Tim / Hiram, regarding your question
<<I was wondering what you thought of HFC systems,and MPEG and SDV over HFC>>. MPEG is transmitted over HFC systems by many cable companies and telcos that are using HFC. Typically, in 750 MHz HFC systems, 100 to 150 MHz in the downstream spectrum are reserved for digital video. In each 6 MHz spectrum block, about 5 MPEG-2 digital channels can be transmitted, or about 80 channels in 100 MHz. The digital channels can be broadcast programming available simultaneously to all subscribers with a digital settop box that are authorized to receive the programming (a pay per view movie or event), or the digital channels can be a customer specific program that is only viewed by one customer (video on demand). Broadcast applications of MPEG-2 over HFC are a very powerful and effective use of bandwidth...as more customers have digital settop boxes or receivers in the future, more and more of the analog spectrum can be migrated to MPEG to increase the quantity of simultaneous broadcast channels.
<<Speeds over HFC??>> If 650 MHz downstream bandwidth on HFC were to be utilized for 64-QAM modulated MPEG-2 transmission, that would be equivalent to over 2 Gbps broadcast to all homes, or about 13 times the amount of content that can be simultaneously transmitted to a home by a 155 Mbps (OC-3) link in a SDV system. Wow.
SDV over HFC?? No, these are two different animals. SDV is a baseband digital fiber to the curb system in a star topology, targeted to a small number of homes per optical transceiver. HFC is a radio frequency based network with a monster downstream broadcast capability to a whole bunch of houses per optical node in a bus topology, with pretty decent upstream communications capacity if it is very well engineered. SDV and HFC don't mix. However, the idea of SDV can be applied to HFC for things like video on demand. I have yet to see anyone figure out how to avoid losing a lot of money on video on demand. Time Warner just threw in the towel on it in Orlando, as have most RBOC trials. The project where I work still has a whole bunch of video on demand customers on HFC, and it really works super, but I'm better off by being ignorant about what it really costs.
Fiber amplifiers do nothing for shared return path bandwidth, and are not used the in last mile of any HFC systems that are using 1310 nm lasers. The key to maximizing return path bandwidth is carefully planning for the worst case number of simultaneous users of the return path, which can be done by very careful engineering. A telco planning for 100% of homes passed having an average of 1.2 to 1.5 active lines per home has a much bigger challenge than a cable company that is looking to skim off a phone line from 10 or 20 percent of homes passed. Two alternatives are used to greatly increase the amount of return path communications that are available. Block converters used with QPSK modulated digital telephony signals, or OFDM modulation of digital telephony signals. OFDM is emerging into the general production stage from ADC these days, and looks like a cleaner and better upstream solution on paper, but I have not seen it first hand. (I have worked with QPSK and block converters on the return path.) Yes, this stuff really does work, and the telephone transmission quality really is better than typical copper pair lines. The group I work with has converted many thousands of regular, everyday POTs telephone customers from copper pairs to an HFC system. But it doesn't come quick, easy or cheap when there is no instruction book to follow on how to design and build a full service HFC network that provides 100% telephone coverage to all homes, with uninterruptable backup remote power systems in the neighborhood that include a natural gas power generator.
As for return speeds and congestion with cable modem service on HFC, there is no reason cable modems should get tied up if the headend and interconnection systems are properly engineered by a networking expert, and the HFC return path is fairly clean. Contention for telecommunications resources has been a historic part of telecommunications networks for a century. Every time you dial a phone, the call path goes through many levels of concentration to reach its destination. Cable modem network engineering can be successful if prudent levels of sharing are observed. Part of the problem may be that this is a new technology. And unfortunately, some of the cable companies that have implemented cable modems have rushed to get the technology out in the homes before they have invested in a first class data com network back at the headend. |
