Into the Land of the Brownfield and MDUs
Having recognized the benefits of fiber optics and passive optical networks (PON) in greenfield deployments, telephone operating companies are becoming increasingly interested in moving the technology into brownfield applications. The challenge lies in the fundamental difference between greenfield and brownfield environments.
In a greenfield situation, the operator has a blank slate upon which they can easily and cost-effectively install the technology of their choosing. In a brownfield scenario, the operator is dealing with some degree of embedded infrastructure. For example, in densely populated downtown Manhattan, digging up the streets and rewiring a building isn’t always an option because of the disruption that would occur.
Now, clearly, a location like downtown Manhattan does not suffer from any lack of competition. But, for the operator who has lower bandwidth twisted-pair fiber into a building, how will they compete with, for instance, the cable operator who already has a “bigger pipe” (coax) into the building? And, can they leverage a PON to reduce their network cost as well?
To accommodate the challenges of The Last 1,000 Feet and offer bandwidth comparable to even the most aggressive competitors, PON vendors are introducing ONTs that leverage VDSL2 to deliver FTTH-quality services. While that may seem counter to PON’s fiber-all-the-way-to-the-subscriber mantra, reusing the existing twisted-pair or CAT3 drop cable has advantages for residential, business, and MDU subscribers alike. This article explores how VDSL2 is being integrated with PON technology, the rationale for using VDSL2, and the advantages and applications of this hybrid PON-VDSL2 technology in brownfield applications.
PON + VDSL2 = PONVDSL2?
In a greenfield setting, when trenches are open, before sidewalks and driveways are in place, when wiring is being installed in a new building, prior to fences being built, and before Aunt Jenny plants her rose garden, taking fiber to the home or business is simple and cost-effective. In fact, studies indicate that a new fiber infrastructure with a PON architecture is comparable to and sometimes less expensive than copper.
However, once the network and those amenities are in place, the equation changes dramatically. The cost and time increases dramatically when providers replace a copper network with fiber all the way to the subscriber. A large part of total cost is the result of replacing the drop wiring. How much cost is related to drop wiring becomes difficult to quantify because it is so variable, depending upon factors such as aerial vs. buried cable, physical length, obstacles, and rights-of-way issues. Estimates for FTTH/FTTB brownfield deployments are as much as 15% to 25% of OSP costs that are associated with drop wiring replacement.
Beyond the financial cost of replacing drop wiring there is also the cost of disruption to consider. Even though their service improves, subscribers can be sensitive to a range of factors such as:
• Tunneling under or cutting through sidewalks and drives.
• Burying fiber across a manicured lawn.
• Controlling pets during work periods.
• Granting access to workers.
• Disrupting service.
Considering both economic and customer satisfaction costs, replacing an outdated copper network can be expensive. One way to control these costs is to take the PON fiber only close to the subscriber, then use the existing drop wiring to deliver services across The Last 1,000 Feet.
To illustrate, Figures 1 and 2 compare a typical PON residential fiber-to-the-home deployment with a fiber-near-the-subscriber deployment. Rather than placing the PON ONT on or inside the subscriber’s home, the ONT is located at a distribution point near the residence, where there is access to the drop wiring. The distance from the home can be up to 1,000 feet for ultra-high speed services or farther, based on the rate/reach performance required as noted in Figures 3 and 4.
The ONT provides the VDSL2 interface to the network and connects to the twisted-pair/CAT3 drop wiring. At the subscriber end of the drop, a VDSL2 modem provides the interface to the customer’s in-home wiring. It is installed just like an ADSL2+ modem. But unlike ADSL2+, the VDSL2 modem delivers more bandwidth, with symmetrical service capabilities up to 100 Mbps.
There are three additional advantages to deploying the PON ONT at the distribution point rather than at the subscriber location:
1. The cost of sharing one high-density ONT over 24 customers is more cost-efficient than placing a dedicated ONT at each subscriber’s location.
2. Power and battery backup capabilities can be better maintained and more closely monitored.
3. Maintenance is directed to a single location, as opposed to numerous locations.
From 50 Mbps to 100 Mbps
Let’s start by considering the service bandwidth needs of residential subscribers. MSOs are currently offering 20 Mbps+ Internet access. Most, if not all, network operators are planning on IPTV offerings in the not-so-distant future, with today’s compression and video delivery techniques requiring 10-15 Mbps per channel for HDTV. VoIP telephony is becoming commonplace, and although bandwidth requirements are small, the ability to provide priority bandwidth undisturbed by Internet or IPTV services is critical. A single residence with Internet access and 2-3 high-definition streams of IPTV, plus voice, could easily need 40 Mbps to 65 Mbps today.
This growing need for bandwidth is also predicted by Nielsen’s Law of Internet Bandwidth, which states that a high end-user’s connection speed grows by 50% per year. (1) Using that model, today’s subscribers, who are now content with connections in the 40-50 Mbps range, will be demanding more than 100 Mbps in 2012.
SOHO businesses, another large battleground for broadband competitors, have significant bandwidth needs as well. While they most often don’t require the downstream bandwidth necessitated by IPTV, their need for higher bandwidth upstream is greater than residential. As business communications and online services increase, so does the need for bandwidth.
As a result, it is reasonable for network operators to plan for bandwidth-per-subscriber demands approaching 100 Mbps – whether business or residential. Indeed, many network operators are formulating such networks today.
The choices of technology to deliver 100 Mbps are limited. Coaxial cable has the capacity, but DOCSIS 3.0 and an HFC or RFoG network is required. ADSL2+, the workhorse of DSL technologies, tops out around 25 Mbps downstream and 3.5 Mbps upstream according to ITU G.992.5. (2)
VDSL2, on the other hand, can provide the necessary bandwidth. Being a DSL technology, its rates are dependent upon the distances transmitted. In the downstream direction, it delivers better than 95 Mbps out to 1,000 feet – well within the length of typical drop wiring. Figure 3 provides a comparison of ADSL and VDSL rates over a range of distances.
VDSL2 can also do the job in the upstream direction. Up to 1,000 feet, data rates of 85 Mbps or better are possible. This upstream bandwidth enables the symmetrical data services more appropriate for business subscribers. Commercial-quality SLAs services can be delivered with the same rich Ethernet capabilities that FTTB PON systems provide with an ONT located directly at the business location.
PON-ing On and On
Certainly VDSL2 is not limited to PON systems; other network technologies supply services over VDSL2. The reason for using PON as the access architecture is the way it fits so well with longer-term network strategies. As noted earlier, network operators already recognize the value of a fiber optic network.
Why is an FTTH or FTTB PON network the logical choice from bandwidth, cost, and competitive perspectives? To begin, the architecture provides virtually unlimited bandwidth for any services imagined for years to come. In addition, its low operating expenses offer increased profits.
Considering all factors, a PON infrastructure offers an access network with service capabilities and cost structure equal to or better than any competitor’s, which allows operators to build a defensive moat around a valuable customer base.
By using a PON access network with VDSL2, the best of both FTTH and DSL worlds is possible. A PON system with FTTH/B can be installed for those subscribers along the cable route that need the added benefits only FTTH/B can provide. At the same time, with the same PON fiber, VDSL2 drops can provide FTTH/B-like services, while using the existing Last 1,000 Feet of drop wiring until the full advantages of true FTTH/B makes sense.
SO-PON and So Forth
The discussion so far has used single-family residential deployments as an example. Indeed, this is a primary application in a segment being hotly contested by telco and MSO network operators. Figure 1 illustrates the residential application.
Small Office/Home Office (SOHO) and Small and Medium Businesses (SMB) applications are also the focus of intense competition, and the VDSL ONT solves many deployment challenges for these applications as well. As with single-family applications, using the existing drop wiring is more cost-effective and quicker than replacing The Last 1,000 Feet of copper with fiber.
In addition, MDU and MTU buildings are particularly difficult to wire with fiber all the way to the subscriber in brownfield settings. Oftentimes, ducting is completely filled with copper distribution wiring with no room for the fiber. This makes a fiber overlay difficult, if not impossible, without extended service interruption. To further complicate matters, the existing distribution cabling is typically controlled by the building owner, so coordinating access to the wiring can be problematic.
By using a VDSL ONT, the PON fiber is brought to the MDU building, and the VDSL ONT is installed close to the demarcation point for the in-building telecom wiring. VDSL2 ports are then spliced into the existing CAT3/twisted-pair wiring, and VDSL2 modems are installed in each residential or business unit.
This tactic is advantageous because the entire building doesn’t need to be rewired and thus there are no rewiring costs. In addition, service interruptions are minimal and broadband services can be installed incrementally as subscribers sign up for service.
What made PON attractive to operators for greenfield applications is now drawing their attention to using this technology in brownfield applications. To address the unique challenges of brownfield applications, PON vendors are leveraging VDSL2 technology in their ONTs to allow high-bandwidth service over the existing copper twisted pair. The result is FTTH- and FTTB-like services without the costs of replacing the most expensive portion of the copper network: The Last 1,000 Feet. This combination of PONs with the new VDSL2 ONTs allows operators to maximize their existing plant while rolling out new, high-bandwidth applications at a fraction of the cost of other network upgrades.
1. Nielsen’s Law of Internet Bandwidth states that: a high-end user’s connection speed grows by 50% per year. Nielsen’s Law is similar to the more established Moore’s Law. Unfortunately, comparing the two Laws shows that bandwidth grows slower than computer power. Moore’s Law says that computers double in capabilities every 18 months: this corresponds to about 60% annual growth. As shown in the table, bandwidth will remain the gating factor in the experienced quality of using the Internet medium. See graph, table, and exposition by Jakob Nielsen, Ph.D., at www.useit.com/alertbox/980405.html.
2. Asymmetric Digital Subscriber Line. See standards/names/stream rates comparison table at http://en.wikipedia.org/wiki/ADSL.