OFC, Baltimore, 7–10 March 2000, paper FA1 (invited)
HFC architectures for broadband two-way access David Piehler Harmonic, Inc., 549 Baltic Way, Sunnyvale, California 94089 Phone. (408) 542-2740; fax: (408) 542-25 J2, email.
[email protected]
This talk will cover the evolution of the hybrid fiber coax (HFC) network from a one-way medium for the broadcast of analog video into a high-speed, two-way data network. Particular emphasis is given to the next generation ofHFC networks.
OFC, Baltimore, 7–10 March 2000, paper FA1 (invited)
HFC architectures for broadband two-way access David Piehler Harmonic. Inc .. 549 Baltic Way, Sunnyvale, California 94089 Phone. (408) 542-2740;fax. (408) 542-251 2, email.
[email protected]
The hybrid fiber coax network is the physical medium over which cable television companies distribute their product. Originally this network was entirely coaxial cable, and the content was entirely analog video signals. The system bandwidth was limited by the high frequency attenuation of the coaxial cable and the bandwidth of the RF amplifiers used. The introduction of linear optical transmitters and optical fiber gave birth to the HFC (hybrid fiber coax) network. In an HFC network, fiber is used as a backbone and for some signal distribution while coaxial cable is used for the "last mile." Until recently, increasing the bandwidth in order to offer more analog video channels was the primary motivation to upgrade an HFC network. More recently, cable companies have been "mining" HFC network to create a two-way, interactive data network. Coax cable has the highest bandwidth of the non-fiber transport mediums. Thus, coaxial cable is the ideal vehicle to carry high bandwidth data into and out of homes . A new requirement for the network is the distribution of different digital content to targeted service areas, along with the broadcast content common to all service areas [1,2]. - The delivery of content that is specific to a single customer or small group, such as internet service, video on demand (VoD), or telephony is called narrowcasting. 1. Optical Narrowcasting
The frequency divi sion-multiplexing format of the RF signal allows the easy mixing of targeted content with broadcast content. Targeted digital data are typically modulated in a 64- or 256-QAM format onto RF carriers at frequenc ies above the broadcast band (usually greater than 550 MHz). The digital content can be mi xed with analog video content by RF combination or by optical combination (Figure 1.) For optical narrowcasting, more than one wavelength is incident on the receiver. The presence of two wavelengths does not confuse the RF receivers in the television, set top box, cable modem, or cable phone since each wavelength carries content at different RF frequencies. The presence of two wavelengths will decrease the carrier to noise ratio, but a proper balance of optical powers and optical modulation indices can minimize this decrease .
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550 600
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550 600
Fig.!. (a) RF insertion of narrow cast content. (b) Optical in sertion of narrow cast content
OFC, Baltimore, 7–10 March 2000, paper FA1 (invited)
2. DWDM in the forward path A CMTS (cable modem termination system) connects the HFC plant to the internet. The optical insertion of digital content from the CMTS can take place either at the hub or the headend . If the CMTS is located at the hub, the CMTS feeds a narrowcast transmitter or transmitters, (usually a 1310 nm DFB laser), and the narrowcast signal is injected into the 1550 nm broadcast signal by coarse WDM. DWDM can be used to enable narrowcasting with bulk of equipment such as CMTSs in the headend. In this case, the digital services can be targeted at service areas at the hub, and injected into the broadcast signal at the hub as shown in Figure 2.
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HEADEND
HUB
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Fig. 2. DWDM used to enable narrowcasting. One headend serves a plurality of nodes.
3. DWDM in the return path The return path signal is usually QPSK or 16-QAM modulated onto RF carriers in the 5 to 45 MHz range. In a typical HFC architecture the return path signals from many cable modems are aggregated into a single RF cable feeding a return path optical transmitter. As the return traffic increases, the number of homes served by a single node (the demarcation point between the fiber and coax plant) must decrease . Simply combining all return path signals in an HFC (or any point to multipoint) system, is unacceptable due to noise funneling. DWDM return path transmitters have been deployed in the hub and in the node, to enable segmentation of the return data streams.
4. DWDM for video on demand True VoD (with pause, rewind, and fast-forward) requires multiple streams of digital video content to be routed to various service areas. There may be a desire to centralize the location of the video servers from hubs to the headend. One solution is to convert the digital video streams (on QAM modulated RF carriers) into baseband digital signals, and use SONET-like transport to distribute the content to the hubs. At each hub the video information in converted from baseband digital back into QAM modulated RF carriers. Figure 3 shows an alternate method [4]. The video server output directly feeds a band of analog DWDM transmitters. The video content is put on to a single fiber, and delivered to designated areas in its native QAM format using wavelength drop filters. From the hub site either RF or optical techniques can add the VoD content to the broadcast content. (Figl.)
OFC, Baltimore, 7–10 March 2000, paper FA1 (invited)
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Video Headend
Head~nd
Wavelength Drop filt~\
Fig. 3. Video on demand architecture using DWDM to transport video streams in their native QAM format .
5. Moving the fiber deeper As new services evolve, there is a desire to create an HFC network that is scaleable and future-proof. One can moves the node closer to the customer, eliminating RF amplifiers on the coax part of the plant, this can result in immediate cost savings from reduced powering requirements [3]. The "mini node" is then a platform for both current services, as well as future services. Without RF amplifiers limiting the bandwidth, the 750-1000 GHz band can be "mined" for future two-way services, while leaving the existing network and services intact. There is also good reason to use the subcarrier multiplexed data format of the HFC network even as fiber moves to the home [5]. Modulation techniques such as 64-QAM are spectrally efficient, carrying for example 27 Mb/s of data in 6 MHz of RF bandwidth. This format is equally good for broadcast services, such as video, as well as two-way interactive services.
References: [1] X. Lu, "Broadband Access over HFC Networks," Optical Fiber Communications Conference, OSA Technical Digest (Optical Society of America, Washington , DC, 1999), paper ThV I. [2] O. Sniezko, "V ideo and data transmi ss ion in evolving HF C network," Optical Fiber COlI/lJlunications Conference, OSA Technical Digest, (Optical Society of America, Washington , DC, 1998), paper WE4. [3] O. Sniezko, et aI. , " HFC arc hitecture in the making," 1999 NCTA Technical Papers, page 20. [4] 1. Yeh , M . Selker, J. Trail , D. Piehler, and l. Levi , " DWDM Arc hitectures for Video on Demand Transport and Distribution," I£££IL£OS Swnmer Topical Meeting: RF Photonics for CATV and HFC Systems, 1999, talk ThB2.2.
[5] T. H. Wood, G. C. Wilson, R. D. Feldman, and J. A. Stiles, "FiberVista: A Cost-Effective Fiber-to-the-Home (FTTH) System Providing Broad-Band Data Over Cable Modems Along with Analog and Digital Video", IEEE Photon. Tech. Lett. , vol. 11, p. 475 , (1999).
