Traffic intensity, GOS and national numbering

Traffic intensity

Traffic intensity is the average number of calls (servers) simultaneously in progress during a period, measured in Erlangs (E):

$$A = \frac{C\,h}{T}$$

where C = number of calls in time T and h = mean holding time. 1 E means one circuit continuously busy. Example: 120 calls of 3 min in one hour give $A = 120 \times 3/60 = 6$ E. Other unit: 1 E = 36 CCS (hundred call-seconds).

Grade of Service (GoS)

GoS is the proportion of calls that are lost (or delayed beyond a limit) in the busy hour because all servers are busy:

$$GoS = \frac{\text{calls lost}}{\text{calls offered}} = \frac{A_{lost}}{A_{offered}}$$

A smaller value means better service; e.g. 0.01 = 1 call lost in 100. It is used with Erlang B tables to find the number of trunks needed.

National numbering plan (ITU-T E.164 format)

A numbering plan gives every subscriber a unique number so that calls can be routed and charged. ITU-T E.164 sets the international format, with at most 15 digits (excluding prefixes):

 |<------------ max 15 digits ------------->|
 +------+----------+--------------------------+
 |  CC  |   NDC    |           SN             |
 +------+----------+--------------------------+
 1-3 dig  |<----- national (significant) no. ----->|

• CC – Country Code: 1 to 3 digits, given by ITU (Nepal 977, India 91, USA 1). First digit is the world zone.
• NDC – National Destination Code: area/trunk code or mobile network code (Kathmandu valley 1; Nepal Telecom mobile 984/985/986, Ncell 980/981/982).
• SN – Subscriber Number: identifies the line within the NDC area.
• NSN (National Significant Number) = NDC + SN; its length is fixed by the national plan (maximum 15 − length of CC).

Prefixes (not part of the number):

• Trunk (national) prefix: dialled before the NSN for calls inside the country, usually 0 (e.g. 01-4xxxxxx in Nepal).
• International prefix: dialled before CC for outgoing international calls, ITU recommends 00.
• Short codes for emergency and services (100 police, 101 fire, 102 ambulance in Nepal).

Example: a Kathmandu fixed line: international format +977 1 4xxxxxx; national format 01-4xxxxxx; local format 4xxxxxx. A Nepali mobile: +977 98X XXXXXXX.

Requirements of a good national plan: unique numbers, enough spare capacity for growth (planned for 30–50 years), easy routing and charging from the leading digits, uniform length where possible, compatibility with E.164 and with number portability.