Cable services that once were considered futuristic, such as interactive video-on-demand, fast Internet access and IP telephony, are quickly becoming today's reality, and their rapid rise is placing increasing demands on the hybrid fiber/coax (HFC) networks that provide them.
Today's advanced networks must provide for reliable two-way communications, scalability to support ever-increasing amounts of home-dedicated bandwidth, and the means to reduce operating costs — while allowing for capacity investments to more closely match both new services and plant demographics.
In addition, these networks must be planned, provisioned and managed in a seamless and transparent way. The transition from analog to digital channels, and the addition of varying SDTV and HDTV formatted channels, should be accomplished in a manner that will not affect the performance of the rest of the system. At the same time, traditional revenue generators such as IPPV and advertising should not be negatively impacted, but instead, allowed to further develop.Making room for new services
From a transport technology standpoint, there are four basic physical dimensions which may be employed to handle this onslaught of new service demands. They are bandwidth, spatial multiplexing, spectral efficiency and wavelength.
The bandwidth of today's advanced networks is currently installed at 750 MHz, but recent traffic analysis studies have shown that strong arguments can be made in favor of installing 870 MHz systems. Spatial multiplexing refers to using "home run" fibers to nodes serving ever-shrinking numbers of homes passed. This allows parts of the forward spectrum to be "reused," enabling larger amounts of bandwidth to be allocated to a single user.
Spectrally efficient methods of transmitting digital information such as 64 QAM and 256 QAM allow higher amounts of information to occupy each hertz of bandwidth. The use of the optical wavelength as an additional dimension to expand the performance and capacity of these advanced multi-service networks is just emerging. Such methods, referred to as optical networking, open up a wide array of possibilities, such as multi-channel transport (DWDM), passive and active optical routing, and optical cross connecting.
Advanced optical networking tools such as Sonet high-performance video transport, mixed format DWDM and WDM overlay technology allow operators to take full advantage of these rapidly deploying advanced services, while maintaining excellent control over equipment and operating costs. These three optical networking tools are shown schematically in Figure 1.
In Sonet high-performance video transport, analog video is digitized in a 10-bit uncompressed format, which is mapped directly into a Sonet OC-3c (concatenated) frame. Sixteen of these streams are electrically multiplexed into an OC-48 rate, and eight of these optical signals can be multiplexed via 1550 DWDM into a nearly 20 Gbps data stream.
Sonet high-performance video transport has advantages over DS-3-based traditional multiplexed Sonet. The OC-3c mapping allows for much higher performance and lower cost than that provided by using video codecs. Sonet high-performance video transport is also preferable to available proprietary video transport systems because full Sonet capability is needed for interfacing to other Sonet systems in the network.
In addition, Sonet high-performance video transport systems have the advantages of traditional Sonet voice and data networks; namely, multiple interfaces such as DS-3, ASI, Video IF, QAM and Ethernet, add/drop capability, and cross-connect capability via time slot interchange.
Mixed format DWDM, illustrated in Figure 3, contains not only baseband transmission formats like Sonet, but analog and spectrally efficient digital formats like QAM transmission as well. In bandwidth intensive services such as VOD, numerous video streams must be transported from the headend to a remote hub and then routed to individual groups of nodes.
Mixed format DWDM at the 1550 nm window allows for multiple groupings of QAM channels over multiple wavelengths to be transported in their home native format, thus leaving the video servers and QAM modulators located in the headend, reducing the amount of equipment located in the hubs.
The multiple wavelengths also allow for passive optical routing of these channels to their respective node locations. Mixed format DWDM is also used in the reverse path; single nodes or groups of nodes are combined at RF in the hubs and retransmitted via DWDM over a single fiber back to the headend.Narrowcast/broadcast separation
WDM overlay technology makes use of the frequency division multiplexed nature of cable signals and the wide wavelength coverage of today's Indium Gallium Arsenide (InGaAs) optical receivers to combine multiple optical wavelengths — each having different RF channels on a single optical receiver.
WDM overlay technology performs the important task of separating the broadcast and narrowcast portions of the network so they may be optimized independently. The broadcast portion of the spectrum may now take advantage of low dollar-per-milliwatt technologies such as EDFAs (erbium doped fiber amplifiers) and high-power YEDFAs (ytterbium, erbium doped fiber amplifiers), while the narrowcast portion of the spectrum can be delivered by low-cost 1310 nm and 1550 nm directly modulated uncooled DFBs. Such a system is illustrated in Figure 2.
In this architecture, the broadcast channels (typically 80 to 110 analog channels) are transmitted from the headend to the node via 1550 nm transmission.
This is also used to transmit from the headend to the hub, where a high-power 1550 nm YEDFA amplifies the optical signal to levels approaching 25 dBm.
After amplification, the narrowcast portion of the spectrum (typically eight to 16 QAM channels plus two to three analog channels) is added via a 1550/1310 nm WDM multiplexer or a 1550/1550 nm DWDM multiplexer. Both the broadcast and narrowcast traffic are transmitted to the node, where they are received by the same photodetector.
The addition of the narrowcast wavelength degrades the broadcast signal carrier-to noise ratio by less than 1 dB. Networks utilizing WDM overlay technology have an added benefit in that they allow costs to be deferred. This means that a portion of the cost of the network is deferred until the service demands on the network require the additional capacity. For the WDM overlay system, the 1310 nm overlay transmitters can be added only when the service or demographic demands require it.
Each of these technologies has its own complementary place in today's HFC network configured for advanced services. Sonet high-performance video transport is used in the primary rings of the network and in regional interconnects; mixed format DWDM for the secondary interconnects; and WDM overlay technology in fiber distribution. Providing products based on these technologies, with the ability to work together in a seamless fashion, is essential for successful delivery of these advanced services.