Determining Trunking Requirements

Using the output from the spreadsheet, the trunking requirements for each site and between each site can be determined. The call-flow rate for the busy hour can be translated to approximate data-network bandwidth requirements. In order to determine the trunking requirements, the busy hour call flow must be converted to a traffic measurement called an erlang. An erlang is equivalent to the amount of traffic that can be handled by a single trunk in one hour. To convert the busy hour calculation to erlangs, divide the 1-h call-flow rate by 60 min and multiply by 15 percent. The 15 percent is added to compensate for call setup and teardown overhead. This is depicted in the following equation.

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This will leave a dimensionless quantity for the busy hour. The final step is to determine what is called the grade of service or GoS. GoS is essentially an estimate of the number of calls that will be blocked. Typical design goals fall between 0.01 and 0.05, which translates to between 1 out of 100 and 5 out of 100 calls being dropped. Luckily the telecommunications industry has provided guidelines for trunk selection, which are based on busy-hour traffic and grade of service. These charts are derived using complex statistical analyses of call distributions and arrival rates. Table 8-1 provides a guideline for trunk selection. It is usually helpful to document the trunk requirements in a simple chart such as Table 8-2. TABLE 8-1

Trunking Requirements Based upon Busy Hour and Grade of Service

Number of Trunks

Grade of Service

.01

.02

.03

.05

.07

.10

1

0.01

0.02

0.03

0.05

0.07

0.11

2

0.15

0.21

0.27

0.35

0.43

0.53

3

0.44

0.56

0.66

0.82

0.94

1.10

4

0.82

1.01

1.15

1.36

1.53

1.74

5

1.28

1.53

1.71

1.97

2.18

2.43

6

1.79

2.09

2.30

2.61

2.86

3.14

7

2.33

2.68

2.92

3.28

3.56

3.89

8

2.92

3.31

3.58

3.97

4.28

4.67

9

3.50

3.94

4.25

4.69

5.03

5.44

10

4.14

4.61

4.94

5.42

5.78

6.22

11

4.78

5.31

5.67

6.17

6.56

7.03

12

5.42

6.00

6.39

6.92

7.33

7.83

13

6.11

6.69

7.11

7.69

8.11

8.64

14

6.78

7.42

7.86

8.47

8.92

9.47

15

7.47

8.14

8.61

9.25

9.72

10.31

16

8.19

8.89

9.36

10.03

10.53

11.14

17

8.89

9.64

10.14

10.83

11.36

TABLE 8-2

Voice Trunking Requirements

Source Destination No. of Trunks

TABLE 8-2

Voice Trunking Requirements

TABLE 8-2

Voice Trunking Requirements

Source

Destination

No. of Trunks

Site A

Site B

2

Site A

Site C

3

Site A

Site D

1

Site B

Site C

Site C Site D 1

NOTES:

• This number needs to be calculated for each remote destination the site will be communicating with.

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