Video delivery is proving to be essential to many Fiber-to-the-Home (FTTH) systems, but there is a lot of competition in the video delivery business. If FTTH is to differentiate itself, it must deliver a video product that no other technology can deliver.
By JAMES O. "JIM" FARMER
Wave7 Optics Inc.
Many people view video delivery as the linchpin of the "triple play" bundled service offering. Traditionally and by technical necessity, cable TV operators, with their hybrid fiber-coax (HFC) plants, have only delivered broadcast analog and digital video. ADSL operators are able to deliver video only via Internet Protocol (IP)--if they can deliver video at all. Modern fiber-to-the-home (FTTH) systems are the first to have the technical capability to deliver both, including high-definition TV (HDTV), if desired.
Broadcast
Broadcast delivery is the oldest of the technologies. Our present analog system traces its roots back to just after World War II. Color was added in the 1950s but didn't enjoy significant consumer penetration until the 1960s. By the end of the 1970s, however, it was universal. Though much work had been done to improve the quality of the broadcast signal, until the advent of digital broadcast, the only new features added were stereo and closed captioning. If you can find a working TV set from 1950, it will still work on today's analog broadcasts.
Digital broadcast TV was developed during the 1990s as a way to deliver HDTV as well as to pack several programs in the channel occupied by one analog program. It is now enjoying widespread deployment in satellite transmission (all DBS service is digital), cable, and over-the-air broadcasting. All HDTV broadcasting is digital, and much standard definition (SDTV) or regular TV is digital. Clearly, the trend is to digital, but the fact that few consumer TVs can directly receive digital signals precludes a fast transition.
Both analog and digital broadcasts can be accommodated in FTTH systems. The signals are sent over an analog optical path, using a wavelength in the 1550-nm band because that band can be optically amplified economically. "Analog" as applied to the optical path should not to be confused with "analog" as applied to TV broadcast. Broadcast signals are sent on RF carriers, high frequency signals upon which the analog or digital TV signal is imposed or modulated. The transmission of many carriers is what necessitates the use of an analog optical system; if the optical system were digital, you would not be able to recover individual RF carriers.
There have been reports of problems with broadcast transmissions on certain FTTH systems, though with proper engineering, these issues can be addressed. The first issue is the need for a certain minimum signal level at the receiver, which is necessary in order to recover signals with an acceptable carrier-to-noise ratio. There is also a maximum signal level you can launch onto the fiber due to a phenomenon called Stimulated Brillouin Scattering (SBS). SBS is a function of the fiber itself, the length of that fiber, and, to an extent, the transmitter. The maximum signal power you can launch and the minimum signal power you need at the receiver define the loss budget. The loss budget also is impacted by the number of times the signal is split and by the length of the fiber. A 32-way split is an aggressive split ratio in many practical cases.
When deploying broadcast video, passive optical networks (PONs) also suffer from crosstalk between the data path and the RF path due to a fiber problem called Stimulated Raman Scattering (SRS). SRS causes crosstalk between a data signal and a broadcast signal transmitted on the same fiber. The spacing between the broadcast and the data optical wavelengths affects the degree of SRS, and here the standards bodies have created a headache for implementers. All FTTH systems place broadcast transmission at about 1550 nm, a necessity to allow for economical optical amplifiers. Such systems use 1310 nm for upstream data, and some standards use 1490 nm for downstream data. At 1490 nm, interference into 1550 nm is quite bad.
Other FTTH systems use the 1310-nm bi-directional wavelength for data and the 1550-nm wavelength for broadcast. These systems have a significant advantage in terms of SRS immunity. The wavelength spacing between 1310 nm and 1550 nm means a very significant reduction in SRS, to the point where it is a non-issue.
IPTV
TV over Internet Protocol or IPTV has been around for several years, primarily as a way to transmit TV signals in DSL systems. With IPTV, you don't put the signals on an RF carrier. Rather, you break the digital video signal into chunks called packets. Each packet is put inside suitable protocols, which always include IP and will include certain higher and lower layer protocols as well. The packets can be sent on the data portion of the FTTH network, which is separate from the broadcast portion. If all video is sent using IPTV, the broadcast portion of the FTTH network can sometimes be eliminated for cost savings. But before doing so, operators must understand the limitations this may impose on their service offerings.
Network efficiency is a key consideration in IPTV. Most Ethernet-based networks can use a protocol called IGMP, which maximizes the efficiency of the network by replicating a signal in the network when it is being used by more than one subscriber. Otherwise, more signal streams would have to be transmitted from the headend, burdening the data network. There is no practical corresponding protocol in ATM systems, making their use for video less efficient--though possibly still practical.
IGMP allows many users to access a program at the same time, just as you would in a broadcast environment. This is called multicasting. IGMP enables the network to duplicate packets going to multiple subscribers in the network. Thus, if 100 people are simultaneously watching a program, it is necessary to generate the stream only once in the CO or headend, and the network will replicate that stream of packets wherever it needs to in order to reach all 100 people. The alternative is to generate the stream 100 times at the CO or headend. For programs intended for only one subscriber, such as video-on-demand, unicasting is used, and IGMP does not apply.
To receive IPTV, a special set top terminal (STT) must be used for each TV. Some manufacturers have built boxes with multiple STTs to reach two or three TV sets. STTs built for DSL delivery work well for FTTH delivery, as they have Ethernet connections for the input signal (regardless of whether the delivery network uses Ethernet or ATM). That said, there is a key disadvantage to using IPTV as opposed to broadcast: Few existing homes have Ethernet connections in the right place for the STT. Getting an Ethernet connection to the TV is a cost issue for the provider, as is the fact that each and every TV (and VCR and PVR and picture-in-picture input) must have its own STT.
The Future
Both broadcast and IPTV will be a part of FTTH, as each brings specific advantages to the consumer and the operator. Broadcast is ideal for popular linear channels, traditional channels designed for the subscriber to view or record at the time the material is transmitted to all subscribers. IPTV is ideal for programming intended for only one subscriber, such as video-on-demand. In between, there are linear programming networks that meet more of a niche market, which could go either way. An ideal world includes STTs that can handle both formats and present a unified program guide to the consumer. Viewers shouldn't have to know or care whether a program is delivered via broadcast or via IPTV. This nirvana is technically feasible, but the pieces are not all in place to make it happen just yet.
James O. "Jim" Farmer is the chief technical officer at Wave7 Optics Inc., headquartered in Alpharetta, GA. He may be reached at www.wave7optics.com.