(Extended version from that published in Communications Technology, June 1, 2009)
By John Downey and Brady Volpe
In Part I of this extended edition (original abbreviated version appeared in CT's March 2009 issue John and I discussed many general DOCSIS upstream issues that should be understood prior to deploying DOCSIS 3.0. In this post we will focus more specifically on DOCSIS 3.0 issues that will occur as you are deploying DOCSIS 3.0 or post -deployment.
DOCSIS 3.0 upstream US Considerations
When DOCSIS 2.0 Upstream speed is exhausted, then DOCSIS 3.0 Upstream can be implemented. Some considerations include:
DOCSIS 3.0 Upstream Issues
As with DS issues, there are also Upstream issues that need to be addressed. US bonding has not been pursued at this point because most people haven’t even exploited D2.0 US capabilities. This does not mean we should avoid the potential issues that will arise. Eventually, we will want to offer US speeds greater than what a single channel modem can offer of ~ 25 Mbps. This will require more US spectrum, D3.0 CMs, and CMTS linecards with US bonding capability. Some of the potential issues are:
The blue trace in Figure 1 below shows the case of strong laser clipping, while the green line represents a flat noise floor from the return path laser with no clipping. Note that this return has four bonded channels in the US.
Using Return Path monitoring tools like PathTrak or Cisco Broadband Troubleshooter (CBT) to view 5-65 MHz for apparent laser clipping as in the above figure is critical for ongoing preventative maintenance. Also needed is an analyzer that can read < 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 cause problems at the input of the CMTS. PathTrak (RPM-3000 card only) also has the ability to demodulate and display live cable modem US constellations as seen in Figure 2 below.
This monitoring system also provides average, maximum, and minimum MER of DOCSIS 3.0 and lesser CMs on each node. In monitor mode it provides the ability to determine relative node quality. It can determine which node has better CM MER than another and during which time of the day MER is the worst, since historical analysis is built-in.
D3.0 CM Transmit Levels
To address the potential issue where a CM today transmits near max power of 54 dBmV for 64-QAM, the specification has changed the CMTS US port level setting to allow it to be 6 dB lower as shown in figure 3 below.
This means the CMTS can be set lower so modems can be placed on those high value taps without changing HE or plant losses. This is at the expense of lower MER/SNR readings. The lowest setting on the CMTS today is -1 dBmV for 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. D3.0 identified this potential issue and forced D3.0 CM vendors to support a transmit level of 3 dB higher than the 2.0 spec. Therefore, 64-QAM has to transmit at least a maximum output of 57 dBmV with a single channel.
To keep the CM inexpensive and act as a constant power device, the max output will be dictated by how many US frequencies are active on the port (known as TCS for transmit channel set) and the highest modulation used. Four channels stacked will be 57-6 = 51 dBmV per ch. So, the overall effect from 2.0 to 3.0 is really a 3 dB difference. The spec lowered the nominal setting allowed on the CMTS so cable operators didn’t need to lower tap faceplates or drop padding on every US port on the CMTS to keep D3.0 CMs online if they maxed out. Most systems will leave the default of 0 dBmV configured and adjust padding appropriately. Cisco has a command to keep CMs online that are maxed out, power-adjust continue 4 (default). Many customers set it to 6 dB to give more room to work with.
When fiber is run deeper into the network as in the case of RF over Glass (RFoG) or DOCSIS Passive Optical Networks (DPON), a new conundrum is raised. Many of these architectures will incorporate 32-way optical splitter/combiners. Having a laser Tx in your house combined with 32 other houses feeding 1 Rx in the HE is addressed with lasers timed with the actual traffic from the house; unlike how it is done today where the US laser is on all the time. Having US bonding and/or load balancing presents a potential issue where an US laser could be transmitting the same time as another US laser. This may be address with the fact that even though multiple lasers are transmitting the same instant in time, if they are carrying different frequencies, then it may be acceptable. The next question, will S-CDMA pose the same problems? This multiplexing scheme allows multiple modems to transmit the same instant in time.
Figure 4 below depicts possible US isolation issues that can occur when an US frequency is narrowcast and another US frequency is introduced across multiple nodes. Creative placement of pads and filters and/or isolation amplifiers can be used to prevent the signal from back-feeding when architectures like these are implemented.
Because of different combining schemes for different nodes as shown in figure 4 above, two USs which are at the same frequency, could interfere with each other. Upstream 2 is being fed from two nodes, while US0 is from one node and US1 is from the other separate node. The 24 MHz signal for US0 will travel to the US2 combiner and back-feed to the fiber receiver number 2, then possibly back-feed to US1 of the CMTS. This 24 MHz signal will be about 40 dB lower than the expected signal if the isolation in each splitter is at least 20 dB. Some ways to increase the isolation are: use amplifiers, add filters, or selectively move attenuation to different points in the network. This situation may also cause issues with load balancing since one port is shared across two other ports. Can US2 load balance with US0 and a different load balance with US1?
Figure 5 above illustrates system plant design variations; not the in-home wiring variations! This is the level that a home user device would need to transmit (if attached directly to the tap) to arrive at the associated amplifier at its recommended level. If step attenuators, EQ taps, or line equalizers with reverse padding are used to make end of line CMs transmit higher, it addresses two concerns. Since more attenuation is added where there was once less loss for ingress, it allows “ALL” CMs to have a better CNR and maybe a better MER/SNR. The loss from step attenuators and these other options attenuates the ingress from those low value taps and houses attached to those taps. These options also solve the potential issue of modems “ramping up” to max power when a power outage occurs. A typical unit with 55 dBmV output would overdrive the system if located at the end of the cable span.
Frequency Expansion & Tap Change-outs
If systems 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 of the active used to be a 29 dB tap, then got dropped to 26, then dropped to 23 all because of CM US transmit levels, very close to 55 dBmV or so. Modems farther away on low value taps transmit much less, sometimes as low as 35 dBmV. To get this delta much closer (tighter bell curve of CM 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:
Since these modems would go from say 55 dBmV to 49 dBmV, we have room for 3.0 CMs with US bonding and/or we could add padding at the node to force all the CMs higher again and get better SNR. If CM transmit levels are still a concern, using a D3.0 CM in 2.0 mode would allow higher US Tx level because the use of a single frequency and a D3.0 CM offers 3 dB higher power. Running D3.0 in low modulation schemes allows higher power as well. Using S-CDMA with more codes may also allow higher transmit power, but will depend on implementation.
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.