HFC access networks take DWDM in new directions
Don Sipes Scientific-Atlanta Inc.
DWDM technologies must be modified to meet the needs of cable-TV companies and carriers with video-intensive traffic patterns.
When it comes to dense wavelength-division multiplexing (DWDM), access networks have requirements that are fundamentally different from their transmission counterparts.
Most trunk or backbone networks employ DWDM with one purpose in mind: to send as much information as possible, as far as possible, on as few fibers as necessary. Optical switching or routing is used only for protection or provisioning and nearly all of the traffic formats are set by standard transmission protocols, such as Asynchronous Transfer Mode (ATM) over Synchronous Optical Network (SONET) or the newly arrived packet-over-SONET (POS) format.
In the access network, on the other hand, traffic originating from multiple sources such as switches, cable-TV headends, and video server banks must be aggregated together, transported to the general serving area, and then routed to residential and business users. Return traffic must follow the same path in the opposite direction.
In addition, these systems must transport this traffic in its native format--in the format used by end-user devices based on analog, QAM, or baseband digital formats. And low cost is a supreme consideration here.
For hybrid fiber/coaxial-cable (HFC) access networks, the traditional DWDM transmission model is being extended to take into account the unique needs of the access network. Three new technologies are being employed: multi-format DWDM, passive optical routing, and DWDM overlay technology.
Three choices
Multi-format DWDM enables traffic modulated in different formats--including analog video, baseband digital, and spectrally efficient digital formats such as QAM 64 and QAM 256--to be transported all on the same fiber.
Passive optical routing uses newly available DWDM devices to separate, redirect, and recombine these diversely formatted signals and route them independently to the end user or groups of end users in a multicast format.
DWDM overlay technology makes use of the frequency-division multiplexing nature of the HFC access network to combine different traffic from different sources at different wavelengths--all on the same photodetector.
The use of multi-format DWDM interconnects, passive optical routing, and WDM overlay technologies offers several system advantages over other approaches.
First and most important is the dramatic increase in bandwidth. An individual user (either business or residential) utilizing advanced services such as high-speed Internet access, telephony, video on-demand, or other interactive services over multiple appliances at the same time, might expect to experience individualized data rates of more than 20 Mbits/sec--in addition to the 550 to 750 MHz of broadcast bandwidth already present. This is a far cry from the data rates available today.
Second, the use of DWDM allows a substantial reduction in the amount of equipment that must be located at the hub site. Since the primary operation at the hub site in a DWDM HFC access network is reduced to passive routing, retransmission, and optical amplification, the size and physical plant requirements for these sites can be greatly reduced. Moreover, as the size of these remote hub sites is reduced, each site becomes much easier to locate, does not require environmental control, and does not require nearly as much maintenance as larger hub sites.
An added benefit of smaller hub sites lies in the fact that while the number of cable modem, digital video, and telephony processors and controllers may stay the same, centrally locating this equipment requires fewer people to maintain the system, yet improves reliability and makes system upgrades easier. Finally, utilizing DWDM in the HFC access network preserves the linear nature of the HFC system and the inherent transparency associated with it. Future upgrades involving new compression schemes, modulation formats, or data protocols can be added without having to change the network.
Attributes of an HFC access network
A DWDM-based HFC access network must be able to deliver high-quality video, Internet access at speeds in excess of 6 Mbits/sec, voice, and highly interactive video services at costs dramatically less than backbone or even metro-based DWDM networks. This demand for low cost requires that access-network traffic avoid any form of bit handling--i.gif., transforming the information from one format to another. An example would be transforming SONET traffic to QAM-modulated data. At an information origination source such as a headend, video server bank, class 5 switch, or data server, the information is placed directly in the format required by the user appliance--be it a computer, telephone, set-top box, or traditional TV set.
This requirement for native-format transmission means that the DWDM network must be transparent to a wide array of transmission formats, from analog video; to spectrally efficient modulated forms of digital transmission like QPSK, QAM 64, QAM 256, or 8 VSB; to baseband digital formats like SONET, Fiber Distributed Data Interface (FDDI), or ATM. A DWDM backbone transmission, by comparison, carries only baseband encoded information.
The additional complexity of the multi-format DWDM HFC access network requires that the transmission be designed to accommodate a wide array of signal-to-noise requirements--from 50 to 55 dB for analog video to less than 20 dB for baseband digital. The sensitivity to noise, distortion, and crosstalk of the analog and spectrally efficient digital transmission channels requires that the system be essentially linear in nature--that is, the linewidths of the transmission sources must be controlled, the gain of the optical amplifiers flattened, wavelength crosstalk kept to less than 32 dB at each component, and fiber nonlinearities essentially nonexistent.
In the DWDM HFC access network depicted in Figure 1, baseband digital traffic for business users and high-speed data traffic are routed onto the multi-format DWDM transmission system to hub locations, where routers break down the traffic to individual users. Interactive video services are QAM 256-modulated and transmitted via 1550-nm DWDM transmitters, where at the hubs they are passively routed to their respective nodes.
Broadcast television is transmitted in the traditional analog VSB format by externally modulated and linearized 1550-nm transmitters. High-power ytterbium/erbium-doped fiber amplifiers (YEDFAs) broadcast the traffic to a number of nodes.
At the hubsites, locally originated traffic is modulated into an appropriate part of the HFC RF spectrum and transmitted via a 1310-nm WDM overlay laser to the node. By channeling the analog, QAM-modulated digital, and locally originated data into different parts of the 50- to 870-MHz spectrum, multiple wavelengths can be received on the same photodetector, thus minimizing cost.
Spectrum allocation and transmission
Figure 2 shows a channel allocation plan for the multi-format DWDM HFC access network. Due to their high power and high sensitivity to noise and distortion, analog channels are placed at the longest wavelength portion of the 1530 to 1560 nm spectrum, approximately at 1558 to 1560 nm. Allowing for an optical isolation guard band of approximately 5 nm (continuing improvements in DWDM passive technology are reducing the width of this required guard band), the QAM-modulated channels are placed next at either 200- or 100-GHz spacing. Baseband digital traffic runs below 1540 nm. These channels are also placed with the same spacing as the QAM-modulated wavelengths.
Since the analog channels typically run at power levels 30 dB higher than the baseband digital channels, issues such as stimulated Brillioun scattering, stimulated Raman scattering, and optical crosstalk from the high-power analog channel to the lower-power baseband digital channels are of prime importance.
Placing the analog channel at the longest-wavelength end of the spectrum is optimum for a number of reasons. First, the 1560-nm end of the band is where the noise performance of optical amplifiers is at their best. For low-noise HFC-grade erbium-doped fiber amplifiers (EDFAs) and YEDFAs, a full 2-dB improvement in noise figure can be achieved by utilizing this portion of the spectrum rather than the 1530-nm portion. Second, since the Stokes transitions involved in Raman scattering have much higher cross-sections than the anti-Stokes, any Raman scattering from the long-wavelength analog channel will fall out of band. Finally, by placing the analog channels at the long-wavelength end of the spectrum and the baseband digital channels at the short-wavelength end, interference from the high-power analog channel onto the lower-powered baseband digital channels is minimized.
For short distances and low QAM channel loadings, directly modulated 1550-nm DWDM transmitters can be used to transport QAM-modulated digital information. For distances typically encountered from the headend or primary hub site to the node (in excess of 50 km) and for channel loading greater than eight QAM channels, a low-cost externally modulated approach is preferred.
The important factors leading to the use of externally modulated transmitters for the DWDM transmission of QAM-modulated signals are related to the linewidth of the transmission scheme. Due to the high dispersion of standard fiber at 1550 nm, directly modulated 1550-nm transmitters in HFC networks have suffered from high levels of dispersion-induced, second-order distortion. The narrow linewidths afforded by externally modulated transmission allow for distortion-free transmission over distances in excess of 100 km. External modulation for DWDM QAM transmission also enables use of the entire 200 MHz allocated for interactive digital services. It also avoids unwanted FM-to-AM conversion, which can arise when a wide-linewidth directly modulated transmitter drifts in wavelength with respect to DWDM multiplexing and demultiplexing filters.
Mixed-format traffic at the hub site
At the hub site, the multi-format DWDM spectrum is separated and interactive channels are routed to their respective nodes. The broadcast portion of the spectrum is split through a passive 1 ¥ n splitter before it is sent to the nodes. The multiwavelength QAM portion of the spectrum is separated through a DWDM demultiplexer before it is recombined with the broadcast signal. Baseband traffic for business and other specialized users is separated and sent to remote routers, servers, and PBX equipment for voice services.
The multi-service, multi-format nature of the HFC access network places special requirements on the passive optics used in this application. They must have low loss, have good port-to-port uniformity, and be stable over a wide temperature. The high carrier-to-noise ratio (CNR) requirements of the analog and QAM portion of the optical spectrum require DWDM components with wide passbands, sharp filter edges, and low crosstalk.
Having mixed-format traffic operating at widely different amplitude levels also places special emphasis on good port-to-port isolation. Isolation levels in excess of 32 dB are necessary in this application. Glass planar waveguide technology for high-count optical splitters, as well as dielectrically coated DWDM components, have been successfully employed. Devices based on Bragg gratings are under evaluation.
Another critical area of concern is passive optics packaging. Because some channels are passively split, some channels are optically routed, and others can involve forms of optical add-drop multiplexing, the quality of the passives` housing and the routing of fibers to and from them takes on increased importance.
Signal detection at the node
After the traffic has passed the hub site, each node has two wavelengths falling on its optical receiver. One wavelength contains the broadcast portion of the spectrum and the other the QAM-modulated narrowcast traffic comprising voice, data, and interactive video services. The broadcast channels are typically 80 to 96 channels of analog video and an additional 10 to 20 broadcast digital channels. The narrowcast spectrum typically involves 10 to 20 digital channels.
To achieve acceptable end-of-line performance, the CNR of the analog channels must be more than 51 dB at the optical node receiver. A well-designed 1550-nm broadcast system can deliver in excess of 52-dB CNR at the node for 0-dBm received optical power. Given the limited number of narrowcast channels, the DWDM QAM overlay transmitter can operate at a much higher-percent OMI than the broadcast transmitter--and thus have a much lower received optical power level at the node.
In a typical scenario, the DWDM overlay transmitter will transmit 16 64-QAM channels with a received optical level of -7 dBm. The effect of the DWDM QAM overlay signals on the analog channels is approximately 1-dB degradation in CNR (see Fig. 3). The prime contribution to the degradation of the analog channels` CNR is from the shot noise contribution from the overlay laser. Other noise sources such as overlay laser RIN and fiber scattering effects are of lesser importance.
Once the number of RF channels on the overlay laser exceeds around 20 channels, two receivers separated by a WDM demultiplexer are required in the node, and the broadcast and narrowcast channels are combined in the RF domain.
Reverse traffic transmission
A method of using DWDM concepts for reverse HFC transmission is illustrated in Figure 4. In this configuration, reverse transmitters operating in the node at 1310 nm relay the 5- to 40-MHz reverse traffic to the node, where it is converted to RF and combined with other nodes as traffic demands require.
The combined RF signal then drives a 1550-nm ITU grid-selected laser, which is multiplexed with other wavelengths from transmitters receiving RF traffic from other groups of nodes. This multiplexed signal is transmitted back to the headend for demultiplexing, demodulating, and then routing to each of the respective service-processing units.
The performance of the 1550-nm DWDM link is sufficiently higher than that of the 1310-nm link, which creates a nearly transparent link--that is, the performance of the node-to-hub segment is preserved nearly intact as it is retransmitted from the hub to the headend.
Multiple services at low cost
Making efficient use of all three of these technologies--multi-format DWDM interconnects, passive optical routing, and WDM overlay technologies--is critical for providing multiple services to end-users at the required low costs.
Several cable operators, most notably Tele-Communications Inc. (TCI), have announced projects to upgrade their networks via some of these techniques. Competitive local-exchange carriers and other nontraditional service providers such as utilities also have reported activities aimed at providing services via DWDM HFC access networks. At the same time, vendors providing complete end-to-end solutions are offering new platforms to take advantage of DWDM`s unique capabilities to deliver interactive services. u
Don Sipes is vice president of technology, transmission networks systems, at Scientific- Atlanta Inc. (Atlanta, GA).