GPON: A migration path to an all IP / Ethernet Network

Of the varieties of passive optical networks available in the market today, only Gigabit PON (GPON) offers the migration path service providers seek for today's network as well as the bandwidth required to deliver new services.

By GARY LEE and EYAL SHRAGA
FlexLight Networks Inc.

Much is being written these days about voice over IP (VoIP), IPTV, video-on-demand over the Internet, and other emerging applications that are currently, or will be, deployed in service provider networks. There is little debate about the market's demand for these services or the likelihood of these services being provided. The impact of companies like Vonage is indicative of early market demand and acceptance.

The recent exploratory tenders by major providers for video and television services over IP are a further indicator of major deployments, as providers seek new revenue streams to both increase top-line revenue to combat ever-tightening profits in the consumer market, as well as decrease customer churn through "sticky" applications like television and next-generation entertainment services.

While there is little doubt IP-based services and the all-IP network are coming, questions about the correct infrastructure, especially on the access network, remain. xDSL, passive optical networks (PONs), and point-to-point fiber networks all have various pros and cons. This paper focuses on the various flavors of PON, and shows that of the varieties available in the market today, only Gigabit PON (GPON) offers the migration path service providers seek for today's network as well as the bandwidth required to deliver on the promise of these new services.

Advantages of a PON architecture

PONs in general offer a number of advantages over legacy infrastructures:

• PONs by definition are passive, meaning they do not require traditional active components between the central office and customer premises. These active components generally require power to operate and are therefore more expensive to initially deploy (i.e., more capital expenses, capex) and also more expensive to maintain in the outside plant (i.e., more operating expenses, opex).
  
• PONs are also by definition usually shared among numerous users, allowing the capital costs of trenching or pulling a single fiber from the central office to the customers to be shared among many users, thereby improving the return on investment (ROI) on capex. Security and bandwidth-sharing mechanisms inherent in the PON protocols ensure that the sharing of the fiber is secure and transparent to the users.
  
• Because a single high-speed optical interface in the central office is now capable of driving fiber-based services to multiple customers, the footprint requirements in the central office for a similar number of customers is much lower compared with other solutions involving point-to-point fibers to each customer.
  
• PONs promise to support both legacy services (POTS, analog video) and broadband services (VoIP, digital video over IP, IPTV, etc).
  
• PONs support the sharing of one common access network (physical and protocols) for all residential customers (for POTS, video, and data services) and many business customers (for T1/E1 and Ethernet services), therefore reducing the number of discrete overlay access networks necessary to provide services to each group.

The different PONs

Although PON technology has been available since the mid-1990s, it has only been over the last few years that the standards have matured and commercial PONs have been implemented.

The original standard for APON, also known as BPON, is characterized by an ATM architecture ratified by the ITU in the mid-1990s. APON supports voice and data using an ATM encapsulation mode, where all services are transported over the PON using ATM.

More recently, an Ethernet-based standard was introduced by the IEEE; it's commonly known as "EPON." All services in EPONs are delivered over Ethernet.

The most recent standard is the GPON standard, also ratified by the ITU. It offers direct support of TDM and Ethernet traffic in their native formats or via an optional ATM encapsulation method. GPON also offers the bandwidth service providers require for next-generation services.

There are some key differences among what each of these PON standards will offer a service provider. These differences are shown in the Table below. In particular, these differences affect IP service deployment, bandwidth, efficiency, and split ratio differences.

IP Service Deployment: As shown in the table, there is a vast difference among the three options in how each PON protocol supports legacy and emerging IP services. APON/BPON uses an ATM layer for its transport of services, which is an inefficient use of network protocols. Therefore, while APON networks have been useful in supporting legacy services, they are likely not the best choice for emerging IP services better served with more efficient protocol support and higher speeds.

EPONs inherently support all IP networks today through their use of Ethernet protocols. They are therefore positioned for all-IP service networks. Their support for legacy services (i.e., POTS, T1, E1, etc.), however, requires some type of proprietary or emerging circuit-emulation over Ethernet services -- something that today's providers trying to provide legacy services with traditional "five 9s" of reliability may not support. Therefore, while EPONs are prima facia an excellent choice for today's providers looking at IP service deployments, they may fall short in the face of higher bandwidth requirements and the need to support legacy services.

GPONs are based on providers' requirements as defined within the FSAN organization (www.fsanweb.org) and the ITU, and therefore may be best aligned with providers' desires for legacy and IP support. As shown in the table, GPONs support legacy services such as T1/E1 in their native formats through re-use of SONET's GFP protocol, reclassified as GEM in the GPON standard. GPON also fully supports all Ethernet protocols as well as VLANs, quality and class of service, IMGP, and other Layer 2+ mechanisms for full support of emerging IP services.

This mix of legacy and IP services support gives providers a migration path to all-IP services by allowing today's networks to be deployed with legacy services (POTS, T1, E1, analog video) yet ensuring a seamless migration to all-IP services. For today's provider, GPON offers the best of all worlds -- a mixture of legacy and IP services as well as a very efficient all IP network.

Bandwidth and bandwidth efficiency: A prime factor for a carrier analyzing the merit of a PON is the overall bandwidth available for services sold to customers. This available bandwidth can also be termed as the "revenue bits" of the network. Revenue bits are derived from the overall network bandwidth with the protocol overhead or tax subtracted from it.

While EPON supports 1.25 Gbits/sec downstream, GPON works at downstream speeds of 2.488 Gbits/sec. Moreover, when the bandwidth utilization is taken into account, EPON performance becomes even worse. With only about 50% efficiency, EPON's revenue bandwidth is limited to 600 Mbits/sec while GPON, with its bandwidth utilization, can allow the service provider to allocate about 2300 Mbits/sec. That's almost four times more user-available bandwidth.

GPON also supports several options for the upstream bit rates, ranging from 155 Mbits/sec to 2.488 Gbits/sec. This variety allows the provider to select different performance according to the bandwidth requirement of its customers.

The Figure below clearly shows GPON's bandwidth superiority over EPON.

PONs for fiber to the home and business are emerging as a clear winner with major carriers around the world for delivering voice, video, and data services. As service providers seek elegant ways to deploy new IP services, only GPON delivers on the promise of an economical network for the delivery of these services while also offering a migration path from today's legacy network to an all-IP architecture.