Table Of Contents
IP SLAs—Analyzing VoIP Service Levels Using the UDP Jitter Operation
Prerequisites for IP SLAs VoIP UDP Jitter Operations
Restrictions for IP SLAs VoIP UDP Jitter Operations
Information About IP SLAs VoIP UDP Jitter Operations
The Calculated Planning Impairment Factor (ICPIF)
Voice Performance Monitoring Using IP SLAs
Codec Simulation Within IP SLAs
How to Configure the IP SLAs VoIP UDP Jitter Operation
Configuring the IP SLAs VoIP UDP Jitter Operation
Configuration Examples for IP SLAs VoIP UDP Jitter Operations
IP SLAs VoIP UDP Operation Configuration: Example
IP SLAs VoIP UDP Operation Statistics Output: Example
Feature Information for the IP SLAs VoIP UDP Jitter Operation
IP SLAs—Analyzing VoIP Service Levels Using the UDP Jitter Operation
First Published: August 14, 2006Last Updated: July 16, 2008This document describes how to use the Cisco IOS IP Service Level Agreements (SLAs) UDP jitter operation to proactively monitor Voice over IP (VoIP) quality levels in your network, allowing you to guarantee VoIP quality levels to your users in IPv4 or IPv6 networks. The IP SLAs VoIP UDP jitter operation accurately simulates VoIP traffic using common codecs, and calculates consistent voice quality scores (MOS and ICPIF) between Cisco IOS devices in the network.
Cisco IOS IP SLAs is an embedded feature set in Cisco IOS software that allows you to analyze IP service levels for IP applications and services, to increase productivity, to lower operational costs, and to reduce occurrences of network congestion or outages. IP SLAs uses active traffic monitoring for measuring network performance. The accuracy of measured data is enhanced by enabling the IP SLAs Responder, available in Cisco routers, on the destination device.
Finding Feature Information
Your software release may not support all the features documented in this module. For the latest feature information and caveats, see the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the "Feature Information for the IP SLAs VoIP UDP Jitter Operation" section.
Use Cisco Feature Navigator to find information about platform support and Cisco IOS and Catalyst OS software image support. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.
Contents
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Prerequisites for IP SLAs VoIP UDP Jitter Operations
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Prerequisites for IP SLAs VoIP UDP Jitter Operations
To use this feature, your networking devices on both ends of the connection must support Cisco IOS IP SLAs. Cisco IOS IP SLAs is an integrated feature set in Cisco IOS software.
Restrictions for IP SLAs VoIP UDP Jitter Operations
This feature uses UDP traffic to generate approximate Voice over IP scores. It does not provide support for the Real-Time Transport Protocol (RTP).
Note
ICPIF and MOS values provided by this feature, while consistent within IP SLAs, are estimates only and are intended only for relative comparisons. The values may not match values determined using other methods.
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Information About IP SLAs VoIP UDP Jitter Operations
To use the IP SLAs VoIP UDP Operation feature, you should understand the following concepts:
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The Calculated Planning Impairment Factor (ICPIF)
The ICPIF originated in the 1996 version of ITU-T recommendation G.113, "Transmission impairments," as part of the formula Icpif = Itot - A. ICPIF is actually an acronym for "(Impairment) Calculated Planning Impairment Factor," but should be taken to simply mean the "calculated planning impairment factor." The ICPIF attempts to quantify, for comparison and planning purposes, the key impairments to voice quality that are encountered in the network.
The ICPIF is the sum of measured impairment factors (total impairments, or Itot) minus a user-defined access Advantage Factor (A) that is intended to represent the user's expectations, based on how the call was placed (for example, a mobile call versus a land-line call). In its expanded form, the full formula is expressed as:
Icpif = Io + Iq + Idte + Idd + Ie - A
where
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ICPIF values are expressed in a typical range of 5 (very low impairment) to 55 (very high impairment). ICPIF values numerically less than 20 are generally considered "adequate." While intended to be an objective measure of voice quality, the ICPIF value is also used to predict the subjective effect of combinations of impairments. Table 1, taken from G.113 (02/96), shows how sample ICPIF values are expected to correspond to subjective quality judgement.
For further details on the ICPIF, see the 1996 version of the G.113 specification.
Note
The full E-Model (also called the ITU-T Transmission Rating Model), expressed as R = Ro - Is - Id - Ie + A, provides the potential for more accurate measurements of call quality by refining the definitions of impairment factors (see the 2003 version of the G.107 for details). Though the ICPIF shares terms for impairments with the E-Model, the two models should not be confused.
The IP SLAs VoIP UDP Operation feature takes advantage of observed correspondences between the ICPIF, transmission rating factor R, and MOS values, but does not yet support the E-Model.IP SLAs uses a simplified ICPIF formula, defined in more detail later in this document.
Mean Opinion Scores (MOS)
The quality of transmitted speech is a subjective response of the listener. Each codec used for transmission of Voice over IP provides a certain level of quality. A common benchmark used to determine the quality of sound produced by specific codecs is MOS. With MOS, a wide range of listeners have judged the quality of voice samples sent using particular codecs, on a scale of 1 (poor quality) to 5 (excellent quality). The opinion scores are averaged to provide the mean for each sample. Table 2 shows MOS ratings and the corresponding description of quality for each value.
As the MOS ratings for codecs and other transmission impairments are known, an estimated MOS can be computed and displayed based on measured impairments. This estimated value is designated as MOS-CQE (Mean Opinion Score; Conversational Quality, Estimated) by the ITU in order to distinguish it from objective or subjective MOS values (see P.800.1 for details).
Voice Performance Monitoring Using IP SLAs
One of the key metrics in measuring voice and video quality over an IP network is jitter. Jitter is the name used to indicate the variation in delay between arriving packets (inter-packet delay variance). Jitter affects voice quality by causing uneven gaps in the speech pattern of the person talking. Other key performance parameters for voice and video transmission over IP networks include latency (delay) and packet loss. IP SLAs is an embedded active monitoring feature of Cisco IOS software that provides a means for simulating and measuring these parameters in order to ensure your network is meeting or exceeding service-level agreements with your users.
IP SLAs provides a UDP jitter operation, which consists of UDP probe packets sent across the network from an origin device to a specific destination (called the operational target). This synthetic traffic is used to record the amount of jitter for the connection, as well as the round-trip time, per-direction packet loss, and one-way delay time (one-way latency). (The term "synthetic traffic" indicates that the network traffic is simulated; that is, the traffic is generated by IP SLAs.) Data, in the form of collected statistics, can be displayed for multiple tests over a user-defined period of time, allowing you to see, for example, how the network performs at different times of the day, or over the course of a week. The jitter probe has the advantage of utilizing the IP SLAs Responder to provide minimal latency at the receiving end.
The IP SLAs VoIP UDP jitter operation modifies the standard UDP jitter operation by adding the capability to return MOS and ICPIF scores in the data collected by the operation, in addition to the metrics already gathered by the UDP jitter operation. This VoIP-specific implementation provides even more useful information in determining the performance of your VoIP network, thereby improving your ability to perform network assessment, troubleshooting, and health monitoring.
Codec Simulation Within IP SLAs
The IP SLAs VoIP UDP jitter operation computes statistics by sending n UDP packets, each of size s, sent t milliseconds apart, from a given source router to a given target router, at a given frequency f. The target router must be running the IP SLAs Responder in order to process the probe operations.
To generate MOS and ICPIF scores, you specify the codec type used for the connection when configuring the VoIP UDP jitter operation. Based on the type of codec you configure for the operation, the number of packets (n), the size of each payload (s), the inter-packet time interval (t), and the operational frequency (f) will be auto-configured with default values. (See Table 3 for specifics.) However, you are given the option, if needed, to manually configure these parameters in the syntax of the udp-jitter command.
Table 3 shows the default parameters that are configured for the operation by codec.
For example, if you configure the VoIP UDP jitter operation to use the characteristics for the g711ulaw codec, by default a probe operation will be sent once a minute (f). Each probe operation would consist of 1000 packets (n), with each packet containing 180 bytes of synthetic data (s), sent 20 milliseconds apart (t).
The IP SLAs ICPIF Value
ICPIF value computation with Cisco IOS software is based primarily on the two main factors that can impair voice quality: delayed packets and lost packets. Because packet delay and packet loss can be measured by IP SLAs, the full ICPIF formula, Icpif = Io + Iq + Idte + Idd + Ie - A, is simplified by assuming the values of Io, Iq, and Idte are zero, resulting in the following formula:
Total Impairment Factor (Icpif) = Delay Impairment Factor (Idd) + Equipment Impairment Factor (Ie) - Expectation/Advantage Factor (A)
This means that the ICPIF value is computed by adding a Delay Impairment Factor, which is based on a measurement of delayed packets, and an Equipment Impairment Factor, which is based on a measurement of lost packets. From this sum of the total impairments measured in the network, an impairment variable (the Expectation Factor) is subtracted to yield the ICPIF.
This is the same formula used by Cisco Gateways to calculate the ICPIF for received VoIP data streams.
The Delay Impairment Factor
The Delay Impairment Factor (Idd) is a number based on two values. One value is fixed and is derived using the static values (as defined in the ITU standards) for Codec Delay, Look Ahead Delay, and Digital Signal Processing (DSP) Delay. The second value is variable and is based on the measured one-way delay (round-trip time measurement divided by 2). The one-way delay value is mapped to a number using a mapping table that is based on a G.107 (2002 version) analytic expression. Table 4 shows sample correspondences between the one-way delay measured by IP SLAs and Delay Impairment Factor values.
Table 4 Sample Correspondence of One-Way Delay to ICPIF Delay Impairment
50
1
100
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150
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200
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The Equipment Impairment Factor
The Equipment Impairment Factor (Ie) is a number based on the amount of measured packet loss. The amount of measured packet loss, expressed as a percentage of total number of packets sent, corresponds an Equipment Impairment Factor that is defined by codec. Table 5 shows sample correspondences between the packet loss measured by IP SLAs and Equipment Impairment Factor values.
The Expectation Factor
The Expectation Factor, also called the Advantage Factor (A), is intended to represent the fact that users may accept some degradation in quality in return for ease of access. For example, a mobile phone user in a hard-to-reach location may have an expectation that the connection quality will not be as good as a traditional land-line connection. This variable is also called the Advantage Dactor (short for Access Advantage Factor) because it attempts to balance an increased access advantage against a decline in voice quality.
Table 6, adapted from ITU-T Rec. G.113, defines a set of provisional maximum values for A in terms of the service provided.
These values are only suggestions. To be meaningful, the use of the factor A and its selected value in a specific application should be used consistently in any planning model you adopt. However, the values in Table 6 should be considered as the absolute upper limits for A.
The default Advantage Factor for IP SLAs VoIP UDP jitter operations is always zero.
The IP SLAs MOS Value
IP SLAs uses an observed correspondence between ICPIF and MOS values to estimate an MOS value. Usage of the abbreviation MOS within the context of this feature should be taken to represent the MOS-CQE (Mean Opinion Score; Conversational Quality, Estimated).
The E model, as defined in G.107 (03/2003), predicts the subjective quality that is experienced by an average listener by combining the impairment caused by transmission parameters (such as loss and delay) into a single rating, the transmission rating factor R (the R Factor). This rating, expressed in a scale of 0 (worst) to 100 (best) can be used to predict subjective user reactions, such as the MOS. Specifically, the MOS can be obtained from the R Factor with a converting formula. Conversely, a modified inverted form can be used to calculate R Factors from MOS values.
There is also a relationship between the ICPIF value and the R Factor. IP SLAs takes advantage of this correspondence by deriving the approximate MOS score from an estimated R Factor, which, in turn, is derived from the ICPIF score. Table 7 shows the resulting MOS values that will be generated for corresponding ICPIF values.
Table 7 Correspondence of ICPIF Values to MOS Values
0 - 3
5
Best
4 - 13
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High
14 - 23
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Medium
24 - 33
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Low
34 - 43
1
Poor
IP SLAs will always express the estimated MOS value as a number in the range of 1 to 5, with 5 being the best quality. A MOS value of 0 (zero) indicates that MOS data could not be generated for the operation.
How to Configure the IP SLAs VoIP UDP Jitter Operation
This section contains the following procedure:
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Configuring the IP SLAs VoIP UDP Jitter Operation
Perform this task to return VoIP scores with IP SLAs VoIP UDP jitter operation statistics.
The VoIP-specific implementation of the IP SLAs UDP jitter operation contains different configuration options than the standard UDP jitter operation. As soon as you specify the codec keyword in the udp-jitter command syntax, you are configuring the VoIP-specific implementation of the jitter operation.
Restrictions
Currently, IP SLAs supports only the following speech codecs (compression methods):
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The following commands, available in UDP jitter configuration mode, are not valid for UDP jitter (codec) operations:
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The show ip sla configuration command will list the values for the "Number of statistic distribution buckets kept" and "Statistic distribution interval (milliseconds)," but these values do not apply to jitter (codec) operations.
Note
The flow-label command defines the value in the flow label field in the IPv6 header for a supported IP SLAs operation and is valid only in IPv6 networks.
SUMMARY STEPS
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DETAILED STEPS
Troubleshooting Tips
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What to Do Next
To view and interpret the results of an IP SLAs operation use the show ip sla statistics command. Checking the output for fields that correspond to criteria in your service level agreement will help you determine whether the service metrics are acceptable.
Configuration Examples for IP SLAs VoIP UDP Jitter Operations
In the following examples, a VoIP UDP jitter (codec) operation is configured, then output from the corresponding show commands is given. This example assumes that the IP SLAs Responder is enabled on the device at 209.165.200.225.
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IP SLAs VoIP UDP Operation Configuration: Example
Router> enablePassword:Router# configure terminalEnter configuration commands, one per line. End with the end command.Router(config)# ip sla 10Router(config-sla)# udp-jitter 209.165.200.225 16384 codec g711alaw advantage-factor 2Router(config-sla-jitter)# owner admin_bofhRouter(config-sla-jitter)# exitRouter(config)# ip sla schedule 10 start-time nowRouter(config)# exitRouter#Router# show running-config | begin ip sla 10ip sla 10udp-jitter 209.165.200.225 16384 codec g711alaw advantage-factor 2owner admin_bofhip sla schedule 10 start-time now...Router# show ip sla configuration 10Entry number: 10Owner: admin_bofhTag:Type of operation to perform: jitterTarget address: 209.165.200.225Source address: 0.0.0.0Target port: 16384Source port: 0Operation timeout (milliseconds): 5000Codec Type: g711alawCodec Number Of Packets: 1000Codec Packet Size: 172Codec Interval (milliseconds): 20Advantage Factor: 2Type Of Service parameters: 0x0Verify data: NoVrf Name:Control Packets: enabledOperation frequency (seconds): 60Next Scheduled Start Time: Start Time already passedLife (seconds): 3600Entry Ageout (seconds): neverStatus of entry (SNMP RowStatus): ActiveConnection loss reaction enabled: NoTimeout reaction enabled: NoVerify error enabled: NoThreshold reaction type: NeverThreshold (milliseconds): 5000Threshold Falling (milliseconds): 3000Threshold Count: 5Threshold Count2: 5Reaction Type: NoneNumber of statistic hours kept: 2Number of statistic distribution buckets kept: 1Statistic distribution interval (milliseconds): 20Enhanced History:When a codec type is configured for a jitter operation, the standard jitter "Request size (ARR data portion)," "Number of packets," and "Interval (milliseconds)" parameters will not be displayed in the show ip sla configuration command output. Instead, values for "Codec Packet Size," "Codec Number of Packets," and "Codec Interval (milliseconds)" are displayed.
IP SLAs VoIP UDP Operation Statistics Output: Example
Use the show ip sla statistics command to display Voice scores (ICPIF and MOS values) for the jitter (codec) operation.
Router# show ip sla statistics 10Entry number: 10Modification time: 12:57:45.690 UTC Sun Oct 26 2003Number of operations attempted: 1Number of operations skipped: 0Current seconds left in Life: ForeverOperational state of entry: ActiveLast time this entry was reset: NeverConnection loss occurred: FALSETimeout occurred: FALSEOver thresholds occurred: FALSELatest RTT (milliseconds): 19Latest operation start time: 12:57:45.723 Sun Oct 26 2003Latest operation return code: OK!Voice Scores:ICPIF: 20 MOS Score: 3.20!RTT Values:NumOfRTT: 10 RTTAvg: 19 RTTMin: 19 RTTMax: 20RTTSum: 191 RTTSum2: 3649Packet Loss Values:PacketLossSD: 0 PacketLossDS: 0PacketOutOfSequence: 0 PacketMIA: 0 PacketLateArrival: 0InternalError: 0 Busies: 0Jitter Values:NumOfJitterSamples: 9MinOfPositivesSD: 0 MaxOfPositivesSD: 0NumOfPositivesSD: 0 SumOfPositivesSD: 0 Sum2PositivesSD: 0MinOfNegativesSD: 0 MaxOfNegativesSD: 0NumOfNegativesSD: 0 SumOfNegativesSD: 0 Sum2NegativesSD: 0MinOfPositivesDS: 1 MaxOfPositivesDS: 1NumOfPositivesDS: 1 SumOfPositivesDS: 1 Sum2PositivesDS: 1MinOfNegativesDS: 1 MaxOfNegativesDS: 1NumOfNegativesDS: 1 SumOfNegativesDS: 1 Sum2NegativesDS: 1Interarrival jitterout: 0 Interarrival jitterin: 0One Way Values:NumOfOW: 0OWMinSD: 0 OWMaxSD: 0 OWSumSD: 0 OWSum2SD: 0OWMinDS: 0 OWMaxDS: 0 OWSumDS: 0 OWSum2DS: 0Where to Go Next
For information about other types of IP SLAs operations and IP SLAs features, see the Cisco IOS IP SLAs Features Roadmap.
Additional References
The following sections provide references related to the IP SLAs VoIP UDP Jitter Operation feature.
Related Documents
Voice over IP (VoIP) codecs
"Understanding Codecs: Complexity, Hardware Support, MOS, and Negotiation"
http://www.cisco.com/en/US/tech/tk1077/technologies_tech_note09186a00800b6710.shtml
Jitter in Packet Voice Networks
"Understanding Jitter in Packet Voice Networks (Cisco IOS Platforms)"
http://www.cisco.com/en/US/tech/tk652/tk698/technologies_tech_note09186a00800945df.shtml
Cisco IOS IP SLAs command-line interface enhancements
Cisco IOS IP Service Level Agreements Command Line Interface, Cisco white paper
Cisco IOS IP SLAs commands
PSTN Fallback for Voice Gateways
"SIP: Measurement-Based Call Admission Control for SIP"
http://www.cisco.com/en/US/docs/ios/12_2t/12_2t15/feature/guide/ftcacsip.html
Standards
Full support for these standards is not claimed.
ITU Telecommunication Standards ("ITU-T Recommendations In Force") can be obtained from http://www.itu.ch. Summary definitions are available from a variety of internet sources.
MIBs
CISCO-RTTMON-MIB
To locate and download MIBs for selected platforms, Cisco IOS releases, and feature sets, use Cisco MIB Locator found at the following URL:
RFCs
RFC 768
User Datagram Protocol
RFC 1889
RTP: A Transport Protocol for Real-Time Applications
1 Full support by this feature for listed RFCs is not claimed.
Technical Assistance
Feature Information for the IP SLAs VoIP UDP Jitter Operation
Table 8 lists the release history for this feature.
Not all commands may be available in your Cisco IOS software release. For release information about a specific command, see the command reference documentation.
Use Cisco Feature Navigator to find information about platform support and software image support. Cisco Feature Navigator enables you to determine which Cisco IOS and Catalyst OS software images support a specific software release, feature set, or platform. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.
Note
Glossary
codec—In the context of IP Telephony, a codec is a compression and decompression algorithm used to transfer voice and video data more efficiently. Voice codec types are typically referred to using the ITU recommendation number that defines the algorithm (for example, "G.711" instead of "PCM").
CS-ACELP—The codec type defined in the reference documents G.729 and G.729A, Coding of speech at 8 kbit/s using conjugate-structure algebraic-code-excited linear-prediction (CS-ACELP).
ITU—The International Telecommunication Union. The ITU is an international organization within the United Nations System where governments and the private sector coordinate global telecom networks and services. The ITU Telecommunication Standardization Sector (ITU-T), responsible for defining standards (Recommendations) covering all fields of telecommunications, is one of the three operational sectors of the ITU. The ITU web site is at http://www.itu.int.
ITU-T—ITU Telecommunication Standardization Sector. The ITU-T is one of the three operational sectors of the ITU, and is responsible for defining standards (called ITU-T Recommendations) covering all fields of telecommunications.
MOS-CQE (Mean Opinion Score; Conversational Quality, Estimated)—The score calculated by a network planning model which aims at predicting the quality in a conversational application situation. Estimates of conversational quality carried out according to ITU-T Rec. G.107, when transformed to a mean opinion score (MOS), give results in terms of MOS-CQE.1
PCM—The codec type defined in the reference document G.711, Pulse code modulation (PCM) of voice frequencies.
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Any Internet Protocol (IP) addresses used in this document are not intended to be actual addresses. Any examples, command display output, and figures included in the document are shown for illustrative purposes only. Any use of actual IP addresses in illustrative content is unintentional and coincidental.
© 2006-2008 Cisco Systems, Inc. All rights reserved.1 Definition from ITU-T Recommendation P.800.1. Used in accordance with the ITU Copyright and Disclaimer Notice.
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