The second criterion refers to the probability that a new incoming call will be rejected or blocked. To avoid the former, broadband networks limit the number of active calls so that packets from established calls will not be lost due to new calls arriving. As in circuit-switched networks, the Grade of Service can be calculated for individual switches or for the whole network.Ritter, M., Phuoc, P., Multi-Rate Models for Dimensioning and Performance Evaluation of ATM Networks, COST 242, Institute of Computer Science, University of Würzburg, June 1994


Maintaining a Grade of Service

The telecommunications provider is usually aware of the required Grade of Service for a particular product. To achieve and maintain a given Grade of Service, the operator must ensure that sufficient telecommunications circuits or routes are available to meet a specific level of demand. It should also be kept in mind that too many circuits will create a situation where the operator is providing excess capacity which may never be used, or at the very least may be severely underutilized. This adds costs which must be borne by other parts of the network. To determine the correct number of circuits that are required, telecommunications service providers make use of Traffic Tables. An example of a Traffic Table can be viewed in ''Figure 1''. It follows that in order for a telecommunications network to continue to offer a given Grade of Service, the number of circuits provided in a circuit group must increase (non-linearly) if the traffic intensity increases.


Erlang's lost call assumptions

To calculate the Grade of Service of a specified group of circuits or routes,
Agner Krarup Erlang Agner Krarup Erlang (1 January 1878 – 3 February 1929) was a Denmark, Danish mathematician, statistician and engineer, who invented the fields of teletraffic engineering, traffic engineering and queueing theory. By the time of his relatively ...
used a set of assumptions that relied on the network losing calls when all circuits in a group were busy. These assumptions are: *All traffic through the network is pure-chance traffic, i.e. all call arrivals and terminations are independent random events *There is statistical equilibrium, i.e., the average number of calls does not change *Full availability of the network, i.e., every outlet from a switch is accessible from every inlet *Any call that encounters congestion is immediately lost. From these assumptions Erlang developed the Erlang-B formula which describes the probability of congestion in a circuit group. The probability of congestion gives the Grade of Service experienced.


Calculating the Grade of Service

To determine the Grade of Service of a network when the traffic load and number of circuits are known, telecommunications network operators make use of ''Equation 2'', which is the
Erlang-B 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 capaci ...
equation. :\mbox=\frac\qquad(2) ''A'' = Expected traffic intensity in Erlangs, ''N'' = Number of circuits in group. This equation allows operators to determine whether each of their circuit groups meet the required Grade of Service, simply by monitoring the reference traffic intensity. (For delay networks, the
Erlang-C The Erlang distribution is a two-parameter family of continuous probability distributions with support x \in independent exponential distribution">exponential variables with mean 1/\lambda each. Equivalently, it is the distribution of the time ...
formula allows network operators to determine the probability of delay depending on peak traffic and the number of circuits.)


References

*{{FS1037C MS188 Telecommunications engineering Teletraffic