DOCSIS 3.1 - How to achieve maximum throughput
The new version of the DOCSIS specification - DOCSIS 3.1, completely changed the principles of DOCSIS operation, increasing the channel capacity by 50%, performance up to 10 Gb / s in the forward channel and up to 2 Gb / s in the reverse - speeds comparable to fiber data transmission.
DOCSIS 3.1 provides more bits per 1 Hz compared to DOCSIS 3.0 with the same signal-to-noise ratio
The DOCSIS 3.1 specification was released and successfully tested under laboratory conditions in 2015. At the beginning of 2016, 5 new cable modems that support the DOCSIS 3.1 standard were certified, providers around the world began to introduce and test equipment of this standard.
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But what makes DOCSIS 3.1 unique compared to earlier versions, and how will testing methods change in this regard? This article discusses the two main technologies used in the latest version of the specification: orthogonal frequency division multiplexing (OFDM) and low density parity check (LDPC) code multiplexing. The article also describes methods for achieving maximum levels of performance.
The easiest way to understand OFDM is to remember how DOCSIS 3.0 works. There, for one direct channel, one carrier frequency is used with a 6 MHz band (8 MHz in Europe). For modulation of this frequency, single-carrier QAM (SC-QAM) is used and the symbols are transmitted at this frequency strictly sequentially. If there are problems with signal reception, then the modulation must be reduced - not only for this frequency, but also for all other channels in the network. This means that the modulation must be optimized for the worst part of the coaxial network.
In contrast to SC-QAM, OFDM uses a bandwidth from 24 to 192 MHz. Up to 8,000 subcarriers with a width of 25 to 50 kHz each can be placed inside this band. ( More precisely, 7680 subcarriers 25 kHz or 3840 subcarriers 50 kHz - approx. Translator ). All subcarriers are synchronized with each other in time and form a single set of characters. These symbols, in turn, are distributed over subcarriers and time slots and transmit code words.
The main advantage of this approach is that the symbols are transmitted simultaneously at different frequencies. This creates some unique opportunities. So, if interference occurred on the same subcarrier, OFDM simply eliminates it by combining adjacent frequencies. This allows you to continue data transfer with an optimal level of performance. ( In addition, this transmission method is much less sensitive to narrowband and impulse noise, as they affect only some subcarriers, whereas in the case of a normal signal, interference affects the whole of its spectrum - comment of the translator )
Since the type of modulation in OFDM is set for a certain period of time, this technology allows you to control the mutual ratio of the phases of subcarriers. If one subcarrier is at its peak, then the next one can be configured in antiphase, i.e. at zero. This reduces the interference between adjacent subcarriers and allows them to use higher levels of modulation and, consequently, increase the overall network throughput. Instead of using one modulation level for the entire range, OFDM allows different modulation levels to be used for each subcarrier. In addition, you can create modulation profiles in such a way as to set individual modulation levels for all subcarriers and to have several such profiles.
Modulation - SC QAM
Dedicated channels with a bandwidth of 6 MHz (8 MHz in Europe).
Each frequency channel is independent of the others.
Characters in one channel are transmitted sequentially.
Modulation is optimized for the worst part of the cable network.
Take one subcarrier as an example. Each profile has its own modulation level (for example, 64 QAM, 1024 QAM, 2048 QAM or 4096 QAM). OFDM can use the highest profile for this HFC network segment. In one segment it will be 4096 QAM, in another it may be 1024 QAM. In the third segment at this frequency there may be too much interference and this part of the spectrum will be excluded from the profile altogether, etc.
Now let's see what happens on this subcarrier in order to understand the operation of all 8000. A separate profile describes a separate subcarrier in order to achieve its maximum performance in each time period.
As indicated above, all subcarriers are combined with each other for the joint transmission of symbols from which the code words are formed. Subcarriers are attached to each character of the code word and their modulation level is described by a profile. Profiles, in turn, are assigned letters (for example, A, B, C, and D). Thus, it turns out that optimization is performed not only for each subcarrier separately, but also for all 8000 subcarriers in the complex.
Instead of optimizing modulation for the worst part of the network, it can be optimized for the best part at any given time. This makes DOCSIS 3.1 a much more efficient technology than its predecessors. Where a channel on DOCSIS 3.0 could transmit 6.3 bits per 1 Hz, DOCSIS 3.1 can reach 10.5 bits per 1 Hz using 4096 QAM modulation. More typically, when multiple modulation levels are used at the same time, DOCSIS 3.1 can reach 8.5 bits per 1 Hz, providing an increase in efficiency of 35% without changes in the HFC network.
Improvements achieved using OFDM would not have been possible without the use of error correction algorithms. DOCSIS 3.0 uses a forward error correction algorithm with a Reed-Solomon code (FEC) and measures the level of bit errors (BER). BER refers to a single carrier, and OFDM uses a lot. Due to the fact that OFDM distributes transmitted data across multiple subcarriers, the use of BER no longer makes sense.
DOCSIS 3.1 uses LPDC instead of FEC. This algorithm works over the entire range and estimates the errors not of individual bits, but of the entire code words. If this error can be corrected, LPDC automatically does this, which allows the use of higher modulation levels and significantly reduces the need to retransmit code words. LPDC brings the channel throughput to the theoretical limits described by the Shannon theorem.
But LDPC has one flaw. Since this algorithm changes the settings in real time, the system can reach maximum values ​​for power and modulation levels by correcting the errors that occur. This means that the network will degrade imperceptibly for the operator and at some point the errors will become uncorrectable, and users will notice a decrease in the quality of service. In order to avoid such a situation, it is necessary to test the system more thoroughly.
For testing to be successful, it is very important to understand what OFDM consists of. All of this is based on the PLC - PHY link channel level, which contains information on how to decode the OFDM signal. Without this level, the modem cannot “see” the OFDM carrier and understand how to decode it. A level above is a pointer to the next code word (next codeword pointer - NCP), which tells the modem which code word should be read next and which profile to use to decode each code word. Next comes Profile A. This is a boot profile that each DOCSIS 3.1 modem must be able to use in order to “understand” higher QAM modulation levels in other profiles.
Profiles - a simplified situation. For simplicity, let's assume that profiles use the same modulation on all subcarriers.
The parameters of power levels, MER and noise in profile A are selected for reliable OFDM operation. If this profile works, then standard profiles B, C and D can be used. Profiles other than them can be created by CMTS and cable modem manufacturers at their own discretion and their number is unlimited.
When transmitting PLC level information, it is important to ensure that there are no uncorrectable codeword errors (CWE). At the PLC level, information transfer should be as reliable as possible, therefore the power level and MER should be strictly in the specified range. For this, the parameters of this level must be strictly fixed - the DOCSIS 3.1 specification restricts the use of BPSK or 16 QAM for PLCs.
If everything works without errors at the PLC level, the NCP parameters are also fixed and should not allow uncorrectable errors (CWE). If there is a loss of messages at this level, the modem will re-request information or, even worse, there will be no connection at all. In DOCSIS 3.1, only QPSK, 16 QAM or 64 QAM can be used to transmit NCP.
Since profile A is bootable, it is assigned lower modulation levels than others: QAM 16 and QAM 64. This is done so that all modems can work even in the worst part of the cable network. A signal with a lower modulation level can operate at lower power levels and MER. As well as the two previous levels, profile A must have fixed parameters and avoid uncorrectable errors. If uncorrectable errors appear, the modem will go into DCOSIS 3.0 mode and there will be no increase in efficiency. Profile A can also work at higher modulation levels, with correctable CWE errors allowed, this is normal, as long as there are no uncorrectable ones.
Profiles - the real situation. OFDM eliminates certain subcarriers and allows each to have different modulation levels for different subcarriers. This optimizes the overall bandwidth of the channel - each profile has its own exceptions.
When all 3 levels are working within the specified limits, you can look at the overall bandwidth. One of the errors at this stage may be the measurement of the signal level in the entire 192 MHz band. It must be remembered that the total power in a given band of the spectrum is equal to the power of a 6 MHz signal, taking into account the bandwidth. Thus, the total OFDM signal power is very different from the power of a single carrier with a width of 6 (8) MHz. In order to more accurately adjust the OFDM signal power, all levels should be measured relative to the signal power with a 6 MHz bandwidth.
OFDM has some more unique characteristics. The levels of the first and last 6 MHz in a given OFDM signal bandwidth will be approximately 0.8 dB less than the levels of the other subcarriers due to a fall in the guard band. This becomes important when standard instruments are used for measurement, or if power is measured in the 6 MHz frequency band allocated from the general range. In addition, the carrier with PLC is about 0.8 dB higher than other subcarriers due to the additional pilot signals and transmitted data. Thus, the overall flatness of the OFDM signal compared to the standard 6 MHz signal will fluctuate within 1.6 dB due to the initial and final drops and the effect of PLC.
In order for OFDM to operate at peak performance, the average power level must not exceed the specified limits, the MER must be good and the noise levels must be minimal. Noise greatly affects the OFDM signal and can lead to the fact that profiles with high modulation levels will not be used at all.
If all these requirements are met, then it becomes possible to use profiles with high modulation levels. It is important that the parameters are fixed within the profile (locked). Profiles with a high level of modulation may have a number of corrected errors (CWE), as this is not as critical as for lower levels, but uncorrectable errors will result in maximum performance not being achieved. For example, if C profile has uncorrectable errors, D and higher profiles will not be able to use higher modulation than C profile. To achieve high modulation levels, the HFC network must be clean and not allow uncorrectable errors to occur (which is also true for earlier versions DOCSIS).
DOCSIS 3.1 uses OFDMA - Orthogonal Frequency-Division Multiple Access for the return channel.
Individual subcarriers in OFDMA can be turned off to ensure backward compatibility with DOCSIS 2 / 3.0 channels
DOCSIS 3.1 solves the main dilemma that operators have been facing for many years: “To spend money on a complete upgrade of the entire cable network or gradually make changes to the existing network?” Using OFDM and LDPC technologies, operators can significantly increase network bandwidth with its minimal upgrade .
A small modernization of the physical structure of the network is enough to increase its efficiency (speed and throughput) by 35% using DOCSIS 3.1. This will also give operators additional time for further gradual upgrades, which, in turn, will provide an opportunity to further increase capacity.
However, operators need to be fairly careful in implementing and testing DOCSIS 3.1. If this is done incorrectly, then there will be no improvement compared to DOCSIS 3.0.
Using the practices described in this article will help ensure that DOCSIS 3.1 is used as efficiently as possible, reducing the number of maintenance crews leaving and providing high quality service to customers.
The next step will be the implementation of the DOCSIS 3.1 Full duplex specification, which will ensure symmetric data transmission at a speed of 10Gb / s in both the forward and reverse channel.
Original article here .
DOCSIS 3.1 provides more bits per 1 Hz compared to DOCSIS 3.0 with the same signal-to-noise ratio
The DOCSIS 3.1 specification was released and successfully tested under laboratory conditions in 2015. At the beginning of 2016, 5 new cable modems that support the DOCSIS 3.1 standard were certified, providers around the world began to introduce and test equipment of this standard.
')
But what makes DOCSIS 3.1 unique compared to earlier versions, and how will testing methods change in this regard? This article discusses the two main technologies used in the latest version of the specification: orthogonal frequency division multiplexing (OFDM) and low density parity check (LDPC) code multiplexing. The article also describes methods for achieving maximum levels of performance.
Orthogonal Frequency Domain Multiplexing
The easiest way to understand OFDM is to remember how DOCSIS 3.0 works. There, for one direct channel, one carrier frequency is used with a 6 MHz band (8 MHz in Europe). For modulation of this frequency, single-carrier QAM (SC-QAM) is used and the symbols are transmitted at this frequency strictly sequentially. If there are problems with signal reception, then the modulation must be reduced - not only for this frequency, but also for all other channels in the network. This means that the modulation must be optimized for the worst part of the coaxial network.
In contrast to SC-QAM, OFDM uses a bandwidth from 24 to 192 MHz. Up to 8,000 subcarriers with a width of 25 to 50 kHz each can be placed inside this band. ( More precisely, 7680 subcarriers 25 kHz or 3840 subcarriers 50 kHz - approx. Translator ). All subcarriers are synchronized with each other in time and form a single set of characters. These symbols, in turn, are distributed over subcarriers and time slots and transmit code words.
The main advantage of this approach is that the symbols are transmitted simultaneously at different frequencies. This creates some unique opportunities. So, if interference occurred on the same subcarrier, OFDM simply eliminates it by combining adjacent frequencies. This allows you to continue data transfer with an optimal level of performance. ( In addition, this transmission method is much less sensitive to narrowband and impulse noise, as they affect only some subcarriers, whereas in the case of a normal signal, interference affects the whole of its spectrum - comment of the translator )
Since the type of modulation in OFDM is set for a certain period of time, this technology allows you to control the mutual ratio of the phases of subcarriers. If one subcarrier is at its peak, then the next one can be configured in antiphase, i.e. at zero. This reduces the interference between adjacent subcarriers and allows them to use higher levels of modulation and, consequently, increase the overall network throughput. Instead of using one modulation level for the entire range, OFDM allows different modulation levels to be used for each subcarrier. In addition, you can create modulation profiles in such a way as to set individual modulation levels for all subcarriers and to have several such profiles.
Modulation - SC QAM
Dedicated channels with a bandwidth of 6 MHz (8 MHz in Europe).
Each frequency channel is independent of the others.
Characters in one channel are transmitted sequentially.
Modulation is optimized for the worst part of the cable network.
Take one subcarrier as an example. Each profile has its own modulation level (for example, 64 QAM, 1024 QAM, 2048 QAM or 4096 QAM). OFDM can use the highest profile for this HFC network segment. In one segment it will be 4096 QAM, in another it may be 1024 QAM. In the third segment at this frequency there may be too much interference and this part of the spectrum will be excluded from the profile altogether, etc.
Now let's see what happens on this subcarrier in order to understand the operation of all 8000. A separate profile describes a separate subcarrier in order to achieve its maximum performance in each time period.
As indicated above, all subcarriers are combined with each other for the joint transmission of symbols from which the code words are formed. Subcarriers are attached to each character of the code word and their modulation level is described by a profile. Profiles, in turn, are assigned letters (for example, A, B, C, and D). Thus, it turns out that optimization is performed not only for each subcarrier separately, but also for all 8000 subcarriers in the complex.
Instead of optimizing modulation for the worst part of the network, it can be optimized for the best part at any given time. This makes DOCSIS 3.1 a much more efficient technology than its predecessors. Where a channel on DOCSIS 3.0 could transmit 6.3 bits per 1 Hz, DOCSIS 3.1 can reach 10.5 bits per 1 Hz using 4096 QAM modulation. More typically, when multiple modulation levels are used at the same time, DOCSIS 3.1 can reach 8.5 bits per 1 Hz, providing an increase in efficiency of 35% without changes in the HFC network.
Low Density Parity Check
Improvements achieved using OFDM would not have been possible without the use of error correction algorithms. DOCSIS 3.0 uses a forward error correction algorithm with a Reed-Solomon code (FEC) and measures the level of bit errors (BER). BER refers to a single carrier, and OFDM uses a lot. Due to the fact that OFDM distributes transmitted data across multiple subcarriers, the use of BER no longer makes sense.
DOCSIS 3.1 uses LPDC instead of FEC. This algorithm works over the entire range and estimates the errors not of individual bits, but of the entire code words. If this error can be corrected, LPDC automatically does this, which allows the use of higher modulation levels and significantly reduces the need to retransmit code words. LPDC brings the channel throughput to the theoretical limits described by the Shannon theorem.
But LDPC has one flaw. Since this algorithm changes the settings in real time, the system can reach maximum values ​​for power and modulation levels by correcting the errors that occur. This means that the network will degrade imperceptibly for the operator and at some point the errors will become uncorrectable, and users will notice a decrease in the quality of service. In order to avoid such a situation, it is necessary to test the system more thoroughly.
Achieve maximum network bandwidth
For testing to be successful, it is very important to understand what OFDM consists of. All of this is based on the PLC - PHY link channel level, which contains information on how to decode the OFDM signal. Without this level, the modem cannot “see” the OFDM carrier and understand how to decode it. A level above is a pointer to the next code word (next codeword pointer - NCP), which tells the modem which code word should be read next and which profile to use to decode each code word. Next comes Profile A. This is a boot profile that each DOCSIS 3.1 modem must be able to use in order to “understand” higher QAM modulation levels in other profiles.
Profiles - a simplified situation. For simplicity, let's assume that profiles use the same modulation on all subcarriers.
The parameters of power levels, MER and noise in profile A are selected for reliable OFDM operation. If this profile works, then standard profiles B, C and D can be used. Profiles other than them can be created by CMTS and cable modem manufacturers at their own discretion and their number is unlimited.
When transmitting PLC level information, it is important to ensure that there are no uncorrectable codeword errors (CWE). At the PLC level, information transfer should be as reliable as possible, therefore the power level and MER should be strictly in the specified range. For this, the parameters of this level must be strictly fixed - the DOCSIS 3.1 specification restricts the use of BPSK or 16 QAM for PLCs.
If everything works without errors at the PLC level, the NCP parameters are also fixed and should not allow uncorrectable errors (CWE). If there is a loss of messages at this level, the modem will re-request information or, even worse, there will be no connection at all. In DOCSIS 3.1, only QPSK, 16 QAM or 64 QAM can be used to transmit NCP.
Since profile A is bootable, it is assigned lower modulation levels than others: QAM 16 and QAM 64. This is done so that all modems can work even in the worst part of the cable network. A signal with a lower modulation level can operate at lower power levels and MER. As well as the two previous levels, profile A must have fixed parameters and avoid uncorrectable errors. If uncorrectable errors appear, the modem will go into DCOSIS 3.0 mode and there will be no increase in efficiency. Profile A can also work at higher modulation levels, with correctable CWE errors allowed, this is normal, as long as there are no uncorrectable ones.
Profiles - the real situation. OFDM eliminates certain subcarriers and allows each to have different modulation levels for different subcarriers. This optimizes the overall bandwidth of the channel - each profile has its own exceptions.
When all 3 levels are working within the specified limits, you can look at the overall bandwidth. One of the errors at this stage may be the measurement of the signal level in the entire 192 MHz band. It must be remembered that the total power in a given band of the spectrum is equal to the power of a 6 MHz signal, taking into account the bandwidth. Thus, the total OFDM signal power is very different from the power of a single carrier with a width of 6 (8) MHz. In order to more accurately adjust the OFDM signal power, all levels should be measured relative to the signal power with a 6 MHz bandwidth.
OFDM has some more unique characteristics. The levels of the first and last 6 MHz in a given OFDM signal bandwidth will be approximately 0.8 dB less than the levels of the other subcarriers due to a fall in the guard band. This becomes important when standard instruments are used for measurement, or if power is measured in the 6 MHz frequency band allocated from the general range. In addition, the carrier with PLC is about 0.8 dB higher than other subcarriers due to the additional pilot signals and transmitted data. Thus, the overall flatness of the OFDM signal compared to the standard 6 MHz signal will fluctuate within 1.6 dB due to the initial and final drops and the effect of PLC.
In order for OFDM to operate at peak performance, the average power level must not exceed the specified limits, the MER must be good and the noise levels must be minimal. Noise greatly affects the OFDM signal and can lead to the fact that profiles with high modulation levels will not be used at all.
If all these requirements are met, then it becomes possible to use profiles with high modulation levels. It is important that the parameters are fixed within the profile (locked). Profiles with a high level of modulation may have a number of corrected errors (CWE), as this is not as critical as for lower levels, but uncorrectable errors will result in maximum performance not being achieved. For example, if C profile has uncorrectable errors, D and higher profiles will not be able to use higher modulation than C profile. To achieve high modulation levels, the HFC network must be clean and not allow uncorrectable errors to occur (which is also true for earlier versions DOCSIS).
What about upstream?
DOCSIS 3.1 uses OFDMA - Orthogonal Frequency-Division Multiple Access for the return channel.
Individual subcarriers in OFDMA can be turned off to ensure backward compatibility with DOCSIS 2 / 3.0 channels
DOCSIS 3.0 and DOCSIS 3.1 Comparison Chart
Conclusion
DOCSIS 3.1 solves the main dilemma that operators have been facing for many years: “To spend money on a complete upgrade of the entire cable network or gradually make changes to the existing network?” Using OFDM and LDPC technologies, operators can significantly increase network bandwidth with its minimal upgrade .
A small modernization of the physical structure of the network is enough to increase its efficiency (speed and throughput) by 35% using DOCSIS 3.1. This will also give operators additional time for further gradual upgrades, which, in turn, will provide an opportunity to further increase capacity.
However, operators need to be fairly careful in implementing and testing DOCSIS 3.1. If this is done incorrectly, then there will be no improvement compared to DOCSIS 3.0.
Using the practices described in this article will help ensure that DOCSIS 3.1 is used as efficiently as possible, reducing the number of maintenance crews leaving and providing high quality service to customers.
What's next?
The next step will be the implementation of the DOCSIS 3.1 Full duplex specification, which will ensure symmetric data transmission at a speed of 10Gb / s in both the forward and reverse channel.
Original article here .
Source: https://habr.com/ru/post/319484/
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