Delivering video over passive optical networks
By GAYLORD HART, NEC Eluminant Technologies--How will video services reach subscribers over PON? Take a closer look at two service models.
By GAYLORD HART
NEC Eluminant Technologies
Service providers are endorsing next-generation, packet-based integrated voice and data access platforms for fiber-to-the-home (FTTH) networks, but video still presents unique challenges because of its bandwidth and transmission requirements. With the advent of interactive TV (ITV) and switched video-on-demand (SVOD), these challenges become even greater. The deployment of video services requires a cost-effective solution that can support evolving services with requirements that may not yet be known.
The market for video services is enormous and growing. There are now over 73 million households subscribing to basic cable TV (CATV) services and over 17 million paying for satellite-delivered TV services. For network operators, video represents a highly attractive broadband service offering in terms of revenue potential, both for content and advertising. This potential is increasing as services such as pay-per-view (PPV) and video-on-demand gain subscribers.
Successful deployment of FTTH networks--from both a technology and financial standpoint--will require the ability to deliver video services effectively along with voice and data services. This "triple-play" of voice, video, and data is an essential revenue component for the future success of service providers. Today, many networks face bandwidth constraints in the last mile, so delivering all of these services is either impossible or requires that service providers operate parallel networks. To compete successfully, service providers will need to transform their access networks to deliver all of these services via a single integrated transport and access solution. Ultimately, FTTH networks offer the best solution.
Several network operators plan to deliver all services through a direct fiber connection to residences and businesses at some point in the future. A passive-optical-network (PON) architecture is one way to accomplish this goal. The key feature of the PON architecture is the elimination of all active electronics in the distribution network. Low-cost passive optical splitters are used to distribute an optical signal over multiple paths to several subscribers using a single fiber or fiber pair. Most PONs use WDM to put both the downstream and upstream optical signals onto a single fiber at different wavelengths.
PONs offer high reliability as well as low operating and maintenance costs. Standards have been established by the International Telecommunication Union (ITU) for ATM-based PONs (G.983 series). The IEEE is developing Ethernet-based PON specifications.
Alternative FTTx models require active electronics in outdoor field locations. In these architectures, an intermediate access multiplexer or a digital loop carrier is needed in the distribution portion of the access network. These architectures use an optical transport link from the central office (CO) or headend out to an active neighborhood node, from which fibers go to individual businesses and homes. The major disadvantage of these approaches is the use of active electronics. Active electronics require battery backup, decrease overall network reliability, and raise field maintenance costs, even when the electronics are enclosed in a controlled environmental vault or in a protected aboveground enclosure. Because a PON eliminates outside plant electronics, an upgrade to a PON, if required, is much easier and less expensive to accomplish. Moreover, active FTTx models face bandwidth constraints due to the intermediate electronics, while PONs essentially can offer unlimited bandwidth over the fiber itself. Current ATM-PON standards support 622-Mbit/sec symmetrical transport shared among up to 32 subscribers, and work is underway to extend that above 1 Gbit/sec. Ultimately, the PON topology is the most cost-effective and flexible approach to building access networks.
From an equipment perspective, PON is rapidly reaching cost parity with traditional access systems based on SONET or hybrid fiber/coaxial (HFC) architectures. The primary barrier to deploying PON is the initial installation cost of the fiber itself. But from a total cost of ownership perspective, PON can provide a much less expensive and more reliable network than traditional topologies in use today. For all these reasons, industry analysts project strong growth for PON over the next few years.
Worldwide PON equipment sales are expected to reach $314 million in 2005, from $66 million in 2001, according to market researcher IDC (Framingham, MA) in an October 2001 report on PON equipment. Once the fiber is there, PON is the lowest-cost approach to delivering services over fiber, noted IDC analyst Sterling Perrin. Data will be a big driver, but video services will add more revenue and make the value proposition stronger for the service provider, according Perrin.
The delivery of television services over PONs is accomplished using one of two approaches: video overlay or in-band SVOD. Both models offer unique benefits and have some disadvantages. In the video overlay model shown in Figure 1, a separate optical wavelength is used on the fiber to transport video services between the optical line terminal (OLT) at the CO, or headend, and the optical-network terminal (ONT) at the residence. In the in-band SVOD model (see Figure 2), video services are transported as digital data in the same digital stream with other voice and data services on the fiber.
Video overlay model
The video overlay approach uses WDM to combine the video and other optical transport signals onto a single fiber. The video wavelength carries a broadband analog radio-frequency (RF) signal containing all the TV signals and is independent of any other wavelength used for the PON itself. Since that is an analog signal, it is typically positioned in the 1550-nm region to minimize attenuation during transport. This analog signal typically comprises several RF channels, which may contain traditional National Television System Committee (NTSC) TV signals, RF-modulated digital TV signals, or both. All these signals are frequency-division multiplexed onto a single broadband RF signal for transport over a single optical wavelength. Because these signals are analog in nature, care must be exercised in system design to ensure signal quality and reliability over the optical transport path. It may be necessary to make tradeoffs in cost, transport distance, or the number of PON splits to ensure adequate performance. The overlay model supports delivery of local broadcast services, satellite-derived TV services, and locally stored SVOD services.
There are two basic overlay methods, both using economical, off-the-shelf technology: the direct broadcast satellite (DBS) approach and CATV approach. In either case, the network architecture of Figure 1 remains the same. The primary difference is in the RF transport component. In the DBS approach, the RF transport consists of quadrature phase-shift keying (QPSK)-modulated digital video signals placed in the RF spectrum between 950 and 1450 MHz. In the CATV approach, the RF transport signal consists of quadrature amplitude modulation (QAM)-modulated digital TV signals and possibly traditional NTSC analog TV signals. The CATV approach typically uses 50-750 MHz for transmission.
The DBS approach uses QPSK modulation, which is more robust and may transport over greater distances or with more splits in the PON, but QPSK also consumes more bandwidth. The CATV approach uses bandwidth-efficient 64 or 256 QAM, but may not transport as far as the QPSK signals. However, both approaches use forward error correction and can be engineered to deliver reliable services over a PON. Similarly, high-definition television (HDTV) signals using the broadcast format may also be carried on the PON. Analog NTSC signals, which are even more susceptible to transport losses, may require significant network engineering and higher costs if they are to be transported over a PON.
At the home, the video optical wavelength is filtered off the fiber, and a separate analog optical receiver in the ONT converts the optical transport signal back into an RF signal for delivery throughout the home over traditional coaxial cable. Depending on the overlay model used, either a DBS receiver or CATV set-top box is used at the home to convert these signals for NTSC delivery into a standard TV. CATV set-tops tend to be more feature-rich than DBS receivers and often support more advanced services such as ITV or Web-surfing on the TV. If traditional NTSC TV signals are delivered in the CATV model, cable-ready TVs may be connected directly without any need for a set-top box, allowing multiple TVs in the home to be easily supported.
For PON architectures using a video overlay, there typically is no reciprocal out-of-band set-top box or DBS receiver return transmission path to allow these devices to communicate back to the CO or headend for services such as SVOD, PPV, or ITV. In traditional DBS systems (and some CATV systems), return communication usually is through a separate telephone connection established via a modem in the DBS receiver (or set-top box). That is not only inconvenient for the subscriber, but it also provides insufficient bandwidth for emerging interactive services. Similarly, many CATV systems are two-way and use an RF return path in the network, but the PON model does not typically support that. The most likely solution is to use an Ethernet connection back to the service provider, using the same mechanism that provides bidirectional data services to the customer, as shown in Figure 1.
In-band SVOD architecture
The second model for providing TV services over the PON uses in-band SVOD, as shown in Figure 2. This approach also supports delivery of local broadcast services, satellite-derived TV services, and locally stored SVOD services. But in this model, there are no continuously broadcast services per se. No video signal transmission to a subscriber occurs unless someone is actually watching or has requested a service. Signal transport is initiated automatically when a viewer selects a channel, triggering a request for service back to the system CO, or headend.
Once requested by a viewer, the video signal is switched from its respective source onto the PON, then travels as data over an optical link from the OLT to the ONT at the subscriber premises. In the case where more than one viewer on the PON is watching the same program, the signal need only be sent once over the PON, and multicast techniques are used to connect to the single transport stream as many subscribers as are viewing this signal. The ONT supports Ethernet-based networking in the home, and the link between the ONT and a subscriber's set-top box will likely be an Ethernet channel.
Because it operates as an on-demand system, the in-band SVOD model is highly bandwidth-efficient. This approach is also more cost-effective than the overlay model, because it does not require a separate optical overlay network and all the RF modulators and ONT receivers associated with it. The in-band SVOD model supports the integration of all service delivery over a single switching, transport, and access technology. However, for both the overlay and in-band PON models, work remains to be done for end-to-end integration and next-generation service delivery over a PON. DBS receivers and CATV set-tops don't typically support an Ethernet-based return path through a separate network. Similarly, current set-tops do not support Ethernet-delivered video services. And back office integration of billing and flow-through switching and provisioning of services needs to be implemented more extensively. However, much of the base work to accomplish these tasks has already been done. SVOD is being delivered in CATV systems and some incumbent local-exchange carrier networks today.
Both PON models offer telephone carriers and CATV operators the power to provide video and interactive two-way communications services over a high-bandwidth, low-maintenance, and low-operating-cost subscriber connection. Of the two models, the video overlay approach can be deployed today using mostly off-the-shelf equipment, though for advanced interactive TV services, additional capabilities are needed. The in-band SVOD approach is perhaps the most attractive but requires further system integration before being ready for wide-scale deployment. That is not far away, however.
Bridging with PON
As CATV operators and telecommunications carriers migrate their networks to deliver true broadband services over a single integrated technology platform, PON is the most powerful and efficient way to bridge the last mile of these networks. Competitive pressures and the ever-growing demand for new services and greater bandwidth will drive carriers to solutions which provide the greatest flexibility at the lowest overall lifecycle cost. PON architectures scale across all services and across residential and business requirements. Apart from technical issues that equipment providers and carriers are already addressing, carriers are beginning to recognize in their economic models that PON architectures not only lower overall operating costs but can increase revenue through the delivery of new services over a single connection to both businesses and residences.
In concert with service providers, equipment vendors are actively at work refining technical solutions to deliver broadband voice, video, and data services to homes and businesses via PON.
Gaylord Hart is vice president of marketing and business development/broadband access technology at NEC Eluminant Technologies Inc. (Chantilly, VA). He can be reached at firstname.lastname@example.org.
Sidebar: How video transport over PON works
For both the video overlay and in-band switched video-on-demand (SVOD) passive-optical-network (PON) video transport models, the last mile portion of the network-the PON-is basically the same.
An optical line terminal (OLT) in the central office, or headend, aggregates digital services for transport over the PON. In the case of the in-band SVOD model, a downstream optical signal for voice, video, and data from the OLT is transmitted over a fiber connection at 1490 nm. In the case of the overlay model, the 1490-nm signal only carries voice and data, and an additional downstream optical signal at 1550 nm is added to the fiber to transport TV services.
Upstream signals are similarly handled using 1310 nm for optical transport. The fiber from the OLT feeds field-located passive optical splitters, which can then provide fiber-delivered services to up to 32 optical-network terminals (ONTs) located at residences or businesses. Broadband services are handed off by the ONT: Twisted pair connections provide POTS, a 10/100Base-T Ethernet link handles data, and a coaxial connection or 10/100 Base-T Ethernet link feeds a set-top box.