Date: Sun, 15 Oct 2000 16:40:08 +0200 From: Len Conrad <lconrad@Go2France.com> To: freebsd-questions@FreeBSD.ORG Subject: Re: Best DSL Router? Message-ID: <5.0.0.25.0.20001015142252.04506b30@mail.Go2France.com> In-Reply-To: <Pine.BSF.4.21.0010150155500.1344-100000@oahu.WURLDLINK.NET > References: <5.0.0.25.0.20001015134404.06dc9710@mail.Go2France.com>
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>ADSL is not full duplex. Then what is it? half duplex? simplex? No, xDSL is full duplex at stated rate(s) on two unconditioned wires. "ADSL (asymmetric digital subscriber line) is called "asymmetric" because most of its two-way or duplex bandwidth is devoted to the downstream direction, sending data to the user. Only a small portion of bandwidth is available for upstream. Published by : Computer Reseller News Date : Monday, September 04, 2000" >ADSL is Assymetrical DSL. SDSL is Full Duplex. You're mixing your terms. xDSL is full duplex. (A)symmetric refers only to the relative values of the up and down signalling rates, not to the media access scheme. Full duplex refers to the simultaneous send and receive, half duplex is only one party sending at a time. This is a one-sender-at-time media access scheme, just like 10BaseT Ethernet or Token Ring or ARCnet. In xDSL, two frequency bands are used to provide two independent "media" so each sender can send simultaneously. >With PacBell ADSL, the upstream 128k is shared with the >downstream 1.5Mbps on a standard phone line connected to a Alcatel DSLAM. There's no band sharing going on. The 128 kbits upchannel has its frequency band and simultanously the 1.5 Mb/s channel has a separate frequency band. They can both send simultaneously, aka "full duplex". >If one was downloading only, one does get 150kbytes/sec which >193kbytes/sec is the max for a 1.544Mbps connection. If you were >uploading, it would be 13kbytes/sec up and the downloads would slow down >to 30kbytes/sec. If there is simultaneous transmission in both directions, as you say there is on the BA DSL (at whatever effective signalling rate), then the xDSL line is full duplex. >This is only a problem if you do both simultaneously Which is a the most common requirement. If you can do both simultaneously, then the channel or service is full duplex. A classic T1/E1 is full duplex but on 4 wires (each pair is simplex), and it can be replaced with xDSL 2 wires, of same speed which is also full duplex. >and what happens is the uploads will take up >the entire 128kbps of course, there's nobody else on the line to share that 128 with, but that's only one channel of the two available. The 128 up channel does not occupy the full up/down bandwidith, only the up bandwith. The down channel is a separate channel and frequency band. Now if BA lines cannot really support full 1.5/128 signalling rates, then that's not an xDSL issue. I've seen some DSL provisoning web sites quote a residential address only 144 kb/sec up and down due to line quality and line length issues. >so there is no room for the Acknowledgement packets to >be sent back for the received packets. yes, there is, the separate down channel is available, in theory, on the full-duplex xDSL line. If in practice, line conditions are so poor that effective operation appears to be half-duplex (ie, sending up destroys sending down), then that's a line issue, and still doesn't mean xDSL is not full duplex. >That's one reason why I mentioned traffic shaping as that seems to >be the only solution. If 128 kb/sec up transmission destroys the capacity of the 1.5 mb/sec down channel such that tcp ACK packets can't be sent down, then there's no effective bandwidth to manage. > > Paradyne MVL does share 768 kbits/sec between the two channels but > > that is afaik unique to MVL, which isn't typical xDSL. > > Yes, but are you uploading and downloading at the same >time. That's a different issue altogether. xDSL is full duplex, period, as you say the Bell Atlantic ADSL is, with asymmetric signalling rates. If the line quality + available CO bandwidth don't allow full duplex data transmission at max (asymmetric) signalling rates, that's not xDSL's pb. "Full duplex" and "(a)symmetric signalling rate" refer to two diffent, independent characteristics. Here's very readable reference work: http://www.paradyne.com/sourcebook_offer/index.html Len http://BIND8NT.MEIway.com: ISC BIND 8.2.2 p5 installable binary for NT4 http://IMGate.MEIway.com: Build free, hi-perf, anti-spam mail gateways To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-questions" in the body of the message
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