1	DEPLOYING VOIP ON THE OUTSIDE PLANT RON HRANAC rhranacj@cisco.com
2	VoIP—Voice Over Internet Protocol A much-too-common myth: “High-speed data works fine in my system, so voice should be no problem!”
3	VoIP—The Philosophy VoIP requires an organizational change: It’s not your father’s high-speed data!
4	VoIP—The Reality High-speed data and voice services can in most cases be successfully deployed on a CATV network if the ENTIRE cable system—headend, distribution network, and subscriber drops—meets or exceeds certain minimum technical performance parameters.
5	Recommended Network Specifications The first is the technical requirements in Part 76 of the FCC Rules www.access.gpo.gov/nara/cfr/waisidx_03/47cfr76_03.html The second is the assumed RF channel transmission characteristics outlined in the DOCSIS^® Radio Frequency Interface Specification www.cablemodem.com/specifications The third is ensuring the HFC plant’s unavailability contribution does not exceed 0.01% as described in the PacketCable™ Availability Reference Architecture www.packetcable.com/specifications
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7	FCC Rules: Part 76 Minimum visual carrier amplitude: 0 dBmV at the subscriber terminal; +3 dBmV at the end of a 30 meter drop. Maximum visual carrier amplitude: Do not overload the subscriber’s receiver or terminal Aural carrier amplitude: 10 dB to 17 dB below the visual carrier Visual carrier amplitude change: No more than 8 dB variation on any channel within any six month interval No more than 3 dB variation during a 24-hour period between any adjacent visual carriers within the cable system bandwidth No more than 10 dB difference between any two channels in 300 MHz systems, +1 dB for each additional 100 MHz bandwidth
8	FCC Rules: Part 76 Aural carrier frequency: No more than +/- 5 kHz from nominal frequency (i.e., for NTSC channels, the aural carrier must be 4.5 MHz +/- 5 kHz above the visual carrier) In-channel frequency response: +/- 2 dB (for 6 MHz NTSC channels this specification must be met from 0.75 MHz to 5.0 MHz above the lower frequency boundary of the channel) Visual carrier-to-noise ratio: 43 dB (relative to a 4 MHz noise bandwidth for NTSC television channels) Visual carrier-to-coherent disturbance ratio (CTB, CSO, XMOD) 51 dB for standard and IRC channelization; 47 dB for HRC channelization Terminal isolation: Minimum 18 dB, and sufficient to prevent reflections caused by open- or short-circuited subscriber terminals from producing visible picture impairments at any other subscriber terminal
9	FCC Rules: Part 76 Low frequency disturbances (hum): The peak-to-peak variation in visual signal level caused by undesired low-frequency disturbances is not to exceed 3% of the visual signal level Chrominance-to-luminance delay inequality: 170 nanoseconds or less Differential gain: Maximum +/-20% Differential phase: Maximum +/-10 degrees Signal leakage (less than and including 54 MHz and greater than 216 MHz): No more than 15 µV/m field strength at a 30 meter measurement distance using a resonant half-wave dipole Signal leakage (over 54 MHz up to and including 216 MHz): No more than 20 µV/m field strength at a three meter measurement distance using a resonant half-wave dipole
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11	DOCSIS^® 1.1 Assumed Downstream RF Channel Transmission Characteristics
12	RF Channel Spacing (Design Bandwidth)
13	RF Channel Spacing (Design Bandwidth)
14	Transit Delay
15	Transit Delay Signals traveling one way from the headend to the subscriber through, say, 18 km of fiber and 1 km of coax: about 95 microseconds (μsec) transit delay The DOCSIS transit delay specification is <0.800 millisecond (msec) one way
16	Carrier-to-Noise Ratio
17	Carrier-to-Noise Ratio
18	Carrier-to-Distortion or Interference Ratio
19	Carrier-to-Distortion or Interference Ratio
20	Amplitude Ripple
21	Amplitude Ripple
22	Amplitude Ripple
23	Group Delay
24	Group Delay
25	Micro-reflections
26	Micro-reflections Micro-reflections—also called reflections or echoes—are caused by impedance mismatches In the real world of cable networks, impedance can at best be considered nominal Impedance mismatches are everywhere: connectors, amplifiers inputs and outputs, passive device inputs and outputs, and even the cable itself Upstream cable attenuation is lower than downstream cable attenuation, so upstream Micro-reflections tend to be worse Anywhere an impedance mismatch exists, some of the incident energy is reflected back toward the source
27	Micro-reflections The reflected and incident energy interact to produce standing waves, which manifest themselves as the “standing wave” amplitude ripple one sometimes sees in sweep receiver displays Higher orders of modulation are affected by micro-reflections to a much greater degree (e.g., 256-QAM vs 64-QAM, 16-QAM vs QPSK) Downstream micro-reflections and group delay may be compensated for using adaptive equalization, a feature available in all DOCSIS modems Upstream micro-reflections and group delay may be compensated for using adaptive equalization, a feature available in DOCSIS 1.1 and 2.0 cable modems Upstream adaptive equalization is not supported by most DOCSIS 1.0 modems
28	Micro-reflections Damaged or missing end-of-line terminators Damaged or missing chassis terminators on directional coupler, splitter, or multiple-output amplifier unused ports Loose center conductor seizure screws Unused tap ports not terminated—this is especially critical on low value taps Unused drop passive ports not terminated Use of so-called self-terminating taps at feeder ends-of-line
29	Micro-reflections Kinked or damaged cable (includes cracked cable, which causes a reflection and ingress) Defective or damaged actives or passives (water-damaged, water-filled, cold solder joint, corrosion, loose circuit board screws, etc.) Cable-ready TVs and VCRs connected directly to the drop (return loss on most cable-ready devices is poor) Some traps and filters have been found to have poor return loss in the upstream, especially those used for data-only service
30	Micro-reflections Here’s an approx. -40 dBc echo at just over 2 µsec This echo easily meets the DOCSIS downstream -30 dBc @ >1.5 µsec parameter Amplitude ripple is negligible, and group delay ripple is slight
31	DOCSIS^® 1.1 Assumed Upstream RF Channel Transmission Characteristics
32	Transit Delay
33	Transit Delay Signals traveling one way from the subscriber to the headend through, say, 1 km of coax and 18 km of fiber: about 95 microseconds (μsec) transit delay The DOCSIS transit delay specification is <0.800 millisecond (msec) one way
34	Upstream RF Channel Characteristics
35	Upstream Digitally Modulated Carrier Amplitude The zero-span method is the easiest way to obtain an accurate amplitude measurement
36	Upstream Carrier-to-Interference
37	Amplitude Ripple
38	Amplitude Ripple In this example, an approx. -23 dBc echo at ~720 ns causes visible amplitude ripple across the 5-40 MHz spectrum Group delay ripple also is present
39	Group Delay
40	Group Delay QPSK typically requires a minimum MER of 10~13 dB, depending on CMTS make/model 16-QAM typically requires a minimum MER of 17~20 dB, depending on CMTS make/model
41	Group Delay Upstream group delay measurements require specialized equipment In this example, group delay is nearly constant (within about 100 ns) between 10 and 35 MHz
42	Group Delay Specialized test equipment can be used to characterize upstream in-channel performance In this example, in-channel group delay ripple is about 60 ns
43	Micro-reflections
44	Micro-reflections In this example, an approx. -23 dBc echo is visible at ~720 ns (0.720 µsec) This echo meets the DOCSIS upstream -20 dBc at <=1.0 µsec parameter Note that the echo is still sufficient to cause amplitude and group delay ripple
45	Micro-reflections Here’s another example: An approx. -33 dBc echo at just over 1 µsec This echo meets the DOCSIS upstream -30 dBc at >1.0 µsec parameter Here, too, the echo is sufficient to cause some amplitude and group delay ripple
46	DOCSIS^® 1.1 Electrical Input to the Cable Modem
47	Downstream Digitally Modulated Carrier Amplitude
48	Downstream Digitally Modulated Carrier Amplitude When measuring the amplitude of a digitally modulated carrier, make certain you are measuring its average power level Use test equipment that performs automated measurements, rather than trying to make error-prone manual measurements that require bandwidth, IF filter, log amplifier and detection corrections
49	Cable Modem Total Input Power
50	Cable Modem Total Input Power
51	Cable Modem Total Input Power A quick way to estimate approximate total power is based on the rule-of-thumb that each time the number of channels doubles (assuming all channels have the same signal level), the total power increases 3 dB (3.01 dB).
52	Other DOCSIS^® RFI Spec Parameters Downstream digitally modulated carrier average power level relative to analog visual carrier levels: -10 dBc to -6 dBc 64-QAM bit error rate: Cable modem post-FEC BER must be less than or equal to 10^-8 when operating at a C/N ratio (E_S/N₀) of 23.5 dB or greater 256-QAM bit error rate: CM post-FEC BER must be less than or equal to 10^-8 when operating at a C/N ratio (E_S/N₀) of 30 dB or greater when the input receive signal level is -6 dBmV to +15 dBmV 33 dB or greater when the input receive signal level is -6 dBmV down to -15 dBmV
53	Downstream Performance: QAM Analyzer
54	Downstream Performance: QAM Analyzer
55	Downstream Performance: QAM Analyzer Check signal level, BER, MER and constellation at upconverter input and output Some external upconverters have a very tight window with regard to IF input and RF output levels vs. BER
56	Downstream Performance: QAM Analyzer Check signal levels and BER at downstream laser input and node output Bit errors at downstream laser input but not at CMTS or upconverter output may indicate sweep transmitter interference, loose connections or combiner problems Bit errors at node output but not at laser input are most likely caused by downstream laser clipping
57	Upstream Performance: Constellations and MER
58	Upstream Performance: Constellations and MER
59	Upstream Performance: Packet Loss
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61	First, Some Definitions Availability: The ratio of time that a service is available for use to total time. PacketCable’s reference model assumes 99.94% end-to-end availability. The HFC network maximum contribution to this is 0.01% unavailability, or 99.99% availability—the so-called four nines. Reliability: Probability that a system or device will not fail during some specified period.
62	PacketCable™ Availability Reference Model
63	Contributors to Network Availability Network architecture System powering Redundancy Status monitoring System maintenance practices Subscriber drop installation quality Service restoration
64	Relative Network Contributors to Outages
65	Network Availability According to an analysis in Modern Cable Television Technology, 2^nd Ed., achieving 99.99% availability requires: Improved HDT and NID reliability Hardened and more reliable powering Shorter cascades of both coaxial equipment and power supplies Reliable status monitoring throughout the network Proactive maintenance High quality drop installations
66	Network Impairments That Affect Service Availability Ingress and impulse noise Improper network alignment Distortions (CPD, hum, CSO, CTB) Poor in-channel frequency response (amplitude tilt and ripple) Group delay Micro-reflections Intermittent connections Sweep transmitter interference Laser clipping—upstream and downstream
67	Downstream Checklist Entire cable network—headend, distribution network and subscriber drops—DOCSIS-compliant Upconverter setup, IF input/RF output levels Downstream laser input levels Avoid downstream frequencies near band edges or rolloff areas Avoid downstream frequencies that may be susceptible to ingress from strong over-the-air signals¹ Forward and reverse properly aligned Frequency response flat Signal leakage and ingress management Good installation practices
68	Upstream Checklist CMTS modulation profile optimized for modulation format in use—for instance, 16-QAM Entire cable network—headend, distribution network and subscriber drops—DOCSIS-compliant Select upstream frequency that avoids diplex filter roll-off area Forward and reverse properly aligned Signal leakage and ingress management Good installation practices
69	DOCSIS Checklist
70	DOCSIS Checklist
71	DOCSIS Checklist
72	DOCSIS Checklist
73	DOCSIS Checklist
74	DOCSIS Checklist
75	References Farmer, J., D. Large, W. Ciciora and M. Adams. Modern Cable Television Technology: Video, Voice and Data Communications, 2^nd Ed., Morgan Kaufmann Publishers; 2004 Hranac, R., “What’s in a Number? Defining Availability is Tricky”, December 1999 Communications Technology www.ct-magazine.com/archives/ct/1299/hranac.htm Thadani, N., “VoIP Availability: Will Your Net Thrive Under Pressure?”, November 2003 Communications Technology www.ct-magazine.com/archives/ct/1103/1103_voip.html Thadani, N., “VoIP Availability – Part 2: Will Your Net Thrive Under Pressure?, December 2003 Communications Technology www.ct-magazine.com/archives/ct/1203/1203_voip2.html
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