Passive vs. Active FTTH

Is the Gap Closing?

The debate over active versus passive optical networking for FTTH deployments is not new, but it has intensified of late as the cost and performance gaps between the two architectures have narrowed.

Proponents of active optical, or active Ethernet as it’s called in FTTH context, are on the offensive, but then — in North America at least — they are the challengers. PON reigns supreme at the pleasure of its Bell company backers and a historically cheaper architecture.

“If you go back 20 years when fiber to the home was first talked about, the PON architecture was an ideal architecture for some very important reasons,” namely that fiber and optical components were very expensive, says Ram Rao, director of market management for World Wide Packets, a manufacturer of carrier Ethernet gear. “Back then, the only way to build a cost-effective infrastructure was to share the fiber wherever possible and to share the optics wherever possible. ... Fast forward 20 years later, and all the cost points have changed.”

Rao says while fiber once was 6 to 8 cents a foot, it is now 0.6 cents, and optical components that were thousands of dollars are now tens of dollars.

In a PON architecture, one fiber is run from the optical line terminal, or OLT, in the central office to a passive splitter in a remote terminal where it is split 32 or 64 times to reach individual homes equipped with optical networking terminals. In this point-to-multipoint scenario, there are no active components in the field, and the fiber is shared.

In contrast, an active Ethernet solution is point-to-point from the central office to the home unless distance requires fiber to be aggregated in a remote cabinet and lit with active optical components from which point-to-point fiber connections are run to a network interface device at each home.

“Certainly, the incremental cost to lay a bundle [of fiber] is not much more than to lay a single fiber,” says Michael Howard, principal analyst and co-founder of Infonetics Research Inc. “That argument says the cost of PON versus active Ethernet [is] closing.”

However, Howard adds, the telcos — namely Verizon Communications Inc. and AT&T Inc. — that have chosen PON say inactive splitters are more reliable and require less attention.

From an operations perspective, the differences between active Ethernet and PON are not significant, says Steve Kemp, senior director of access product marketing for Alcatel N.A. “The real issue is the outside plant because that’s the most expensive part to put up,” he says.

Indeed, experts say much of the cost of active Ethernet is tied up in having electronics in the field, which need to be housed in an enclosure with power and cooling. Some telcos have reported a 15 to 20 percent premium to deploy active Ethernet for these reasons, says Ron Hartkeymeyer, solutions marketing director for FTTP at Calix.

Where the OLT resides in the central office as it does in many European deployments or in the basement of an MDU, active Ethernet has proven cost effective. “The best time to dedicate facilities is when it’s sitting at one point of concentration at the bottom of a building,” says Kemp.

Splitting the optical signal also impacts the link budget, limiting the distance for GPON to 12 miles, he adds. Active Ethernet, in contrast, can reach up to 80km from the central office to the remote terminal and up to 20km from the remote terminal to the subscriber, according to Matt Roman, senior product manager for Pannaway Technologies Inc. This, Roman says, is a key point for less dense deployments common for rural operators, which are warming to active Ethernet. (See related article, Rural ILECs Get Active, in the Added Insight section at www.xchangemag.com/addedinsight.)

In addition to distance gains, active Ethernet also claims greater throughput to users based on its point-to-point architecture. “They all say they are giving 100mbps to the home,” says Alcatel’s Kemp. “No, they aren’t. Once the thing leaves the building, it’s only going over gigE. Divide that by 100 [customers].”

The argument, then, is the bandwidth delivered to the user is, like PON, limited by the backhaul from the remote terminal. Some argue that PON offers a better ratio with 2.4gbps feeding 32 users versus 1gbps allocated across a 48-port active Ethernet box. “GPON runs at 2.4gbps. Any one unit on that PON can be provisioned for 64kbps up to a full gigabit,” says Calix’s Harkeymeyer, explaining while the bandwidth is shared, it’s also dynamic whereas active Ethernet is locked down on the last leg. “They would need five 1gig pipes to serve 48 users [at 100mbps],” he says.

In fact, active Ethernet vendors are beginning to move in this direction. “When the other guys say that bandwidth doesn’t exist, that’s just not true. We are delivering 10 gigabits to our OLTs,” says Philip Yim, executive vice president of global marketing and R&D for Allied Telesyn. “We have more capacity being fed to us than any other vendor; we can deliver nonblocking 100mbps capacity to end users.”

Analyst Howard says the “most obvious advantage” of active Ethernet is the ability to change the equipment on either end of the fiber to get more bandwidth.

The GPON model, of course, breaks down when you start looking at delivering 100mbps on a sustained basis, says World Wide Packets’ Rao. “They have to find a way to get away from the shared architecture and create a point-to-point connection. They end up talking about WDM PON,” he says, explaining this provides for a wavelength to each home. “The relevance of the [shared point-to-multipoint] protocol is completely lost at this point.”

If the default is a scalable point-to-point connection, he adds, “at the end of the day that is going to be Ethernet.”

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