Very rarely does a technical advance promise late-night infomercial-type results. Even rarer does that advance rely on repositioning existing technology in a way that is even simpler and less expensive than its original use.
That unlikely scenario may play out in the cable industry. CableLabs, the industry’s research and development consortium, is working to transition a long-haul transmission technology – coherent optics – for use in parts of the network far closer to end users.
In a long blog post at the organization’s site posted in February, two researchers – distinguished technologist Dr. Alberto Campos and VP of Wired Technologies Dr. Curtis Knittle – wrote that using coherent optics in access networks has the potential to increase capacity by 1,000 times compared to analog approaches. They say that they have already demonstrated increases by 50 times over a distance of 80km.
The R&D is being driven by need. Cable operators and other telecom providers are being pushed by the tremendous and growing demands for bandwidth. Accommodating that growth is becoming harder to do.
The options are not ideal. Previously, the best option was simply to use dormant fibers that were dropped into the trenches or attached to poles during construction. However, “lighting dark fibers” is fading as a solution because much of that fiber has been used for various other purposes, such as redundancy, business services and cell tower backhaul. Other options exist, but they are costly (such as opening up roads to lay more fiber).
CableLabs thinks that it has an answer to the challenge. A bit of explanation is required, however:
There are two ways in which lasers distribute signals in the access network. One is analog, which involves modulating a continuous lightwave. The other, incoherent optics, is digital. It sounds simplistic, but in this approach ones and zeros are sent simply by turning the laser on and off. Yes, just like a tiny, amazingly fast flashlight.
Further complicating this situation is that its newest transmission standard, version 3.1 of the Data over Cable Service Interface Specification (DOCSIS 3.1), is a bit of a blaster. It uses a tremendous amount of power, which means that more cushioning has to be provided to protect fragile digital signals. Thus, relatively few “digital carriers” – frequencies in which digitized data signals are embedded – are possible.
The innovation that CableLabs is championing is repositioning a technique that is common in the long-haul portion of the network. The nature of coherent optics allows more carriers to be used. In addition, the technology enhances the capacity of each digital carrier in three ways:
In addition to the binary on/off transmission, it enables transmission of varying amplitudes. Secondly, it allows use of both vertical and horizontal polarizations and, finally, it uses two phases. Each of these is subtly different. The bottom line of this high-level physics is that they each multiply the amount of data that can be transmitted and add up to the gaudy numbers cited by Campos and Knittle.
The advantage that coherent approaches have in the distribution network compared to their use in the core of the network involves distance. The core of the network stretches over great expanses. Sending digital optical signals long distances creates two challenges: signal dispersion and the need for amplification. Equipment that compensates for both of these is part of the long-haul networking landscape. Carriers know how to account for both issues. However, these problems are absent in the shorter access networks. Not having a problem is better – and cheaper – than having one and dealing with it cleverly.
CableLabs is in the process of putting together a working group to further explore the efficacy of using coherent technology deep in access networks. The process, due to the reality that the technology already is well understood, may be available relatively quickly.