FTTP comes in an array of sizes and flavors that significantly complicates the process of seamlessly transforming the embedded copper infrastructure to fiber.
By MIGUEL ALONSO
Calix
Service providers want to minimize risk and maximize flexibility as they manage the transformation from copper-based access networks such as ADSL2+ to fiber to the premises (FTTP). Despite impressive advances in both copper-based access networks (ADSL2+ and VDSL2) and video compression technologies (MPEG-4, WM-9), it is hard to argue that the future of the access network is anything but fiber.
That said, FTTP comes in an array of sizes and flavors that significantly complicates the process of seamlessly transforming the embedded copper infrastructure to fiber. The transformation is somewhat akin to changing out the engines on an in-flight aircraft: fraught with the potential to get it wrong and perilous for those who do. For this reason, many service providers are adopting an approach to fiber transformation that is aggressive and prudent at the same time; they are building flexibility into their access networks and ensuring that they can deliver services such as IPTV over any type of access media.
Questions service providers need to consider
The first issue service providers face is: what kind of fiber access network will they employ? Will it be broadband passive optical network (BPON), extended BPON, Gigabit PON (GPON), or even what is increasingly (and oddly) referred to as "active optical" or "active Ethernet" (In some parts of the world, primarily Japan, Ethernet PON (EPON) has also been an option. However, EPON has seen limited deployment elsewhere, and almost none in North America, due to the lack of multiservice capability and limited distance support).
For all of the acronyms being bandied about, the question really comes down to a choice between point-to-point or point-to-multipoint topologies. Both passive and active optical networks have active components at either end of the fiber and are passive in between; they differ in topology, not in the presence or absence of active components. Most carriers today are opting for a multipoint PON approach because it reduces the number of upstream optical line terminal (OLT) ports required to implement fiber, thereby minimizing cost. Virtually all service providers deploying fiber access networks today take the BPON approach simply because it is available, it works, and it enjoys a high level of interoperability.
A second critical question facing service providers anticipating fiber builds is: from where will they drive FTTP? Service providers can drive any type of FTTP from central offices (COs), remote terminals (RTs), and even nodes close to the subscriber. Many service providers are pushing fiber deeper into their networks to shorten copper loop lengths and provide sufficient feeder bandwidth for more advanced services such as IPTV. These remote fiber-fed locations become logical points from which to deploy FTTP.
Two trends will influence the future of fiber access networks. First, as service providers deploy more and more bandwidth-hungry services such as HDTV over IP, they will need more downstream bandwidth. Further, as interactivity increases, users will need more upstream bandwidth. BPON's average, per subscriber downstream bandwidth of roughly 20 Mbps will be increasingly viewed as inadequate, and pressure will mount to migrate to GPON, which offers an average of 80 Mbps to downstream subscribers. Second, as the overall capital cost of various network connections declines, the business case will sway increasingly in favor of point-to-point architectures. Precisely the same trend occurred with regard to local area network (LAN) infrastructure within businesses in the 1990s: early LANs distributed the cost of expensive router ports using shared media hubs; as the cost of the upstream LAN ports declined (and traffic requirements increased) LANs gradually transformed to one user per upstream switch port.
Capitalizing on BPON today
Does this mean that service providers should wait for GPON to be broadly and reliably deployable or for active Ethernet to drop in price? Unless service providers are satisfied running their business with a much smaller number of subscribers than they have today, the answer is no. As the clock ticks, service providers are seeing their subscriber bases erode. Under these circumstances, waiting for the "next big thing" is rarely the optimal strategy. Indeed, service providers are under enormous pressure to stem subscriber erosion by delivering advanced video services.
DSL taught the industry that the "next big thing" tends to take longer to mature and to deploy on a large scale than anticipated. After roughly a decade, DSL is finally rolling out in really big numbers. BPON is just getting started, and early GPON deployment is probably at least 12-18 months away. In all likelihood it will be several years before GPON is deployable in scale-years in which customers will disappear. The prudent course is to capitalize on BPON today.
IP eases migration to FTTP
Fortunately, a critical yet rarely recognized benefit of IP is that it decouples services from media. The IPTV video bit stream, for example, is encoded in MPEG-2/4 and delivered over UDP/IP sessions. These sessions have no awareness of the underlying layers and are therefore able to operate over any type of access media-copper, fiber, even wireless. IP is more frequently viewed as a convergence engine, but in telecom access networks, this service/media decoupling provides far greater value. This decoupling means that service providers can begin delivering broader service mixes over existing ADSL2+ and early BPON access networks without sacrificing the ability to migrate subscribers to GPON and active Ethernet at a later point.
In fact, the decoupling of services and media allows each to evolve independently. As compression algorithms improve, as middleware solutions advance, or as interactivity on the network increases, service providers can upgrade their service bundles without altering underlying media. Similarly, as VDSL2, GPON, or active Ethernet become available and affordable, service providers can begin to deploy them in their infrastructures without altering their service mix and without being forced to retrofit ADSL2+ or BPON subscribers.
Flexible access platforms enable decoupling
Today, flexible access platforms allow service providers to selectively deploy broadband fiber and copper access connections from any location. These platforms let service providers move forward aggressively with new access technologies while preserving their option to move to other technologies at the appropriate time. IP-based service delivery similarly allows service providers to aggressively move forward with deeper, richer information, communications, and entertainment services without waiting for the "optimal" underlying access technology. Such platforms also give carriers much more latitude in marketing their services and much more operational efficiency.
Miguel Alonso is director of product management at Calix (Petaluma, CA). He may be reached via the company's Web site at www.calix.com.