11

Voice Traffic Engineering Voice Traffic Engineering & Management& Management

22

PSTN and PBX networks are designed PSTN and PBX networks are designed with 2 objectives:with 2 objectives:

Maximize usage of their circuits Maximize usage of their circuits

Minimize the cost.Minimize the cost.

33

In telecommunications, calls being handled by In telecommunications, calls being handled by a switching system are referred to as a switching system are referred to as traffictraffic.. Analogous to road way traffic. Must know how Analogous to road way traffic. Must know how

many lanes are needed, but with little waste.many lanes are needed, but with little waste.

The job of a traffic engineer is to balance the The job of a traffic engineer is to balance the trade-off between cost and service.trade-off between cost and service. Must be able to handle traffic sufficiently during Must be able to handle traffic sufficiently during

busy hours and no more.busy hours and no more. Designs based on historic data.Designs based on historic data. how many trunks are enough?how many trunks are enough?

44

Other questions to consider.. Other questions to consider..

How many customer service agents are needed to deliver a How many customer service agents are needed to deliver a particular level of service in a call center.particular level of service in a call center.

How long callers have to wait with a certain number of How long callers have to wait with a certain number of agents.agents.

Both the agents and the circuits are analogous in Both the agents and the circuits are analogous in telephony traffic engineeringtelephony traffic engineering

55

EfficiencyEfficiency is defined as the percent of time the is defined as the percent of time the server is working (carrying traffic) as opposed server is working (carrying traffic) as opposed to waiting for a call. to waiting for a call. This is also known as This is also known as occupancyoccupancy..

For a given load, the more the servers, the less For a given load, the more the servers, the less blocked or held calls. blocked or held calls.

The rate at which traffic arrives is seldom The rate at which traffic arrives is seldom uniform.uniform.

Principles Governing Network DesignPrinciples Governing Network Design

66

Principles of Network DesignPrinciples of Network Design

More servers than necessary provide good service, More servers than necessary provide good service, but at an excessive cost. but at an excessive cost.

Too few servers and both service and cost deteriorate, Too few servers and both service and cost deteriorate, resulting in loss of customers and decreased agent resulting in loss of customers and decreased agent productivity.productivity.

The analyst’s job is to reach an optimum balance The analyst’s job is to reach an optimum balance between costs and service.between costs and service.

77

Principles of Network DesignPrinciples of Network Design

Networks can be classified as Networks can be classified as Loss SystemsLoss Systems, , or or Delay SystemsDelay Systems..

Loss systems - calls are blocked or rejected Loss systems - calls are blocked or rejected without being served. without being served.

Service is measured by the % of calls blocked.Service is measured by the % of calls blocked.

Delay systems are queued calls, such as the ACD Delay systems are queued calls, such as the ACD systems. systems.

Service is measured by % of calls that are delayed Service is measured by % of calls that are delayed within a certain time interval.within a certain time interval.

88

Traffic Engineering FundamentalsTraffic Engineering Fundamentals

To administer a telecommunications system, 3 To administer a telecommunications system, 3

variables are required:variables are required: Grade of Service (GoS) Grade of Service (GoS) Traffic load Traffic load Quantity of serversQuantity of servers Knowing 2 can determine the third.Knowing 2 can determine the third.

Systems should not be designed to handle highest Systems should not be designed to handle highest busy hour nor non-random traffic, such as that busy hour nor non-random traffic, such as that experienced during major catastrophes or call in experienced during major catastrophes or call in contestcontest

99

Traffic Engineering FundamentalsTraffic Engineering Fundamentals

Grade of Service (GoS) refers to the % of calls Grade of Service (GoS) refers to the % of calls that encounter blockage.that encounter blockage. As a business objective, must determine amount of As a business objective, must determine amount of

blockage that can be tolerated.blockage that can be tolerated. If the GoS is too high (zero blockage), many circuits If the GoS is too high (zero blockage), many circuits

will be underutilized.will be underutilized. Too low means too many busy signals encountered.Too low means too many busy signals encountered. Most business situations call for between 1% and 5% Most business situations call for between 1% and 5%

blockage.blockage.

1010

Traffic Engineering FundamentalsTraffic Engineering Fundamentals

Traffic LoadTraffic Load is the amount of traffic during busy hour. is the amount of traffic during busy hour.

Busy Hour – Busy Hour – The Average Busy Hour (ABH) is the average of the busy The Average Busy Hour (ABH) is the average of the busy

hour over a several day period.hour over a several day period.

Used to determine the capacity needs.Used to determine the capacity needs.

If the busy hour of each day varies, the average is called the If the busy hour of each day varies, the average is called the Average Bouncing Busy Hour (ABBH).Average Bouncing Busy Hour (ABBH).

1111

Traffic Engineering FundamentalsTraffic Engineering Fundamentals

Busy HourBusy Hour At the busy hour, more blockage will occur, At the busy hour, more blockage will occur,

It is not feasible to design circuits to handle the absolute peaks. It is not feasible to design circuits to handle the absolute peaks. Therefore, some blockage is expected.Therefore, some blockage is expected.

If some seasons have busier times than others, the study If some seasons have busier times than others, the study should be taken then.should be taken then.

Between the holidays or registration week at a college.Between the holidays or registration week at a college.

A common definition for seasonal busy hour is the average A common definition for seasonal busy hour is the average amount of traffic during the 10 highest days of the year.amount of traffic during the 10 highest days of the year.

1212

Traffic Engineering FundamentalsTraffic Engineering Fundamentals

Traffic Load can be distinguished between Traffic Load can be distinguished between Offered loadOffered load vs vs Carried loadCarried load. . Carried LoadCarried Load is what is shown on Billing Reports is what is shown on Billing Reports

Talk time.Talk time. Offered Load Offered Load also includes setup times.also includes setup times.

Talk Time + Setup TimeTalk Time + Setup Time Call setup time is the time used to signaling time and Call setup time is the time used to signaling time and

ringing, as the circuit is occupied during that time, but ringing, as the circuit is occupied during that time, but no billing is recorded.no billing is recorded.

1313

Engineers use Engineers use Offered Load (talk + setup)Offered Load (talk + setup) to to engineer a engineer a Circuit Group.Circuit Group.

The Traffic Load is the amount of traffic The Traffic Load is the amount of traffic during busy hour.during busy hour.

Once engineers know the Traffic Load and the Once engineers know the Traffic Load and the required service level (amount of acceptable required service level (amount of acceptable blockage, also called GoS), the number of blockage, also called GoS), the number of circuits required could be found by looking at circuits required could be found by looking at a traffic design table.a traffic design table. There are several type of traffic design tables, for There are several type of traffic design tables, for

specific characteristics.specific characteristics.

1414

Traffic Design TablesTraffic Design Tables 3 types of tables are used, depending on the 3 types of tables are used, depending on the

given situation. Each table uses a different given situation. Each table uses a different distribution theoremdistribution theorem Erlang A tableErlang A table and assumes Block Calls and assumes Block Calls HeldHeld

(BCH), which assumes that callers immediately (BCH), which assumes that callers immediately redial upon receiving a busy.redial upon receiving a busy.

Erlang B tableErlang B table and assumes Block calls and assumes Block calls ClearedCleared (BCC), which assumes the caller either waits (BCC), which assumes the caller either waits longer to redial or does not redial.longer to redial or does not redial.

Erlang C tableErlang C table and assumes Block called and assumes Block called DelayedDelayed (BCD), in which the calls are queued until a circuit (BCD), in which the calls are queued until a circuit becomes available.becomes available.

1515

Sources of traffic usage informationSources of traffic usage information

Many PBX systems provide traffic usage data.Many PBX systems provide traffic usage data. Most LECs can perform a Most LECs can perform a busy studybusy study on selected on selected

trunk groups. trunk groups. shows the number of items that callers attempted and shows the number of items that callers attempted and

encountered a busy.encountered a busy. If no busies are logged, too many trunks have been If no busies are logged, too many trunks have been

assigned.assigned. Call Accounting System – a PC connected to a Call Accounting System – a PC connected to a

SMDR port of the switch provides info. SMDR port of the switch provides info. It can then exported and parsed to a spreadsheet as in the It can then exported and parsed to a spreadsheet as in the

next figure.next figure. No call setup time info is provided in this report.No call setup time info is provided in this report.

1616

Traffic Usage is measured in CCS or Erlangs. Traffic Usage is measured in CCS or Erlangs.

Erlang equals 1 hour of traffic usage. Erlang equals 1 hour of traffic usage. Can be converted to minutes by multiplying by 60.Can be converted to minutes by multiplying by 60.

CCS (Centi* Call Seconds) is used for smaller CCS (Centi* Call Seconds) is used for smaller increments of measurement. increments of measurement.

1 CCS is worth 100 seconds. 1 CCS is worth 100 seconds. C is the Roman Numeral for 100C is the Roman Numeral for 100

1717

1 Erlang=60 minutes=3600 seconds=36 CCS, 1 Erlang=60 minutes=3600 seconds=36 CCS, 1 CCS=1.67 minutes.1 CCS=1.67 minutes.

The typical resident generates about 3-5 CCS The typical resident generates about 3-5 CCS per call and the typical business is about per call and the typical business is about double that.double that.

Knowing desired GoS and traffic load, the Knowing desired GoS and traffic load, the number of circuits is found using traffic tables.number of circuits is found using traffic tables.

1818

Example: using the first type of table, we state that 1% blocking is acceptable, and Example: using the first type of table, we state that 1% blocking is acceptable, and 4 Erlangs of traffic are measured during the ABBH, 10 trunks are needed. If 4 Erlangs of traffic are measured during the ABBH, 10 trunks are needed. If 5% blocking is acceptable, only 8 trunks are needed.5% blocking is acceptable, only 8 trunks are needed.

Remember, 4 Erlangs = 4 hours of traffic (not necessarily via 1 circuit)Remember, 4 Erlangs = 4 hours of traffic (not necessarily via 1 circuit)

1919

Busy signals from blocked calls can be returned in Busy signals from blocked calls can be returned in one of 2 ways when considering a PBX:one of 2 ways when considering a PBX: By the carrierBy the carrier By the ACD, which can be throttled, controlling the By the ACD, which can be throttled, controlling the

number of calls coming into the call center.number of calls coming into the call center. If calls in queue > 30 and agents logged on <25 and oldest call >2 If calls in queue > 30 and agents logged on <25 and oldest call >2

minutes, then busy.minutes, then busy.

The key question is “will the caller call back or go The key question is “will the caller call back or go somewhere else?” depends on their motivation and somewhere else?” depends on their motivation and alternatives.alternatives. Some ACDs can capture the ANI and return a busy signal. Some ACDs can capture the ANI and return a busy signal.

The ANI is stored in a database, so knowledge of whether The ANI is stored in a database, so knowledge of whether or not they call back can be known.or not they call back can be known.

2020SMDR (Station Message Detailed Report)

2121

2222

Avaya Definity G-3 PBX

2323

All too many times, PBX are trunked improperly due to poor planning.

Often management wants zero busy signals, so over trunking and excessive costs are incurred. However, when planning is used, you are often working with incomplete data, shooting at a moving target.

Assumptions are based on History, which may be incorrect assumptions and history may not repeat itself. Therefore, frequent measurements should be performed and adjustments made accordingly. Must stay alert to factors that could change the pattern, such as business activities, number of employees, internal work flow, acquisitions, etc..