""Logan, Harold"" wrote in message news:[EMAIL PROTECTED] > be given no credibility whatsoever. I'm neither a wireless guru nor an RF > engineer. > > The problem of ethernet and contention-based media has been solved for the > most part by full duplex ethernet and layer 2 QoS. As far as the hosts on > the network are concerned, the media is always available. "Congestion" is > not encountered until data hits a switch, at which point a layer 2 (or layer > 3, depending on how cool the switch is) QoS policy can be enforced, > packets/frames can be prioritized, and life is good. > > I wonder then if it would be possible to microsegment a wireless network, as > Priscilla suggests, using different ranges of the RF spectrum?
many things are possible, but with radio, as opposed to wire, you have a different set of problems. I'm not going to claim I have all this down yet either. Wireless experts I know all seem to have several years experience in military radio and satellite communications and operate in a different world than us wireheads. ;-> consider how many radio stations there are in a given area. why? why can't you have a radio station every couple of clicks? because a radio signal uses a certain amount of spectrum to transfer intelligence. 98.7 on your radio dial isn't exactly and only 98.7 megahertz. the signal includes frequencies to either side. if two stations' signals overlap, you have problems. radio spectrum is limited. for good reason. it must be rationed so that more stations can broadcast good signal. 802.11 is nothing more than radio, pure and simple. the unregulated ISM band is subject to the same laws of physics as are broadcast radio bands. you can't have two signals operating at the same frequency in the same area, or the signals interfere with eachother. DSSS permits some flexibility, but not much. There are 11 channels in the US available under 802.11b however, you have to have proper channel separation or the frequency overlap will cause problems, same as if you have two radio stations whose signals are in the same frequency range. >Most wireless > equipment I've worked on operates spread spectrum so that interference on > one channel doesn't cause a total data loss. If anyone on the list has used > the army SINCGARS radios, they (as well as most other military > communications equipment, I'm sure) incorporate frequency-hopping as well as > scrambling to make jamming and interception more difficult. Rather than hop > frequencies, would it be possible to have AP's that listen on all available > 802.11 bands, but "segment" the hosts into collision domains by having them > only transmit on certain channels? I would guess that for one thing, the military bands are not as crowded as the ISM bands. secondly, my quick look on google seems to indicate that even among the military bands, certain frequencies are reserved for certain activities. secondly, considering that the best in radio receivers these days still can't sort it out when two radio broadcasts on close frequencies collide, it would be a good guess that this would be problematic. the laws of physics still aplly. as RFC 1925 states, "No matter how hard you push and no matter what the priority, you can't increase the speed of light." Radio is not wire. Wire contains signal, meaning keeps it contained within the medium. it has been know to happen that data cables laid close to eachother have bled into eachother, causing interference on both cables. I believe that twisted pair construction has pretty much eliminated this problem. wireless radio cannot contain much of anything. the signal spreads out. nature of the beast. > > Does that sound doable or am I talking out of my arse? =) I've been wrong about "trekkie tech" many many times, so who knows? on the other hand, be careful what you wish for. I for one would not enjoy living in a world where we all had to be connected every moment of every day. > > Hal > > > -----Original Message----- > > From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] > > Sent: Monday, July 21, 2003 6:06 PM > > To: [EMAIL PROTECTED] > > Subject: Re: Bandwidth, QoS, and Contention networks [7:72645] > > > > > > Quick addendum: > > > > As Howard mentioned, the problem with a contention medium > > isn't only how > > much bandwidth is available but also how quickly a station > > can access it. If > > the stataion listens before it sends and can't send for long > > periods of time > > because the medium is not free, this is bad news for voice and video. > > > > Shared Ethernet has gone the way of LocalTalk, (almost?) but > > shared wireless > > is gaining popularity, as Chuck has been saying. I haven't > > seen any studies > > yet that address medium access delay on wireless networks > > based on load and > > access characteristics, but maybe they will get published at > > some point, > > (although it's pretty unpredicatable with bursy data sources, > > of course). > > > > Suffice it to say, "microsegmentation" for wireless networks will be > > necessary to some extent, just like it was with shared > > Ethernet, depending > > on delay and delay variance requirements of the applications. > > > > Priscilla > > > > "Chuck Whose Road is Ever Shorte wrote: > > > > > > ""Zsombor Papp"" wrote in message > > > news:[EMAIL PROTECTED] > > > > "Chuck Whose Road is Ever Shorte wrote: > > > > > after viewing the presentation, you tell me - is > > > > > this not saying > > > > > that 5 megabits is more than adequate for voice, video, etc? > > > > > > > > I don't think that was the point of the presention. > > > Regardless, I can tell > > > > you that 5Mbps is enough for voice. :) For video, it depends > > > on the > > > quality. > > > > > > 30 frame per second video over ISDN requires 3 ISDN lines ( 6 > > > B-channels ) > > > for a total of 384K. I've spoken about video over WAN links on > > > several > > > occasions with the video guy in my group. He tells me he likes > > > to reserve > > > 500K over T1's typically. > > > > > > Howard, if your reading, I just looked over my unsent drafts of > > > the question > > > regarding bandwidth, and the point I failed to raise in this > > > posted thread > > > is that of global synchronization. One of the major benefits of > > > such QoS > > > mechanisms as RED and WRED is that the phenomenon of global > > > synch can be > > > controlled, meaning a more efficient use of bandwidth ( no > > > periods of > > > congestion followed by periods of low activity because of the > > > TCP backoff > > > mechanisms ) > > > > > > Now supposing, even in a contention medium, I could fine tune > > > my queueing > > > such that I no longer suffered from global synch. I set my > > > voice queues and > > > my delay sensative queues such that sufficient badwidth was > > > available, and I > > > used RED or WRED for the general queue. I'm wondering if there > > > are studies > > > done, papers in the IETF working groups demonstrating that > > > given proper > > > queueing mechanisms that less bandwidth is required or > > > necessary? > > > > > > Cisco does offer downstream only QoS on their wireless product > > > line. Not > > > sure I understand the mechanism completely, but I have to > > > believe it is > > > based on enough solid study such that given a reasonable > > > design, there would > > > be less concern for voice in the mix. Recall that Cisco will > > > soon be > > > releasing their own wireless IP phone, and the whole point of > > > it is to > > > provide untethered mobility throughout an enterprise. > > > > > > > > > > > > > > > > > > I'm just asking - if people are more productive, > > > > > despite the obvious lack of bandwidth, and > > > > > despite the step back to a contention medium, > > > > > is there something to be said about the > > > > > perceived need for 100 megabits to the desktop? > > > > > > > > I agree with Fred, such a perception is probably misguided in > > > most cases. > > > > Most people are very happy even with their 1.5Mbps DSL line. > > > But all this > > > > depends on what you want to do. Full-screen DVD quality video > > > won't work > > > > over DSL. Even online gaming could use more than 1.5Mbps. > > > FWIW, I've heard > > > > that in Korea, there is a serious market for dedicated > > > 100Mbps connections > > > > to the *home* due to wide-spread online gaming (I don't know > > > if this is > > > > true, I find it a bit hard to believe). > > > > > > > > > can't comment much regarding the needs of on-line gaming, but > > > the guy > > > teaching the wireless class two weeks ago said he was an avvid > > > on line gamer > > > and that his DSL was plenty fine for what he did. He also said > > > he had a > > > couple friends around his neighborhood who did on-line gaming > > > via a sireless > > > AP that he set up for their use. Don't know the particular > > > game, so I can > > > say as to whether it is the same one you play. > > > > > > > > > > > Also consider that pure 10Mbps Ethernet interfaces are > > > getting pretty > > > rare; > > > > most of the Ethernet interfaces are 10/100. So in a campus > > > network, in > > > most > > > > cases, there is no real reason to not have 100Mbps to the > > > desktop. > > > > > > indeed. and with three teenage boys around the house, I am > > > happy to sell > > > lots of these things in order to keep the refridgerator full. > > > ;-> yes, Mr > > > Customer, you never can tell when your users will need this > > > bandwidth, what > > > with internet radio, lots of databases out there on the web, > > > and all the > > > e-mail attachments people need to read to get their work done. > > > ;-> > > > > > > > > > > > > > > > > > Thanks, > > > > > > > > Zsombor > > [EMAIL PROTECTED] Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=72887&t=72645 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
