SCTE New Jersey Chapter 9/13/07

SCTE New Jersey Chapter 9/13/07
VoIP Testing SCTE New Jersey Chapter 9/13/07

Regional Sales Engineer Sunrise Telecom 814.692.4294
My Business Card Larry Jump Regional Sales Engineer Sunrise Telecom

Today’s Agenda DOCSIS Troubleshooting Pyramid RF Impairments
DOCSIS Problems VoIP Impairments

Troubleshooting Methodology
SCTE: Practical Voice over Internet Protocol (VoIP) Troubleshooting Methodology Troubleshooting any DOCSIS network begins with a BOTTOM UP approach. Today, typically 80% of the problems are RF and 20% are IP/DOCSIS related. VoIP Lives Here Communications between CMs, MTAs, subscribers & IP servers Communications between CMTS & CMs only The DOCSIS Troubleshooting Pyramid is tool used to illustrate inter-relations of the RF plant, DOCSIS protocol and IP protocol. When we think about troubleshooting VoIP, we are actually saying that VoIP sits at the top of the pyramid. If something is broken with DOCSIS protocol (the CMTS or cable modem) then the top of the pyramid will crumble. Similarly if the RF plant is not functioning properly, the whole pyramid will collapse completely. To troubleshoot a VoIP network one has to think about the network in its entirety, not just the top of the pyramid. Notes ___________________________________________________________ What we fix today

Building Blocks of Troubleshooting
SCTE: Practical Voice over Internet Protocol (VoIP) Building Blocks of Troubleshooting Evolution of RF Troubleshooting RF impairments have typically been viewed as the root cause of all network problems in cable networks Since two-way data services have been introduced, cable operators have evolved the RF plant to higher and higher standards Although RF impairments are still readily present in cable networks, it is important to acknowledge that other impairments exist which must be addressed DOCSIS & IP Protocols > 20% of Impairments VoIP is transported by the DOCSIS network. A reliable DOCSIS network is critical to a reliable VoIP network. If DOCSIS is not working, forget about VoIP! DOCSIS consists of three main elements; the RF network, DOCSIS protocol and the IP network. Many people tend to think first about the RF network when troubles arise in DOCSIS and/or VoIP system – and with good reason, nearly 80% of impairments are RF-based. In the case that problems are not caused by the RF network, however, this can lead to costly troubleshooting in the wrong area. A critical first step in proper troubleshooting technique is using the proper tools to determine if a problem is RF-based or if the problem is protocol-based. This for certain will pay off in time, dollars, and customer satisfaction. Notes ___________________________________________________________ DHCP / TFTP / ToD / DNS / CMS / etc. servers Modem and MTA configuration files, CMTS configs

Build a Solid RF Foundation for the Pyramid
SCTE: Practical Voice over Internet Protocol (VoIP) Build a Solid RF Foundation for the Pyramid Sweep/balance for alignment Return path node certification and maintenance Home installation quality of workmanship and materials Conform to DOCSIS Specifications for signal quality

SCTE: Practical Voice over Internet Protocol (VoIP)
Forward Sweep Headend or Hub Site Forward Signal Path H L R Ensures Unity Gain in the Forward path. Unity Gain minimizes distortions on all forward signals. Ensures that entire frequency band arrives at the customer’s receive site with equal quality. Ensure good sweep and 95% of the other problems go away! In the forward path, signals originate at the headend and are transmitted to many customer drops. One output feeds many inputs. In the forward system the outputs of like devices are typically set to same output level. Each amplifier compensates for the cable and passive loss before it. This principal (Gain = Loss) is Unity Gain. Level changes in the forward path occur due to changes in cable loss caused by temperature. Automatic level or gain controls (ALC or AGC) are located in the amplifiers. A section of the alignment procedure for the forward path involves setting the operating point for the automatic control circuit. This procedure include measuring the absolute level of the pilot channels used in the system and comparing the levels to the calculated levels for the current temperature. If the pilot frequencies are entered into the sweep table as measured frequencies, the automatic control system can be adjusted in the normalized sweep mode eliminating the need to change measurement modes.

Return Sweep Return Signal Path H L R Ensures Unity Gain in the Return path. Unity Gain minimizes distortions on all return signals. Ensures that entire frequency band from all subscribers arrives at the headend or hub site with equal quality. Ensure good sweep and 95% of the other problems go away! In the forward path, signals originate at the headend and are transmitted to many customer drops. One output feeds many inputs. In the forward system the outputs of like devices are typically set to same output level. Each amplifier compensates for the cable and passive loss before it. This principal (Gain = Loss) is Unity Gain. Level changes in the forward path occur due to changes in cable loss caused by temperature. Automatic level or gain controls (ALC or AGC) are located in the amplifiers. A section of the alignment procedure for the forward path involves setting the operating point for the automatic control circuit. This procedure include measuring the absolute level of the pilot channels used in the system and comparing the levels to the calculated levels for the current temperature. If the pilot frequencies are entered into the sweep table as measured frequencies, the automatic control system can be adjusted in the normalized sweep mode eliminating the need to change measurement modes.

Downstream Measurements

Modulation Error Ratio
SCTE: Practical Voice over Internet Protocol (VoIP) Modulation Error Ratio MER is used as the single figure of merit for DVB-C standards. It includes distortions such as CCN, CSO, CTB, laser compression, etc…. The sum of all evils. A 256 QAM picture tiles at 28dB A minimally good MER is 31 dB for 256 QAM at the back of the customer’s set. H

Modulation Error Ratio (MER)
SCTE: Practical Voice over Internet Protocol (VoIP) Modulation Error Ratio (MER) Modulation Error Ratio (MER) is a measure of the phase and voltage variation. Symbol Error And expressed mathematically by: Modulation Error Ratio (MER) measures the signal quality of any digital signal. It is very effective at doing this because it looks at the voltage and phase of each symbol of a digital signal after the symbols have been down-converted from RF and demodulated to baseband. A symbol is simply a collection of multiple “bits” of data. In QPSK modultion, used in the example above, each symbol represents two bits of data (22). A symbol represents four bits of data in 16-QAM modulation (24) and six bits in 64-QAM (26). How many bits are in each symbol for 256-QAM? Notes ___________________________________________________________ Finally, aggregate MER is just the sum of multiple MER measurements: Each point represents one symbol (2-bits) in QPSK of data or one phase position. The distance from the circle is the error.

DOCSIS Compliant Downstream
SCTE: Practical Voice over Internet Protocol (VoIP) DOCSIS Compliant Downstream Analog Measurements CNR ( ≥35 dB per DOCSIS spec) CSO ( ≥41 dB per DOCSIS spec) CTB ( ≥41 dB per DOCSIS spec) Analog (Digital) Measurements BER (post-FEC 10-8 or less per DOCSIS spec) Modulation Error Ratio (MER) Constellation Analysis Digital Channel Power The upstream is usually the focus of RF troubleshooting in DOCSIS networks due to the many sources of RF impairments, but the downstream MUST NOT BE IGNORED! Because spectrum analysis will not reveal impairments under digitally modulated, RF up-converted DOCSIS channels, it is important to use Modulation Error Ratio (MER) and Forward Error Correction (FEC) analysis to determine if impairments are present under DOCSIS carriers. These tests analyze measure signal quality on a demodulated signal and provide much more information on the actual signal quality of a digital signal than does spectrum analysis. Further, measuring digitally modulated signals with digital power meters is crucial for determining the true RF power of that signal. Notes ___________________________________________________________ 1 error in 100 Million bits

Typical BER/MER Requirements
SCTE: Practical Voice over Internet Protocol (VoIP) Typical BER/MER Requirements 64 QAM 256 QAM BER MER MER Quality 10-10 >35 >35 Excellent Good Marginal 10-5 <23 <28 Fail Based upon the DOCSIS specification of 10-8 Post-FEC received BER at the cable modem, the above MER represent a “rule-of-thumb” for end-of-line measurements. For 64-QAM at a subscriber’s premise one must measure at least 27 dB MER at the input of a cable modem / eMTA in order to be DOCSIS compliant. Below 27dB MER Post-FEC errors will occur causing lost voice frames and degraded MOS scores. Similarly, 31 dB is the minimum required MER for 256-QAM at the input of the cable modem / eMTA. A good rule of thumb is to always add at least 3 dB of headroom above the minimum requirements. Therefore, 30 dB MER is recommended for 64-QAM and 34 dB for 256-QAM. Notes ___________________________________________________________ __________________________________________________________ NOTE: Set-top boxes can tolerate some Post FEC errors, but cable modems cannot.

Downstream Digital Measurements
SCTE: Practical Voice over Internet Protocol (VoIP) Downstream Digital Measurements

Constellation Analysis
SCTE: Practical Voice over Internet Protocol (VoIP) Constellation Analysis Noise CW Interference Phase Noise Downstream constellation analysis provides a powerful tool for analyzing impairments in the RF which may or may not have occurred during RF transport of the signal. Headend impairments MUST be identified and corrected at the source (the headend) as not amount of field troubleshooting will resolve the problem. Noise, CW (coherent wave or interfering signals) Interference, and Compression are all examples of RF impairments that could occur from sources in the HFC network. Phase Noise and I/Q Gain or Phase Error are typically caused by improper setup or operation of headend QAM modulators or up-converters. Seeing low MER values in the field and the associated constellation diagram with Phase Noise and I/Q Gain or Phase Error types of displays is a clear indication that action should be taken at the headend to remedy the situation, not in the field. Notes ___________________________________________________________ __________________________________________________________ I/Q Gain Error I/Q Phase Error Compression

Equalizer Stress Digital receivers use adaptive equalizers to negate the effects of signals arriving other than the desired signal. Signals can arrive ahead of or after the desired signal. In a cable system, the majority of signals are reflections and micro-reflections that arrive after the desired signal. Cable modems and digital set top boxes must be able to handle pre and post (DELAYED) signals at levels defined by DVB standards. If the equalizer is pushed beyond those limits, errors will occur. By using the Velocity of Propagation, the distance to the source of the reflection can sometimes be located. If the reflections occur before the next upstream amplifier, they are simply amplified and passed downstream thereby eliminating the ability to perform fault detection based on reflection time. Equalizer stress is used more as a figure of merit for the margin available to the set top box or cable modem.

Cable Modem Downstream
SCTE: Practical Voice over Internet Protocol (VoIP) Cable Modem Downstream

Downstream Frequency Response
SCTE: Practical Voice over Internet Protocol (VoIP) Downstream Frequency Response DOCSIS Specifies <.5 db peak to valley per MHz

Intermittents While you can’t measure a problem that isn’t happening, there may be clues Any reading that is not normal for your system may be suspect Example -Lower than normal MER/BER Erodes headroom and error margin – any degradation will cause issue Higher than usual ingress/noise Check Ingress in Reverse AND Forward Path Any CPD CPD levels are often variable – may be minor now – major tomorrow Use Statistical Measurements to monitor over time

Cable Modem Downstream
SCTE: Practical Voice over Internet Protocol (VoIP) Cable Modem Downstream Stats Mode Measurements Graphed over time MER and Pre and Post BER measured over time

Upstream Certification

DOCSIS Compliant Upstream
SCTE: Practical Voice over Internet Protocol (VoIP) DOCSIS Compliant Upstream Linear Impairments such as: Micro-reflections <= 0.5 µsec (per DOCSIS spec.) <= 1.0 µsec -30 > 1.0 µsec Amplitude ripple (0.5 dB/MHz per DOCSIS spec.) Group Delay (200 ns/MHz per DOCSIS spec.) Non-linear Impairments such as: Common Path Distortion (CPD) Return Laser Clipping Transient Impairments such as: Ingress & Impulse Noise (CNR > 25 dB per DOCSIS) MER / Pre & Post-BER Channel Characterization The upstream RF requirements are also defined by the DOCSIS specification. Unlike the downstream, however, there are many more impairments in the upstream and this is usually the Achilles heel of any DOCSIS and VoIP network. Micro-reflections and amplitude usually coincide with one another and are the effect of poor return loss due to improper terminations or loose connectors. Group delay is especially noticeable at the return path spectrum band edges and is created by the diplex filter roll-off in the RF amplifiers. For 16-QAM, it is recommended that group delay of 100 ns/MHz or better is more appropriate if pre-equalization is not enabled in the cable modems. Laser clipping is devastating to VoIP! Especially legacy FP lasers, which should be replaced with newer DFB lasers that support more than one or two DOCSIS channels in the upstream. Notes ___________________________________________________________ __________________________________________________________

Effects of Over-Driving a Laser
SCTE: Practical Voice over Internet Protocol (VoIP) Effects of Over-Driving a Laser The return path of a DOCSIS network is specified as 5-42 MHz (5-65 MHz Euro-DOCSIS) and RF amplifiers typically transport up to 42 MHz due to the diplex filters. Return path optical transmitters transport frequencies up to 200 MHz. For this reason it is important to look at the return path in the headend all the way to 200 MHz when using a spectrum analyzer. This will enable the user to observe if the laser is operating in a non-linear (or clipping) mode. The presence of frequencies above 42 MHz and a mirror image of the “hump” and DOCSIS channel on the right hand side is a clear indication that the laser is operating in a non-linear mode and creating distortion components. When distortion components of this nature are present in an optical component, one can be certain that the laser is also clipping signals, causing lost symbols and thus dropped VoIP frames. This will provide a clear indication of a poor VoIP channel and dissatisfied VoIP customers. Notes ___________________________________________________________

CPD – Note the 6 MHz Marker Delta!
SCTE: Practical Voice over Internet Protocol (VoIP) CPD – Note the 6 MHz Marker Delta! Because the channels in the forward system are 6 MHz apart, the sum and difference frequencies occur at 6 MHz intervals as well. 6 MHz 6 MHz 6 MHz This screen shot represents a very clear example of high levels of CPD. The DOCSIS channel in this illustration is well placed as it falls centered between two CPD interferers. If the DOCSIS channel were centered on top of a CPD interferer, there would likely be severe packet loss, causing significant VoIP impairments. CDP under a DOCSIS carrier may result in post-FEC errors and low MER when it becomes severe enough. Notes ___________________________________________________________ __________________________________________________________

Upstream Characterization
SCTE: Practical Voice over Internet Protocol (VoIP) Upstream Characterization While passive measurements will allow one to view some ingressors into the HFC network, some impairments require active signal injection to measure upstream DOCSIS system parameters Such a method requires a 16-QAM (or up to 64-QAM for DOCSIS 2.0) source injected in the field with a spectrum analyzer in the headend for QAM demodulation. This will enable the user to measure the HFC plant for DOCSIS compliancy in areas of Group Delay, Micro-Reflections, and Amplitude Ripple. In addition, one can measure MER, pre and post-BER, and digital power of a controlled digital source over an extended period of time for time-lapsed measurements. Notes ___________________________________________________________ 16-QAM TRANSMITTER SPECTRUM ANALYZER with QAM DEMOD

Upstream Laser Compression - Constellation
SCTE: Practical Voice over Internet Protocol (VoIP) Upstream Laser Compression - Constellation While we have seen laser clipping shown through looking at second and third harmonics of the laser on a spectrum analyzer, through upstream characterization, we can also see that laser clipping will also distort a 16-QAM carrier. This capture represents the laser compressing the outer-most (highest voltage points) of the 16-QAM signal as it is clipped by the return path laser in the fiber node. Almost no distortion is occurring to the center most symbols, because they are at a much lower voltage level than the outer symbols and thus are not being “clipped” by the laser. Note the MER of 26.5 dB. The 16-QAM source used before hitting the laser had an MER of >40 dB. It was the laser that degraded the MER. The injection point was a tap adjacent to the node. Notes ___________________________________________________________

Micro-Reflections Micro-reflections are an indication of a mismatch in the network. A problem because the mismatches reflect the incident signal back towards the source causing standing waves (or ripples) in the amplitude of the frequency response where these 2 signals collide Micro-reflections can best be thought of as return-loss in an HFC network. The cable modem transmits a signal to the CMTS and part of that signal is reflected back into the input of the cable modem. If the signal is high enough in power, it will corrupt the next transmission that the cable modem makes, effectively corrupting the next set of outgoing data. Think of “echo” when you are talking on the phone. Sometimes if the echo is so bad, its hard to hear yourself think and you loose your train of thought. The same principal applies to a cable modem. Notes ___________________________________________________________

Group Delay Group delay occurs at the roll-off points of the diplex filters and its effect gets worse with more filters in the cascade. (remember there are 2 for every active) Group delay affects MER; to achieve minimum MER levels for 16-QAM group delay must be reduced to a certain level While diplex filters are single greatest contributors to group delay in the RF network, don’t think you are safe just because you running your modems below 38 MHz! 16-QAM and higher order modulations require tighter Group Delay requirements. Every “sloppy” connector, kink in your coax, and loose RF component in your RF amplifiers and contributing a little to Group Delay. There are many HFC networks that fail DOCSIS compliancy at 25 MHz! Notes ___________________________________________________________

Which means? As different frequencies pass through a Cable System, some will move faster than others— Group Delay t SYSTEM Filters & Traps SYSTEM Filters & Traps 5 MHz 10 MHz 15 MHz 20 MHz 25 MHz 30 MHz 40 MHz 35 MHz 20 MHz 15 MHz 10 MHz 5 MHz 25 MHz 30 MHz 35 MHz 40 MHz 10 MHz 15 MHz 20 MHz 25 MHz 30 MHz 35 MHz 40 MHz 5 MHz Notes ___________________________________________________________ T I M E

Group Delay DOCSIS specifies 200 nSec/MHz But <70 nSec/ MHz is recommended for VoIP in 16-QAM modulation Here is a one system that exceeds the 200 ns/MHz DOCSIS spec. It is doing so well below the 38 MHz diplex filter range as well. Normally 35 MHz would be considered a very safe place to run cable modems. After further examination it was determined that this plant had Group Delay problems at all frequencies and was the reason that cable modems where intermittently dropping offline in 16-QAM modulation. This system was in the process of implementing VoIP with high frame loss statistics. Notes ___________________________________________________________ 240.6 ns/MHz at 35 MHz!!!

16-QAM with Group Delay Upstream pre-qualification of a DOCSIS channel is highly encouraged before deployment of a DOCSIS upstream channel on a new frequency. Using a QAM generator in the field and a spectrum analyzer in the headend with QAM demod capability, provides the ability to determine if a channel is able to support a DOCSIS channel. In this example, the test channel is centered at 41 MHz in a 5-42 MHz return path. The group delay is in excess of 80 ns and the flatness exceeds 4 dB. In spite of this, the MER is 28.4 dB for the 16-QAM signal, which is much more than the required 18 dB MER for that given modulation level. This is also further confirmed by the lack of any pre- and post-FEC errors. Notes ___________________________________________________________ __________________________________________________________

Time Lapsed Analysis of US Channel
SCTE: Practical Voice over Internet Protocol (VoIP) Time Lapsed Analysis of US Channel MER Pre-FEC A snapshot of the previous screen does not tell the whole story. It is always critical to perform time-based analysis of any upstream measurement as diurnal plant changes, transient ingress, and RF load-based traffic can greatly impact the performance of the HFC network. For this reason making RF , MER and pre-/post-FEC measurements over an extended time. The slide above shows three traces. The top trace is MER plotted against time, which shows no degradation during the sample period. The second trace shows pre-FEC errors (correctable by the CMTS) during the sample period. The final trace represents post-FEC errors which are errors that are NOT correctable by the CMTS. In this case, the CMTS discards the VoIP frame directly degrading call quality and the MOS score. Notes ___________________________________________________________ Post-FEC

Return Path Monitoring Upstream monitoring allows problem detection over time and most often before the customer sees the problems Early Fault Detection and Faster Node characterization for VoIP and other return services

Moving Up the Troubleshooting Pyramid
SCTE: Practical Voice over Internet Protocol (VoIP) Moving Up the Troubleshooting Pyramid Now we can move from RF diagnostics to DOCSIS CALL SIGNALING & VOICE TRAFFIC DQoS–SERVICE FLOWS And DOCSIS Measurements Now that we have reviewed a number of common RF impairments we can move up the DOCSIS troubleshooting pyramid and discuss DOCSIS and IP impairments. Once again it is important to stress that troubleshooting need not necessarily begin at the RF level. One must first analyze the type of problem and determine the best approach first. Top-down troubleshooting may be more appropriate in some situations and in others bottom-up may be more appropriate. Notes ___________________________________________________________ __________________________________________________________

Cable Modem Connect

Cable Modem Detail

Signaling and Service Flows
There are 2 types of traffic in a VoIP call, signaling and the actual voice transmissions The signaling sets up the call path and tears it down after the call is completed. Signaling also provides dial tone, ring, and ring back. Service flows are simply a system of prioritizing digital transmissions. Some service flows take priority over others. Service flows only exist between the CMTS and the CM. Many systems use best effort data transmission for signaling and QoS for voice. A QoS service flow always gives voice traffic priority and allocates separate bandwidth for that traffic.

Call Signaling and Service Flows
SCTE: Practical Voice over Internet Protocol (VoIP) Call Signaling and Service Flows MTA CM CMTS CMS CMTS CM MTA Call Signaling (best effort) Call Signaling (best effort) Service Flow Add Service Flow Add RTP Call Flow (QoS) Call signaling occurs in a Best Effort traffic environment, which means that it must compete with all other traffic types. Call signaling is not guaranteed by DOCSIS 1.1 QoS. DOCSIS 1.1 service flows are established which do setup the appropriate QoS for the Real Time Protocol (RTP) traffic that is associated with voice packets. This ensures that voice traffic is not in competition with Best Effort traffic and does have a higher priority in routing for QoS. Once the call is terminated the service flows are terminated (deleted) and available for future calls. Notes ___________________________________________________________ __________________________________________________________ Call Signaling (best effort) Call Signaling (best effort) Service Flow Del Service Flow Del

Some Common “DOCSIS” Call Preventers?
SCTE: Practical Voice over Internet Protocol (VoIP) Some Common “DOCSIS” Call Preventers? Call Signaling Fails to Go Through “Best Effort Service” competes with other traffic Usually TCP/IP signaling will go through, but customer may not wait for dial tone or digits CMS receives excessively delayed digits from DOCSIS contention region – REQuest – Grant period used by other best effort services such as Vonage, gamaing, etc. Remedy  Establish dedicated QoS for Call Signaling with (10 kbps) per eMTA, drawback is uses US BW Call Disconnects After Ring Notes ___________________________________________________________ eMTA and CMTS unable to establish DQoS Bad eMTA, not PacketCable certified or bad PacketCable certificate in eMTA eMTA CODEC or configuration file mis-configured CMTS out of Service Flows – Failure to delete inactive SIDs

Test Best Effort & Service Flow Channels
SCTE: Practical Voice over Internet Protocol (VoIP) Test Best Effort & Service Flow Channels Test the IP Best Effort Service Flow Packet Loss, Latency & Jitter Best Effort Verification for Call Signaling STEP 1 STEP 2 Test the Upstream & Downstream Voice Quality VoIP MOS & R-Factor tests Using DOCSIS Service Flow (QoS) All test equipment vendors have handheld meters with head-end server applications designed specifically for the purpose of testing Best Effort and Service Flows. The handheld meter registers with the DOCSIS network as a cable modem. Once connected, communication with the head-end server begins. The communication simulates VoIP traffic on the upstream and downstream DOCSIS channel. Frame loss, delay and jitter are measured. A MOS score is calculated referenced to the E Model R-Factor value. Low MOS scores can be correlated to one or more impairments of packet loss, delay and/or jitter. Measurement of Best Effort signaling traffic and service flow signaling are equally important, because if communication with the CMS cannot be established the call will never occur. Notes ___________________________________________________________ __________________________________________________________

Testing Best Effort Call Signaling
SCTE: Practical Voice over Internet Protocol (VoIP) Testing Best Effort Call Signaling Test the performance of your systems “Best Effort” services Good for testing network performance for call signaling Will your eMTA establish communication with the CMS? Measures Packet loss Latency Jitter Best Effort service flow tests the signaling between the MTA and Call Management Server If the MTA cannot establish communication with the MTA, then a no-dial tone scenario can exist or a delayed dial-tone may exist. This is also a case for erroneous or mis-dialed calls Best Effort tests ensures that the Best Effort window is not in an over-capacity situation or that RF impairments do not exist which are so severe that excessive packet loss is present Notes ___________________________________________________________ Courtesy Sunrise Telecom Broadband Note: Does not use QoS (DOCSIS Service Flow)

Moving Up the Troubleshooting Pyramid
SCTE: Practical Voice over Internet Protocol (VoIP) Moving Up the Troubleshooting Pyramid Now we can move from DOCSIS to IP CALL SIGNALING & VOICE TRAFFIC DQoS–SERVICE FLOWS And DOCSIS Measurements Now that we have reviewed a number of common RF impairments we can move up the DOCSIS troubleshooting pyramid and discuss DOCSIS and IP impairments. Once again it is important to stress that troubleshooting need not necessarily begin at the RF level. One must first analyze the type of problem and determine the best approach first. Top-down troubleshooting may be more appropriate in some situations and in others bottom-up may be more appropriate. Notes ___________________________________________________________ __________________________________________________________

Building Blocks of Troubleshooting
SCTE: Practical Voice over Internet Protocol (VoIP) Building Blocks of Troubleshooting IP traffic is quickly becoming an “impairment” when it reaches excessive levels, overwhelming the Cable Modem Termination System (CMTS) Excessive CMTS utilization causes VoIP subscribers to experience poor call quality, data users experience slow downloads/uploads, and in worst case scenarios the data network comes to a halt! Conventional RF monitoring tools cannot identify IP traffic utilization Capacity planning tools become essential How do we manage the traffic on the network and our subscribers before the CMTS becomes over- subscribed? Excessive traffic in the upstream or downstream on a DOCSIS network is so critical that we can now safely call it an IMPAIRMENT. The impact of traffic over-utilization on a CMTS (or even on the IP back-bone) will cause packet jitter, delay and frame loss. All of these will quickly degrade the MOS score of your VoIP network and result in dissatisfied subscribers. Excessive traffic can come from either too many VoIP users or too many high speed data (HSD) users accessing the network at the same time. A common rule of thumb is never exceed 70% loading of the CMTS. This number may need to be adjusted depending upon demographics (say around a college campus). Notes ___________________________________________________________

Throughput Test Throughput may be an indication of high traffic loading

Two Categories of VOIP Testing
SCTE: Practical Voice over Internet Protocol (VoIP) Two Categories of VOIP Testing 1. Network Components Latency Jitter Lost Packets 2. Voice Quality Mean Opinion Score R Factor If the network tests are taken care of, then usually you don’t have a problem. This is what we do. The other tests are voice quality tests and the networks are what the quality of speech.

Latency Measurement Latency is simply the transit time between one network element to another End to end delay LATENCY = TIME Toll Quality <150 mSec

Types of Delay or Latency
SCTE: Practical Voice over Internet Protocol (VoIP) Types of Delay or Latency Transportation Delay Delay caused by the packet to get through the network components such as routers and gateways. This is controlled by the system architecture. Propagation Delay Simply the time it from one place to the other. Packetization Delay This is the time it takes to build the voice packets in the MTA Jitter Buffer Delay Delay caused by the jitter buffer

Latency Causes Voice must be digitized, optionally compressed, processed for echo canceling, and packetized. Voice packets may take multiple hops. Result is that voice over IP networks has more delay than traditional circuit switched approaches. Solutions Run with very small packetization period (10ms). Minimize processing delays in system components. Minimize number of hops from source to destination. Give voice packets priority.

How is Latency Measured?
SCTE: Practical Voice over Internet Protocol (VoIP) How is Latency Measured? Directing ping packets through a UGS pipe (service flow) provides a good platform for testing latency to the network side of the CMTS Network Entity PING PING PING PING PING PING PING The configuration file can be configured so that if ping packets are sent, they would go through the USG service flow. DOCSIS Analyzer QoS Service Flow

VoIP Issue - Jitter Jitter is simply variations in Latency
Delay in routers vary with current traffic load. Voice packets can take different routes. Net result is a variability in delay, this is called jitter. In other words the packets don’t always arrive in the order they were sent. Voice is a real time communication so the packets must play out in the order they were transmitted Solution. Jitter buffer at the playout side. Received packets are placed here first before playout. Builds in a standard delay to allow packets to arrive, get buffered up, then played out. User hears no gaps between delayed packets. Give voice packets priority.

Jitter = Change in Latency
UGS Service Flow Packet Sequence In Packet Sequence Out 3 2 4 1 5 1 2 3 4 5 Transmitted packets evenly spaced in time Received packets unevenly spaced in time Packets are experiencing changes in transit time (Latency) Delta Latency = Jitter Most gateways use buffering to compensate for jitter, but buffering can contribute to latency

Jitter Buffer To Listener 1 2 3 4 5 1 2 3 4 5
Incoming voice packets 1 2 3 4 5 Voice packets have sequence numbers so a buffer can reconstruct the incoming traffic Buffer plays out packets in sequence order Jitter Buffers cause delay To Listener 1 2 3 4 5

Latency - Numbers vary but….. Preferred <60 mSec Maximum 150 mSec
What’s Good? What’s bad? Latency - Numbers vary but….. Preferred <60 mSec Maximum 150 mSec Very annoying about 250 mSec Jitter Preferred equal to or <20 mSec Maximum should be <30 mSec

VoIP Issue - Lost Packets
There is no time for retransmission! Packet loss can occur due to traffic or physical layer problems Solutions – PLC Packet Loss Concealment If single packet is lost, replay the last packet because the human ear is very forgiving. After one replay, play white noise.

A 1% packet loss will cause the call to break up
VOIP & Lost Packets VOIP is very sensitive to lost packets A 1% packet loss will cause the call to break up A 3% packet loss will drop the call completely Some telcos spec 0.5% packet loss or better for High Speed data/voice circuits

Voice Quality Tests MOS – Mean Opinion Score
Actual listeners grade performance on a scale of 1-5 A more modern method uses a MOS server and send voice data packets to the server for analysis. The server also sends voice data back to the instrument which also performs analysis Using this method, both upstream and downstream are analyzed separately. R Factor is essentially the same thing only rated on a different scale based on 100 instead of 5

Factors Affecting Voice Quality
Latency, Jitter and Lost Packets Noise Delay Echo Intelligibility

Cable Modem VoIP Measurements
SCTE: Practical Voice over Internet Protocol (VoIP) Cable Modem VoIP Measurements Cable Modem or Ethernet VoIP Tests Downstream Upstream CMTS Round Trip Select Test Length

Knowledge Check What is the primary source of impairments in a DOCSIS network NCS signaling is sent in a DOCSIS 1.1 service flow for guaranteed QoS. True or False Notes ___________________________________________________________ DOCSIS specifies that the CM / eMTA achieves a Post-FEC BER to be less than or equal to what

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SCTE New Jersey Chapter 9/13/07

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