Ethernet switched optical networks vs. passive optical networking

By BILL ZAKOWSKI, Amedia Networks -- Ethernet switched networks balance the cost of another powered element with greater flexibility, reach, and security.

Jul 20th, 2004

Ethernet switched networks balance the cost of another powered element with greater flexibility, reach, and security.

By Bill Zakowski
Amedia Networks

Will today's fiber-to-the-premises (FTTP) access networking solutions deliver on the vision they promise? This is an important question with many service providers poised to expand content-on-demand offerings or introduce next-generation services such as home video conferencing and voice over Internet Protocol (VoIP).

Service providers must examine the differences between Ethernet switched optical networks (ESONs) and passive optical networks (PONs) and understand the ramifications of those differences on ultra-broadband services in terms of performance, scalability, reliability, and security.

Application needs of today, tomorrow

The promise of FTTP is enabling next-generation "triple play" management and routing capabilities, designed to provide customers with video, data, and VoIP services. What do service providers require in an FTTP solution today? Just as importantly, what will they require tomorrow?

Today, up to 10 Mbits/sec of bandwidth is needed for a single personal computer, and this number is expected to grow in the next year. Broadband Internet users, both residential and business, are demanding increased bandwidth for regional virtual LANs (VLANs), critical because they allow geographically dispersed communities of interests to share information with a reduced threat of information theft.

Video is another application that demands more bandwidth due to the tremendous amount of data needed to display rich full motion on a monitor or TV. Standard definition (SD) and high definition (HD) video channels currently require 4 and 19 Mbits/sec of bandwidth, respectively. With several televisions and TVs in a typical home, it is easy to accumulate the need for 40 Mbits/sec of bandwidth per household.

Increased bandwidth is also needed to support VoIP. Regular telephone use and conferencing features also add additional bandwidth requirements.

It is not unreasonable to conclude that a typical user may start at 20 Mbits/sec and grow to more than 50 Mbits/sec in a short period of time. Verizon, in its March 16 Telecommunications Conference, projected the bandwidth needs highlighted in this table:

ApplicationBandwidth (Mbits/sec)
Video Conferencing2
File Sharing 3
Distance Learning4
Multi Player Gaming5
Real Time Video10
Hi Definition Video19
Network Hosted software>25

It's not just a matter of provisioning raw bandwidth, however. With demand for upstream applications (video communications for conferencing, sharing of digital home movies, and security surveillance) on the rise, symmetrical access is a critical need. Service providers require access networks with equally high amounts of bandwidth available in both the upstream and downstream directions, at guaranteed service levels.

Beyond costs

ESONs contain an active electronic element, a switch aggregator, between the central office (or head end) and the customer premises; PONs do not. PONs use a technique called power splitting. Because no power is required to operate the power splitter and outside plant maintenance is theoretically eliminated, the operating costs of PONs would seemingly be lower. But there are significant disadvantages.

PON systems can serve only 32 homes from each fiber run, and all PON subscribers share available network bandwidth. For example, a 1-Gbit/sec link shared among 32 subscribers yields approximately 31 Mbits/sec per subscriber. Currently, the only way to increase the available bandwidth is to reduce the number of homes served. Reducing the number of subscribers to 16, for example, will double the bandwidth to 62 Mbits/sec.

ESONs can provide 100 or 1,000 Mbits/sec of bandwidth per subscriber in either the upstream or downstream direction. A single ESON system can serve up to 48 homes on each fiber run ¿ and isolate information streams and faults to each subscriber. Up to 50,000 homes can be served from a single core switch in the central office.

Furthermore, PON systems have limited reach and typically can only extend about 25 km. They cannot isolate information, users, or equipment faults. This can drive up maintenance costs and expose users and content providers to theft. ESON systems, on the other hand, can extend up to 60 km from the central office, can completely isolate information streams, and remotely isolate and locate faults. These capabilities lower operational costs, reduce theft of services, and increase system availability.

The question of security

In a passive network the entire downstream bandwidth is transmitted to the power splitter and delivered to each subscriber. The customer-premises equipment (CPE) contains optical transceivers that switch on and off during the allocated time slots and select their content. This ideally prevents them from accessing content not intended for them. However, it may be possible for a rogue CPE to steal content designated for another subscriber.

In an active system only the content destined for a particular CPE is delivered to that subscriber. All subscriber content is transmitted to the aggregation switch and then only distributed to the appropriate subscribers. At the aggregation switch, all content is converted from an optical signal to an electrical signal, separated from other content, and converted again to an optical signal and delivered appropriately over separate fibers to each destination. Even if rogue CPE is installed in an active network, no content is delivered to it.

Weighing the tradeoff

The tradeoff carriers face is one additional active or powered element for every 48 subscribers with an ESON versus the PON's power splitter with its inherently lower failure rate but no ability to isolate faults, switch local traffic, or provision narrow or uni-cast transmissions.

ESONs support greater bandwidth levels to more homes than PONs and provide guaranteed bandwidth either downstream or upstream. They have intelligence near the subscriber that can reduce latency, flexibly add bandwidth, isolate faults, switch, schedule and queue traffic, and maximize the bandwidth utilization between the switch aggregator and central office. In addition, active Ethernet networks reduce the likelihood of content theft and have higher system level availability and reliability due to fault isolation and remote monitoring capabilities.

For this combination of reasons, ESON-based access solutions are of increasing interest to service providers surveying the FTTP marketplace.

Bill Zakowski is vice president of business development, Amedia Networks (Holmdel, NJ).

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