The Need for Speed
With Verizon’s FiOS menacing the market share of cable operators, DOCSIS 3.0 has come to the rescue. DOCSIS 3.0 offers greater than four times the downstream and upstream data throughput than prior DOCSIS versions. This directly feeds into the business case for many cable operators to compete with FiOS and to improve efficiency. Upgrading to DOCSIS 3.0 is the only way to offer higher data rates than what DOCSIS 2.0 can provide. (DOCSIS 3.0 = >150 Mbps, while DOCSIS 2.0 = ~37 Mbps downstream and 27 Mbps upstream, aggregate speed.)
While DOCSIS 3.0 offers more than four times the downstream and upstream data throughput, it is far more complex to deploy. Therefore, the adage "an ounce of prevention is worth a pound of cure" has never been truer. Planning for DOCSIS 3.0 now is imperative to mitigate potential issues when it’s finally deployed. We must understand and address potential issues for timely deployments as well as economical reasons.
Our analysis examines challenges and issues to be considered for a DOCSIS 3.0 deployment. It will be divided into two parts, downstream and upstream. DOCSIS 3.0 upstream bonding is further down the road, and we have DOCSIS 2.0 to help curtail the speed hunger for a while. For this reason, we will start with DOCSIS 3.0 downstream bonding and tackle DOCSIS 2.0 and 3.0 upstream issues in another article.
We need to add more downstream capacity in the form of DOCSIS 3.0, and it’s time now to consider upgrading. DOCSIS 3.0 uses a technique called channel bonding to unite multiple downstream channels to enable a single DOCSIS 3.0 modem to get four or more times the downstream throughput that is available in a DOCSIS 2.0 system. Bonding in DOCSIS 3.0 is logical; data being transmitted is spread among several individual channels. The "haystacks" aren’t bonded into one gigantic channel.
DOCSIS 3.0 downstream channel bonding is being deployed today. While North America is just getting started (excluding Canada), Asian countries have deployed more than half a million downstream bonding modems in the last few years.
Potential pitfalls
As with any new technology, there are some trade-offs and potential pitfalls that we must understand and plan for in advance. The following will discuss some of these issues.
1. Why it’s needed: This can range from competitive pressure to higher tiers of service to more customers signing up.
2. Frequency stacking levels and placement: What is the edge quadrature amplitude modulation (QAM) modulator maximum output with four channels stacked, and do the channels have to be contiguous?
3. Isolation concerns: Whenever applications have different service groups, we have overlaid networks. Signals destined for one node could "bleed" over to another.
4. Downstream frequency expansion to 1 GHz: Amplifier upgrades are occurring now. It’s best to roll the truck once. Think about diplex filters, spacing, taps, etc.
Note: DOCSIS 3.0 cable modems support a minimum of four upstream and four downstream channels. This means bonded channels could be fewer, but the modem must have the capability to support at least four or more.
DOCSIS 3.0 terms
To level set, it’s best to understand some new DOCSIS 3.0 terminology.
Local downstream: a downstream from the cable modem termination system (CMTS)
Remote downstream: a downstream from an edge QAM modulator
Edge QAM modulator: a device that is fed IP digital data and outputs many RF QAM channels for video and data (DOCSIS)
Primary downstream: a downstream that includes DOCSIS messaging
Secondary downstream: a downstream meant for bonding only
Combiner group domain (CGD): an MxN media access control (MAC) domain, where the old terminology for a MAC domain was one downstream and N upstreams (Note: Keep in mind that more upstreams in a MAC domain require more MAPs and "eats" into the downstream throughput. Approximately every upstream port uses 0.25 Mbps of downstream capacity or more for MAPs.)
"Wideband": a term generically used to describe DOCSIS 3.0 downstream bonding, even though originally a proprietary term
Modular CMTS (M-CMTS): an architecture, not necessarily DOCSIS 3.0, that requires a DOCSIS Timing Interface (DTI) server and edge QAM modulator with DTI capability. This provides downstream load balance of legacy modems within an MxN domain. This means DOCSIS 1.x and 2.0 modems could move between multiple downstreams without forcing a move to a new set of upstreams.
Example overlay
Figure 1 graphically shows fiber nodes vs. frequency/spectral allocation. This is a way to show how connectors and frequencies are shared across fiber nodes. Each symbol represents a physical connector except the dark blue symbols, which represent primary frequencies and how they are associated with a local primary. The light blue symbol represents one edge QAM modulator connector overlaid on eight nodes. This allows load balancing of legacy modems between two downstream frequencies and bonding on four downstream frequencies with DOCSIS 3.0 modems. This requires five downstream frequencies on the plant and three upstream frequencies. Pros and cons follow.
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The "pros" include:
1. Four bonding frequencies per edge QAM modulator connector to offer greater than 150 Mbps throughput.
2. Only one edge QAM modulator connector is required per eight nodes.
3. Basic service allows two downstreams for two nodes with dynamic channel change (DCC) support.
4. Upstream load balancing of DOCSIS 2.0 modems is achieved for more upstream capacity sharing.
5. One upstream connector is shared across two nodes for diminishing DOCSIS 1.x modems.
"Cons" include:
1. Requires an M-CMTS architecture with added devices such as a DTI server.
2. Requires five downstream and three upstream frequencies.
3. May need to push DOCSIS 3.0 modems to the remote downstream to have full four-channel bonding.
Dynamic channel change (DCC) technique 4 can be used to load-balance basic subs across the local downstream and one edge QAM channel primary without re-registering. DCC support requires a 1.1 config file. If modems have a downstream frequency in their config file, use "load balance exclude static strict" so only dynamic load balancing takes place.
Figure 2 shows the physical wiring for the setup shown in Figure 1.
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Figure 2 illustrates how downstreams are potentially combined with edge QAM modulator downstreams and then split to feed multiple service groups or fiber nodes. Downstream 4 is not used, and its associated upstreams are used in the four MAC domains. Upstream connectors use internal frequency stacking on even connectors 0-14 and external stacking on connectors 16-19.
Issues to address
DOCSIS 3.0 downstream has some inherent issues to address.
One of those issues is frequency assignments. The CMTS may be limited to 860 MHz or 1 GHz, and spectrum availability may be scarce in the actual cable plant.
The edge QAM modulator connector is typically limited to a contiguous 24 MHz passband or four channel slots. Keep in mind that Annex A may only be three 8 MHz channels vs. four Annex B 6 MHz channels.
The actual modem limitation may be 50 or 60 MHz passband. DOCSIS 3.0 modems require at least 60 MHz passband. Example: Having the local frequency at 117 MHz and the edge QAM modulator remote at 740 MHz will not work since the modem only has a 60 MHz passband.
The M-CMTS architecture requires a DTI server and upstream ports from the CMTS.
The DTI specification has a distance limitation of 200 meters between the CMTS and edge QAM modulator. There are ideas of utilizing global positioning system (GPS) to sync multiple time servers to allow the edge QAM modulator to be in a hub site and the CMTS in the headend.
One downstream being remote and another being local can cause a difference in time offsets. Load balancing using DCC tech 4 is difficult when the time offset delta is greater than 10 time offset ticks from one downstream to another. This may require edge QAM modulator "tweaking." Also, DOCSIS 1.1 config files are required for DCC to work. Another load balance concern could be level differences if the downstream frequencies are too far apart. Also, there’s no need for DOCSIS 3.0 modems to load balance since they share a bigger pipe.
What about economics? How do you deal with frequency-stacked downstreams if not using them all? Some vendors may offer software licensing to pay just for what you use.
Resiliency is another topic to address. If one downstream frequency goes bad in the field, how will modems recover or react? Today, it may be the case that the bonding modem will come back up online and act as a DOCSIS 2.0 modem in regards to speed. Work is currently underway by many CMTS vendors to create dynamic bonding groups and allow a modem to bond across the frequencies it determines to be good, but that information needs to be relayed back to the CMTS.
Testing and maintaining multiple downstream channels may not be a big issue since the physical channels have not changed for DOCSIS 3.0. However, handheld test equipment with built-in modems will have to be updated to support bonding.
Another overlooked issue is edge QAM modulator max output power per channel. Downstream channel bonding max power with four frequencies stacked on one connector is limited to 52 dBmV per channel. Some vendors’ products may be capable of higher per-channel levels; consult equipment documentation. The DOCSIS 1.x/2.0 downstream specification is 61 dBmV max output with only one channel per connector. This needs to be addressed in the headend combining and splitting so all channels that are combined reach the fiber-optic transmitter at the proper level and flat.
Isolation and level issues
Figure 3 depicts possible downstream isolation and level issues that can occur when a downstream frequency is narrowcast and another downstream frequency is introduced across multiple nodes. Isolation amplifiers can be used to prevent too much loss from the four-way splitter and keep the signal from back-feeding when architectures like these are implemented.
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Keep in mind that isolation is related to the combiner devices and how close of a match to 75 ohms the common leg is and where it is connected. The two-way splitter could have poor isolation because of poor return loss of the fiber transmitter or the CMTS upstream port. The problem with an isolation amplifier is that an active device could be a single point of failure. Also, can the isolation amp handle a high input of power of as much as 50 dBmV or so?
It may be best to combine the edge QAM modulator downstreams closer to the transmitter and use 6-series cable to alleviate headend combining loss. Also, if 16 or more frequencies are stacked on one connector, the device may need internal tilt to overcome headend cabling tilt. Sixteen frequencies contiguous will be about 100 MHz. If a typical allocation is 650-750 MHz, this could be 1 dB of tilt. Higher frequency selections will have less of an effect.
One more thing: 16 frequencies stacked will have less power per channel at the output, about 44 dBmV. It may be wise to design the headend combining and splitting for a worst-case scenario.
Spectrum limitations
The DOCSIS 3.0 downstream spec indicates 1 GHz support, but some equipment may not support that. For example, before the spec was finalized, a downstream bonding modem was introduced with three-channel bonding capability.
The SA DPC2505 is widely used, but only uses three separate downstream tuners and is specified to 930 MHz. This allows the channels to be almost anywhere in the downstream spectrum, but not fully DOCSIS 3.0 certified. The DPC2505 needs all three downstreams for 111 Mbps. This can be done with Annex B and/or Annex A channels, but requires more spectrum if Annex A is used.
Modem steering
Directing modems cannot be overlooked. You may have to restrict legacy embedded multimedia terminal adapters (EMTAs) to the local downstream for linecard redundancy because the edge QAM modulators on the market today may not support redundancy.
For overlay architectures, it may be necessary to force legacy modems to only register on the local downstream or move to a specific downstream frequency. Because DOCSIS 3.0 modems today only support four channels of bonding, it may be necessary to force a 3.0-capable modem to initialize on a remote/edge QAM modulator downstream for channel allocation and speed requirements. Allowing a 3.0 modem to use the local downstream for primary traffic may only leave three channels for bonding and not the intended speed.
The following attribute commands are used with a Cisco CMTS to achieve the previously mentioned criteria. Refer to other CMTS vendors’ documentation for similar functionality and commands.
To keep all D3.0 CMs on the e-qam modulator primary: "cable service attribute ds-bonded downstream-type bonding-enabled enforce"
To force basic subs to register on the local downstream: "cable service attribute non-ds-bonded downstream-type bonding-disabled"
To force basic subs to re-range on a specific downstream frequency: "cable service attribute non-ds-bonded legacy-ranging downstream-type frequency "
These options may be warranted when overlaying DOCSIS 3.0 service with another service from the same CMTS or a different CMTS.
We can assign specific upstream ports to be sent as uniform channel descriptors (UCDs) for specific downstreams. So, the local downstream can be a 2×5 MAC domain/CGD, but only specific upstream ports sent as UCDs for the local downstream or edge QAM channel primary.
Example CLI:
"interface Cable7/0/0"
"downstream Modular-Cable 1/0/0 rf-channel 0 upstream 1 3"
"downstream local upstream 0 2 4"
Summary
One must understand that deploying DOCSIS 3.0 has many parts: understanding the issues, improving the plant, and then maintaining and monitoring the plant to continue to support DOCSIS in general. So if the need for speed exists, preparation for DOCSIS 3.0 is imperative in order to have successful future deployments.
John J. Downey is a broadband network engineer for Cisco Systems. Brady Volpe is director of system engineering and design verification for JDSU.