April 15, 2014

VoIP, The PSTN Killer, Won't Kill Local Loops

Telecom engineers continue to amaze.  

With only the primitive transistors of 1960 they encoded voice for digital transmission via channel banks and T-1 circuits.  In short order carrier networks, both transmission and switching, became all-digital.  That left the copper local loops as the remaining analog portion of the PSTN, providing plain old telephone service (POTS) to traditional phones.  

When ISDN attempted to extend digital transmission to every customer device the electronics of that era limited the capacity on most pairs to 2 Mbit/s or less--more often 160 kbit/s for a maximum-length residential line in a standard serving area around a central office.  That isn't enough for today's World Wide Web and streaming video (which is why ISDN mostly faded away).

So the folks in the back room have sent out another level of capability.  Building on decades of Moore's Law, R&D steadily improved the electronics for Digital Subscriber Line (DSL) technology to  squeeze more and more bandwidth from the copper.  Advances came in several forms:

  • Many carrier frequencies on a pair.  Orthogonal Frequency Division Multiplexing (OFDM) put hundreds of virtual modems in parallel, each on its own carrier frequency, over one wire pair.  
  • More efficient coding.  How a bit appears on the wire changed from simple on/off signals (T-1) to 4-level signals (ISDN), to adding a phase change (quadrature coding in modems).  The number of bits per baud (how many bits each digital symbol conveys) went from 1 to 64 and may go higher. 
  • Improved signal-to-noise (SNR) ratio.  Echo canceling, first applied to voice, does wonders for data too.  
  • Interference canceling. The latest is highly adaptive "vectoring" among all the pairs in a cable.  

The term vectoring isn't instantly clear, but it's sometimes compared to noise-canceling headphones that actively counter unwanted sounds.  Each wire pair in a cable, or at least in a binder group, in effect monitors all the other pairs for potential interference.  When a signal on an one pair affects the signal on another pair the result is crosstalk, which makes it harder for the receiver at the customer end to read the data correctly.  

To implement vectoring, a controller in the central office DSL Access Multiplexer (DSLAM) monitors the signals on all pairs in the local loops and calculates how they affect each other.  The controller adjusts the transmitted signal on each pair to counteract offending interference from all the other pairs.  Yes, that is computationally intense.  Cutting crosstalk means that the signal received by the customer is "cleaner" and more easily understood.  That in turn means the signal can be weaker yet still work reliably which extends the reach and/or raises throughput on each pair.

As true for all DSL, the product of 'bit/sec speed' times 'cable distance' is roughly a constant that is determined by the Signal to Noise Ratio (SNR).  Less noise, including crosstalk, means higher speeds and/or longer reach at a given speed.  Recently Alcatel-Lucent briefed consultants on some new equipment that applies vectoring to local loops.  The following chart reports their tests that show more than double the throughput compared to previous DSL methods.


Throughput of 100 Mbit/s over DSL at 1000 ft and 50 Mbit/s at half a mile should get existing copper loops serious consideration when customers demand more capacity.  They will:  for streaming video on demand, video conferencing, and other unified communications functions.  

But it gets better, at least for the carriers, because progress in speed x distance comes with simpler management.  Most of the DSLAM configuration downloads from a central server.  The crosstalk canceling functions (vectoring) are dynamic so need no setup.  

Next step will be dealing with random external noise from electric motors, switches, welding, etc.  This "impulse noise" is the subject of a recommendation, G.imp, in development at the International Telecommunications Union. 

The sum of these improvements will give customers more capacity for new uses.  People will get the speeds and service qualities they want.  Under central control, a Software Defined Network (SDN) will use the increased bandwidth, or what portion the customer has arranged to pay for, to set up services requested by software applications or devices like IP telephones.

Don't hold your breath for this kind of activity on your lines, but if you breathe normally for a few years you may just have it.


If the facts and my understanding are in line, you just explained how the existing copper facilities can be utilized to handle the increase in bandwidth demand at the bottle-neck (end-user), saving countless amounts of time to market via long interval fiber builds as well as the Cap-X related to bringing high band width to customers outside of the "fiber-loop".

Please correct me if I am wrong but this may be thought of as one of the most efficient and beneficial local loop "recycling projects" projects ever?

Yes, vectoring and other techniques (impulse noise protection, etc.) increase the monetary value of the existing copper infrastructure by increasing the digital throughput capacity. It could mean that the higher value will justify (economically) preserving the twisted pairs in future. And if they are there for DSL, why not use them for analog voice as well? Some carriers might slow their migration to "all IP" networking for voice.

I would like to clarify that, for recent DSL standards such as ADSL2 (G.992.3), ADSL2+ (G.992.5), VDSL2 (ITU-T G.993.2) and Vectored VDSL2 (ITU-T G.993.5), the largest number of "bits per baud" is 15, which translates into 2^15=32,768 possible constellation points for each OFDM sub-carrier.

Also, a simple explanation on vectoring can be found here.

Finally, the ITU-T recommendation for addressing impulse noise problems is called G.inp and was ratified a few years ago.

Thank you for the article.

Thanks for the correction on G.inp. The 64-bit per baud signaling isn't in production yet; the complexity of the constellation may mean it won't be practical. I appreciate the link to the paper on vectoring; it's good to have such explanations handy. To make sure everyone is aware, PDF copies of the DSL recommendations are free downloads here; search for a word or document number.

Very informative article and a few comments above! Thanks for sharing!

Even though local loop will be there a lot of years, I think POTS is dying because old local exchange can not be maintained anymore. Maybe voice will be included as a POTS port in some modems or simply shifted by mobiles.

[Ed note: Luis also pointed out an error in the URL for ITU docs. This has been corrected.]

The basic premise, that POTS is dying, is based on the way that POTS is defined. It is sometimes referred to as a technology and sometimes as a service. Telephone service was tariffed as a service that provided certain characteristics for a certain rate. What is referred to as POTS was defined as providing service in the normal voice frequency band. Even when the service is provided over a digital carrier, it still has defined characteristics that are the same as when it is provisioned over copper loops. The end-user did not get a choice in whether it was provisioned over copper or digital carriers, as long as it met the specified parameters.

Then we started tariffing technologies, such as ISDN and DSL and even FTTH. Other providers offer cable modem service or fixed wireless service. This has created marketing opportunities for carriers to position the service as better or faster based on the technology they offer.

I believe this is where we got off track and made things much more complicated for the end-user consumer. Services are voice, both local and long distance; broadband; and video, whether IPTV or CATV or some other flavor. Other services utilize these services such as OTT, alarms services, etc.

Service Providers should offer services with specific characteristics, such as 10Mbps symmetrical, 1Gbps/256Kbps, 1Gbps symmetrical, HDTV/SDTV, voice at different levels of quality, etc. Services should also be differentiated by whether they are fixed or mobile. As an example, satellite providers are able to provide certain services, however, in many cases they must do so at a lower quality, such as voice; however, they can provide some video services at a high level. A customer who requires low latency would sign up with a Service Provider who offers a service that would meet their requirements, regardless of the technology used.

While I believe that fiber based services will be the long-term winner, fiber is not a service and customers want and need services, not technology. Service Providers should be able to offer the same service over different technologies and compete based on service quality and price. One of our clients who has converted their entire network to FTTH has 5 circuits left on copper because the customer requirements will require additional equipment to convert the services back to older technology at the customer premises.

There are three things required in any company that builds and operates infrastructure: Money, Material and People. If the U.S. decided tomorrow that every home in the country would have fiber within five years, there would not be enough of these three resources to make it happen and the cost would escalate drastically because of the shortages that would occur. The transition to a modern telecommunication network needs to continue, but it will not occur overnight.

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