Configuring Cisco IOS IP SLAs UDP Jitter Operations for VoIP
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Contents
Configuring IP SLAs UDP Jitter Operations for VoIPLast Updated: August 27, 2012
This document describes how to configure an IP Service Level Agreements (SLAs) User Datagram Protocol (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 devices in the network.
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Information About IP SLAs UDP Jitter Operations for VoIP
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
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. The table below, 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. 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. The table below 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 SLAsOne 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 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 SLAsThe 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 the table below for specifics.) However, you are given the option, if needed, to manually configure these parameters in the syntax of theudp-jitter command. The table below 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 ValueICPIF value computation with Cisco 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 FactorThe 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. The table below shows sample correspondences between the one-way delay measured by IP SLAs and Delay Impairment Factor values. The Equipment Impairment FactorThe 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. The table below shows sample correspondences between the packet loss measured by IP SLAs and Equipment Impairment Factor values. The Expectation FactorThe 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 Factor (short for Access Advantage Factor) because it attempts to balance an increased access advantage against a decline in voice quality. The table below, 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 the table above 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 ValueIP 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. The table below shows the resulting MOS values that will be generated for corresponding ICPIF values.
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 IP SLAs UDP Jitter Operations for VoIPConfiguring and Scheduling an IP SLAs VoIP UDP Jitter OperationDETAILED STEPS Troubleshooting Tips
What to Do NextTo add proactive threshold conditions and reactive triggering for generating traps (or for starting another operation) to an IP SLAs operation, see the "Configuring Proactive Threshold Monitoring" section. operation) To display and interpret the results of an IP SLAs operation, use the show ip sla statistics command. Check the output for fields that correspond to criteria in your service level agreement to determine whether the service metrics are acceptable. Configuration Examples for IP SLAs UDP Jitter Operations for VoIP
Example IP SLAs VoIP UDP Operation ConfigurationThe following example assumes that the IP SLAs Responder is enabled on the device at 209.165.200.225. Router> enable Password: Router# configure terminal Enter configuration commands, one per line. End with the end command. Router(config)# ip sla 10 Router(config-sla)# udp-jitter 209.165.200.225 16384 codec g711alaw advantage-factor 2 Router(config-sla-jitter)# owner admin_bofh Router(config-sla-jitter)# exit Router(config)# ip sla schedule 10 start-time now Router(config)# exit Router# Router# show running-config | begin ip sla 10 ip sla 10 udp-jitter 209.165.200.225 16384 codec g711alaw advantage-factor 2 owner admin_bofh ip sla schedule 10 start-time now . . . Router# show ip sla configuration 10 Entry number: 10 Owner: admin_bofh Tag: Type of operation to perform: jitter Target address: 209.165.200.225 Source address: 0.0.0.0 Target port: 16384 Source port: 0 Operation timeout (milliseconds): 5000 Codec Type: g711alaw Codec Number Of Packets: 1000 Codec Packet Size: 172 Codec Interval (milliseconds): 20 Advantage Factor: 2 Type Of Service parameters: 0x0 Verify data: No Vrf Name: Control Packets: enabled Operation frequency (seconds): 60 Next Scheduled Start Time: Start Time already passed Life (seconds): 3600 Entry Ageout (seconds): never Status of entry (SNMP RowStatus): Active Connection loss reaction enabled: No Timeout reaction enabled: No Verify error enabled: No Threshold reaction type: Never Threshold (milliseconds): 5000 Threshold Falling (milliseconds): 3000 Threshold Count: 5 Threshold Count2: 5 Reaction Type: None Number of statistic hours kept: 2 Number of statistic distribution buckets kept: 1 Statistic distribution interval (milliseconds): 20 Enhanced 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. Example IP SLAs VoIP UDP Operation Statistics OutputUse the show ip sla statistics command to display Voice scores (ICPIF and MOS values) for the jitter (codec) operation. Router# show ip sla statistics 10 Entry number: 10 Modification time: 12:57:45.690 UTC Sun Oct 26 2003 Number of operations attempted: 1 Number of operations skipped: 0 Current seconds left in Life: Forever Operational state of entry: Active Last time this entry was reset: Never Connection loss occurred: FALSE Timeout occurred: FALSE Over thresholds occurred: FALSE Latest RTT (milliseconds): 19 Latest operation start time: 12:57:45.723 Sun Oct 26 2003 Latest operation return code: OK ! Voice Scores: ICPIF: 20 MOS Score: 3.20 ! RTT Values: NumOfRTT: 10 RTTAvg: 19 RTTMin: 19 RTTMax: 20 RTTSum: 191 RTTSum2: 3649 Packet Loss Values: PacketLossSD: 0 PacketLossDS: 0 PacketOutOfSequence: 0 PacketMIA: 0 PacketLateArrival: 0 InternalError: 0 Busies: 0 Jitter Values: NumOfJitterSamples: 9 MinOfPositivesSD: 0 MaxOfPositivesSD: 0 NumOfPositivesSD: 0 SumOfPositivesSD: 0 Sum2PositivesSD: 0 MinOfNegativesSD: 0 MaxOfNegativesSD: 0 NumOfNegativesSD: 0 SumOfNegativesSD: 0 Sum2NegativesSD: 0 MinOfPositivesDS: 1 MaxOfPositivesDS: 1 NumOfPositivesDS: 1 SumOfPositivesDS: 1 Sum2PositivesDS: 1 MinOfNegativesDS: 1 MaxOfNegativesDS: 1 NumOfNegativesDS: 1 SumOfNegativesDS: 1 Sum2NegativesDS: 1 Interarrival jitterout: 0 Interarrival jitterin: 0 One Way Values: NumOfOW: 0 OWMinSD: 0 OWMaxSD: 0 OWSumSD: 0 OWSum2SD: 0 OWMinDS: 0 OWMaxDS: 0 OWSumDS: 0 OWSum2DS: 0 Additional ReferencesRelated Documents
Standards
1 Full support by this feature for listed RFCs 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.
MIBsRFCs
2 Full support by this feature for listed RFCs is not claimed.
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Feature Information for IP SLAs UDP Jitter Operations for VoIPThe following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature. Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.
Glossarycodec --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.3 PCM --The codec type defined in the reference document G.711, Pulse code modulation (PCM) of voice frequencies . Cisco and the Cisco logo are trademarks or registered trademarks of Cisco and/or its affiliates in the U.S. and other countries. To view a list of Cisco trademarks, go to this URL: www.cisco.com/go/trademarks. Third-party trademarks mentioned are the property of their respective owners. The use of the word partner does not imply a partnership relationship between Cisco and any other company. (1110R) Any Internet Protocol (IP) addresses and phone numbers used in this document are not intended to be actual addresses and phone numbers. Any examples, command display output, network topology diagrams, and other figures included in the document are shown for illustrative purposes only. Any use of actual IP addresses or phone numbers in illustrative content is unintentional and coincidental. 3 Definition from ITU-T Recommendation P.800.1. Used in accordance with the ITU Copyright and Disclaimer Notice. © 2012 Cisco Systems, Inc. All rights reserved.
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