docsis 3.1 leaves the lab and hits the field with midco · 2019-12-08 · maintenance (pnm) tools...

© 2016 Society of Cable Telecommunications Engineers, Inc. All rights reserved.

DOCSIS 3.1 Leaves The Lab And Hits The Field With Midco

A Technical Paper prepared for SCTE/ISBE by

Stacy Druse Network Engineer Professional

Midco 3901 N Louise Ave

Sioux Falls, SD 57107 (605) 357-5729

[email protected]

Jason Miller Technical Marketing Engineer

Cisco 2413 Bluff Court

Mandeville, LA 70448 (408) 894-2067

[email protected]

© 2016 Society of Cable Telecommunications Engineers, Inc. All rights reserved. 2

Table of Contents

Title Page Number

Introduction ________________________________________________________________________ 3

Why Migrate To DOCSIS 3.1? __________________________________________________________ 3

Midco Prepares For DOCSIS 3.1 Downstreams ____________________________________________ 4

DOCSIS 3.1 Downstream Overview _____________________________________________________ 5

DOCSIS Downstream Speeds __________________________________________________________ 6 1. DOCSIS 3.0 Speeds ____________________________________________________________ 6 2. DOCSIS 3.1 Speeds ____________________________________________________________ 7

Optimizing OFDM Parameters _________________________________________________________ 10

Midco Deploys DOCSIS 3.1 In the Field _________________________________________________ 11

Midco Field Trial – Summary To Date ___________________________________________________ 13

Conclusion ________________________________________________________________________ 14

Abbreviations ______________________________________________________________________ 14

Bibliography & References ___________________________________________________________ 15

List of Figures

Title Page Number

Figure 1 – Midco Fargo, ND DOCSIS 3.1 Trial 11

List of Tables

Title Page Number

Table 1 - OFDM Downstream Data Rate Estimate 9

Table 2 – Guard Bands (Taper Regions) per Roll-Off Period Sample 10

Table 3 – CM Minimum CNR Performance in AWGN Channel 13


Introduction Midco, a U.S. multiple system operator (MSO), is recognized as a technical leader with a history of successful early deployments of new technology such as DOCSIS 2.0 64-QAM upstreams and DOCSIS 3.0 channel bonding. The company has consistently been ranked as a top Internet service provider (ISP) by PCMag and others for customer broadband speeds. Midco plans to continue this leadership by offering gigabit services to all customers through a migration to DOCSIS 3.1 by 2017.

This paper covers technical drivers related to why Midco elected an early migration to DOCSIS 3.1 for gigabit services instead of continuing to expand their DOCSIS 3.0 offering. It focuses on their specific deployment scenario in their first trial market as well as other follow-on markets that took place before July 2016.

The paper also covers some technical aspects of DOCSIS 3.1 that became more apparent during actual testing. This includes calculated channel speeds and configuration parameters that were optimized by Midco to obtain improved performance in its field trial.

There is also a commentary on the field trial, detailing what went well and what items proved to be a bit more challenging.

Why Migrate To DOCSIS 3.1? Midco operates cable systems across South Dakota, North Dakota, Minnesota and parts of Wisconsin. Broadband services have always been an important offering for Midco and a way to differentiate their services from their competition. The current tiers of 75 megabits per second (Mbps), 120 Mbps, and 200 Mbps are provided using DOCSIS 3.0 (D3.0) channel bonding. The typical deployment bonds eight 6 MHz downstream channels and three 6.4 MHz upstream channels. A DOCSIS service group usually consist of two fiber nodes each with a dedicated return receiver on the converged cable access platform (CCAP).

Midco prides itself on consistently topping the fastest Internet service provider (ISP) rankings, such as PCMag.com, which measure not only speed performance but also the ability to consistently deliver those speeds. With this in mind, a gigabit offering is a natural progression and Midco is committed to offering gigabit services to all customers by the end of 2017. The question now becomes how to best offer this type of service accompanied by other services, specifically video and voice. Although fiber to the home (FTTH) technology would provide the capacity needed, and will be offered as an option for select customers, the time and cost to convert from an existing hybrid fiber coax (HFC) infrastructure is too high and not economically feasible for Midco.

If the choice is to leverage the current HFC infrastructure, then one is left with a choice in offering gigabit services: allocate new spectrum and grow the current D3.0 implementation beyond eight downstream channels or use DOCSIS 3.1 (D3.1) on this newly allocated spectrum to provide the additional capacity. The latest D3.0 modems today scale to handle up to 32 downstream channels providing roughly 1.2 gigabit per second (Gbps) aggregate capacity. This aggregate capacity may be enough for a handful of modems to obtain 1 Gbps speeds but not at scale. In the past, the method to grow D3.0 capacity was to provide multiple bonding groups and allow load balancing between them. For example, four downstream channels provide approximately 150 Mbps of capacity to a group of modems. When the combined data


requirements of the modems exceed 150 Mbps, an additional 24 MHz of forward spectrum (4 x 6 MHz downstream) could be used to offer a second group of four downstream channels increasing the aggregate capacity to 300 Mbps. Obviously the speed per modem will still be capped at 150 Mbps if using a four channel modem. This same expansion technique can be applied to 32 channel modems; however, with forward spectrum being a coveted resource, it becomes much more difficult to grow capacity in 192 MHz-wide (32 downstream channels) chunks. Also, 1 Gbps is not viewed by Midco as a speed “destination” but rather as a mile marker along the way to future speeds offerings, which will continue to grow. D3.0 modems are likely not going to grow beyond 32 downstream channels.

Enter DOCSIS 3.1! A technology that runs over the current HFC plant, D3.1 provides higher order modulations to deliver more bits over a given amount of spectrum, all while providing an easier way to incrementally grow capacity to 1 Gbps and beyond at scale. To achieve higher modulations on the same HFC plant, D3.1 relies on orthogonal frequency division multiplexing (OFDM) and an updated forward error correction (FEC) method known as low density parity check (LDPC). These changes mean that spectrum allocated to D3.1 cannot be used by older D3.0 modems.

Midco will cap the existing spectrum used by D3.0 modems and allocate all future spectrum to D3.1. Since spectrum is a finite resource shared with video, the sooner this cap takes place, the easier it is to find space for the D3.1 migration.

Midco Prepares For DOCSIS 3.1 Downstreams Midco added 144 MHz of D3.1 capacity (equivalent to 24 channels on their HFC plant) in addition to their spectrum dedicated to the current D3.0 service to offer a gigabit downstream tier. They will continue to use D3.0 for upstream traffic, bonding three to four 6.4 MHz channels. This paper focuses on D3.1 downstream technology and implementation since that was what was tested and trialed.

As mentioned previously, D3.1 downstream channels use OFDM, LDPC and higher order modulations, so only D3.1 modems are able to leverage this new capacity. Midco does not plan on swapping out all D3.0 modems for D3.1 modems immediately, so it will be necessary to maintain existing D3.0 channels in addition to allocating 144 MHz for D3.1. Current D3.1 modems can support two 192 MHz OFDM downstream channels along with 32 D3.0 downstream. These modems can receive traffic on both D3.1 and D3.0 channels simultaneously assuming the traffic is coming from the same CCAP equipment. This enables a D3.1 modem to achieve gigabit speeds using the combined capacity of the D3.0 and D3.1 channels.

In addition to setting up a lab for testing, the main prerequisite was to free up 144 MHz of forward spectrum in the HFC plant for this new service. Most capacity was made available as a result of an analog video reclamation project. Future capacity beyond 144 MHz will come from advanced video compression and a longer-term migration to a passive plant architecture. Midco uses proactive network maintenance (PNM) tools to monitor their HFC network and resolve most issue before they impact customers. Throughout the trial, no HFC plant issues were encountered.

The Fargo, ND market was chosen for the initial field trial because it had ample spectrum available. The system has 1 GHz forward capacity and 5 – 85 MHz in the return. Although in future locations, the D3.1 modems will use existing D3.0 capacity as well as new D3.1 capacity, in Fargo, eight new D3.0 channels were added in addition to the 144 MHz D3.1 capacity so testing would not impact any current D3.0


modems. Testing would include extended, sustained data transfers over 1 Gbps that would not represent typical customer traffic patterns.

Training and other items for Midco engineering / operations were also done but are beyond the scope of this paper.

DOCSIS 3.1 Downstream Overview An overview of DOCSIS 3.1 technology will assist the reader in understanding the upcoming sections of this paper. If the reader is already familiar with D3.1, then certainly skip this section and read on!

DOCSIS 3.1 provides faster speeds by increasing the downstream channel size beyond 6 MHz and the modulation order beyond 256-QAM. It also allows multiple downstream profiles on the same channel to enable the CCAP equipment to run higher modulations to modems on a cleaner part of the plant and lower order modulations to others.

The D3.1 downstream width is configurable from 24 MHz up to 192 MHz. The wider the channel, the faster the symbol rate, and the higher speeds we can achieve. However, one single 192 MHz channel could be a bit problematic if we continued to use a single carrier as is common practice today. If there was noise in one part of the spectrum, it might be necessary to reduce the entire channel’s modulation to limit bit errors. Instead, within the 192 MHz block of spectrum, we take advantage of a technology called OFDM where we use a number of small subcarriers (either 25 kHz or 50 kHz wide). This way, we can adapt to impairments on a much smaller channel basis as required, using a lower order modulation on an impaired portion of the spectrum with higher order modulations elsewhere.

There are 3,840 - 50 kHz subcarriers or 7,680 – 25 kHz subcarriers within 192 MHz of spectrum. All these individual carriers would normally require guard bands if not for the OFDM technology. With OFDM, there is an integer number of subcarriers within the symbol period. In the frequency domain, this results in the peaks of subcarriers lining up with the nulls of other subcarriers. The subcarriers are then said to be orthogonal to each other where one subcarrier has no impact on another. Guard bands between subcarriers synchronized within the spectrum block are no longer necessary and only required on the edges where they come in contact with other channels. If the spectrum isn’t continuous, then additional guard bands are required within the OFDM band around these channels as well.

In addition to current D3.0 64-QAM and 256-QAM, support for modulations of 512-QAM, 1024-QAM, 2048-QAM and 4096-QAM are required for D3.1 OFDM channels. These provide for 9, 10, 11, and 12 bits per symbol respectively. Optionally, modulation can run as high as 16384 QAM, however current CCAP and cable modem software did not support this high of modulation at the time of testing.

Running these higher order modulations over the same HFC plant designed for 256-QAM requires a more robust FEC system to correct bit errors before they are experienced by users. LDPC FEC is now used in D3.1 and is more robust than the R-S FEC used in D3.0. Another type of FEC called BCH is also in the downstream. Interleaving is done in D3.1 in both the time and frequency domains, further improving robustness. LDPC’s effectiveness at error correction comes at a cost, however. The overhead associated with it is 12.25% compared to 4.69% for R-S FEC used in D3.0 (assuming full length codewords).

There is also overhead associated with the cyclic prefix (Ncp) which is used to minimize intersymbol interference caused by micro-reflections. A configurable number of samples from the end of the OFDM


symbol are replicated and placed at the beginning of the symbol as overhead. Any echoes created by micro-reflections impact these overhead samples as opposed to actual traffic. The cyclic prefix configuration can vary in set values from 192 to 1024 samples.

Finally, with OFDM, some subcarriers need to handle overhead functions not related to transporting user’s data traffic. These include the physical layer link channel or PHY link channel (PLC) used to signal channel parameters to modems; pilots used for frequency and phase tracking, and for estimation of channel frequency response as part of adaptive equalization; (the pattern of pilots in the PLC band denote the location of the PLC); subcarriers that are nulled out as part of guard bands or exclusion bands; and next codeword pointer (NCP) messages used to identify when one FEC codeword ends and the next one begins.

DOCSIS Downstream Speeds As mentioned previously, the main driver for Midco’s move to D3.1 is increased downstream speeds. It is therefore fundamental to understand the actual speeds we can achieve with OFDM downstream channels and how this compares to DOCSIS 3.0. Speed testing served as the measuring point for how effective D3.1 was working. Note that a D3.0 downstream channel or video QAM channel is now referred to as single carrier QAM (SC-QAM) since the OFDM channel is comprised of many subcarriers. SC-QAM and D3.0 downstream are used interchangeably in this paper.

The usable speed of a channel is found by multiplying the symbol rate by the modulation order (how many bits per symbol are carried by each signal) and then subtracting DOCSIS overhead.

Formula 1 - usable data rate = symbol rate x modulation order x (1 – overhead %)

1. DOCSIS 3.0 Speeds

With DOCSIS 3.0 standards and earlier, we have options for 256-QAM and 64-QAM in the downstream. We can find the modulation order by taking the log2 of the QAM modulation order. A 256-QAM signal carries 8 bits of information per symbol [log2(256) = 8] while a 64-QAM signal carries 6 bits of information per symbol [log2(64) = 6]. (Note: If you don’t have “log2” function, you can take the natural log (ln) of the QAM modulation order and divide by ln(2)).

The symbol rate is determined by the usable amount of channel spectrum. Signals do not drop off instantaneously at the edge of the channel so we need to allow for filter roll off to prevent the signal interfering with adjacent carriers. Usable spectrum then varies with the value for this roll off, alpha (α), and the formula channel width / (1+ α). The alpha specification used in Annex B for North America is 12% for 256-QAM. Therefore the symbol rate for a 6 MHz downstream channel running 256-QAM is 6 MHz / (1 + 0.12) = 5.36 megasymbols / second.

Overhead for the downstream physical layer (PHY) in DOCSIS 3.0 and earlier standards is attributed to R-S FEC, MPEG-2 encapsulation, and Trellis coded modulation (TCM).

In Annex B downstreams, R-S FEC uses 128 / 122 code where there are 6 symbols of overhead for every 128 symbols which is 4.69%. MPEG encapsulation has 4 bytes of overhead for every 188 byte frame, resulting in 2.13% of overhead. Trellis coded modulation for 256-QAM uses 19/20 for 5% overhead. Note that some items like FEC sync have relatively low overhead (40 bits out of every 78,848) and thus


only minor impact. We multiply the efficiency of each item (1 – overhead %) to determine the total efficiency of 88.57%. Total DOCSIS PHY overhead is then 1 – 88.57% or 11.43%. For our calculations, this is rounded up to 12% to factor in for some DOCSIS MAC overhead present in all downstream channels as described below.

Above the PHY layer, overhead can vary dramatically based on configurations and frame size. Ethernet frames will have 18 bytes of overhead whether the frame size is 46 or 1500 bytes so the overhead percentage can vary from under 2% to almost 40%. There will also be overhead associated with IP packets. Typical traffic generating test equipment like the unit used in our testing will count overhead associated with Ethernet and IP framing and thus will report values reflecting the usable data rate (speed after DOCSIS overhead). Therefore, we did not attempt to estimate this overhead.

Items associated with DOCSIS MAC control messages do take away from the usable bit rate and are not factored in by our test set. For D3.0 and below, 250 kilobits per second (kbps) overhead for messages like MAC domain descriptor (MDD) on a non-primary downstream channel may be typical. Since we round overhead to 12% these will already be factored into our calculation. For primary channels which carry upstream channel descriptor (UCD) and bandwidth allocation MAP messages, the additional overhead is usually 250 kbps per upstream channel (e.g. eight upstream channels in a MAC domain will take up 2 Mbps of overhead on each primary downstream channel).

Using formula 1 for a 6 MHz downstream channel running 256-QAM we get:

Usable date rate = (5.36 M symbols / sec x 8 bits / symbol) x (1 – 12% overhead) = 37.7 Mbps

A bonded DOCSIS 3.0 channel would then equal the number of channels bonded multiplied by the usable data rate of each channel.

Formula 2 – DOCSIS 3.0 bonded channel usable data rate = number of channels x [symbol rate x modulation order x (1-overhead %)]

2. DOCSIS 3.1 Speeds

Although the technology is significantly different in D3.1, the way we calculate channel speed is similar to what was just described for D3.0. We use the same concept as in formula 2: number of channels x [symbol rate x modulation order x (1-overhead %)].

Modulation order is easy to determine. The downstream modulation options in DOCSIS 3.1 go beyond 256-QAM and include 512-QAM, 1024-QAM, 2048-QAM and 4096-QAM (8, 9, 10, 11, 12 bits per symbol respectively).

The symbol rate is also easy to determine. The DOCSIS 3.1 OFDM channel is composed of many small subcarriers, either 25 kHz or 50 kHz in width, which are synchronized and spaced orthogonal to each other to prevent interference. This enables the use of the entire subcarrier spectrum. Guard bands for roll off are only required at the edges of the OFDM channel or if exclusion zones are applied around legacy carriers inside the channel. This means that within our OFDM channel, the symbol rate will be the full 25 kHz or 50 kHz subcarrier.

There might be a few ways to consider overhead for DOCSIS 3.1 but it is easiest to divide it into two types. First, there is overhead that decreases the number of subcarriers available for data traffic such as


pilots, guard bands, and other items that consume entire subcarriers. Second, there is overhead that impacts every subcarrier like LDPC and the cyclic prefix (Ncp).

Within OFDM there will be subcarriers used for overhead functions and not available for user data. The synchronization of subcarriers mentioned above uses pilot carriers. Continuous pilots are spaced at set locations within the spectrum block and the number of subcarriers is configurable between 48 and 120 (8 additional pilots are part of the PLC and not configurable). This configuration is relative to a full 192 MHz wide channel and will scale down if the channel size is reduced. There is a PLC, used to signal channel parameters to modems, that is 400 kHz wide. There are also scattered pilots that move over time throughout the channel and occur once every 128 subcarriers. If they were to land on a continuous pilot, the scattered pilot is not used.

It is possible to run multiple OFDM downstream profiles to optimize the channel. To enable this, we use a NCP message to identify when one FEC codeword ends and the next one, possibly running a different profile, begins. Each NCP is 3 bytes long and there may be a number of them in each OFDM symbol plus a cyclic redundancy check (CRC). The number of subcarriers needed for NCP will vary since the modulation order is configurable to QPSK, 16-QAM or 64-QAM. We also have LDPC associated with the NCP that takes even more overhead. If there were two NCP messages running 16-QAM, we would require 36 subcarriers (includes two NCP plus a CRC).

The roll-off period for the edges of the OFDM channel are configurable in D3.1 in set values between 64 and 256 samples. The larger the roll-off period in time, the smaller the guard band requirement in the frequency domain. There are recommended guard bands settings (also called taper regions) in Appendix V of the DOCSIS 3.1 PHY specification. The Cisco CCAP equipment used in testing followed these recommendations. Subcarriers that fall in the guard band are nulled so as not to interfere with adjacent carriers. The D3.1 PHY specification requires that the roll-off period be less than the cyclic prefix.

For the second part of overhead that applies to each subcarrier, there is LDPC FEC and cyclic prefix. LDPC FEC is more robust than R-S FEC used earlier but does require more overhead. With the recommended 8/9 code, we will need to transmit 16,200 bits for every 14,216 bits of traffic (note BCH correction codes are used with LDPC and included in this calculation as is the codeword header). With 1984 bits of overhead in 16,200 bit of traffic, FEC now creates 12.25% overhead. The cyclic prefix configuration can vary from 192 to 1024 samples. (A 204.8 MHz sampling rate is used which corresponds to the OFDM spectrum of the fast Fourier transform (FFT) of either 4096 for 50 kHz subcarriers or 8192 for 25 kHz subcarriers and beyond the scope of this paper). With 50 kHz subcarriers, the overhead with cyclic prefix can vary from 4.48% up to 20% (overhead is half this with 25 kHz subcarriers). MPEG-2 encapsulation and Trellis coded modulation are not done in the D3.1 downstream so are not part of the DOCSIS PHY overhead.

If we assume we are using 50 kHz subcarriers, 1024-QAM and a cyclic prefix of 256, then each subcarrier would provide ~415 kbps (50 k symbols / sec x 10 bits / symbol x (1-17% overhead)). We can then just multiply this value by the number of subcarriers available to carry data to estimate the speed of the OFDM channel.

There are many configuration settings that will impact the number of usable subcarriers making it difficult to calculate this number with a simple formula. Not only can values within a channel vary, like the number of pilots, but the entire channel width can vary. Some things like the number of NCP messages can vary and can only be estimated. Therefore speed estimates for an OFDM downstream is best done using a spread sheet like the one shown below. This only provides an estimate of data speed


after removing the D3.1 PHY parameters but was close enough for our testing. You can email the authors of this paper if you are interested in obtaining a copy. Highlighted values can be changed based on configuration settings. The spread sheet assumes that the modulation order is applied to all subcarriers in the channel. It also only attempts to exclude overhead associated with D3.1 PHY parameters not with DOCSIS MAC control traffic.

Table 1 - OFDM Downstream Data Rate Estimate

4K FFT 8K FFT

Size of channels (MHz) ‐ 24‐192 MHz1 144 144 MHz

FFT size (4K or 8K FFT) 4096 8192 subcarriers

Subcarrier spacing 50 25 kHz

Roll‐off 192 192 samples2

Cyclic prefix (Ncp) 256 256 samples2

Ncp overhead 6% 3%

Guard band on upper and lower edge (MHz)3 1.325 1.1625 MHz

Number of active subcarriers 2827 5667 subcarriers

PLC overhead (number of subcarriers) 8 16 subcarriers

Continuous Pilot Scaling (48 ‐ 120 subcarriers) 48 48 subcarriers

Continuous Pilots (include pilots for PLC) 44 44 subcarriers

Scattered Pilots (estimate) 22 44 subcarriers

Num of NCP (estimate >0) 2 2

QAM order of NCP (QPSK, 16QAM, 64QAM) 6 6 bits / sym

NCP overhead (including CRC) 24 24 subcarriers

FEC overhead 12% 12% 8/9 code

Data QAM order (bits per symbol) 10 10 bits / sym

Data Rate (Mbps) 1127 1178 Mbps

Overhead % based on active subcarriers 20% 17%

1 If using exclusion bands, reduce channel size by amount of spectrum excluded for data rate2 sampling rate is 204.8 MHz (based on OFDM spectrum ‐ FFT size x subcarrier width)

3 Note that guard bands are based on Appendix V of D3.1 PHY spec based on roll‐off period samples

OFDM Downstream

With an accurate estimate of the data rates we could obtain with an OFDM channel, we now set about optimizing the configuration options. For convenience, Appendix V of the D3.1 PHY specification is included as Table 2 – Guard Bands (Taper Regions) per Roll-Off Period Sample below so you can populate the guard band settings.


Table 2 – Guard Bands (Taper Regions) per Roll-Off Period Sample

Appendix V CCAP Proposed Configuration Parameters (Informative)

Optimizing OFDM Parameters Once we put together the spread sheet in Table 1 - OFDM Downstream Data Rate Estimate to determine data rates of an OFDM channel, we could go about optimizing the configuration parameters to maximize channel speeds in the lab and then confirm such a setting would work in the field trail.

We would be deploying a 144 MHz OFDM channel in the field test so this value would be fixed (even if we varied channel sizes between 24 MHz and 192 MHz for testing in the lab). Pilot scaling was already at the minimum and initially some modem firmware had issues with 25 kHz subcarriers so we focused more on roll-off and cyclic prefix. We also used 16-QAM for the NCP modulation since 64-QAM was not supported on all modem firmware at the time.

A high roll-off setting in time would reduce the guard band size requirement in the frequency domain. Guard bands (or taper region) could be as small as 1 MHz per edge or as high as 3.575 MHz as shown in Table 2 – Guard Bands (Taper Regions) per Roll-Off Period Sample. Smaller guard bands means more spectrum available for data subcarriers. So at first thought it made sense to maximize that setting. However, the roll-off setting must be lower than the cyclic prefix setting. And a higher cyclic prefix, although making the channel more robust to intersymbol interference, can create much more overhead per subcarrier.

Speeds for a 144 MHz channel would be optimized by running the lowest cyclic prefix setting our plant could support (minimizing overhead per subcarrier) even if it meant losing more channel spectrum to larger guard bands. We would start off running a cyclic prefix of 192 samples and roll-off of 128 which created a 1.875 MHz guard band on each edge. We would increase the cyclic prefix if needed in deployments to deal with micro-reflections and then increase the roll-off period to reduce guard bands accordingly.


Note that for channel widths below 96 MHz, the channel speed would be optimized by increasing the number of subcarriers using a smaller guard band even though overhead on each increases with higher cyclic prefix. A cyclic prefix of 256 and roll-off of 192 would provide the highest data rate.

An OFDM channel profile was deployed that included multiple data modulation profiles. This used a control profile of 256-QAM as the base (used for all DOCSIS control traffic and also available for user data traffic). Three additional data profiles were also included. The first was 1024-QAM and would be the default used by modems for data traffic. The second and third were 2048-QAM and 4096-QAM. Via OFDM downstream profile test (OPT) messages, the CCAP equipment gathers the receive modulation error ratio (RxMER) from each modem for each subcarriers. Based on this information, the CCAP equipment then recommends which of the modulation profiles for the channel would best fit each modem. Mixed data modulation profiles where different subcarriers in the same data profile could run different modulation orders were supported but not required in our field test.

As mentioned earlier, interleaving takes place in both time and frequency to help LDPC better correct bursty noise. The frequency domain interleaving is not configurable but the time domain interleaving can be set from 1 – 32 for 50 kHz subcarriers and 1 – 16 for 25 kHz subcarriers. These changes do no impact channel speed but could introduce delay. The default value of 16 was used for testing and proved adequate.

Midco Deploys DOCSIS 3.1 In the Field In March 2016, Midco started the D3.1 field trial in Fargo, ND. As shown in Figure 1 – Midco Fargo, ND DOCSIS 3.1 Trial, a 144 MHz OFDM downstream channel was added in vacant spectrum starting at 558 MHz. This spectrum was located adjacent to video QAM channels below 558 MHz and D3.0 SC-QAM channels above 702 MHz. D3.1 modems proved very difficult to obtain at this time so testing was limited to only a handful of demo units at employees’ homes as well as in the headend.

Figure 1 – Midco Fargo, ND DOCSIS 3.1 Trial

A D3.1 bonding group was configured to include the 144 MHz OFDM channel and eight D3.0 SC-QAM downstream channels. The modem would share the four 6.4 MHz D3.0 bonded upstream channels with existing modems on this return segment. This would provide downstream capacity of approximately 1430 – 1650 Mbps depending on modulation orders we could achieve. D3.0 modems in this market already had sixteen downstream channels available for bonding.

In one home, a speed test set was installed to enable extended remote speed testing. The test equipment used two gigabit Ethernet interfaces connected to the modem so maximum downstream speed could go


beyond 1 Gbps. Modulation orders of 1024-QAM and 2048-QAM worked well. LDPC correctable codeword errors were observed on the modem but no uncorrectable errors were seen. Testing of 4096-QAM initially failed but after an upgrade to modem firmware, we were able to successfully run this modulation as well. Codeword correctable error counts were high proving that LDPC was working on correcting at least one bit error in almost every codeword (typically >85% for 4096-QAM). The test modems in Fargo have been running 4096-QAM since. The Fargo test modems were often on relatively short coax runs with one amplifier at most.

Although the cyclic prefix of 192 provided the fastest speeds in the lab for a 144 MHz channel, it proved to be too aggressive for some modems in the field when testing started in Sioux Falls. A change to a cyclic prefix of 256 increased the average modem RxMER value by 1 – 3 dB. This increase in overhead from the cyclic prefix was partially offset by increasing the roll-off sample to 192 which reduced the guard band size. The net impact on OFDM data rates for this change was less than 10 Mbps, so this setting became the default for all modems.

Some modems with longer coax runs or amplifier cascades of four or more, saw additional RxMER improvements when the cyclic prefix was set to 512. However, even when the roll-off was increased to the maximum of 256, this setting reduced the OFDM channel speed by approximately 60 Mbps depending on modulation order. This setting was only used when required on service groups with higher amplifier cascades.

The default data modulation profile selection algorithm used by the CCAP equipment was based on table7-41 in the D3.1 PHY specification and is included below as Table 3 – CM Minimum CNR Performance in AWGN Channel. As shown, a suggested value of 41 dB CNR or higher is recommended to run 4096-QAM and 37 dB CNR or higher to run 2048-QAM. This seemed to be a bit too conservative for our HFC plant and was set 2 dB lower (39 dB for 4096-QAM and 35 dB for 2048-QAM). The algorithm also can ignore a percentage of subcarriers in its calculation. The default of 2% also seemed conservative so was adjusted to 10%. These changes were made since modems could receive data running at high speeds with no uncorrectable FEC errors running modulations higher than would have been recommended based on RxMER values. These values will continued to be optimized with future testing.


Table 3 – CM Minimum CNR Performance in AWGN Channel

Midco Field Trial – Summary To Date This paper represents information from the Fargo and Sioux Falls field trial through June 2016.

Support for 1024-QAM was always expected in DOCSIS 3.1 but being able to reliably run modulations up to 4096-QAM on the existing HFC plant was a pleasant surprise. Eight D3.0 downstreams combined with 144 MHz OFDM using 4096-QAM provides approximately 1650 Mbps of downstream capacity for D3.1 modems. The test set behind our Fargo modem was able to run sustained downstream rates of 1570 Mbps when using an Internet mix of packet sizes (the modem was provisioned without downstream speed limits).

LDPC FEC combined with interleaving proved to be very effective at error correction. Although it was typical to count codeword correctable error rates of 85% or higher when using 4096-QAM, uncorrectable codeword errors were rarely seen.

D3.1 test modems proved more difficult to obtain than expected especially for the lab phase that started in September 2015. Modem performance and features supported were highly dependent on firmware. This was not totally unexpected with such a new technology but did sometimes make isolating problems more difficult. It was also sometimes difficult to locate the most current modem firmware. In general, modems based on one silicon provider’s chipsets seemed to perform best with the configuration used by Midco so initial field testing used only modems utilizing that provider’s silicon as implemented by multiple vendors. Correctable and uncorrectable codeword error counts from the modem were critical in determining the highest modulation order we could support. SNMP MIB values for this information was inaccurate which limited gathering this information only from modems that supported telnet access. That should be resolved in future firmware releases.


Future plans around OFDM optimization include moving to 25 kHz subcarriers to reduce the number of subcarriers required by overhead functions and to use 64-QAM for the NCP modulation.

More D3.1 modems will need to be deployed to get a better sampling of plant performance. It is also expected that optimization of cyclic prefix, roll-off values and maybe other values such as interleaving will continue as a better understanding on the HFC plants impact on OFDM is known.

The RxMER values returned for each subcarrier by every modem proved to be a very good tool to provide a detailed view of HFC plant performance over the OFDM spectrum. This will no doubt be added to future PNM applications.

Conclusion Midco can reliably run modulation orders of 1024-QAM, 2048-QAM and 4096-QAM on their existing HFC plant proving that DOCSIS 3.1 is an effective way to provide gigabit broadband downstream speeds and beyond. LDPC proved very effective at correcting codeword errors. However, high counts of correctable codeword errors will now be the norm.

Downstream speeds of 1570 Mbps were achieved behind a D3.1 cable modem using an Internet mix of packet sizes running over a 144 MHz OFDM channel combined with eight D3.0 SC-QAM downstream channels.

Additional D3.1 modems need to be deployed to get a better sampling of the plant performance and allow continued OFDM parameter optimization; however, initial results are encouraging. It is also important to understand if performance will vary over time with changes in environmental conditions.

Moving forward, Midco will continue to grow DOCSIS 3.1 capacity as needed to accommodate broadband requirements.

Abbreviations

BCH codes Error correcting codes invented independently by Raj Bose, K Ray-Chaudhuri, Alex Hocquenghem (BCH initial of last names)

bps bits per second CCAP converged cable access platform CRC cyclic redundancy check D3.0 DOCSIS 3.0 D3.1 DOCSIS 3.1 DOCSIS Data-Over-Cable Service Interface Specification FEC forward error correction FFT fast Fourier transform FTTH fiber to the home HFC hybrid fiber coax Hz hertz IP Internet protocol ISP Internet service provider


LDPC low density parity check ln natural log MAC media access control MDD MAC domain descriptor MPEG Moving Picture Experts Group MSO multiple system operator NCP next codeword pointer Ncp cyclic prefix sample OFDM orthogonal frequency division multiplexing OPT OFDM downstream profile test PHY physical layer PLC physical layer link channel PNM proactive network maintenance QAM quadrature amplitude modulation R-S FEC Reed-Solomon forward error correction RxMER receive modulation error ratio SC-QAM single carrier quadrature amplitude modulation UCD upstream channel descriptor

Bibliography & References CM-SP-PHYv3.1-I09-160602: Data-Over-Cable Service Interface Specifications, DOCSIS 3.1, Physical Layer Specification

CM-SP-MULPIv3.1-I09-160602: Data-Over-Cable Service Interface Specifications, DOCSIS 3.1, MAC and Upper Layer Protocols Interface Specification

CM-SP-PHYv3.0-I12-150305: Data-Over-Cable Service Interface Specifications, DOCSIS 3.0, Physical Layer Specification

docsis 3.1 leaves the lab and hits the field with midco · 2019-12-08 · maintenance (pnm) tools...

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