7.10 Working with Call Scripts

Call scripts are collections of parameters set to recommended values that you assign to call groups when you design your VoIP Quality assessment. Each default call script is configured for assessing a typical voice over IP implementation based on a supported codec. For more information, see Section 7.11.2, Reviewing Codec Types.

When you create connectors in the Design view, you can quickly configure parameters for an assessment by choosing a call script from the list.

You can customize a call script, although you must assign it a name to distinguish it from the default call script it was based on. A call script you modify and rename is later available in the Call Scripts List dialog box.

You can export call scripts from other assessments, including assessments you created with an earlier version of Vivinet Assessor, and then import them into new assessments. For more information, see Section 10.3, Exporting and Importing Assessments, Scripts, and Definitions.

For more information, see the following topics:

7.10.1 Adding a Call Script

By default, call scripts correspond to codecs, as indicated by their names. Default call scripts cannot be changed or deleted; instead, you must add new call scripts based on the default scripts. Once you add a call script with customized settings, you assign it a name to distinguish it from the default call script it was based on. Any call script you modify and rename is later available in the Call Scripts List dialog box and can be edited.

To add a call script:

  1. From the Assess VoIP Quality view, click Call Scripts on the Options menu.

  2. To add a call script, click Add. Complete the fields as discussed below and then click OK.

  3. To change a call script, select a non-default call script in the list and click Modify. Complete the fields as discussed below and then click OK.

  4. To copy a call script, select a call script in the list and click Copy. Complete the fields as discussed below and then click OK.

    Field

    Description

    Call Script Name

    By default, the same as the codec. Enter a new name to distinguish a call script you modified.

    Codec

    The type of codec used on your network. Choose one of the supported codecs from the list. For more information, see Section 7.11.2, Reviewing Codec Types.

    Packet Loss Concealment

    Packet Loss Concealment (PLC) is enabled by default in version 3.2 of Vivinet Assessor; it was disabled by default in previous versions. Most G.711 codecs implement this feature. For more information, see Section 7.11.2, Reviewing Codec Types.

    Use silence suppression

    Emulates the effects of silence suppression (also called voice activity detection) on the line during the VoIP Quality assessment. Disabled by default; when enabled, uses a 50% voice activity rate. For more information, see Section 7.11.5, Understanding Silence Suppression.

    Voice activity rate

    The percentage of time during a simulated call that talking seems to occur. Silence suppression means that during periods of silence in a conversation, no data is sent. The activity rate determines how much actual voice data the simulated call contains. Default is 50% voice activity; accepted values are 1% to 100%, inclusive.

    Override delay between voice datagrams

    Determines the datagram size to be used in the assessment. VoIP applications break voice data into chunks based on delay, or the amount of time, in milliseconds (ms), between successive datagrams. For the G.723 codecs, the default value is 30 ms; for all other codecs, the default is 20 ms. Values entered must be between 10 and 200 ms. Vivinet Assessor may adjust values slightly after you enter them so that no partial buffers are sent. For more information, see Section 7.11.4, Setting Datagram Sizes.

    QoS name (optional)

    Emulates the effects of a Quality of Service (QoS) scheme on call quality. If you enable a quality of service, you must select a pre-configured QoS definition that determines how the traffic will be marked. QoS is not supported by all endpoint operating systems. For more information, see Section 7.11.7, Reviewing Quality of Service.

    Advanced

    Reveals more call script parameters for advanced users to configure.

    Initial delay type

    Introduces delays between the start of each call in a set of calls. Instructs the endpoints to delay sending RTP datagrams for slightly different amounts of time while preserving the interval specified in the schedule. The type of initial delay refers to the distribution algorithm the endpoints use to calculate it. The initial delay parameter and the algorithm choices are explained in Section 7.10.2, Setting the Initial Delay.

    Select Constant Value to set your own initial delay value. All the endpoints in all call groups will then pause for the same amount of time before starting to send call traffic.

    Additional fixed delay

    Lets you add a delay value from a known, constant source. For example, if you are testing equipment that adds 10 milliseconds of delay to each datagram, enter 10 ms here. Range is 0-300 ms.

    Jitter buffer

    Emulates the effects of jitter buffering on your VoIP network. Jitter buffers may be configured based on time (called an “absolute” jitter buffer) or based on number of datagrams (a “frame-based’ jitter buffer). All call scripts have a default jitter buffer of 2 voice datagrams. For more information, see Section 7.11.6, Understanding Jitter Buffers.

  5. To delete a call script, select a non-default call script in the list and click Delete.

  6. Click OK.

7.10.2 Setting the Initial Delay

You use the Initial delay type call script parameter to introduce variations among simulated calls or streams of background traffic in order to realistically emulate traffic on a VoIP network. To better represent the random way in which data is sent over a VoIP network, the initial delay parameter instructs the endpoints to delay the start of calls (or, for background traffic, the sending of TCP/IP datagrams) for slightly different amounts of time while preserving the scheduled call duration you specified in the Schedule view. For more information, see Section 7.5, Scheduling a VoIP Quality Assessment.

When you configure an initial delay value in a call script, you actually introduce variations in the length of each call as well, which means that the amount of data sent by the calls will not be exactly the same. No matter what initial delay you set, all calls stop running at the same time, when the call duration timer expires.

The call duration also controls the amount of TCP/IP data sent by Background Traffic connectors. The variability introduced by setting an initial delay for background traffic affects the timing of SEND commands. Just as with calls, when the call duration timer expires, all background traffic stops running at the same time.

For example, you set an initial delay of between ten and 25 seconds using a Uniform distribution. Calls at the lower limit of the distribution will send data for as much as 15 seconds longer than those at the upper limit of the distribution because the delay before they start to send data will be 15 seconds shorter, yet they will stop sending data at the same time as the calls with longer initial delays.

Despite any initial delay that is configured, all calls are connected immediately. The endpoints then either sleep or begin to send data according to the initial delay value and distribution selected.

The type of initial delay refers to the distribution algorithm the endpoints use to calculate the delay. The initial delay distribution choices are as follows:

Algorithm

Description

Uniform

Distribution of initial delay times between the upper and lower limit you set is completely uniform. Any number within the upper and lower limits is as likely to be used for the delay value as any other number.

Normal

Distribution of initial delay times between the upper and lower limits you set is a normal distribution, plotted with a bell curve. The Marsaglia-Bray algorithm is used to generate the normal distribution.

Poisson

Distribution of initial delay times between the upper and lower limit is a Poisson distribution; most values occur within +/-3 standard deviations with respect to the average. The incomplete gamma function is used to generate this distribution.

Exponential

Distribution of initial delay times between the upper and lower limit you set is an exponential distribution. If you plot the (descending) times against the number of occurrences, the graph's maximum will be at the upper limit and the minimum will be at the lower limit.

For more information, see the discussion of the Initial delay type field in Section 7.10.1, Adding a Call Script.