CONCLUSIONS
DSL technologies provide transport of high speed digital information through the installed telephone lines. Since more and more customers are asking for high-speed connections and since new services are requiring even higher data rate, there is an actual need in DSL to extend rate and reach.
The DSL lines that are deployed today are already operating close to the Shannon limit, which means that the achievable data rate is mainly determined by the transmission medium, i.e. loop and noise environment. This implicates that no miraculous solutions are to be expected that would dramatically increase the achievable data rate without adaptations to the loop and/or noise environment.
The 2 most important strategies to increase bit rate and/or reach of DSL lines guided by the Shannon transmission limit, are the remote-terminal (RT) deployment and the
Dynamic Spectrum Management (DSM).
For the RT deployment an evolution towards shorter loops is envisioned by installing remote terminals closer to the customer premises. The increase in bandwidth is exploited by introducing new xDSL flavours such asADSL2+ and VDSL.
For DSM, instead, it is possible to increase the achievable data rate without cable plant modifications, but designing ADSL as a Multi-User System.
As mentioned in the introduction, the Single-User System design used today is conservative in the sense that realistic deployment scenarios often have interference much smaller than the worst-case noise.
Moreover, the same power-spectral-density constraint is applied to all modems regardless of their geographic locations. The lack of a mechanism to allocate
different amounts of power to different users in different locations can be problematic because of the near-far problem.
We have focused our analysis on the first level of DSM applied on an ADSL realistic network.
Designing ADSL as a Multi-User System, DSM level-1 has the following
advantages:
• Thanks to power allocation algorithm that assures coordination among all the lines in a binder, DSM level-1 assures partial crosstalk avoidance and will give a gain in rate/reach respect to the actual reference ADSL.
• DSM level-1 (together with level-2) is currently the only technique with the potential to increase the rate/reach, to a certain extent without the need of modifications to the cable plant.
• With respect to the higher levels, DSM level-1 is much faster in computation and doesn’t need any centralized controller (only a knowledge of the conditions needed to implement optimally the algorithm): in DSM level-1 the actual transmit PSD’s are computed in each transceiver, hence the multi-user power control is distributed and no central agent needs to be configured.
• A good approximation of the algorithm can be implemented in the actual modems with only small changes to the software.
On the other hand DSM level-1 will emphasize a terrible disadvantage:
• Instability problem: in order to obtain minimum power, DSM level-1 uses lower values for margin settings and this could deteriorate the stability of the lines.
The algorithm used in DSM level-1 is an iterative waterfilling algorithm combined with the minimum power constraint, subject to a certain setting of target bitrates.
It is based on the commonly known water-filling power loading algorithm applied in ADSL today, but the spectral mask limitation has been removed (allowing boosting on long lines) and it aims at minimizing the power consumption in order to avoid excessive crosstalk interference (this boils down to flat power back-off on short lines). This algorithm optimises the overall bit rate of users, under a total power constraint on each user. This process converges to a Nash equilibrium. That means that the users, under a power constraint, move towards a stable point, where each modem’s power allocation is the optimal response to another modem’s power allocation.
Convergence of the algorithm is based on bitrate (close enough to the target bitrate). A set of target bitrates is used and this is implemented using the minimal power needed to insure the chosen bitrate values at the given margin. Coordination is unnecessary as long as the attempted data-rate-tuple is in the achievable rate region of the iterative waterfilling.
The target bitrates set must be chosen with good criteria. DSM level-1 doesn’t need a Spectrum Management Center (SMC), since Iterative water-filling has the advantage that it is distributed over all DSL transceivers (so no central controller is required). Although this SMC could be helpful in order to determine which combinations of bit rates and noise margins are achievable on the lines in a particular cable binder. So DSM level-1 needs an SMC in the sense that the operator needs to know the rate region in order to set correct values for the target bitrates. The SMC should be able to store this information and to use in order to have gains from DSM.
The implementation of the waterfilling algorithm has a flat PSD: thanks to the flat PSD discovery, a flat PSD is chosen as practical implementation of optimal bitloading. This reduces the complexity of the algorithm.
The algorithm we have implemented in our performance simulations for DSM level-1 has the following scheme:
DSM Algorithm
While (number of iterations defined by change improvements. First iteration is always done)
For each user (for i= 1 to numbers of lines active) find target Bitrate (as the max attainable bitrate among the set of target bitrate values)
While rate ≠ target bitrate
waterfill PSD level) (maximize the rate with the actual power constraints=>find
calc max bitrate (with PSD constraint found with waterfilling) If rate too high
decrease power/
increase margin
If rate too low
increase power/
decrease margin
end (ok: rate=target for user i: he has the min power for the actual noise conditions)
end
until newBi-oldBi or newGi-oldGi for each user < threshold
This algorithm differs from the distributed power algorithm since:
• The iterations are done for all the modems together at the same time and not one after another one.
• We use a tuple of target bitrates in which each line has to set the maximum attainable one, so we don’t know the rate region, but we want to find it.
Due to that, the convergence of the algorithm is not assured, but we have the following advantages:
• since the modems iterate all together, the algorithm is closer to reality: if for example a noise comes up, each modem will react on it, and they don’t do this in a coordinated way (one after another one).
For the performance simulation, the margin for DSM is fixed and the bi/gi determine the end of the iterations: iterations stop if the differences between new bi/gi and old
ones are lower than certain thresholds that identify improvements in bitallocation.
To test the maximum gains that DSM level-1 can have, we have done Performance
Simulations using a fixed noise margin equal to the target noise margin.
Using this algorithm, we have investigated if DSM level-1 is a valid proposal
towards which DSL should move in the nearly future: we have quantified how
much improvement it can add with respect to the actual reference ADSL in the actual networks and we have studied in detail the behaviour of the lines that apply DSM
level-1.
One noteworthy point is our particular choice of inputs in the simulations: we have used real cable scenario information coming from statistical data and a set of target bitrates that are normally chosen today by operators.
This is a substantial difference from the simulations normally conducted up till now: no worst cases are used anymore, but the real scenarios are analyzed and no optimal conditions for the implementation of the algorithm are satisfied, but results on how the algorithm adapts itself to the real data are analysed.
Since DSM is “dynamic” in the meaning that it adapts to the conditions, this choice seems to us the best one for our purpose of giving a good view of DSM level-1 performance in the real ADSL actual networks. However, we have added detailed comments to all the results, in order not to give only sterile data presentation, but general advices extendible to any network.
Thesis contributions
Significant conclusions are summarised in the following points.
With respect to the actual reference bitallocation for ADSL, DSM level-1:
• presents gains that increase linearly with the number of lines that apply DSM in a binder. Generally significant gains are present only with full DSM (when all the lines apply it).
• grants the privilege of variety in profiles: an operator can increase the customer base on long lines by adding a low cost profile at a low service bitrate, without loosing in performance for the short lines.
• has higher gains when the noises are worst: with only AWGN and with DSL G9961 the gains are low (2% for Europe and 5% for America), while with SHDSL and HDSL we can gain up till 8% (for Europe) and till 16% (for North America) of increase in coverage.
• has a small variation in gains respect to the number of noise sources.
• for the combination of noises has gains that follow the behaviour of the worst source noise present in the combination. For the ETSI FB model combination, indeed, the gains are almost the same of the ones found for 15 SHDSL noise sources.
• generally takes its gains from long lines thanks to Boosting on long lines and Power Backoff applied by the short loops. Therefore, bigger gains are present for the American distribution since it has more long lines with respect to the European distribution.
• can upgrade a certain number of customers from a lower service bitrate to the one immediately higher, granting privilege for an upgrade on long loops. • has very big gain if it would be deployed together with a Remote Terminal
scenario: Power Backoff applied by the RT lines creates spectacular gains to the CO lines even if the last one don’t use DSM level-1.
• has good gains in loop length reached when we consider the statistical average reached for each tier. The gains are slightly lower if we use plot with
• can have more stability problems depending on the margin settings, the topology used and on the selection of the target bitrates.
Using the results coming from the coverage network and loop length reached, we can conclude that, as expected, DSM level-1 doesn’t do miracles against the physical limitation of attenuations in the lines (small improvements in loop length reached), but, reducing crosstalk, it can sensibly increase the coverage of the network (big gains in number of customers added) up till 16% in a CO scenario and more than 20% in a CO/RT scenario. Generally, DSM level-1 is suited for all the scenarios in which the actual ADSL has its lower performances.
From our investigation, we can conclude that DSM level-1 is a valid proposal towards which ADSL should move in the nearly future in order to increase the rate/reach in the actual fields.
Further works
Since DSM level-1 is the first level out of three of the DSM techniques and since the drawback encountered is the possibility for instabilities of the lines, several areas are still open to further improvements:
• Studies on performance of sublevels of DSM level-1 can be easily conducted using the Dynamic Simulation tool presented. Off course the gains will be lower than the ones found with full DSM level-1.
• Studies on the higher levels will be conducted with improvements of the tool. Gains are expected to be higher thanks to the coordination of the lines.
