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.edumep@AQUA.WHOI.EDU (Michael E. Pare) (10/10/89)
Most cable TV companies use a sub-split format with reverse (toward the headend) frequencies only ranging from 5-30MHz and the forward (away from headend) strating at 54MHz to top end (300, 400, 450 etc.). This severly limits the useable bandwidth for data since the reverse direction is so limited. Most broadband equipment utilizes IEEE standard frequencies which start above the 54MHz level for the reverse direction. They are designed for mid-split systems which most private CATV plants are (universities etc.). Even if you limited the channeling to 6MHz in each direction (limited daat bandwidth) everyone on the system would have to use 'like' equipment in order to talk on the system. You'd probably have to have the CATV company coordinate the efforts. This assumes you can find the hardware and would it be worth it for a 6MHz bandwidth?! Also, in many metro areas there is usually more than one CATV company which certainly breaks up the network within the metro area. These companies have no intentions of linking together. But this is just my limited thoughts on using CATV as a MAN.
kwe@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.