TELEPHONE LOAD AND QUALITY OF SERVICE

The intensity of telephone traffic is measured in erlangs (Erl). One erlang corresponds to 60 minutes of occupation per hour. So, when it is said that the traffic is 2 erlangs, it means that the total time of occupation of some device (or group of devices) per hour is 120 minutes. Of course, one line cannot be occupied for more than 60 minutes per hour. Therefore, to service traffic exceeding 1 erlang, a group of 2 or more communication lines is required.

The flow of telephone calls is a random process with a random nature of call arrivals and connection durations. For one subscriber line (SL) of a PBX, the normal load is considered to be 0.2 erlangs. That is, during the busiest hour, on average, one SL is occupied for 12 minutes. For local communication in urban conditions, it is assumed that there are 4 three-minute conversations (two incoming and two outgoing).

Service quality is characterized by the probability of the calling subscriber receiving a rejection (a BUSY signal) when attempting to establish a connection at any stage of the call. This probability is called losses. Overall, for a call within the city network, losses amount to up to 0.025, meaning one call out of forty in the busiest hour receives a BUSY signal before reaching the called subscriber's telephone set.

On the segment from PBX to the telecom operator, losses are not more than 0.005. That is, when exiting to the city from PBX, rejection is allowed for no more than one call out of two hundred.

To determine the necessary number of external connecting lines for PBX, one should know the number of its subscribers having urban connections and set an acceptable loss norm.

With acceptable losses on the PBX - operator segment equal to 0.005 and a specific load per subscriber of 0.2 erlangs, the necessary number of connecting lines for different values of PBX subscriber capacity is provided in the table.

PBX ㅤㅤㅤㅤㅤㅤㅤㅤㅤ Lines to
subscribers ㅤㅤㅤㅤㅤ the operator

30ㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤ 13

60ㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤ ㅤ 21

120ㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤ36

240ㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤ63

360ㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤ90ㅤ

For small stations, the load is assumed to be no more than 0.15 erlangs per subscriber line (SL). According to the technical standards for small PBXs with capacities up to 16 SLs, the following ratio should be observed.

PBXㅤㅤㅤㅤㅤㅤㅤLines to theㅤㅤㅤㅤㅤLosses
subscribersㅤㅤ ㅤoperator, minㅤㅤㅤㅤ

8ㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤ2ㅤㅤㅤㅤㅤㅤ ㅤ 0.2ㅤㅤㅤㅤㅤ

16ㅤㅤㅤㅤㅤㅤㅤㅤ ㅤㅤ4ㅤㅤㅤㅤㅤㅤ ㅤ0.1ㅤㅤㅤㅤㅤㅤ

128ㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤ16ㅤㅤㅤㅤㅤㅤㅤ0.25ㅤㅤㅤ

ㅤㅤㅤㅤㅤ

From this table, it is evident that the loss standards are not being met. However, if the loss standards were to be adhered to, the total traffic intensity should be reduced to the values indicated below.

Total intensity,ㅤㅤㅤㅤㅤㅤLines to theㅤㅤㅤㅤㅤ Lossesㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤ
Erlㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤoperatorㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤ

0.14ㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤ ㅤ8/2ㅤㅤㅤㅤㅤㅤㅤ0.005ㅤㅤㅤㅤㅤㅤㅤㅤㅤ

0.8ㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤ16/4ㅤㅤㅤㅤㅤ ㅤ0.005ㅤㅤㅤㅤㅤㅤㅤ

8ㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤ128/16ㅤㅤㅤㅤㅤㅤㅤ0.005ㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤ

So, reducing the number of external lines compared to the optimal is only possible by restricting access to external communication for certain subscribers or by deteriorating service quality.