by Jean-Pierre Lartigue
Debate over the next generation of broadband services is no longer reserved for industry conferences and technical committees. Beyond the service provider community-in homes, businesses, government departments, and among international corporations-awareness of broadband has grown to such a degree that subscriber expectations for broadband and triple-play connectivity may soon begin to outpace the upgrades already made to many access networks.
Fibre deployed deep in the access network is a clear imperative for providing broadband services, but it faces the intense challenge of expensive and lengthy civil works in the last mile. Due to the high upfront costs and associated risk on return, network operators are hedging their bets by reusing and improving copper performance where possible, minimising capital investment and riding the wave of innovation toward 50 Mbit/s and beyond, based on the capacity of VDSL2. This is the fibre-to-the-node, or FTTN, approach. However, in the densest parts of their networks, operators are building fibre-to-the-home (FTTH) networks that will accommodate subscriber bandwidth requirements well into the foreseeable decades.
Combining full IP-based connectivity and the latest FTTH innovations, gigabit passive optical network (GPON) is increasingly appearing as the key mature FTTH technology that more than 25 operators such as Verizon, Singtel, Hanaro, NTELOS, several Swedish city nets, and France Telecom are using to solve their broadband demand conundrum. In comparison to BPON and EPON, and in particular active point-to-point approaches, GPON boasts an overwhelming range of benefits that enable rapid, flexible, mass-market fibre deployments at the lowest possible cost of ownership and rollout.
The advantages of GPON to network operators can be summarised as follows.
Mass-market ready. GPON is a full IP-based technology capable of unbeatable long-reach coverage (up to 60 km). This provides subscriber connectivity under a wide range of deployment scenarios and geographic topologies.
Service flexibility. GPON is the fibre access technology best positioned to offer subscribers multiple service streams from multiple service providers because of its gigabit capacity at next-generation service-aware edge and aggregation nodes. The availability of 10× to 20× more bandwidth that can be shared dynamically, coupled with an efficient Layer 2 protocol, provides the highest available throughput, bar none. This enables service providers to offer a wider range of service options than point-to-point fibre, which is capped at 100 Mbit/s per subscriber at comparable economics, or EPON, which offers far less bandwidth on average because of its lower line rates. In addition-and as Verizon is proving with the largest GPON rollout in the world-multiple overlaid optical signals can be sent over the same fibre plant in the downstream direction, enabling a twofold bandwidth improvement for video and streaming Internet traffic.
Easy, modular planning and rollout. The basic technology premise for GPON is the optimised deployment of shared plant deep into the access network, with optical splitters (each serving usually 32 to 64 subscribers) distributed remotely across the access loop. This enables a straightforward, pay as you grow network-planning approach with predictable costs and outcomes. Moreover, GPON’s standardisation and ongoing interoperability is driving healthy, beneficial competition among component, system, and customer premises equipment/optical division multiplexer vendors-rapidly driving down costs as volume ramps and technology matures. In this perspective, GPON follows the same path as xDSL with similar commitment from key access vendors.
Highest density and availability. GPON offers key operational gains with higher fibre densities than have ever been possible before. Based on a typical scenario where a single central office is serving 16,000 subscribers, a GPON deployment requires only one fibre rack and two hardware racks occupying limited floor space of 11.25 m2, consuming 4,800 W of power (see Figure 1). Conversely, an equivalent point-to-point deployment requires space for 16,000 fibre connections (one per subscriber premises) provisioned using 24 racks, with an additional 24 racks for electronics. The required total floor space amounts to more than 100 m2, with power consumption exceeding 50,000 W!
With space and power at a premium at many central offices, GPON’s advantages are clear. Furthermore, GPON benefits from a completely passive outside-plant multiplexing scheme in which small-form-factor optical splitters are used to decrease the fibre count between subscribers and the access node. Indeed, the passive nature of a GPON network means electronics are required only at either end of the circuit, with nothing but fibre in between. This eliminates outside plant powering and minimises truck rolls.
Lowest FTTH total cost of ownership. Less central office hardware, cabling, and power and more efficient use of duct and fibre all make the PON approach-principally GPON- a very attractive and feasible architecture for FTTH. GPON also achieves far higher bandwidth speeds and efficiencies than either BPON or EPON, particularly when the opportunities for overlaying analogue services over RF are factored in.
In the scenario of a large city with 1 million households, the capex for point-to-point fibre is 30% higher than for GPON and the opex is approximately 50% higher-more so when statistical probabilities of repair based on point-to-point’s higher fibre count are factored in (see Figure 2). Detailed deployment scenarios show that upfront investment with point-to-point architectures is 40% more important than with GPON. The economic advantages of GPON improve even more for suburban areas or small/medium-sized cities.
Compelling evolutionary path. As 2006 draws to a close, 25 of the world’s top operators have already endorsed GPON by selecting it for their deployment plans. Looking forward at the evolutionary roadmap for the next 5 years, it is already acknowledged that the GPON industry standard (under the successful drive of FSAN and ITU) will be enhanced significantly while other approaches struggle to catch up. Data rates will continue to grow to 10 Gbit/s as optical multiplexing techniques become more sophisticated. Within the next few years, integrated CWDM and DWDM capabilities should allow sufficient capacity and bandwidth for one wavelength per subscriber.
Sharing of FTTH infrastructure. In a growing number of Organisation for Economic Co-operation and Development (OECD) countries, partnerships between operators and local communities are becoming commonplace to make possible the deployment of FTTH to every citizen (i.e., especially in rural underserved or suburban expensive-to-serve areas). Today, PON is largely used when there’s an obvious economic requirement to share trenches, ducts (e.g., the Netherlands), or dark fibre (e.g., Japan) or provide triple-play IP bitstream wholesale services (e.g., Sweden). When sharing dark fibre, detailed analysis shows that GPON presents operational advantages over point-to-point architectures by avoiding complex and costly collocation of multiple operators’ active equipment or by enabling the reuse of existing, size-constrained duct infrastructure.
Ultimately, major operators’ decisions to deploy FTTH will be determined by the local topology, legacy network migration strategies, and the business case for today’s services, with an eye on tomorrow’s bandwidth needs. As the successor to DSL, GPON, with its propensity to support open civil infrastructure, its all-IP technology, its roadmap to higher speeds and higher splitter densities at lower marginal cost, and its wide adoption and support that will further drive down component costs, is continuing to define the broadband access ‘tipping point’ with ever more clarity.
Jean-Pierre Lartigue is vice president, marketing and communications, Alcatel-Lucent Access Division (www.alcatel-lucent.com).