Traffic Requirements of Digital Telephone Switching Systems


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Individual exchanges shall be dimensioned to meet both the general traffic requirements given in this section and the specific requirements for number of lines and trunks, number of Busy Hour Call Attempts (BHCA), and other characteristics peculiar to individual exchanges.

Traffic Requirements of Digital Telephone Switching Systems

5.24.2 Grade of Service

a) Dial Tone Delay
Is the time elapsed from off-hook of a telephone set to the initiation of dial tone, and at the worst case shall not exceed 3 s 98.5% of the time.
The number of call initiations encountering dial tone delays of more than 3 seconds shall not exceed 1.5%. This value will be calculated as an average, taking measurements during the busy hours of business days during the busy season.
b) The average dial tone delay shall not exceed 600 milliseconds.

5.24.3 Traffic Carrying Units

Traffic carrying units (such as trunks) shall provide a grade of service not less than 0.005.
5.24.4 Functional Units – Service Circuits
Functional units shall provide a grade of service not less than 0.001.
Service circuits shall provide a grade of service not less than that specified in paragraph 5.24.2 (Dial Tone Delay).

5.24.5 Post-dial Delay

The post-dialing delay is one of the principal measures of performance of a signaling system, and is defined as the amount of time it takes after the calling subscriber completes dialing until ring-back tone is received.
The average post-dialing delay (for digital systems) shall not exceed 1 second, including the circuit operation and translation time. However, should be noticed that the ITU-T Q.709 “Hypothetical Signaling Reference Connection” specifies no more than 2.2 seconds of post-dial delay for 95% of calls.

5.24.6 Blocking Probability

The following maximum probabilities shall be met under traffic conditions specified throughout in sections 5.5 – 5.22, and shall not include losses that might occur at line modules.
The line unit blocking factor for the digital exchange, shall be 0.5% of total calls or less.
a) The intra-office matching loss shall not exceed P = 0.005
b) The outgoing matching loss shall not exceed P = 0.005
c) The incoming matching loss shall not exceed P = 0.005
d) The register’s loss shall not exceed P = 0.001
e) The switching network matching loss shall not exceed P = 0.005

5.24.7 Availability

The digital exchange shall have full availability to all outgoing trunks in a group. The availability of incoming trunks is a function of the originating exchange design (outgoing trunks of originating exchange).

5.24.8 Overload Conditions

During operation of the exchange it will be assumed that overload conditions exist when either the offered traffic rises to 10% above normal load, or the number of call attempts rise to 20% above normal load. Overloads may be extended for a period of an hour or longer, and the exchange must be able to provide service during such periods. Calculations or simulation results must show how the characteristics in paragraphs 5.24.2, 5.24.3, and 5.24.4 are affected under up to 10% excess traffic in 5% increments; plus up to 20% excess call attempts in 5% increments. During overload conditions none of the above matching loss characteristics shall increase by more than a factor of two (2).
The average daily busy hour traffic shall be assumed to be equal to the average of the traffic for ten average busy hours on ten different days.

5.24.9 Load Control under Emergency Conditions (Priority Service)

Under emergency conditions, service to priority subscribers shall be maintained by load control. The load control must be initiated and carried out automatically by the system via keyboard and a Video Display Terminal (VDT) from the exchange maintenance center, or from remote location. There will be at least two categories of priority subscribers. All switches must be equipped with emergency features remotely controlled by a terminal over a line or a derived circuit. Safeguards must be provided to prevent incorrect or accidental actuation of these emergency features, and the remote terminal must be acknowledged by an answer signal indicating that the emergency condition has been set up.

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