How to Define Telephone Traffic?
Basic Concept of Telephone Traffic:
Telephone traffic is originated by the individual needs of different subscribers and so it is beyond the control of telephone administration. Any and every subscriber can originate a call at any and every moment without giving any previous information and the duration of calls is also not previously known. Although the individual telephone traffic originates at random, the average telephone traffic for a particular exchange follows the general pattern of activity in the exchange area. Normally there is a peak in the morning, a dip during lunch period followed by an afternoon peak. In some localities, the traffic has a seasonal characteristics, for example at a holiday resort. A typical 24 hours variations in calling rate is shown below.
2.1 Whatever be the nature of variation of traffic, a telephone engineer is interested in maximum traffic that occurs in an exchange.
The hour in which maximum traffic usually occurs in exchange is known as Busy Hour.
Busy Hour Traffic is the average value of maximum traffic in the busy hour. In computing Busy Hour Traffic the seasonal effects are also taken into account. Sometimes it is convenient to refer to the Busy hour calling rate (BHCR). Busy hour calling rate is the number of calls originated per subscriber in the busy hour. This provides a simple means for designing the exchange with respect to the number of subscribers. It also provides probable growth of traffic to the estimated growth in a number of subscribers. The busy hour calling rate may vary about 0.3 for a small country exchange and 1.5 or more for a busy exchange in the business area in a city.
When the volume of traffic is quoted in terms of a number of calls that originated in a given time, this is insufficient to determine the consequent occupancy of lines and equipment. Therefore, measurement of traffic should not only consider a number of calls but also their duration. The duration during which equipment and circuits are held when a call is made is called HOLDING TIME. Normally, it is average holding time per call for the particular item of equipment that is taken into account, so far as the caller is concerned the useful time is during the conversation only. However, the total time during which equipment and circuits are held when a call is made also includes, the period during which call is being established and time is taken to release the equipment after the call has concluded.
2.2 Measurement of Telephone Traffic.
The total cost of providing telephone service can be roughly divided into that charge which is constant and independent of the volume of traffic and those, which are determined by the amount of traffic. The cost of the subscriber’s line and instrument and certain individual equipment in the exchange is totally independent of the volume of traffic. The quantity of common switching equipment required is almost entirely dependent on the volume of traffic. The quantity of such equipment is dependent not only on a number of calls but also on the duration of calls. Therefore to determine the quantity of switching equipment in automatic exchange or staffing in manual exchange telephone traffic may be measured in terms of both the number of calls and the duration of calls.
For certain purpose, it is sufficient to specify a Traffic Volume which is a product of a number of calls occurred during the time concerned by their average duration. however, for the purposes of automatic exchange, a more precise unit of traffic flow is required. this is called Traffic Intensity. Traffic intensity is the average number of calls simultaneously in progress. The unit of traffic intensity is Erlang.
A traffic intensity of one erlang is obtained in any specified period when the average number of calls simultaneously in progress during that period in unity. The specified period is always one hour and is taken as being the busy hour unless some other period is indicated.
There is a more precise way to define traffic intensity. The average Traffic Intensity during a specified period T, carried by a group of circuits or equipment, is given by the sum of the holding times divided by T. The holding times and period T all being expressed in the same unit.
Sometimes it is stated that the average traffic intensity is equal to the average number of calls, which originate during the average holding time. All the above three definitions give the same numerical result.
The foregoing relationships may be expressed symbolically as follows.
Let S be the sum of holding times during a given period T, both expressed in hours. Then by definition.
A = S/T
Where A is the average traffic intensity. Let C be the total number of calls during the period T then the average holding time ‘t’ hours per call, is given by
Then A = S/T
Can also be written as
A = Ct/T
It also follows that when the average call duration is known, the average call intensity can be obtained by determining the number of calls occurring during the period T. Also because A is equal to average number of calls simultaneously in progress, an approximate value of A can be obtained by counting the number of occupied circuits or equipment at uniform interval during the time T and finding the average value.
2.3 Grade of service.
Owing to the fact that calls originated in a pure chance manner, it is likely
that during the busy hour some calls may fail to mature due to the insufficiency of switching equipment. To ensure that the number of calls so lost is reasonably small, it is the standard practice switching equipment such that on the average not more than one call out of every 500 in the busy hour is lost at each switching stage, with the provision that loss does not fall below 1 in 100 with a 10 percent increase of traffic.
This allowable loss has termed the grade of service and is usually represented by the symbol ‘B’ with one lost call in 500 the grade of service is written as
B= 1/500 or B= 0.002
The Grade of service is a factor employed for dimensions of the exchange equipment.
A few typical problems are Worked out below to illustrate how the terms and definitions of telephone traffic are actually applied in practice.
If the calling rate per line per day in an exchange of 5000 lines is 6.0 and the proportion of the traffic that occurs in the busy hours is 12 percent, what is the busy hour’s traffic in Erlangs, assuming an average holding time of 2.5 minutes per call?
Calling rate per line per day = 6.0
Capacity of the exchange = 50000 lines
Total number of calls made in a day = 5000 x 6
Number of calls originated in the hours = 30,000 x 12/100
Holding time of a call = 2.5 minutes
Busy hour traffic = C x t/60
= 3600 x 2.5/60
= 150 Erlangs or T.u.s.
A group of selectors observed for ten busy hours carried an average of twenty Erlangs and the total number of calls lost was twelve. The calls had an average duration of two minutes. What grade of service was given?
Traffic carried by the selectors in one busy hour = 20Erlangs
Average holding time = 2 minutes
Total number of calls carried in one busy hour = 20 x 60/2
Number of calls lost in ten busy hours = 12
The average number of calls lost in one busy hour = 12/10 = 1.2
Total number of calls offered in busy hour = 600 + 1.2
Grade of service = Number of calls lost
number of calls offered
Say, = 0.002
- Scanning Method
This is the practical method for measuring traffic in SPC switches.
Here the observation of traffic is not continuous. The group of equipments is scanned at regular intervals and the traffic flow is calculated.
A=1/S ∑ Fv
or A= I/S [f1+f2+f3+……..+fs]
where A=Tele traffic intensity in Erlangs
S=Number of scans made on the group.
Fv=The number of occupied devices found in the vth scan
A group of equipment was scanned for ascertaining the traffic flow. The scanning was done once in 5 seconds for one minute. The number of occupied devices in each scan is as follows
1st scan=4,2nd scan=3,3rd scan=2
4th scan=3,5th scan=1,6th scan=3
7th scan=2,8th scan=4,9th scan=3
10th scan=5,11th scan=4,12th scan=2
Calculate the intensity of traffic.
Duration of observation = 60 s
Frequency of scanning = 5 s
Number of scans = 12
A = 1/S [ f1+f2+f3+…….+f12]
= 1/12*36 = 3 Erlangs
2A. Quantitative Indicators For Quality of Service
The quality of service of a telecommunications network is characterized by the level of satisfaction of the customers connected to it. There are a number of technical and customer service indicators that determine the quality of service. Technical performance indicators encompass reliability (fault rate and time to clear faults), connectivity (dial tone delay and call completion rates) and operator response time for booking calls (manual operations). Specific technical performance indicators are:
(a) fault rate, that is the number of faults per the main line per year;
(b) the average number of lines faulty any day as a percent (%) of total main lines;
(c) percent (%) of faults cleared by the next working day;
(d) dial tone delay, that is time (in seconds) before dial tone received after the call is originated;
(e) call completion rates, that is percent (%) of originated calls successfully completed; and
(f) time to answer for operator service.
The number of faults per the mainline per year defines the frequency of breakdown of the telephone lines. For a well constructed and well-maintained network, the average number of faults per the mainline per year should be 0.2 or less; that is the telephone line should not be out of order more than once in five years. Because the figure is normally small in industrialized countries, this indicator is often expressed in faults per 100 main lines. The actual situation in developing countries is much worse, with the average number of faults in some countries exceeding three faults per mainline per year.
The number of lines faulty on any day as a percent (%) of total lines in service is an important performance indicator for the company because it actually represents the percentage of the network that is not generating revenues at any particular time. This indicator is closely related to the fault rate and the time to clear.
The time to clear faults is normally expressed in terms of the percentage of reported faults cleared within a given time. The significant time frame normally applied is “by the next working day”.
Call Completion Rate
The Call Completion Rate (CCR) measures the percentage of originated calls successfully completed. The CCR, which is normally measured during the peak traffic hour, is an indication of the probability of establishing a connection at the end of dialing. In practice, dialing can commence only after the dial tone is received; hence, connectivity also depends on the availability of a dial tone, the ability of the network to establish a transmission path between the calling and the called party and to switch the call to the called party. The network components involved for a local call are:
(a) the customer premises equipment (terminal equipment such as a telephone and indoor wiring);
(b) the local cable network; and
(c) the local switching equipment.
For domestic long distance calls, in addition to the above equipment, long-distance switching equipment, and transmission media and equipment are required while for international calls, international switching equipment and transmission media and equipment are required. Hence, the CCR for the international calls depends on the quality of the total network – local, domestic long-distance and international.
A successful call could be defined in two ways. First, the call could be considered as successfully completed only if the called party answers and communication (voice, data, fax, etc.) are established. Another interpretation of a successful call could be establishing a connection successfully to the called number although the called party may not answer. In respect of telephone calls, the called party may not answer because of a number of reasons including:
(a) called party is not available near the phone and hence the phone keeps on ringing without an answer. In the age of answering machines, the probability of not receiving an answer is low; and
(b) called line is busy and therefore the telephone at the called number does not actually ring. The probability of this happening is also being reduced through the use of the “Call Waiting” facility by many users.
The CCR reflects directly the degree of congestion in the network and indirectly the fault rate. The CCR depends on the equipment available to switch and transmit the signaling messages. The equipment may not be available either because of under dimensioning in which case the available equipment is not adequate to handle the traffic or faulty equipment which would cause the same effect. In many developing countries, the poor CCR is mainly due to faulty switching equipment; however, because of poor maintenance, the outside plant network could also contribute to the poor CCR.
In the international network, the CCR has been further categorized into:
(a) Answer Bid Ratio (ABR);
(b) Answer Seizure Ratio (ASR); and
(c) Congestion (CONG).
The ABR is the ratio of successful calls to total originating international calls. The ratio is the measure of effective international calls, reflects the performance of the total international network between the calling and called country and hence is the CCR for the entire international network or the probability of a call being successful. The ASR is the ratio of successful calls to total incoming international calls. It is a measure of the performance of the called country’s telephone network and hence reflects its CCR. The CONG is the percentage of calls lost due to congestion in the international network. It is a measure of the inadequacy in the number of international circuits between the two countries.