PON’s Split Personalities

Service providers continue to face rising pressure to meet growing user demands for more bandwidth at cheaper rates -- a tough combination for the market to deliver. From YouTube and online gaming to the explosion of social networking platforms, the modern Internet is straining residential broadband resources. For example, according to Tech Crunchies, 25 percent of American households watch TV online, up from 20 percent just last year. For service providers, this presents a unique market opportunity to design networks that can accommodate the continuously increasing demand for higher bandwidth.

When investing in a new fiber-based access infrastructure, the communications industry makes technology decisions that aim to support the society for a long time. Today, selecting a technology that scales in its most vital sense will be the key to success for service providers and vendors alike. Scaling isn’t just about raw bandwidth, but about service density and how flexible the technology is to accommodate future enhancements, driven by market competition or regulatory framework.

In this article, we’ll outline how system vendors can design Passive Optical Network (PON) systems that are flexible enough to support different service provider business models and scalable enough to grow with residential users’ bandwidth requirements. To begin, we’ll review challenges currently facing the service provider industry in delivering effective Fiber-to-the-Home (FTTH) PON solutions. We’ll also consider specific product design features in addition to an overall strategy.

More Than Dueling Challenges
Without a doubt, service providers have a tough job; if only users knew the legwork and back-end infrastructure that enable their IPTV and YouTube viewing, online gaming, and Twitter applications. Beyond the unrelenting demand from users for bandwidth-hungry applications, service providers are also up against constantly evolving industry standards, based on Institute of Electrical and Electronics Engineers (IEEE) for EPON and Carrier Ethernet, International Telecommunication Union (ITU) for GPON, Internet Engineering Task Force (IETF) for IP/MPLS features, and Third Generation Partnership Project (3GPP) mobile technologies for evolution toward a unified approach to fiber access and mobile backhaul. In addition, system designs need to support local variations; the fiber access EPON standards in Japan, for example, vary from those in China, primarily regarding encryption.

As we approach 2010, we’re seeing increased interest around regulating the Internet. Broadband stimulus in the U.S. and regulation regarding fiber unbundling in Europe has put the spotlight on how technologies can support open access models, defining how alternate service providers can reach end-users through the new fiber access network deployed by the few dominating access players in each market.

Split Architecture for OLT Systems
Faced with a world of changing requirements, Optical Line Terminal (OLT) system designs for the Central Office must include features that incorporate the evolving demands of users and standards alike. The OLT obviously needs to support first mile PON specific standards, but it must also process the aggregated end-user flows for service and end-user provisioning in accordance to a broader set of access and Carrier Ethernet standards.

With the highly competitive PON controller market, system vendors are looking for feature-rich and highly scalable PON controllers that have passed tests carried out by standard bodies and service providers. But they are also looking for programmable solutions that allow increased interoperability between vendor versions for parts of the PON technology, particularly around OAM functions. In addition, the OLT must support advanced traffic management for upstream and downstream traffic, enable different business models, and support trunk cards connecting to the metro network with higher capacity. (See Figure 1.)

Figure 1. System vendors developing ON OLT should leverage a unified design approach for the complete FTTx market.

This calls for a natural split architecture where PON controllers are complemented with programmable devices optimized for Layer 2 to Layer 4 Carrier Ethernet packet processing, both on the line cards accessing the network terminals and on the trunk card accessing the metro network.

A split design enables system vendors to optimize design potential. PON controllers are access-specific chips that integrate a number of EPON or GPON MACs supporting an equivalent number of ONU/ONT Customer Premises Equipment. A programmable Ethernet switch integrates GE and 10GE MAC for uplink to the metro core. End-user packet processing can be aggregated from several PON controllers to allow a common user plane for different access types, such as PON, Point-to-Point Ethernet, and FTTN/VDSL2+ deployments. Programmable Ethernet switches feature a shared memory switch and integrated traffic management to store bursty traffic to the connected end-user terminals. A programmable pipeline of processor cores enable any type of data plane implementation, including OAM offload, user session management, PPPoE, DHCP, and multicast session snooping and filtering. All integrated into a single chip, this gives plenty of headroom for custom designs and enables future enhancements to the data plane in support of new Carrier Ethernet standards.

Matching Features for PON Controller and Programmable Ethernet Switch
PON controllers and programmable Ethernet switches make a good match for OLT designs. (See Figure 2.)

Figure 2. PON controllers and Programmable Ethernet switches make a good match for OLT designs.

While there is a range of PON-specific technical details to ensure a high quality of the shared PON resources, the programmable Ethernet switch empowers the PON line card with a range of Layer 2 to Layer 4 forwarding principles and session management capabilities. While the PON controller supports forward error correction on the first mile, the programmable Ethernet switch comes with an integrated traffic manager with deep packet buffering capabilities for assured and fair user experience. Also in OAM, the 2 chips complement each other. The PON controller focuses on ONT loopback, configuration, and first mile link monitoring, while the programmable Ethernet switch supports a full 802.1ag and ITU Y.1731 implementation for link and Ethernet service OAM -- important in open access environments. (See Figure 3.)

 
Figure 3. High level line card design for OLT system featuring PON controller and programmable Ethernet switch.

Coming Together
A unified and programmable access approach leverages packet processing over multiple access platforms, including Ethernet, PON, and Fiber-to-the-Network (FTTN) -- Very High Bitrate Digital Subscriber Line (VDSL) deployments. By taking a unified and programmable approach to FTTx network design, service providers can modify the network components as needed rather than deploy new technology and provision services quickly,  significantly extending the lifetime of equipment.

As the PON standard bodies (IEEE and ITU-T) have started to push for application-level standards, there is a growing assortment of uncertainties. Which application will gain acceptance by the service provider industry as a whole, and will there be specific interests by key PON players like Verizon, KDDI, and China Telecom?

With strong growth in Japan, China, and the United States, PON technology is looking to fulfill the promise of faster and cheaper broadband services. With advances in broadband services and new enhancements for packet-based mobile backhaul coupled with a changing regulatory landscape, system vendors are looking for cost-effective and programmable designs.

Because design decisions made now have such far-reaching implications for the next generation of standards and users, any new equipment deployment or upgrade must be carefully inspected and tested. In making design decisions, service providers can combine the best of both worlds by splitting the architecture between the optimized PON controllers and programmable Ethernet switches for a unified and programmable approach that delivers on the promise of next-generation broadband systems.

Endnotes
Institute of Electrical and Electronics Engineers (IEEE): www.ieee.org
Internet Engineering Task Force (IETF): www.ietf.org
International Telecommunication Union (ITU): www.itu.int
Tech Crunchies: http://techcrunchies.com
Third Generation Partnership Project (3GPP): www.3gpp.org/

About the Author
Per Lembre is Director of Product Marketing, Xelerated. He has more than 10 years of experience in product management and marketing in the data networking and communications industry. Xelerated is a fabless semiconductor company recognized as being the only network processor vendor to have combined the efficiency of an ASIC with the programmability of traditional network processors. Xelerated's products target network equipment for the metro, access and high-end enterprise markets. For more information, email per.lembre@xelerated.com or visit www.xelerated.com.

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