Your Guide to Troubleshooting VoIP
Contents
1. VoIP quality has set up a Network Instruments 10 100 1000 Probe Appliance on the core switch to monitor all call manager activity and any VoIP traffic traversing the link He has configured Observer to send him an e mail whenever any of the following conditions arise e MOS falls to 3 5 or less UN it e Jitter levels crossing the MPLS mesh exceed 20 ms S a e Delay levels crossing the MPLS mesh exceed 80 ms i i g F directions at once Any of these conditions are indications that VolP quality is threatened Given the topology involved the most likely source of problems is the MPLS mesh routers which are under the service provider s control By digging deeper into the statistics the analyzer provides you can determine why the MOS is falling and what is causing jitter delay or packet loss Coleman Jennings Senior Network Engineer AAA East Central If jitter is the problem a good place to start is by comparing jitter levels against bandwidth utilization to see if there is any correlation The analysis shown below taken from Network Instruments Observer Expert shows just such a correlation F g lea er af Wa E i Station 1 to Station 2 Station 2 to Station 1 Bandwidth Utilization HAH ci Comparing Jitter to Bandwidth Utilization As this scenario shows such a link between spikes in bandwidth and jitter could mean it is time to invest in more bandwidth or time to put more controls on employee Inter2. problem resolution Utilize Expert Analysis on VolP communication to quickly pinpoint the source of common VolP problems 10 Baseline network traffic For comprehensive understanding of VolP traffic capture and store long term network data Only with critical trending data can you accurately perform baselining activities Baselining validates VolP performance helps future capacity planning efforts and provides long term understanding of VolP health CI Travel s VoIP Savings High phone bills were eating up CI Travel s profits They decided to take advantage of VolP technology to reduce per call expenses The new VolP phones while dramatically reducing per call costs came with new problems To make the investment pay off Ingram required tools to monitor and troubleshoot the VolP exchange So far Observer s VoIP capabilities have helped cut CI Travel s phone bill by 25 to 30 percent Paul Ingram Director of IT Cl Travel isa 150 million division of Cruise International Summary of VoIP statistics and quality metrics The following table summarizes the statistics and quality measurements discussed in this paper both defining what is measured and describing its relevance Jitter Delay Packet loss Bursts Gaps Average call setup teardown Codec Mean Opinion Score MOS R factor NETWORK INSTRUMENTS Jitter measures the variability of delay in packet arrival times In spite of
3. the jitter buffers used to counteract jitter at excessive levels it can interfere with smooth playback and cause packets to be dropped The amount of time it takes a packet to reach its destination Whenever packets travel a network some delay is inevitable For real time telephone conversations there is a one way delay budget of approximately 150 ms The percentage of packets that did not reach their destination Periods characterized by high rates of packet loss The burst percentage is the percentage of time that the call experienced high rate packet loss the burst density is the actual percentage rate of packet loss during bursts Periods characterized by low rates of packet loss The gap percentage is the percentage of time that the call experienced low rate packet loss the gap density is the actual percentage rate of packet loss during the gaps An average of how long it is taking the call manager to open and close calls The compression decompression method that was used for the call Starting with a theoretical perfect score of 5 excellent impairment factors such as codec delay jitter and packet loss are used to calculate how a typical user would rate voice quality Similar to MOS this scale ranges from 1 100 Corporate Headquarters NETWORK INSTRUMENTS WHITE PAPER By using triggers to notify you when jitter levels are reaching a point that threaten voice quality you can examine your router
4. East Coast o Office How Call Quality Relates to User Satisfaction R MOS 100 Very Satisfied 90 Satisfied 80 Some Users Dissatisfied 70 m Many Users Dissatisfied Nearly All Users Dissatisfied 50 Not Recommended 0 MOS and R factor are used to gauge user satisfaction with call quality MOS levels under 3 5 and R factor below 80 mean trouble NETWORK INSTRUMENTS WHITE PAPER Where to place probes on such a network depends on what you want or need to see If you need access to all local conversations on either coast including both call setup and actual voice data use a SPAN session on the access layer switch to mirror VoIP traffic to the analyzer Assigning all VoIP traffic to a dedicated VLAN makes this fairly straightforward Capturing Local IP Phone Traffic Shows e Any phone s communications with its local call manager e Both sides of the full duplex connection between local phones talking to each other e Both sides of the full duplex connection between phones located on opposite coasts West Coast VoIP Call Manager il VolP Call Manager Analyzer z Probe a Connection visible to analyzer A Connection hidden from analyzer East Coast Office What you will not be able to see from this probe is any communications between the East Coast and the call manager located on the West Coast If you are more interested in a coherent view of calls between the West Coast and East C
5. Increasing jitter buffer size deployment and had can help but only to a point plans for implementing other critical network Because no network can guarantee a perfectly steady stream of packets under real world applications conditions VolP phones use jitter buffers to smooth out the kinks A jitter buffer is simply a First In First Out FIFO memory cache that collects the packets as they arrive forwarding We really needed them to the codec evenly spaced and in proper sequence for accurate playback some type of network Jitter buffering and packet loss concealment analyzer to monitor gt and maintain network performance Network T Jack King d Director of I T Jitter buffer Codec an a7 Makd gt gt 01001101 VoIP packets can arrive at the receiving phone out of sequence late early or not at all IP phones use a jitter buffer to reconstruct the packet stream at the receiving end duplicating missing packets or filling in with white comfort noise when necessary While a jitter buffer can successfully mask mild delay and jitter problems severe jitter can overwhelm the jitter buffer which results in packet loss see below Increasing the size of the jitter buffer can help but only to a point A jitter buffer that increases overall round trip delay to 300 ms will make normal conversation difficult Packet Loss As mentioned above packet loss can be the result of the jitter buffer being overwhelmed Other reasons
6. NETWORK INSTRUMENTS WHITE PAPER Your Guide to Troubleshooting VoIP VoIP s extreme sensitivity to delay and packet loss compared to other network applications such as web and e mail services presents a real challenge A basic understanding of VoIP traffic and of the quality metrics provided by VoIP monitoring tools will help keep your network running smoothly NETWORK INSTRUMENTS www networkinstruments com NETWORK INSTRUMENTS WHITE PAPER Your Guide to VoIP This white paper guides you through the essentials of VoIP troubleshooting including A major Midwest health common problems and the metrics you should employ to fix and prevent them You ll learn provider recently undertook a how these metrics relate to one another and to overall VoIP health You ll also learn the best major VoIP deployment practices for keeping your VoIP network flowing smoothly your time to resolution low and end users happy The hospital used Observer Expert to conduct network assessments to benchmark network performance Network Jitter and Delay before implementing VoIP Real time voice communications are sensitive to delay and variation in packet arrival times Codecs require a steady dependable stream of packets to provide reasonable playback quality Packets arriving too early too late or out of sequence result in jerky jumbled Ml playback This phenomenon is called jitter We were in the middle of a significant VolP
7. ctives place your analysis tools to ensure optimal visibility of VolP communications 5 Implement VLANs to isolate and monitor VoIP issues Organize your VoIP traffic by VLAN user groups This practice will greatly simplify problem resolution 6 Monitor rollouts to ensure a positive user experience Determine whether users are receiving a positive experience by reviewing cumulative VoIP metrics codecs and other network performance variables during VoIP deployment By evaluating VLAN setups and overall link utilization you can judge overall network performance and quickly make adjustments during implementation 7 Compare jitter to overall network bandwidth utilization to understand response time When jitter becomes a problem look at the big picture A correlation between jitter and bandwidth usage means the problem is overall network usage If there is no direct correlation excessive jitter might be caused by isolated network factors that require further investigation 8 Set up your analyzer to proactively monitor VoIP activity Utilize monitoring and notification tools to soeed problem resolution Determine norma or acceptable levels of activity for your network and its users Then set up thresholds within your analyzer to alert you when thresholds are broken or in danger 9 Automate problem resolution Expert Analysis functionality eliminates unnecessary trial and error when troubleshooting VolP issues by automating
8. factor falls below 80 it s likely that you have more than a few dissatisfied users If you see these statistics trending downward it s time to examine more detailed analysis to determine what is going wrong Network Instruments LLC 10701 Red Circle Drive e Minnetonka MN 55343 USA toll free 800 526 7919 telephone 952 358 3800 fax 952 358 3801 www networkinstruments com 2007 Network Instruments LLC All rights reserved Network Instruments Observer GigaStor NI DNA and all associated logos are trademarks or registered trademarks of Network Instruments LLC All other trademarks registered or unregistered are sole property of their respective owners October 2007
9. hitecture Observer s VoIP Expert will monitor VoIP traffic even over wireless networks NETWORK INSTRUMENTS WHITE PAPER Top 10 VoIP Best Practices Our sales engineers have put together this valuable list of the best practices to use when implementing or troubleshooting VoIP Keeping these steps in mind will help you get the most from your VoIP network 1 Understand and measure call quality components There are a variety of metrics you can use to assess VoIP call quality including jitter MOS R Factor gap density burst density Quality of Service prioritization and compression techniques Ensure you are accurately analyzing VoIP communication by learning how to measure these attributes 2 Implement Quality of Service prioritization Incorrectly set QoS precedence for VoIP traffic leads to delays in packet delivery and reduced call quality 3 Conduct site surveys The more you know about your network the better prepared you are to properly integrate VoIP Conduct a site survey to review current WAN bandwidth levels traffic flows and existing switches for bottlenecks and choke points Then identify or determine specific needs through testing and modeling 4 Deploy analysis tools strategically for maximum visibility Placing network analysis consoles and probes on your network requires a clear understanding of VoIP traffic patterns Are you concerned with monitoring VoIP traffic locally over WAN links both Depending on your obje
10. include landline media failure and poor wireless signal quality The latter can be a big problem with VoFi Voice over WiFi service Regardless of the source VoIP phones and gateways attempt to conceal this type of signal degradation by duplicating packets to fill in the missing data As with jitter these techniques can maintain voice quality only to a point Packet loss on data networks has long been characterized as a bursty phenomenon which is another way of saying it never rains it pours Networks tend to either sporadically drop single packets these periods are called gaps in packet loss or large numbers of contiguous packets in a burst Packet loss concealment techniques typically have no problem handling packet loss during gap periods it is the sustained bursts you must watch out for Call Management Problems If the VoIP call manager sometimes called the VoIP server is overwhelmed with requests or its connection to the network is impaired call setup delays can reach the point where users abandon calls before they are able to connect to the other party If IP phones are misconfigured or their IP connection to the server is impaired calls remain open in the call queue long after the parties have disconnected NETWORK INSTRUMENTS WHITE PAPER Managing VoIP Quality You can manage only what you can measure Managing a VoIP deployment therefore requires some hard numbers beyond subjective user assessments of
11. ity Plotting jitter in milliseconds Monitor Quality of Service QoS Observer s VoIP Expert reports QoS levels by call packet and protocol This analysis also shows percentage of VoIP utilization compared to other network traffic allowing you to plan network upgrades Evaluate Jitter Observer s VoIP Expert displays the statistical variance of packet arrival times known as jitter measured in timestamp units or RTP time units Measure Bursts and Gaps Observer s VoIP Expert provides comprehensive metrics for monitoring bursts or periods of high packet loss and gaps or periods of little packet loss Burst and gap percentages quickly indicate the quality of a call Track and Decode VoIP and Video Observer offers complete VoIP and video decodes including H 323 Session Initiation Protocol SIP MGCP and SCCP Cisco skinny Observer also supports Avaya CCMS Nortel UNIStim and Mitel systems Reconstruct and Review VoIP Calls Capture and reconstruct calls for playback so you can hear the same call quality issues experienced by users Compare VoIP to Network Performance Use Observer to identify whether jitter or delay is being caused by other applications on the network Observer will track VoIP issues along with overall network performance Monitor VoWLAN Observer s VoIP enhancements are automatically available across multiple topologies thanks to the Network Instruments Distributed Network Analysis NI DNA arc
12. ll down on specific conversations for an in depth view VoIP Expert Analysis Features Convenient at a glance summaries e VoIP Traffic Summary e Call Summary e Voice Quality Scoring e Precedence QoS The Network Instruments tools let me reconstruct an entire VoIP conversation and hear it real time This tool set allows me to have eyes into a customer s network Ivan McDuffie NEC Unified Solutions NETWORK INSTRUMENTS WHITE PAPER Detailed views with in depth Call Detail Records e Addresses e Number of packets packet bytes packet loss e Start time initial setup duration duration Jitter ms e Current jitter maximum jitter e MOS R factor e QoS for each call e Number of packets that arrive out of order e Detailed analysis for packet loss and delay e Gap and burst measurements e Long term Call Detail Records trending Quickly Solve Problems with over 50 VoIP Experts Observer offers over 50 event based and threshold based VoIP Experts to immediately flag ets Eyes pa to siti problems for faster problem resolution Examples include Sre OHNI OOD e Alarms for unacceptable jitter level e Lost packets Alterations in the QoS stream Wao Q0 Q00 4507079 92118 c 008 BT 16 lo 343 10 000 Manage VoIP Audio Quality Observer displays individual Mean Opinion Score MOS and R factor for individual calls and as an average Place alarms on this score to proactively manage overall VoIP qual
13. net usage for applications such as streaming media and peer to peer file sharing unrelated to business If there isn t an obvious correlation between jitter and bandwidth utilization the depth of data provided by an all purpose network analysis and monitoring tool can help you dig deeper for the correct diagnosis For example if VolP traffic across an MPLS mesh is subject to excessive jitter it could be the result of route flapping on the service provider s routers An analyzer can confirm and document this Armed with the hard data provided by analysis you could then contact the service provider so they can address the problem If delay across the mesh exceeds the contractual obligations of the Service Level Agreement SLA the provider may owe your organization some refunds for service failure in addition to being responsible for fixing the problem NETWORK INSTRUMENTS WHITE PAPER Troubleshooting Connection Problems When a user can t get a dial tone or if there are excessive delays in ringing the other party s phone examining a graphical display of how the call is progressing between the parties and the call manager can indicate what is going wrong Network Instruments VoIP Expert displays just such a diagram simply right click on any call or connection stream Because differing protocols dictate differing phone call manager interactions some knowledge of the protocol is necessary for detailed troubleshooting But even if
14. note that VoIP support means more than just decoding the packets of various VoIP protocols it also means being able to track and display network delay jitter and packet loss and to distill this information into overall quality scores both per call and in aggregate VoIP Support means more than just decoding the packets of various VoIP protocols And to be really useful to the enterprise the tool should also track store and analyze long term trends This is so that you can understand what is normal VoIP performance and maintain a database of Call Detail Records CDRs from which you can generate reports for management or service providers The VoIP monitoring tool should also be capable of automatically notifying you when selected statistics indicate a developing problem On all of these counts Network Instruments Observer meets the requirements VoIP Points of Visibility In switched environments where to deploy an analyzer or probe for maximum visibility isn t necessarily obvious Complicating matters for VoIP is the fact that each call includes both client server communications between IP phones and the call manager during setup and tear down and peer to peer the streams of voice data passed between the parties For example consider the following VoIP network deployment VoIP Call n Manager Access _ 8 Core Switch West Coast SS Office e Core Switch _ R Access To Sf
15. oast including all call manager communications use a SPAN session to mirror both the uplink traffic between the core and MPLS mesh and all traffic flowing to and from the call manager This will give you a coherent view of inter office calls along with all call manager communications both local and remote Office VolP Call Fi Manager Observer wins hands down Christian Wilson Network Administrator Select Comfort Access Switch a amp VoIP Call apr won ortS Manager SP age p Ca Man Fi Analyzer Probe Connection visible to analyzer ead Connection hidden from analyzer East Coast Office NETWORK INSTRUMENTS WHITE PAPER With a probe deployed in this manner you will not be able to see the peer to peer voice traffic between local phones For complete coverage connect probes to both the core and access layers at each site Another alternative is to deploy probes at the core 24 7 365 monitoring the access layer with a portable analyzer or software probe only to troubleshoot local call problems as needed VoIP Network Analysis How can VoIP analysis help manage quality By closely monitoring the network conditions that affect VoIP you can begin to address developing infrastructure problems before they result in user complaints or downtime Tracking Network Performance Consider the East Coast West Coast example described in the previous section Ken the administrator responsible for ensuring
16. quality although these are obviously important as well Beyond monitoring the network parameters discussed in this paper having an overall quality score such as a Mean Opinion Score MOS or R factor score can also be a useful VoIP network health index VoIP monitoring tools calculate the MOS and R factor scores using a formula known as the E model Using the statistics it has collected from the network the analyzer calculates how much the various impairment factors such as codec compression jitter delay and packet loss would affect the typical user s perception of call quality VoIP Specific vs All Purpose Monitoring Tools There are a number of different options on the market for managing VoIP quality mainly falling into three categories e Dedicated VoIP tools originally developed for the telecom industry These tools are great for testing IP phone and gateway designs but not as good at solving deployment problems on a live network e Network protocol analyzers that have added VoIP Support by licensing technology developed for the telecom industry and integrating it into their product line e Network monitoring tools that approach VoIP quality management from an IT administrator s point of view rather than from that of telecom engineer To the IT administrator managing VoIP quality is just another network task This makes the third approach the all purpose network monitoring tool often the most practical choice But
17. s for problems or contact your service provider and help them solve the problem As with jitter using automatic notifications to actively manage levels of delay can prevent the problem from escalating to the point where users complain Sporadic packet loss is usually insignificant However sustained bursts see the next item can affect quality VoIP phones have no trouble masking a lost packet here and there by duplicating the previous packet or filling longer silences with white noise But users will notice sustained bursts If VoIP traffic has been assigned proper QoS and has enough bandwidth the most likely culprit is media failure Usually not significant as packet loss concealment technologies are usually successful in masking the effects of low level packet loss Contrast with bursts described above A spike in these statistics can indicate a problem with the call manager or its connectivity to the network Different codecs are capable of different levels of quality sound reproduction Higher compression comes at the cost of lower quality but may be necessary given the bandwidth available to the call If it seems as if the codecs in use are using more compression than necessary or not enough given the amount of bandwidth available perhaps the VoIP phones can be reconfigured to use a different codec These are useful as quick overall indicators of VoIP health If the average MOS falls below 3 5 or the average R
18. you lack a detailed knowledge of the protocol the Connection Dynamics display highlights which party isn t responding or which party is responding slowly Call manager IP phone A IP phone B Picks up handset Call manager turns on dialtone and displays message on IP phone A Dials number Acknowledgement that phone B is ringing Picks up handset Ringing IP phone B O Conversation begins An example of a Connection Dynamics display showing a VolP call using the SCCP protocol It is easy to see how such a diagram is essential to efficiently troubleshoot VolP connection problems Better manage and troubleshoot VoIP across your network using Observer s VoIP Expert available in Observer Expert and Observer Suite Whether you re interested in the big picture or a specific conversation Observer offers real time statistics Expert VolP analysis and reports to help you address all levels of VolP traffic Over 70 VolP specific metrics Understanding VolP performance is the key to accurate troubleshooting Observer provides visibility into the network application and VolP traffic to quickly resolve issues VoIP Expert summaries provide easy understanding of overall VoIP network health Over 70 VoIP specific metrics like call quality can be tracked on an aggregate or per call basis Take advantage of over 50 VoIP Expert events for immediate problem identification Once you ve identified the issue dri
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