Grade of service
Encyclopedia
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.
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 of frequencies, or in the case of digital data transported via analogue circuits, may include equalization for phase
. Criteria for mobile quality of service
in cellular telephone circuits include the probability of abnormal termination of the call.
The Grade of Service is one aspect of the quality
a customer can expect to experience when making a telephone call. 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.
For a Lost Call system, the Grade of Service can be measured using Equation 1.
For a delayed call system, the Grade of Service is measured using three separate terms:
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.
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:
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.
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 formula allows network operators to determine the probability of delay depending on peak traffic and the number of circuits.)
Telecommunication
Telecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...
engineering, and in particular teletraffic engineering
Teletraffic engineering
Telecommunications traffic engineering, teletraffic engineering, or traffic engineering is the application of traffic engineering theory to telecommunications...
, the quality of voice service is specified by two measures: the grade of service (GoS) and the quality of service
Quality of service
The quality of service refers to several related aspects of telephony and computer networks that allow the transport of traffic with special requirements...
(QoS).
Grade of service is the probability
Probability
Probability is ordinarily used to describe an attitude of mind towards some proposition of whose truth we arenot certain. The proposition of interest is usually of the form "Will a specific event occur?" The attitude of mind is of the form "How certain are we that the event will occur?" The...
of a call
Telephone call
A telephone call is a connection over a telephone network between the calling party and the called party.-Information transmission:A telephone call may carry ordinary voice transmission using a telephone, data transmission when the calling party and called party are using modems, or facsimile...
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
Busy hour
Busy hour: In a communications system, the sliding 60-minute period during which occurs the maximum total traffic load in a given 24-hour period....
when the traffic
Traffic
Traffic on roads may consist of pedestrians, ridden or herded animals, vehicles, streetcars and other conveyances, either singly or together, while using the public way for purposes of travel...
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.
On the other hand, the quality of service which a single circuit
Telecommunication circuit
A telecommunication circuit is any line, conductor, or other conduit by which information is transmitted.A dedicated circuit, private circuit, or leased line is a line that is dedicated to only one use...
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
Equalization
Equalization, is the process of adjusting the balance between frequency components within an electronic signal. The most well known use of equalization is in sound recording and reproduction but there are many other applications in electronics and telecommunications. The circuit or equipment used...
for amplitude over a specified band of frequencies, or in the case of digital data transported via analogue circuits, may include equalization for phase
Phase (waves)
Phase in waves is the fraction of a wave cycle which has elapsed relative to an arbitrary point.-Formula:The phase of an oscillation or wave refers to a sinusoidal function such as the following:...
. Criteria for mobile quality of service
Mobile QoS
Quality of service mechanism controls the performance, reliability and usability of a telecommunications service. Mobile cellular service providers may offer mobile QoS to customers just as the fixed line PSTN services providers and Internet Service Provides may offer QoS...
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.The Grade of Service is one aspect of the quality
Quality of service
The quality of service refers to several related aspects of telephony and computer networks that allow the transport of traffic with special requirements...
a customer can expect to experience when making a telephone call. 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.
For a Lost Call system, the Grade of Service can be measured using Equation 1.
For a delayed call system, the Grade of Service is measured using three separate terms:
- The mean delay
– Describes the average time a user spends waiting for a connection if their call is delayed. - The mean delay
– 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.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, A.K. Erlang 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 equation.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 formula allows network operators to determine the probability of delay depending on peak traffic and the number of circuits.)