Traffic intensity in CCS, CM and CS

Traffic intensity is the average number of calls (or occupied circuits) in progress at the same time during a given period, usually the busy hour. It equals the total occupancy time of all circuits divided by the length of the period:

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

where $C$ = number of calls in period $T$ and $h$ = mean holding time. It is a dimensionless quantity, measured in erlang (E). One erlang means one circuit kept busy continuously for the whole hour (or two circuits each busy half the time, etc.).

Traffic volume is the total holding time, $V = C h$ (erlang-hours or call-seconds). Traffic intensity = traffic volume / period.

Measurement units

Besides the erlang, several older units count the call time during the busy hour.

1. CCS (hundred call seconds)

• 1 CCS = 100 call-seconds of occupancy in one hour.
• One circuit busy for the full hour = 3600 call-seconds = 36 CCS.
• So 1 E = 36 CCS. Used mainly in North America.

2. CM (call minutes)

• Total call minutes of occupancy in the busy hour.
• One circuit busy for 60 minutes = 60 CM, so 1 E = 60 CM.

3. CS (call seconds)

• Total call seconds in the busy hour; 1 E = 3600 CS.

4. Others: EBHC (equated busy hour call, 1 EBHC = 2 min of occupancy, so 1 E = 30 EBHC) and TU (traffic unit, same as erlang).

Example: a trunk group carries 120 calls in the busy hour with mean holding time 3 min.

$$\begin{aligned} A &= \frac{120 \times 3}{60} = 6\ \text{E} \\ &= 6 \times 36 = 216\ \text{CCS} \\ &= 6 \times 60 = 360\ \text{CM} \\ &= 6 \times 3600 = 21600\ \text{CS} \end{aligned}$$