GPON vs. HFC: The next technology battle

Telephone companies are aggressively marketing the superiority of their GPON-based FTTP networks versus the cable multiple-system operators' HFC/DOCSIS networks. But just how much of an advantage does GPON technology provide?

LIGHTWAVE
http://lw.pennnet.com
7/23/2007, Alan Marks, Alcatel-Lucent

As traditional telecom service providers begin to upgrade their fiber access networks using GPON technology, it is important to understand how these networks will be positioned against their likely main competitors, the cable multiple-system operators' (MSOs) upgraded hybrid fiber-coax (HFC) networks. The cable MSOs have spent $100 billion over the last 9 years to upgrade their HFC networks, but will that be enough to achieve service parity with the telcos' emerging GPON-based implementations? To help answer this question, there are several issues to consider, including: Basic network topology. Bandwidth evolution and next-generation high-speed Internet. Video services evolution.

Network topology overview

From a network topology perspective, HFC networks are characterized by active coax plant (with trunk and line amplifiers) connected to fiber nodes that perform electrical/optical conversion. Network powering is provided from the fiber nodes to drive the coax amplifiers. The fiber nodes are connected to a distribution hub using a single fiber for the upstream and a single fiber for the downstream. At the hub locations, electrical/optical conversion is performed, and the subscribers are connected to services. In today's modern HFC networks, the fiber nodes typically pass 500 households.

For the purposes of making a comparison with a GPON network topology, we can also consider a near-term and relatively modest cost upgrade of this topology to reduce the node size to 125 households passed (HHP). In this case, each 125 HHP coax leg has its own downstream receiver at the node (and transmitter at the hub), which are wavelength-division multiplexed (WDM) over a single fiber from hub to node. In the upstream, the returns from each coax leg can be multiplexed and digitized, sent over a single fiber, and demultiplexed at the hub.

In either case, the subscribers are sharing the spectrum that is available on the coax. A 500 HHP coax leg will share 50 to 750 MHz of downstream spectrum and 5 to 42 MHz of upstream spectrum. A 125 HHP coax leg also will share 50 to 750 MHz of downstream spectrum and 5 to 42 MHz of upstream spectrum.

GPON, by comparison, uses a completely passive outside-plant (OSP) architecture without any OSP powering requirements. It is estimated that OSP powering alone can cost nearly $3 million yearly for an HFC network that passes 1 million homes.

In GPON, subscribers are connected to a port on the access node through a passive splitter, which is fed by a single fiber from the access node (or optical line terminal�OLT). WDM technology is used to provide a wavelength for upstream (1,310 nm), a wavelength for downstream (1,490 nm), and an optional wavelength (1,550 nm) for supporting cable-TV RF overlay video. In GPON, the splitter can serve either 32 or 64 subscribers, but in most deployments, a 1:32 split is used to extend the optics reach.

In a GPON system, 32 (or 64) subscribers share the entire transmission capacity of individual wavelengths for upstream and downstream and, optionally, for additional cable-TV broadcast downstream.

In general, a GPON network topology provides several key advantages over an HFC network topology: Greatly reduced operations and maintenance costs, thanks to a completely passive OSP with no powering requirements. Greater transmission capacity with dedicated wavelengths for upstream, downstream, and downstream cable-TV overlay. Smaller node size, with a simple method for reducing node size further�by reducing the split�if necessary.

Bandwidth comparison

From a high-speed Internet (HSI) perspective, GPON has an enormous amount of bandwidth that can be delivered in both the upstream and downstream. In the past couple of years, services evolution and bandwidth requirements have become increasingly unpredictable. New Internet applications such as YouTube and Slingbox and new video services, including high-definition TV (HDTV), HD-video-on-demand (HD-VoD), and network DVR, have emerged and created new demand for both upstream and downstream bandwidth. GPON provides 2.488 Gbits/sec of downstream bandwidth over the 1,490-nm wavelength; 1.244 Gbits/sec of upstream bandwidth over the 1,310-nm wavelength; and can provide more than 1 GHz of cable-TV broadcast over the 1,550-nm wavelength. This allows for a maximum sustained bandwidth under full system load (100% simultaneous usage) of more than 75 Mbits/sec in the downstream and nearly 40 Mbits/sec in the upstream (using a 1:32 split and 1,550-nm overlay for broadcast video).

Figure 1 depicts the maximum sustained bandwidth under full system load for a variety of conditions, including different splits and the type of video delivery mechanism used.

Figure 1. Maximum sustained bandwidth for GPON under full system load.

DOCSIS-based HFC networks, by comparison, do not have the ability to match GPON with respect to the maximum bandwidth under full system load. As illustrated in Figs. 2 and 3, the maximum bandwidth that can be delivered under full system load�even when using DOCSIS 3.0 technology�is orders-of-magnitude less than what can be delivered using GPON. From a downstream perspective, this is mainly due to the fact that the downstream spectrum is dominated by a variety of different video services, and there is only a limited amount of bandwidth available for HSI service. From an upstream perspective, there is a limited amount of bandwidth available due to the way the coax spectrum is divided. Also, it is not always possible to use the lower portion of this spectrum due to noise issues.

Figure 2. Downstream bandwidth under full system load for DOCSIS-based HFC.

Figure 3. Upstream bandwidth under full system load for DOCSIS-based HFC.

Moreover, not only is the bandwidth delivered in a DOCSIS-based HFC network considerably less than in a GPON network, but the cost per bit is significantly higher in a DOCSIS/HFC network than in a GPON network. The table provides a breakdown of the cost/bit comparison.

Clearly, GPON is superior to DOCSIS-based HFC from a bandwidth efficiency standpoint. The traditional MSO data architecture, which costs on the order of hundreds of dollars/Mbit/sec will not be competitive with GPON networks deployed by GPON-based competition, such as Verizon's FiOS. Using next-generation data architecture technologies such as modular CMTS (M-CMTS), edge QAM, and DOCSIS 3.0 will help cable MSOs reduce the cost/bit, but it will still be an order-of-magnitude higher than the GPON cost/bit.

Video services support

From a video services perspective, GPON allows service providers to support a traditional cable-TV mode of operation, an end-to-end IPTV offering, or a hybrid of traditional cable TV and IPTV.

In today's HFC networks, cable operators use a significant amount of the downstream coax spectrum for analog broadcast TV (typically more than 500 MHz). Digital video services use nearly the entire remaining spectrum (voice over IP and HSI use a small portion of this same spectrum). With the introduction of new video services (HDTV, VoD, network DVR) the broadcasting of all the analog channels and all the digital channels to every subscriber is causing a bandwidth problem (see Fig. 4). However, as shown at the bottom of Figure 4, cable operators are introducing so-called "switched digital broadcast" (SDB) technology to more efficiently use spectrum only for channels that subscribers are watching.

Figure 4. Spectrum utilization for video services in HFC networks.

By comparison, a GPON network that uses the 1,550-nm wavelength for cable-TV RF overlay video can support more spectrum for broadcast services and can either provide traditional VoD services on these channels or use the 1,490-nm wavelength in an IPTV model, while leaving plenty of bandwidth for HSI services (see Fig. 5).

Figure 5. Video services support in GPON with 1,550-nm overlay and IPTV VoD.

However, the 1,550-nm overlay model, which, again, is based on cable-TV technology, requires a video return channel mechanism that is compatible with cable-TV set-top boxes and does not require additional wiring in the home. With GPON, this is accomplished using Multimedia over Coax Alliance (MoCA) technology, which is integrated into GPON optical network terminals (ONTs) as well as residential gateways in the home and set-top boxes from leading cable vendors. The upstream communications from these devices is sent to the ONT using MoCA and is passed on by the ONT upstream on the 1,310-nm wavelength.

In addition to 1,550-nm overlay video, GPON can also support a pure IPTV model, which supports broadcast services in a switched digital broadcast model over the 1,490-nm wavelength, while supporting VoD and other interactive video services in an end-to-end IPTV model.

In general, video services support in GPON is considerably more robust than in HFC. And GPON systems can retain compatibility with existing cable-TV headends and new cable set-top boxes by using the 1,550-nm overlay with MoCA-supported ONTs.

In summary, as services migrate to "everything on demand" and individual streams are sent to different devices in the home, the migration to IPTV over GPON will provide a lower cost/bit than QAM-based schemes on HFC networks. As this happens, the video economics begins to look a lot like the high-speed Internet economics, where GPON is the most efficient network for delivering large amounts of dedicated IP traffic.

Alan Marks is product marketing manager at Alcatel-Lucent. He may be reached via the company's web site at www.alcatel-lucent.com.