'X' marks the spot for FTTx access architectures

By Stuart Benington, Tellabs -- With several varieties of access technology available, choosing the right one requires the matching of capabilities with requirements. FTTH and FTTC provide more flexibility and greater life spans than competing alternatives.

Sep 23rd, 2005

With several varieties of access technology available, choosing the right one requires the matching of capabilities with requirements. FTTH and FTTC provide more flexibility and greater life spans than competing alternatives.

By Stuart Benington

In early-2003, BellSouth, SBC Communications, and Verizon issued a joint request for proposal (RFP) from manufacturers of passive optical network (PON) access network technologies. Yet despite this apparent unity, these service providers have not reached a consensus regarding the depth of fiber deployment in "brownfield" local access networks.

Verizon proceeded with a fiber-to-the-premises (FTTP) PON deployment, which already passes more than 1 million homes. BellSouth deployed a fiber-to-the-curb (FTTC) PON approach. SBC took a fiber-to-the-node (FTTN) initiative that will leverage its investment in copper-based digital subscriber line (DSL) infrastructure.

In short, the "x" remains firmly in place within the phrase "fiber-to-the-x."

Despite issuing a joint RFP, these three service providers decided against a "one-size-fits-all" approach. Instead, they favored various architectures that fit each company's unique needs. These needs are based on end-user services, technological capabilities, existing installed infrastructure, and capital and operational costs.

Market and service drivers

In 2005, several regulatory hurdles to broadband investment were removed in the United States. Now, telephone and cable companies face an imperative to vie for each other's customers, while protecting their own, with the bundled "triple play" of voice, data, and video services over a single subscriber connection. Only through triple-play service bundling can service providers position themselves to increase average revenue per user (ARPU) and reduce churn.

The question remains: How much access network bandwidth is enough to support all of these bundled services?

Bandwidth-hungry video services are playing a large part in determining total per-home bandwidth requirements. In addition to offering broadcast video service on par with cable incumbents, successful service providers will need the capacity to support advanced video services. These services include video on demand (VoD), Internet Protocol TV (IPTV) multichannel broadcasts, and IPTV VoD.

Service providers also must consider several other factors. These include homes with multiple TV set-top boxes, subscriber demand to watch one channel while recording another, and subscriber demand to watch multiple channels with a picture-in-picture format. Additionally, channel-change speeds and other aspects of the user's experience must meet the expectations of traditional TV viewers -- a challenge for IPTV. Finally, the demographics of the subscriber area come into play in determining the viability of varying FTTx architectures. This includes population density, geographic reach from the central office, and consumer demographics per household.

High-definition TV (HDTV) imposes a further consideration. To carry one HDTV channel using MPEG-2 compression requires up to 15 Mbits/sec of bandwidth. Two popular compression technologies -- standards-based MPEG-4 and Microsoft's proprietary Windows Media 9 -- can help, but still require 7.5 to 13 Mbits/sec for each HDTV channel, depending on the resolution and content.

Assuming two set-top boxes per home and simultaneous recording or picture-in-picture, four HDTV channels compressed via MPEG-2 will require a total of 60 Mbits/sec. Using MPEG-4 to deliver IP-based HDTV, the requirement decreases to 52 Mbits/sec (assuming 13 Mbits/sec per channel) or to 30 Mbits/sec (assuming 7.5 Mbits/sec per channel).

If the provider delivers IPTV (switched video carried as an IP-over-Ethernet data stream) and allocates an additional 20 Mbits/sec for data and voice over IP (VoIP), then total bandwidth required for triple-play services is 50 to 72 Mbits/sec.

DSL-dependent FTTN and FTTC architectures

Service providers must select next-generation access equipment based on per-home bandwidth requirements as well as other factors including cost.

The most commonly deployed DSL technology, Asymmetric DSL (ADSL), provides approximately 8 Mbits/sec of total capacity over twisted-pair copper phone lines. A single HDTV IPTV, telephone, and data service would overload it.

Consequently, service providers are exploring three technology upgrades, in some cases simultaneously:

  • IPTV with video compression to reduce video bandwidth requirements
  • advanced DSL equipment to increase available copper loop bandwidth
  • deep-fiber technologies that shorten the copper loop lengths of distance-sensitive DSL access equipment, including FTTP technologies that replace the copper loop with fiber. These can also be coupled with an RF video overlay if an operator is uncomfortable with IPTV's economics or technical maturity.

Combining compression with ADSL2+ or emerging Very High Data Rate DSL 2 (VDSL2) upgrades can support competitive IPTV video offerings. ADSL2+ raises per-home capacity to as high as 20 Mbits/sec on copper loops as long as 5,000 ft. VDSL2 technology achieves data rates ranging from 50 Mbits/sec to 100 Mbits/sec on copper loops fewer than 2,000 ft long. In terms of costs, ADSL2+ and VDSL migrations require replacement or upgrade of both network equipment and customer-premises ADSL modems.

Most ADSL deployments employ FTTN architectures, with central offices serving remote DSL access multiplexers (DSLAMs) located at optical nodes 4,000 to 5,000 ft from customer premises. In low-density applications, FTTN can enable incremental ARPU while limiting capital expenses, assuming well-conditioned copper loops. This architecture supports faster Internet data services and one HDTV or up to four standard-definition VoD channels.

However, delivering multiple HDTV and VoD channels as well as high-speed Internet access requires shortening copper loops to within 500 ft (FTTC) or using fiber all the way to the home (FTTP). Currently, only 25% of North American DSL lines fit the 500-ft profile.

Powered or "active" FTTC offers the additional advantage of supporting an optical RF overlay that devotes the 1,550-nm wavelength to hundreds of broadcast video channels. With FTTC, an optical signal is sent to an active remote optical network unit (ONU) about 500 ft from the home. DSL delivers voice, data, and IPTV services from the ONU to the home and an optional coax cable delivers the RF overlay, if implemented. The RF overlay gives the provider immediate parity with cable video services.

Millions of FTTC lines in the United States already carry voice and data. These lines can be upgraded to support video by increasing the capacity of the active optical link and upgrading the DSL technology that drives the last 500 ft of copper. FTTC, when combined with VDSL2 upgrades, enables bandwidth in excess of 50 Mbits/sec -- enough to support more than four HDTV channels and many standard-definition VoD channels, as well as higher-bandwidth Internet data services.

FTTP architectures

In FTTP implementations, there are two variants gaining momentum in North America: Broadband PON (BPON) and Gigabit PON (GPON). BPON equipment (currently the most common in U.S. FTTP deployments) delivers 622-Mbit/sec downstream capacity from the central office and 155-Mbit/sec upstream capacity. That capacity can be shared across multiple homes or devoted to a single location.

Bandwidth per home is then determined by the splitter architecture. While a 32-home split would support 19.4 Mbits/sec downstream per home, a 16-home split would support 38.9 Mbits/sec per home; an eight-home split would support 77.8 Mbits/sec per home; and a four-home split would support 155.5 Mbits/sec. These splits can be changed to accommodate application demands over time, perhaps requiring upgrades to the optical network terminal (ONT) at the home. Split changes also may require upgrades at the serving office. However, the ability to change splits precludes the need to over-engineer capacity in the near term, and the ability to change splits produces maximum flexibility to increase capacity as ARPU opportunities arise.

A common BPON video implementation is one in which the RF overlay wavelength delivers broadcast TV and in-band IPTV all the way to an ONU powered at the subscriber's home. This passive approach maximizes bandwidth to each customer while removing active equipment in the network.

The available downstream bandwidth can range from 20 Mbits/sec to more than 100 Mbits/sec depending on the PON downstream rate and split ratios. The FTTP approach supports more than four HDTV VoD and many standard definition VoD channels.

GPON technology is a natural evolution of BPON. With GPON, upstream and downstream bandwidth increases substantially in anticipation of consolidated triple-play services over in-band IP streams. Downstream GPON bandwidth is either 1.2 or 2.4 Gbits/sec, while upstream is 1.2 Gbits/sec. Depending on area demographics, this increase can result in per-household bandwidth of up to 100 Mbits/sec, comparable to the gigabit evolution of FTTC architectures. In this regard both FTTP and FTTC are fully capable of accommodating any foreseeable end-user service bundle in which an operator chooses to invest.


For incumbent service providers, brownfield access network upgrades present sensitive competitive and cost issues. The marketplace demands the ability to support growing triple-play service bundling. Yet determining long-term triple-play bandwidth requirements remains as much art as science.

Each service provider's existing access infrastructure and customer base make each deep-fiber decision unique. Protection of existing investments must balance the accommodation of competitive services.

Over time, total cost of ownership and flexibility to grow bandwidth per home (essentially option value for new revenue) constitute the most important factors for all service providers. The bandwidth benefits of FTTC and FTTP provide the best flexibility for operators and consequently the best outlook for top-line growth, as well as a longer life span than copper. In the end, this is the enabler for future-proof investment protection of today's and tomorrow's high-bandwidth services.

Stuart Benington is director of portfolio marketing at Tellabs (Naperville, IL). In this role, he is responsible for identifying and targeting opportunities and driving marketing strategy for achieving corporate profit and loss. He holds a bachelor's degree in physics from Purdue University and an MBA from Northwestern University.

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