Date: Sun, 15 Oct 2000 10:36:23 -1000 (HST) From: Vincent Poy <vince@oahu.WURLDLINK.NET> To: Len Conrad <lconrad@Go2France.com> Cc: freebsd-questions@FreeBSD.ORG Subject: Re: Best DSL Router? Message-ID: <Pine.BSF.4.21.0010151017100.1344-100000@oahu.WURLDLINK.NET> In-Reply-To: <5.0.0.25.0.20001015142252.04506b30@mail.Go2France.com>
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On Sun, 15 Oct 2000, Len Conrad wrote: > >ADSL is not full duplex. > > Then what is it? half duplex? simplex? It can be Adaptive Full Duplex, remember how the Telebit Packetized Ensemble Protocol (PEP) and USRobotics High Speed Technology (HST) protocol worked? It's not true Full Duplex. > No, xDSL is full duplex at stated rate(s) on two unconditioned wires. True but ADSL is shared with voice while HDSL and SDSL isn't on the same two 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" That's not the only reason. ADSL is shared with Voice by all the CLEC's and LEC's. Other forms of DSL could be asymmetric like the Covad 1.5down, 384k up offering but is not shared by voice. > >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. Well, I remember I asked Netopia this question since I had the Netopia R7100C router and wanted to convert it to a R9100C router, and I asked them the same question and this was their answer: Long answer - A 1.5 Mbitps connection should be around 192 kbytes per second download, while a 128kbitps should be around a 16 kbyte per second upload. ADSL is, by definition, asymmetrical and asynchronous, which means that there is only one pipe that is shared in both directions. So, the speeds that you are showing are actually in line with the line speeds you mentioned. > >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". They can but how do you explain that the downstream band slows down whenever the upstream band is in use? Remember, Netopia makes the xDSL routers so they are familiar with the technology that both Copper Mountain and Alcatel uses. > >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. Not always, remember even with 10BT Ethernet, even though it's half duplex, it can still send and receive at the same time, just slow. > >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. Yes, a classic T1 is HDSL and full duplex. ADSL is Assymmetrical and Assynchronous. Both HDSL, SDSL and T1/E1's are Synchronous. > >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. Yes, but you still haven't explained why the up channel would slow the down channel down. > 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. We're actually talking about SBC's PacBell here and not BA. BA is worst than you think since it's 640k/90k only. The sites you see are talking about IDSL which is 144kb both ways or 160k both ways and offered only by Rhythms, Northpoint and Covad and not the LEC's. > >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. Well, if you look at a traditional T1, there is 193ksec available in both directions so you are able to send the ACK packets back. On a ADSL circuit, the downstream is faster than the upstream by a big magnitude. So when you upload, it will normally use up the entire 16k/sec available which means it'll slow the downloads down because there is no room for the ack packets to be send back in the upstream link to acknowledge the packet is received. Ofcourse, if you can knock down the upstream to only do about 13k, then you will have the 3k worth of room to send the acknowledgement packets back. It's like someone sends you a packet and waits for a response but it's delayed because the upstream pipe of the destination is full and traffic shaping is the only way out of it. There is a huge discussion going on at the ba.internet newsgroup and the people there do know what they are talking about. Instead of talking about it here, it might be better to bring this discussion to the newsgroups instead where it'll benefit. > >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. Actually there is. If the entire 16k in the up is taken by the uploading, how will the ack packets make it back to the other end to ack that the package is received? > > > 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. Where was I talking about BA ADSL? I was referring to PacBell ADSL all along. > Here's very readable reference work: > > http://www.paradyne.com/sourcebook_offer/index.html There are different implementations of ADSL. I'm not sure what BA uses but PacBell uses Alcatel's and GTE uses Orckit/Fujitsu. Like I mention above, Netopia should know what they are talking about and so should Cisco engineers. Netopia, Cisco and 3Com does make Routers compatible with CopperMountain for SDSL and Alcatel, Orckit for ADSL. Paradyne which is either still AT&T or formally AT&T would be history since if Lucent was that great, other companies would not have put it down in water. Cheers, Vince - vince@WURLDLINK.NET - Vice President ________ __ ____ Unix Networking Operations - FreeBSD-Real Unix for Free / / / / | / |[__ ] WurldLink Corporation / / / / | / | __] ] San Francisco - Honolulu - Hong Kong / / / / / |/ / | __] ] HongKong Stars/Gravis UltraSound Mailing Lists Admin /_/_/_/_/|___/|_|[____] Almighty1@IRC - oahu.DAL.NET Hawaii's DALnet IRC Network Server Admin To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-questions" in the body of the message
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