dyer@spdcc.COM (Steve Dyer) (10/10/89)
Has anyone ever investigated using a couple of channels from a town's
cable TV system as the basis of a metropolitan-area network? I'm completely
ignorant of the technology and its limitations, but I'd think that something
like this *might* work with broadband modems and the right link-level hardware.
For connection to a regional network also located in the same town
(e.g., NEARnet), it might be preferable to lots of relatively low speed
leased lines.
I'd appreciate hearing from people who know a little bit more about
the issues and whether or not this is completely unrealistic.
--
Steve Dyer
dyer@ursa-major.spdcc.com aka {ima,harvard,rayssd,linus,m2c}!spdcc!dyer
dyer@arktouros.mit.edu, dyer@hstbme.mit.edukwe@bu-cs.BU.EDU (kwe@bu-it.bu.edu (Kent W. England)) (10/11/89)
In article <160@ursa-major.SPDCC.COM> dyer@ursa-major.spdcc.COM (Steve Dyer) writes: >Has anyone ever investigated using a couple of channels from a town's >cable TV system as the basis of a metropolitan-area network? [...] >For connection to a regional network also located in the same town >(e.g., NEARnet), it might be preferable to lots of relatively low speed >leased lines. > Broadband LAN technology was designed to exploit cable TV technology for building high speed LANs over a much larger area than an Ethernet. The desire was to exploit a high speed medium and achieve economies of scale by reusing an existing technology. I don't know whether there was a thought of actually having broadband LAN technology and cable TV co-exist on the same cable system, but it is technologically feasible to do so. Now for the gotchas: 1) Cable TV cable plants are not necessarily designed to support two-way high quality communication as required by a LAN (MAN). What the viewer sees as degraded, but watchable, television translates into useless bandwidth for data. The cable TV operator must support the cable plant to meet data requirements. If you call him and say that the system isn't working, he might respond that all the TV signals look fine, so the trouble must be in your equipment. I know of one campus where the data guys try to run a LAN channel pair on a system operated by TV people, and they are unable to convince the TV people that there are serious signal problems. So much for exploiting existing technology. 2) The IEEE 3 channel broadband repeater technology is too restrictive for use in a MAN scale network. It takes too many channels and does not have the necessary diameter. You must use a *proprietary* vendor technology to build a MAN sized network. Something like the Ungermann-Bass Buffered Repeater or one of the Applitek devices would work. To my knowledge, there are no standard interoperable broadband technologies except for the unacceptable IEEE standard. 3) You want the same channels for data that the cable TV operator wants for TV. He will not give you channel 2 for data; you must go somewhere out on the fringes to find some channels. Applitek is the only vendor that I know that understands this situation. Most of the others use standard TV channels, the same ones defined by the IEEE. So, if you can convince Cablevision to support data, and they hire techs who know how to maintain broadband to data standards and they don't charge more than the phone company would, then maybe you've got something. I have heard that some companies have made private arrangements with local cable operators to use channels for data, but I don't know what arrangements they have made for service. I understand Manhattan Cable offers broadband LAN service to customers in Manhattan, NY, so it can be done. Specifically speaking of NEARnet, the NEARnet people would have to deal with no less than three major cable operators to create a MAN within route 128. The size of the market is nebulous and issues of maintenance and such would be quite difficult to resolve; in my opinion, not worth the trouble. NEARnet handles low-recurring cost, high bandwidth MAN services using Ethernet-on-microwave, not cable TV. Sorry. There does not seem to be a high bandwidth, low capital, low recurring network to suit your needs. (But then, doesn't everyone want lots of bandwidth cheap?) Why the cable industry failed to understand and exploit anything other than entertainment TV escapes me. Why has it taken the phone company so long to understand LAN technology? These paradigm shifts take time, I guess. --Kent England, Boston University (and NEARnet) Disclaimer: These views are not official views of Boston University or NEARnet; they simply reflect the idiosyncratic and distorted view of the speaker and are due entirely to spending too much time in the networking business.
pat@hprnd.HP.COM (Pat Thaler) (10/11/89)
Kent W. England writes: > Now for the gotchas: > > 1) Cable TV cable plants are not necessarily designed to > support two-way high quality communication as required by a LAN (MAN). > What the viewer sees as degraded, but watchable, television translates > into useless bandwidth for data. The cable TV operator must support > the cable plant to meet data requirements. If you call him and say > that the system isn't working, he might respond that all the TV > signals look fine, so the trouble must be in your equipment. I know > of one campus where the data guys try to run a LAN channel pair on a > system operated by TV people, and they are unable to convince the TV > people that there are serious signal problems. So much for exploiting > existing technology. This is very true. In fact the degradation might not even be noticeable when watching the TV signal. > > 2) The IEEE 3 channel broadband repeater technology is too > restrictive for use in a MAN scale network. It takes too many > channels and does not have the necessary diameter. You must use a > *proprietary* vendor technology to build a MAN sized network. > Something like the Ungermann-Bass Buffered Repeater or one of the > Applitek devices would work. To my knowledge, there are no standard > interoperable broadband technologies except for the unacceptable IEEE > standard. > I think Kent is refering to the IEEE 802.3 10BROAD36 standard which runs Ethernet over broadband at 10 Mb/s and uses three channels. There is also an IEEE 802.4 broadband token bus standard. It allows 5 Mb/s on one channel or 10 Mb/s on two. Both of these are intended as LAN or campus backbones, rather than public MANs. There has been some discussion of MANs operating over cable for purposes such as home shopping. However, most MAN work seems to be going to fiber for the higher data rate. Cable has a lot of bandwidth, but it is already used and not available for data. I think maintenance costs are lower on fiber than to maintain data-quality broadband. Pat Thaler