i. Off-hook time (OHT) is the amount of time a telephone is off-hook or in use.
j. Off-hook factor is the representation in percentage of the off-hook time.
k. Tandem is traffic that flows through a switch but does not begin or terminate at that switch. A
node will have most of its traffic in the form of tandem service as it is mainly a hub or a point at which
traffic is relayed or redirected throughout the network. LES can also have some tandem service.
Tandem service will affect the way in which TGC sizing is calculated.
l. Average call rate (ACR) is the average number of calls made by a switch during a one-hour
period during the busiest period of the day.
2.
Grade of service (GOS).
a. GOS is defined as the number of calls completed divided by the number of calls attempted. As
planners, we must target our network for a GOS of 90 percent; this translates into a 10 percent blockage
rate.
b. A blockage rate is needed to protect the switch from becoming overloaded and crashing. This is
what happened in Germany when Siemens activated the European Telephone Service (ETS). Siemens
did not program a blockage rate and when the American Forces began using ETS, it quickly became
overloaded and crashed. It then took weeks to recover and reprogram with a protection blockage rate.
A 10 percent blockage ratio will prevent total breakdown of the switch. When a switch hits this
blockage level it will automatically take precautions to ensure that high priority users have continuous
access to the network. Some of this protection is in the form of restricted access to the switchboard and
preemption for the priority users. These programmed precautions are done in the initial setup or
programming of the switch in its data base.
3.
Switch traffic profile.
a. The switch traffic profile is the percent of a switch's total traffic that falls into any of the four
categories listed below:
(1) Local.
(2) Incoming.
(3) Outgoing.
(4) Tandem.
b. You can determine the traffic profile from the metering reports generated by the switch. Tactical
units normally require their switchboard operators to produce their metering reports at least twice daily.
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