Speeding Upstream

The first part of this article (in CT‘s March 2009 issue) discussed downstream potential issues, while this one focuses on the potential issues associated with upstream deployments. In particular, this article covers the critical upstream areas that one should be aware of when getting ready to deploy or already deploying DOCSIS 3.0.

The business objective for many operators today is to provide faster speeds to compete with Verizon’s FiOS. These speeds are for downstream and upstream. DOCSIS 2.0 can provide approximately 37 Mbps on the downstream and 27 Mbps on the upstream, aggregate speed. Some per-cable modem speeds are approaching these peak rates and exceeding them. The only way to offer higher rates than what DOCSIS 2.0 can offer is to upgrade to DOCSIS 3.0. Instead of reducing node sizes, which can exceed $10,000 per node split, the business case can quickly be made to migrate to DOCSIS 3.0.

DOCSIS 3.0

As with downstream issues (discussed in Part 1), upstream issues also need to be addressed. Activating multiple frequencies per upstream connector on a DOCSIS 3.0 cable modem has different maximum power per channel vs. a DOCSIS 2.0 cable modem. The maximum transmit power for a DOCSIS 2.0 cable modem using 64-QAM (quadrature amplitude modulation) for one channel is 54 dBmV. The DOCSIS 3.0 maximum upstream per-channel power is 57 dBmV for 64-QAM, a 3 dB power increase over DOCSIS 2.0.

The upstream passband has also changed in the DOCSIS 3.0 specification. Frequency assignments were 5-42 MHz (65 MHz for Euro-DOCSIS) for DOCSIS 1.x and 2.0, but it has been extended to 85 MHz for DOCSIS 3.0. The option of going higher is good for future spectrum re-allocation and avoiding known bad frequencies on the upstream. Some things need to be considered, though, and they include diplex filters, line equalizers, step attenuators, and customer premises equipment (CPE) overload. If incorporating any of these devices in the plant, they may need to be replaced in the future to accommodate the new frequency split. Also, can current CPE such as set-top boxes and TV sets handle a potentially high level of "noise" from a modem at 40 MHz or higher? For example, NTSC television set intermediate frequency (IF) is 41-47 MHz, and upstream signals in this range may cause interference to TVs and VCRs connected directly to the cable.

Significant consideration must be given to the total RF power loading that will now be realized with upstream channel bonding in DOCSIS 3.0 modems. In previous DOCSIS specifications, only one upstream channel was present. For DOCSIS 3.0, at least four upstream DOCSIS channels will be transmitting at the same time, possibly with a 6.4 MHz bandwidth each, resulting in nearly 26 MHz of contiguous upstream channel loading. This is a lot of power hitting the return path fiber-optic transmitter.

The probability of laser clipping is increased, especially if one has legacy Fabry-Perot (F-P) lasers in the return path fiber nodes. It is a good idea to upgrade to distributed feedback (DFB) lasers, which have significantly more dynamic range. As well, one should have a comprehensive plan to monitor laser clipping in the return path. Using a return path monitoring system capable of looking above 42 MHz will enable one to see second- and third-order harmonics of a laser in clipping. Remember, any burst noise above the diplex filter (that is, 42 MHz) coming out of the return path filter is usually indicative of return path laser clipping.

The blue trace in Figure 1 shows the case of strong laser clipping distortion above 42 MHz, while the green line represents a flat noise floor above 42 MHz from the return path laser with no clipping. Note that this return has four bonded channels in the upstream.

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Using return path monitoring tools to view 0.5-85 MHz for possible laser clipping as in Figure 1 is critical for ongoing preventive maintenance. Also needed is an analyzer that can read less than 5 MHz for AM radio or ham radio ingress, which can quickly leak into the network and contribute additional power to the return laser, causing clipping as well as possible problems at the input of the cable modem termination system (CMTS).

Flap list

Cable flap-list monitoring is used for cable modem issues caused by upstream noise impairments and timing issues. The following configurations are recommended as best practices.

>cable flap-list miss-threshold 5

Modems are polled every 20 seconds (15 when linecard redundancy is configured) and correlate with a "hit" when the three-way maintenance "handshake" is successful. If a poll is missed, the CMTS will go into a fast mode and poll every second. If five consecutive polls are missed, the flap count increments by one, and the miss count would increment by five. This can be correlated with T3 timeouts from the cable modem log.

>cable flap-list power-adjust threshold 2

If the modem has power adjustments of 2 dB or higher during one station maintenance interval, the flap count and power adjust count increment by one.

>cable flap-list insertion-time 120

If the modem sends initial ranging two or more times within two minutes, the flap count increments by one. This does not necessarily mean a modem going offline and online. It could be a modem that goes through "init" states many times.

Some recommendations for flap-list monitoring include:

1. Periodically poll the flap-list at an appropriate interval of every 30 minutes or so.
2. Perform trend analysis to identify modems that are consistently in the flap-list.
3. Clear the flap-list periodically (daily?) to recalibrate.
4. Query the billing and administrative database for cable modem media access control (MAC) address-to-street address translation and generate appropriate reports and work orders. Cable modems in a specific area with lots of flaps can indicate a faulty amplifier or feeder lines.

Note: The bottom line is correctable and uncorrectable forward error correction (FEC) errors. If correctable FEC error counts are incrementing, then eventually it will lead to uncorrectable FEC errors, which equals packet drops. If uncorrectable FEC errors are incrementing much faster than correctable FEC errors and/or signal-to-noise ratio (SNR) seems good, then it could be an impulse event like laser clipping, impulse noise, or sweep interference.

DOCSIS 3.0 transmit levels

To address the potential issue where a cable modem today transmits near max power of 54 dBmV for 64-QAM, the specification has changed the CMTS upstream port receive level setting to allow it to be 6 dB lower, as shown in Table 1.

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This means the CMTS upstream input can be set lower so modems can be placed on those high-value taps without changing headend or plant losses. This is at the expense of lower modulation error ratio (MER)/SNR readings. The lowest setting on the CMTS today is -1 dBmV for a 6.4 MHz wide channel. The range allowed on the CMTS is dictated by DOCSIS 2.0 and lower and says -1 to + 29 dBmV for 6.4 MHz and related to channel width, also known as symbol rate or baud. DOCSIS 3.0 identified this potential issue and forced DOCSIS 3.0 cable modem vendors to support a transmit level of 3 dB higher than the DOCSIS 2.0 specification. Therefore, 64-QAM has to transmit at least a maximum output of 57 dBmV with a single channel.

Frequency expansion and tap change-outs

If operators are looking to do 1 GHz amplifier upgrades, they should look into removable diplex filters in case we change to 5-85 MHz, which DOCSIS 3.0 supports. Also, the truck roll for this is expensive, so doing some tap change-outs now would be optimum. The first tap off the active used to be a 29 dB tap, then got dropped to 26, then dropped to 23, all because of cable modem upstream transmit levels, very close to 55 dBmV or so. Modems farther away on low-value taps transmit at much lower levels, sometimes as low as 35 dBmV. To get this delta much closer (tighter bell curve of cable modem transmit levels), we need to either add loss (attenuation) to low value taps, or maybe we could just change the first few taps off the actives to cable simulator taps. So the tap would look like a 32 dB tap at 1 GHz, but maybe 17 dB at 5 MHz. This solves two issues: downstream level and tilt hitting the house, and upstream maximum transmit levels.

Since these modems would go from, say, 55 dBmV to 49 dBmV, we have room for 3.0 cable modems with upstream bonding, and/or we could add padding at the node to force all the cable modems higher again and get better SNR (MER). If cable modem transmit levels are still a concern, using a DOCSIS 3.0 cable modem in DOCSIS 2.0 mode would allow a higher upstream transmit level because the use of a single frequency and the fact that a DOCSIS 3.0 cable modem offers 3 dB higher output power. Running DOCSIS 3.0 in low modulation schemes allows higher power as well. Using synchronous code division multiple access (S-CDMA) with more codes may also allow higher transmit power, but will depend on implementation.

Summary

It is important to recognize that customers and competition are driving the need for increasingly faster speeds in our DOCSIS networks. DOCSIS 3.0 provides an effective migration path for customer satisfaction, exceeding competitive pressures and enabling the delivery of new services and penetration of new markets. With more than four times the data throughput of its predecessors, DOCSIS 3.0 is apt to be nearly an order of magnitude more complex to deploy in such a manner as to take full advantage of its capabilities. Careful planning, proper plant maintenance and a well-developed continued preventive maintenance program from the onset will help pave the road to a speedy and fiscally fruitful deployment.

John J. Downey is a broadband network engineer for Cisco Systems. Brady Volpe is director of system engineering and design verification for JDSU.