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FTTP - Light fantastic
The addition of video services to fibre-to-the-premises (FTTP) networks has implications for the central office that service providers can’t afford to overlook, says Pat Thompson
As the demand for new high-bandwidth services pushes fibre closer to the consumer, a lot of attention is given to choosing the right FTTP architecture to make it possible. The majority of that effort focuses on the outside plant (OSP), including the distribution and access elements of the passive optical network (PON). After all, this is the portion of the network that will deliver the ‘triple play’ of voice, data, and video demanded by customers.
But it’s equally important for any FTTP network to achieve the same flexibility and reliability as the transport network. Since new video services, such as high definition television (HDTV) and video-on-demand (VoD), are the biggest bandwidth drivers for filling the fibre pipes, it’s important to consider video’s overall implications for the network – and it all begins in the central office (CO).
FTTP and the CO
Before discussing the unique challenges video presents, however, it’s important to first discuss the overall implications of a typical FTTP infrastructure on the CO. The most important function of the network planner is to make informed decisions during the early stages of deployment – decisions that will create a cost-effective and reliable network for years to come. With that in mind, planners must find ways to minimise capital expenses and long-term operational costs while achieving the highest possible level of flexibility in the network infrastructure. Within the CO, that goal is achievable in a number of areas.
Architectural decisions involve connection strategies between optical line terminal (OLT) equipment and OSP fibres. Other considerations include easy test access and WDM positioning. A key requirement for providing flexibility evolves from ensuring full crossconnect capability. With all OLTs, as well as OSP fibres, connected at the fibre distribution frame (FDF), easy access and significant long-term network flexibility is achieved. This enables easy adds, moves, and changes to the FDF.
Another critically important architectural decision involves placement of the video WDM within the CO environment. The video WDM combines the voice and data signals with video signals onto a single fibre – a key element of FTTP deployment. Again, with expense and flexibility in mind, it makes the most sense to place the video WDM in the crossconnect FDF line-up, using patch cords to connect the OLT equipment to the inputs of the video WDM.
A crossconnect patch cord connects the video WDM common port to the designated OSP port. This provides the immediate advantage of requiring just three connector pairs while still maintaining maximum flexibility. With the video WDM located at the FDF and all OLT patch cords routed directly to the FDF, even greater flexibility is provided. Any OLT is easily combined with any other OLT, regardless of its CO location.
The addition of video signals presents significant challenges to the configuration of the CO. For example, the video overlay onto the FTTP network adds additional fibre cable management requirements. Also, in order to transport the video feed to multiple PONs, additional optical splitting is necessary. Optical path protection switches are particularly valuable where the video signal enters the service office from the video serving office.
From the video OLT, video signals pass through several erbium-doped fibre amplifiers (EDFAs) used to amplify and split the signal. Each EDFA output will be further split by additional optical splitters to maximise the video output, allowing the most PONs to be served by the fewest number of EDFAs. Each EDFA can have up to four outputs, each with its own optical splitter, depending on signal strength.
The use of optical splitters is critical, and there are several placement options. For instance, the splitters could reside in either the OLT equipment frame or the fibre frame. Placing the optical splitter in the fibre frame enables the best flexibility. For example, if a particular PON is located a considerable distance away, a stronger video signal would be required and the signal should not be split. By having the optical splitter in the fibre frame, a patch cord can be run from the EDFA to the fibre frame, thus by-passing the optical splitter and allowing a stronger video signal to go to that PON. This flexibility allows video signals of various power levels to reach PONs at various distances. These optical splitters would reside in the fibre frame in a chassis very close to the WDM chassis on the 1550nm input side.
Assuming the office providing the video service is not the same office where the video signal originates, optical protection switching may also be useful. Through diverse path routing, both a primary and protect video feed enters the optical protection switch in the video OLT equipment frame. The primary video feed throughputs to the video OLT, but should that signal drop below a pre-set power threshold, the system automatically switches to the redundant path, or protect video feed. The diverse path routing takes place at the transmission side where a 1x2 splitter creates two diverse signals. This basically provides cost effective SONET-like protection without all the electronics by using a splitter and an optical switch.
Several important cable management considerations that apply in general to the FTTP network architecture will become even more critical when it comes to video signals. Since video signals are usually high-power analog, they require considerations for the use of angle polished connectors, connector cleaning techniques, and other cable management practices that contribute to signal quality.
Every network designer wants to get the most out of the existing electronics. In FTTP, that equates to serving the most PONs while achieving the highest network flexibility for the least amount of expense. But the constantly-changing network still requires every new FTTP design to readily adapt to the future.
Test access for the future
Testing the FTTP network is a serious challenge for service providers. While traditional FDF solutions required breaking into a patch and taking the network out of service, new advanced FDF solutions are emerging that enable remote test and monitoring functionality. This will substantially reduce troubleshooting and fault isolation time – resulting in better efficiency, reliability, and cost savings.
By placing an optical NxN switch between the test equipment and the access port on the fibres, any fibre can be tested with any test equipment from the network operations centre (NOC). For example, if contact is lost with several optical network terminals (ONTs), an optical time domain reflectometer (OTDR) trace can be performed over the particular fibre to isolate the fault. Performance monitoring tests can be accomplished without having to dispatch a technician to the frame to manually perform testing.
Built-in diagnostics can identify problems within the electronic equipment, but to see what’s happening within the fibre requires specific test equipment and non-intrusive access points. FTTP networks are designed as point-to-point connections from the OLT to the customer. If there is a failure in that network, the customer is out of service – there is no redundant path available. Therefore, the ability to restore the network quickly and easily is absolutely critical. The addition of this single NxN switch provides technicians with quick, easy, reliable access to the network – all of which greatly reduces network outage time and saves money.
Since the one constant in telecommunications has always been change, any assumption that the network will remain static can result in significant long-term reliability and flexibility issues. Therefore, informed decisions made by network planners today – from the CO to the customer’s home – will significantly impact the future operational characteristics of their FTTP network.
Pat Thompson is a program manager for ADC , headquartered in Minneapolis, MN, USA
www.adc.com
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