VoIP Testing

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

2. My Business Card Larry Jump Regional Sales Engineer Sunrise Telecom 814.692.4294 ljump@sunrisetelecom.com

3. Today’s Agenda • DOCSIS Troubleshooting Pyramid • RF Impairments • DOCSIS Problems • VoIP Impairments

4. 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 betweenCMTS & CMs only What we fix today

5. 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 • DHCP / TFTP / ToD / DNS / CMS / etc. servers • Modem and MTA configuration files, CMTS configs

6. 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

7. Forward Signal Path H L R Forward Sweep Headend or Hub Site • 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!

8. Return Signal Path H L Return Sweep • 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! R

9. Downstream Measurements

10. 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

11. Modulation Error Ratio (MER) is a measure of the phase and voltage variation. Symbol Error Modulation Error Ratio (MER) And expressed mathematically by: • 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.

12. 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 1 error in 100 Million bits

13. Typical BER/MER Requirements 64 QAM 256 QAM BER MER MER Quality 10-10 >35 >35Excellent 10-8 27-34 31-34Good 10-6 23-26 28-30 Marginal 10-5 <23 <28Fail NOTE: Set-top boxes can tolerate some Post FEC errors, but cable modems cannot.

14. Downstream Digital Measurements

15. Constellation Analysis Noise CW Interference Phase Noise I/Q Gain Error I/Q Phase Error Compression

16. 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.

17. Cable Modem Downstream

18. Downstream Frequency Response DOCSIS Specifies <.5 db peak to valley per MHz

19. 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

20. Cable Modem Downstream • Stats Mode Measurements Graphed over time • MER and Pre and Post BER measured over time

21. Upstream Certification

22. DOCSIS Compliant Upstream • Linear Impairments such as: • Micro-reflections-10 dBc @ <= 0.5 µsec (per DOCSIS spec.) -20 dBc @ <= 1.0 µsec -30 dBc @ > 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

23. Effects of Over-Driving a Laser

24. 6 MHz 6 MHz 6 MHz 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.

25. Upstream Characterization 16-QAM TRANSMITTER SPECTRUM ANALYZER with QAM DEMOD

26. Upstream Laser Compression - Constellation

27. 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

28. 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

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

30. Group Delay • DOCSIS specifies 200 nSec/MHz • But <70 nSec/ MHz is recommended for VoIP in 16-QAM modulation 240.6 ns/MHz at 35 MHz!!!

31. 16-QAM with Group Delay

32. Time Lapsed Analysis of US Channel MER Pre-FEC Post-FEC

33. Return Path Monitoring Early Fault Detection and Faster Node characterization for VoIP and other return services

34. Moving Up the Troubleshooting Pyramid • Now we can move from RF diagnostics to DOCSIS CALL SIGNALING& VOICE TRAFFIC DQoS–SERVICE FLOWS And DOCSIS Measurements

35. Cable Modem Connect

36. Cable Modem Detail

37. 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.

38. 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 (best effort) Call Signaling (best effort) Service Flow Del Service Flow Del

39. 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 • 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

40. Test Best Effort & Service Flow Channels • Test the Upstream & Downstream Voice Quality • VoIP MOS & R-Factor tests • Using DOCSIS Service Flow (QoS) • Test the IP Best Effort Service Flow • Packet Loss, Latency & Jitter • Best Effort Verification for Call Signaling STEP 1 STEP 2

41. 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 Courtesy Sunrise Telecom Broadband Note: Does not use QoS (DOCSIS Service Flow)

42. Moving Up the Troubleshooting Pyramid • Now we can move from DOCSIS to IP CALL SIGNALING& VOICE TRAFFIC DQoS–SERVICE FLOWS And DOCSIS Measurements

43. 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?

44. Throughput Test

45. Two Categories of VOIP Testing • 1. Network Components • Latency • Jitter • Lost Packets • 2. Voice Quality • Mean Opinion Score • R Factor

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

47. 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

48. 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.

49. 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 DOCSIS Analyzer QoS Service Flow

50. 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.