MSD - Traffic Study Report Layout
Traffic Study contains three reports for the Trunk Groups. The reports are Engineering Interval Summary
for Trunk Group, Busy Hour Study Analysis for Trunk Groups and The Trunk Group Day Totals.
The Engineering Interval Summary report gives an overall view of how your Trunk Groups are Trunked
and the GOS (Grade of Service) at which they are operating. This report will give you a view of your
current GOS and trunking. For Trunk Groups this is calculated using the Erlang B table, which is
described later in this document. The recommendations for your Trunk Groups are found in the proposed
section of this report. These recommendations are based on the target GOS you want to achieve. The
report also gives you a view of the number of calls that have been handled with the usage and average
call length and blockage during the study period.
The Busy Hour Study Analysis report shows the 10 busiest intervals during the study period. This report
breaks down the number of calls, usage and average call length, during the interval. It also report the
proposed number of required to get your desired, or target GOS and the current GOS.
The Trunk Group Day Totals reports give an overall look at each Trunk Groups by day. The number of
calls handled, total usage and average call length are included in this report.
Based on the traffic collected from the CBX, the following are recommendations for sizing trunks in your
switch based on the industry standard of blockage:
Trunk Group Daily Engineering Interval Summary Report
Header
Comments
Group
Name of the Trunk Group
Calls
# of Calls received on Trunk Group
Usage Sec’s
Total amount of time in seconds that the Trunk was used
ACL Sec’s
Average Call Length in seconds
Proposed
Trunks
Recommended # of Trunks
GOS
Recommended Grade of Service
Blocked
Estimated # of calls that would be blocked based on Proposed Trunking and
GOS
Current
Trunks
Actual # of Trunks
GOS
Actual Grade of Service
Blocked
Estimated # of calls that are blocked based on Current Trunking and GOS
Trunks
Number of Trunks to add or remove based on Proposed vs. Current.
Trunk Group Daily Busy Interval Report
Page 1
Header
Comments
Date
Date of Activity
Time
Busy Hour Recorded
Day
Day of the Activity
Rank
Where that particular Busy Hour ranks in conjunction with other busy hours
for that day.
Handled Calls
# of calls handled for that busy hour
Usage Sec’s
Amount of time in seconds used in that busy hour
ACL Sec’s
Average Call Length in seconds for that busy hour
Proposed
Trunks
Recommended # of Trunks
Blocked
Estimated # of calls that would be blocked based on Proposed Trunking
Current
Trunks
Actual # of Trunks
Blocked
Estimated # of calls that are blocked based on Current Trunking
+/-
Number of Trunks to add or remove based on Proposed vs. Current.
Trunks Remove/Add
Number of Trunks to add or remove based on Proposed vs. Current.
Trunk Group Day Totals
Header
Comments
Trunk Group
Name
Name of Trunk Group
Number
Number of Trunk Group assigned by the switch
# of Trunks
# of Trunk for that Trunk Group
Call Volumes
Completed
Incoming
# of incoming calls answered
Outgoing
# of outgoing calls made
Calls Blocked
Number
Page 2
Estimated # of calls blocked
%
Estimated % of calls blocked
Total Attempts
(Completed Incoming + Outgoing + # of Calls Blocked)
Totals Usage Sec’s
Amount of time in seconds that the trunk was used
Average Call Length
In
Average incoming call length in seconds
Out
Average outgoing call length in seconds
Total
Total Average Call Length using Incoming and Outgoing
All Trunks Busy
# of second all trunks were busy
Traffic Engineering Primer
Overview
Definition of Traffic
The term traffic is used to describe the method of counting the number of calls and their
duration for the purpose of sizing the number of facilities needed, and to provide a
certain level of service to carry a given number of hours of traffic.
Traffic measurements are used to determine the carrying capacity of our switch when all
traffic and functions are at their greatest demand.
Traffic can be measured by multiplying the number of calls within a specific time period
by the average length of calls.
The following formula is an example of this process: C x A = T
C
A number of calls in a given period. Let’s say 50 calls.
A
The average call length. Let’s say 3 minutes.
T
The amount of traffic
50 calls x 3 minute average call length = 150 minutes of traffic
150 minutes divided by 60 (minutes/hour) = 2.5 hours of traffic
What is Traffic Analysis?
Traffic Analysis is the study of three (3) CBX system characteristics:
1.
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Statistics including: number of times all trunks were busy in one day, number of
calls that were placed over a trunk in one day, time of day the CBX placed the
most calls, many other telephone traffic patterns.
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2.
Facilities including: trunks, attendant consoles
3.
Grade of Service including: portion of calls that are allowed to reach a busy tone
during a study period applies to groups of facilities.
What is the Purpose of Traffic Engineering?
The attempt to balance performance with economics, to achieve the greatest utilization
of facilities while maintaining the cost at a level that is beneficial to the user.
"The art and science determining the minimum quantities of equipment and trunks which
will carry the required traffic with an acceptable grade-of-service". Emerson C. Smith,
Glossary of Communication.
Traffic Measurement
Units of Measurement
1. Traffic is measured in number of hours of usage. Better expressed as how
many hours of traffic are offered in one hour of time over a single facility.
Logic tells us that a maximum of one hour of traffic can be carried in a one
hour time period.
2. In engineering, one hour of traffic in a one-hour time frame is called an
Erlang. It was so named after the mathematician who created many modern
traffic theories.
a. Erlangs relate traffic to the number of circuits in an system and the
utilization of those circuits.
b. Therefore: 1 Hour of Traffic = 1 Erlang
3. The next most commonly used traffic measurement is CCS or Centum Call Seconds.
This method of measurement, which is defined as 100 seconds, is used in expanded
traffic statistics. Many of the industry standards charts and tables are written in CCS
per line.
a. 60 seconds x 60 minutes = 3600 seconds in 1 Hour
b. 1 CCS = 100 seconds, in order to determine how many CCS are in 1 hour
(1 Erlang), divide 3600 seconds by 100 = 36 CCS.
c. Therefore: 1 Hour = 36 CCS
d. Should we have need to convert CCS to minutes, the same principals
apply:
Remembering to keep seconds as the common denominator:
1 CCS = 100 seconds
1 min. = 60 seconds
100 seconds divided by 60 seconds (in a minute) = 1.67 minutes
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e. Therefore: 1 CCS = 1.67 Minutes or 100 Seconds
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4. Examples of source data:
Depending upon where we acquire our traffic information and data, it
becomes necessary to convert from one method of measurement into another
in order to engineer the most accurate design.
Expanded traffic Statistics
AT&T standards tables,
Request For Proposal..............................CCS
Traffic Theory
Erlang B charts
Erlang C charts, etc.
AT&T Offered Traffic Tables.................Erlangs
Phone Bills
SMDR/CDR......................................Minutes/Hours
Understanding traffic measurements and the conversion methods is vital in
performing engineering designs correctly.
Units of Measure Conversion Chart
CCS
CCS
1 CCS
1 CCS
36 CCS
Erlangs
1 Erlang
1 Erlang
1 Erlang
Minutes
1 Minute
1 Minute
=
=
=
=
Centum Call Second
100 Seconds
1.67 Minutes
1 Hour or 1 Erlang
=
=
=
36 CCS
60 Minutes
3600 Seconds
=
=
60 Seconds
.6 CCS (60 sec./100 sec.)
Traffic Theories
Background
Mathematicians develop traffic theories that predict how people will use their telephones.
These theories give us a systematic approach to sizing facilities and switches.
The tables and formulas that we use in traffic engineering are based on theories; on the
probability that someone will pick up their phone and make a call. So the traffic
predictions we make are not correct 100% of the time.
The traffic theory used by Unify, and the industry in general, for trunk traffic analysis is
Erlang B.
Erlang B
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Overview

Developed by A. K. ERLANG who was working on a method called 'statistical
equilibrium'.

Erlang assumed that a trunk group might have any number of calls in
progress at any given time.
Assumptions

Assumes that blocked calls are cleared, but will never be offered to that
trunk group again. If a call finds all trunks busy, it either hangs up or is
routed to another trunk group. From the standpoint of that particular trunk
group, the call is cleared.
This caller may hang-up and re-call but is seen then as a new call being
offered to this trunk group. Retrys are not part of the Erlang B theory.
This theory may sound familiar to you, it is in fact, the principal behind
Route Optimization without Queuing. In Route Optimization, a call is received
and if the facility is busy that call is overflowed to the next trunk group.

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Depending on blockage, offered and carried traffic can differ significantly.
Erlang B Carried Traffic Capacity Tables
No. of
Trunks
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
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(c) Copyright ROLM Company 1979
10.00% 5.00%
2.00%
1.00%
0.50%
0.10%
No. of
Trunks
B.10
B.05
B.02
B.01
B.005
B.001
0.10
0.05
0.02
0.01
0.01
0.00
1
0.54
0.36
0.22
0.15
0.10
0.05
2
1.14
0.85
0.59
0.45
0.35
0.19
3
1.84
1.45
1.07
0.86
0.70
0.44
4
2.59
2.11
1.62
1.35
1.13
0.76
5
3.38
2.81
2.23
1.89
1.61
1.14
6
4.20
3.55
2.88
2.48
2.15
1.58
7
5.04
4.32
3.55
3.10
2.72
2.05
8
5.89
5.10
4.26
3.74
3.32
2.55
9
6.76
5.91
4.98
4.42
3.94
3.09
10
7.64
6.72
5.72
5.11
4.59
3.65
11
8.53
7.55
6.48
5.82
5.25
4.23
12
9.42
8.39
7.25
6.54
5.93
4.83
13
10.33
9.24
8.04
7.28
6.63
5.44
14
11.24
10.1
8.83
8.03
7.34
6.07
15
12.15
10.97
9.63
8.79
8.06
6.71
16
13.07
11.84
10.44
9.56
8.79
7.37
17
13.99
12.72
11.26
10.33
9.53
8.04
18
14.92
13.60
12.09
11.12
10.28
8.72
19
15.86
14.49
12.92
11.91
11.04
9.40
20
16.79
15.38
13.76
12.71
11.80
10.10
21
17.72
16.28
14.6
13.51
12.57
10.80
22
18.66
17.18
15.45
14.32
13.35
11.51
23
19.61
18.08
16.30
15.14
14.13
12.23
24
20.55
18.99
17.15
15.96
14.92
12.96
25
21.50
19.90
18.02
16.79
15.72
13.69
26
22.45
20.81
18.88
17.62
16.52
14.42
27
23.40
21.72
19.75
18.45
17.32
15.17
28
24.35
22.64
20.62
19.29
18.13
15.91
29
25.30
23.56
21.49
20.13
18.94
16.67
30
26.26
24.48
22.37
20.98
19.75
17.42
31
27.21
25.41
23.25
21.83
20.57
18.19
32
28.17
26.33
24.13
22.68
21.40
18.95
33
29.13
27.26
25.02
23.54
22.22
19.72
34
30.09
28.19
25.91
24.39
23.05
20.50
35
31.05
29.12
26.80
25.25
23.89
21.27
36
32.02
30.06
27.69
26.12
24.72
22.06
37
32.98
30.99
28.58
26.98
25.56
22.84
38
33.94
31.93
29.48
27.85
26.40
23.63
39
34.91
32.86
30.38
28.72
27.24
24.42
40
35.88
33.80
31.28
29.59
28.09
25.21
41
36.84
34.74
32.18
30.46
28.94
26.01
42
43
44
45
46
47
48
49
50
37.81
38.78
39.75
40.72
41.69
42.66
43.63
44.61
35.68
36.63
37.57
38.52
39.46
40.41
41.36
42.31
33.08
33.99
34.89
35.80
36.71
37.62
38.54
39.45
31.34
32.22
33.10
33.98
34.86
35.75
36.63
37.52
29.79
30.64
31.50
32.35
33.21
34.08
34.94
35.80
26.81
27.61
28.42
29.23
30.04
30.85
31.66
32.48
43
44
45
46
47
48
49
50
Erlang B Carried Traffic Capacity Tables
(c) Copyright ROLM Company 1979
No. of 10.00% 5.00%
2.00%
1.00%
0.50%
0.10%
No. of
Trunks
Trunks
B.10
B.05
B.02
B.01
B.005
B.001
51
45.58
43.26
40.36
38.41
36.67
33.30
51
52
46.55
44.21
41.28
39.30
37.54
34.12
52
53
47.53
45.16
42.20
40.20
38.41
34.94
53
54
48.50
46.11
43.12
41.09
39.28
35.77
54
55
49.48
47.06
44.04
41.99
40.15
36.59
55
56
50.45
48.02
44.96
42.88
41.02
37.42
56
57
51.43
48.97
45.88
43.78
41.90
38.25
57
58
52.41
49.93
46.81
44.68
42.78
39.08
58
59
53.38
50.88
47.73
45.58
43.65
39.92
59
60
54.36
51.84
48.65
46.48
44.53
40.75
60
61
55.34
52.80
49.58
47.38
45.41
41.59
61
62
56.32
53.75
50.51
48.29
46.30
42.43
62
63
57.30
54.71
51.43
49.19
47.18
43.27
63
64
58.28
55.67
52.36
50.09
48.06
44.11
64
65
59.26
56.63
53.29
51.00
48.95
44.95
65
66
60.24
57.59
54.22
51.91
49.84
45.80
66
67
61.22
58.55
55.15
52.82
50.72
46.64
67
68
62.20
59.51
56.08
53.73
51.61
47.49
68
69
63.18
60.48
57.01
54.64
52.50
48.34
69
70
64.16
61.44
57.95
55.55
53.39
49.19
70
71
65.14
62.40
58.88
56.46
54.29
50.04
71
72
66.12
63.36
59.82
57.37
55.18
50.89
72
73
67.10
64.33
60.75
58.29
56.07
51.75
73
74
68.08
65.29
61.69
59.20
56.97
52.60
74
75
69.07
66.26
62.62
60.12
57.86
53.46
75
76
70.05
67.22
63.56
61.04
58.76
54.31
76
77
71.03
68.18
64.50
61.95
59.66
55.17
77
78
72.02
69.15
65.44
62.87
60.56
56.03
78
79
73.00
70.11
66.38
63.79
61.45
56.89
79
80
73.98
71.08
67.31
64.71
62.35
57.75
80
81
74.97
72.05
68.25
65.63
63.26
58.61
81
82
75.95
73.01
69.20
66.55
64.16
59.48
82
83
76.94
73.98
70.14
67.47
65.06
60.34
83
84
77.92
74.95
71.08
68.39
65.96
61.21
84
Page 10
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
Page 11
78.91
79.89
80.88
81.86
82.85
83.83
84.82
85.79
86.78
87.77
88.75
89.74
90.73
91.72
92.70
93.69
75.92
76.88
77.85
78.82
79.79
80.76
81.73
82.70
83.67
84.64
85.62
86.59
87.56
88.53
89.50
90.48
72.02
72.96
73.91
74.85
75.80
76.74
77.69
78.63
79.58
80.52
81.47
82.42
83.37
84.31
85.26
86.21
69.31
70.24
71.16
72.09
73.01
73.94
74.86
75.79
76.72
77.65
78.57
79.50
80.43
81.36
82.29
83.23
66.87
67.77
68.68
69.58
70.49
71.40
72.30
73.21
74.12
75.03
75.94
76.85
77.77
78.68
79.59
80.50
62.07
62.94
63.81
64.68
65.55
66.42
67.29
68.16
69.03
69.91
70.78
71.66
72.53
73.41
74.29
75.17
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
Trunk Group Efficiency
Increasing Trunk Group Size

Intuitively, we want to believe that if one trunk carries some amount of traffic, two trunks
will carry twice as much--or that each trunk can carry the same amount of traffic.

As the number of trunks increases, the amount of traffic each trunk can carry approaches
one Erlang.

The conclusion we can draw is that larger trunk groups are more efficient because there is a
greater probability of finding a trunk that is available
Example of "Larger Trunk Groups Are More Efficient"
# of Trunks
40
20
10
Carried Traffic (1)
28.7
11.9
4.4
% Utilization (2)
71.75%
59.50%
44.40%
Note:
1. Using Erlang B tables at B.01
2. Carried Traffic / # of Trunks
Increased Blockage
Increasing the blockage on a trunk group will also increase the efficiency of each trunk in the group.
Example of Increasing the Blockage
Traffic Carried
20
20
20
Blockage
B.01
B.05
B.10
# Trunks Required
30
27
25
Glossary
Busy Hour for Engineering
The engineering method to be used for this trunk group. There are two choices:
1. Average busy hour: The mean value associated with the busy hour for each day of the traffic study.
2. Maximum busy hour: The busy hour during the study period with the highest traffic values.
Grade of Service
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A traffic engineering parameter expressing the probability of blockage in the busy hour. For example, a
B.01 grade of service determines subsequent calculations based upon 1 call out of 100 being blocked
during the Busy Hour as determined by an Erlang B traffic model.
All of the blockage values listed in this study have been calculated using the standard traffic formula,
ERLANG B. This formula is widely used and has been found to provide a high degree of accuracy. The
blockage values listed represent the number of calls which would be blocked from a trunk group out of
every one hundred calls attempted.
With route optimization, a call that is blocked from one trunk group may be offered to another. Route
optimization is a CBX feature that automatically selects the most economical route for an outgoing call
based upon routing tables stored in memory. When no trunks are available in the selected route, the call
may be offered to another route, thus making blockage on that trunk group transparent to the caller. For
certain trunks (e.g. FX and WATS) a higher blockage is desirable. Unless specified by the customer, the
following blockages are used to calculate the trunks required based on the traffic study:
Central Office (CO) in, out, or bothway
Inbound WATS (8000)
Outbound WATS
Foreign Exchange (FX)
Tie Lines
Direct Inward Dial (DID)
B.01
B.01
B.05
B.10
B.10
B.01
Peg Count
Increments of traffic values, such as how many incoming calls.
A definition taken from: Telephony's Dictionary: Telecommunication Words and Terms by Graham
Langley:
“A count of seizure, or attempts at seizure, of telephone trunks, circuits, or switching equipment
expressed as calls per hour per line or of calls handled by an operator during a specified time
interval.”
Analysis
Fundamental data for traffic analysis:
 Number of trunks
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
Type of trunks

Number of calls

Busy hour

Traffic patterns

Blockage
Utilization
The number of hours of traffic (in hours or expressed as a percentage of the total possible hours of
traffic) that a trunk group carried for that hour. For example, ten trunks can carry 10 hours of traffic if all
trunks were busy for the whole hour. If the trunks carried 5 hours of traffic, they are utilized 50%. The
Erlang B table is used to determine that maximum allowable utilization to support a particular blockage
level.
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