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This chapter explains how to calculate the number of trunks and ports required to carry voice and provision the amount of bandwidth necessary for your organization. It contains the following sections:
Traffic Engineering, as it applies to traditional voice networks, is the process of determining the number of trunks necessary to carry your organization's calls. For a Voice-over-IP (VoIP) network, an added goal is to provision the appropriate amount of bandwidth necessary to carry both your organization's calls and its data traffic.
This section briefly describes some of the technical issues to consider when engineering traffic.
There are two main types of connections: lines and trunks.
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Note Because DID trunks do not provide a dial tone, they cannot be used for outgoing calls from the PBX to the CO. |
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Note In telephony, a switch is any device that connects individual phones to phone lines (such as a PBX) or connects telephony devices to other telephony devices (in a CO). |
In telephony, media are characterized in the following two ways:
When measuring network traffic, it is important to consider all of the following types of traffic:
In the US, voice traffic is typically measured in one of two ways:
Therefore, 1 Erlang = 36 CCS per hour.
As defined in the "Designing Campus Data Networks" section of Chapter 4, "Data Networking Fundamentals," a campus is a building or a group of buildings all connected to one corporate network that comprises many LANs.
When planning and implementing a converged network, never lose sight of the fact that if you own all of the cables in your campus network, solving any problems with these media is your responsibility. Typically your telephone company responds only to problems beyond the demarcation point (the jack or other connection that ties your system to the telephone company).
Consider the following issues when planning the voice aspects of your converged network:
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Note If the cost of laying additional cable is prohibitive, a wireless networking solution may be appropriate. Contact your Cisco sales representative for additional information. |
Step 2 Gather voice traffic data.
Step 3 Categorize traffic by group.
Step 4 Calculate the number of trunks.
Step 5 Choose the proper combination of trunks.
Step 6 Convert PSTN traffic to IP traffic.
Complete the following steps to forecast growth (referring to the "Growth Forecast Example"):
Step 2 Determine how many employees you have.
Step 3 Calculate the ratio of phones to employees by dividing the number of phones by the number of employees.
Step 4 Forecast your annual growth rate by projecting the number of employees you intend to hire on an annual basis (expressed as a percentage over the next five years).
Step 5 Use the annual growth rate forecast to calculate how many employees you expect to have at the end of each year for the next five years.
Step 6 Calculate the number of phones you expect to require at one-year intervals by multiplying the projected number of year-end employees by the forecasted annual growth rate.
The following example shows how to define and forecast growth for a small company:
Contact your telephone service provider to get the following information (for two weeks of traffic):
Start by separating traffic according to its direction (inbound or outbound). Next, group outbound traffic in terms of the distance (such as local, local long distance, intra-state, inter-state, and so on). Organizing traffic by distance is important because most tariffs are based on distance.
Determine the purpose of the calls. Categories could include fax, modem, call center, 800 customer service, 800 voice mail, and telecommuter.
This section explains how to calculate how many trunks you need. If you are setting up a new system or have insufficient information, complete the following steps (referring to the "New System Example"). If you are modifying a legacy system, see "Legacy System Busiest Hour" later in this chapter.
Step 2 Identify a target GoS. A GoS of 0.01 is standard for most calls. Use 0.05 for tie line services and 0.10 for long distance.
Step 3 Calculate the number of Erlangs as follows:
(No. of users) x CCS = Total CCS
Total CCS / 36 = no. of Erlangs
Step 4 Based on the above calculations, determine how many trunks you need.
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Note CCS is typically dependent on what type of business you are in. The hotel and hospital industries, for example, use a CCS of 4, while finance companies use a CCS of 8. If the CCS is unknown, use a value of 6. |
Suppose that you expect the following traffic patterns:
Assuming that you have 75 end-users and that you are using the default CCS (6), make the following calculations:
Assuming that you are using a GoS value of 0.01 for DID and CO lines, you calculate that you need 11 IT lines and 11 OT lines, for a total of 22 lines:
40%IT | 40% x 12.5E = 5.00 E | 11 DID lines |
40%OT | 40% x 12.5E = 5.00 E | 11 CO lines |
Totals: | 10.00E | 22 analog lines |
Based on the Erlang B table (see "Traffic Engineering Tables"), you find that you need approximately 18 trunks, and that you must provision either one T1 "supertrunk" with DID, CO, and long-distance services, or 18 analog trunks.
Using the Erlang B table (see "Traffic Engineering Tables"), looking up 13.14 E with a GoS of 0.01, you calculate that you will require 28 or 29 trunks (depending on whether you round up or down from 13.14).
Consider the following two rules when optimizing the network for cost:
Knowing how much traffic you have to deal with at peak loads (in Erlangs) and how the traffic flows will determine how many and what type of trunks are required to support your organization's calls. If the calling pattern suggests only local calls, then you might require direct connection to the central office (CO). Extensive long-distance dialing might require a dedicated T1 connection to an Inter Exchange Carrier (IEC or IXC) for long-distance services. An inbound call center that is revenue-generating or provides customer service and support might also require a dedicated T1 connection. Or, several small companies sharing a leased facility might require several small groups of COs.
The last calculation you need to make is to equate Erlangs of carried traffic to packets per second (pps). (If your system includes Asynchronous Transfer Mode [ATM] links, this calculation is made in cells per second [cps] instead of packets per second.) The following example illustrates one way to do this:
Next, apply modifiers to these figures based on the actual conditions. Types of modifiers to apply include packet overhead, voice compression, voice activity detection (VAD), and signaling overhead. Packet overhead can be used as a percent modifier. For example:
Voice compression and voice activity detection are also treated as multipliers. For example, use a 0.125 multiplier for conjugate structure algebraic code excited linear prediction (CS-ACELP [8 Kb]). For VAD, use a 0.6 or 0.7 multiplier.
Signaling overhead is an additional consideration. In particular, you need to factor in the Real Time Control Protocol (RTCP) and H.225 and H.245 connections.
With the information you have collected, you can apply traffic distribution to the trunks to see how that distribution affects bandwidth. Traffic distribution is based on busy-hour and average-hour calculations.
Suppose that for the busy hour and the average hour, the distribution of traffic per trunk is 2.64 Erlangs and 2.2 Erlangs, respectively. If one pulse code modulation (PCM) voice channel requires 64 kbps, then the busy hour will have the following amount of traffic:
and traffic during an average hour will be:
Therefore, 2.2 Erlangs of traffic carried over IP using voice compression requires the following bandwidth:
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Note You may need to account for other modifiers as well, such as call setup and tear down signaling overhead, Layer 2 overhead, and voice activity detection (if used). |
Posted: Mon Oct 2 13:21:37 PDT 2000
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