gjphw@mhuxo.UUCP (WYANT) (07/03/86)
While considering the purchase of a 1200 baud modem, I began wondering
if there is some optimum data rate for modem operations over the U.S.
telephone lines. I have seen a few advertisements for 2400 baud modems,
and read mention of 9600 bps modems becoming available. Where does
the retransmission rate due to lost bits negate the higher data
transmission rate? What might serve to be the most efficient data rate
using ordinary (unconditioned) telephone lines? Would the long distance
digital telephone trunks be a more serious limiting factor than local or
short haul analog lines?
Any suggestions?
Patrick Wyant
AT&T Bell Laboratories
Naperville, IL
*!ihnp4!{mhuxo,ihwld}!gjphwsandersr@ecn-pc.UUCP (Robert C Sanders) (07/07/86)
In article <1207@mhuxo.UUCP> gjphw@mhuxo.UUCP (WYANT) writes: > > While considering the purchase of a 1200 baud modem, I began wondering >if there is some optimum data rate for modem operations over the U.S. >telephone lines. I have seen a few advertisements for 2400 baud modems, First; almost ALL of the 1200 baud manufacturers, esspecially the onces who first designed them, offer 2400 baud modems. There are many available. >and read mention of 9600 bps modems becoming available. Where does >the retransmission rate due to lost bits negate the higher data >transmission rate? Yes, there are 9600 baud modems out. Two major manufacturers are marketing them (net- help me with the names!). From the articles that I have read (PC World, BYTE, MAC World), the modems use a mathematical coding scheme to encode the data to acheive an equivilant 9600-baud thoughput. Then, they adjust themselves by testing the line and determining the optimum speed -- they will drop to lower rates (8400, 5600, 4800, etc) in a continuously variable manner to get the maximum speed realiably. Because of this constant variability and change, both ends have to use the type modem from the same manufacturer, unless of course you set the modems to use the industry standard speeds of 2400, 1200, or 300. These modems, if I remember right, are Hayes command set compatible, with extensions for their own capabilities. - bob -- ------------ Continuing Engineering Education Telecommunications Purdue University "Time is a mouse that requires constant feeding..." -- me ...!ihnp4!pur-ee!pc-ecn!sandersr
halloran@unirot.UUCP (Bob Halloran) (07/08/86)
In article <542@ecn-pc.UUCP> sandersr@ecn-pc.UUCP (Robert C Sanders) writes: >In article <1207@mhuxo.UUCP> gjphw@mhuxo.UUCP (WYANT) writes: >>and read mention of 9600 bps modems becoming available. Where does >>the retransmission rate due to lost bits negate the higher data >>transmission rate? > >Yes, there are 9600 baud modems out. Two major manufacturers are marketing >them (net- help me with the names!). From the articles that I have read >(PC World, BYTE, MAC World), the modems use a mathematical coding scheme to >encode the data to acheive an equivilant 9600-baud thoughput. Then, they >adjust themselves by testing the line and determining the optimum speed -- >they will drop to lower rates (8400, 5600, 4800, etc) in a continuously >variable manner to get the maximum speed realiably. Because of this constant >variability and change, both ends have to use the type modem from the same >manufacturer, unless of course you set the modems to use the industry standard >speeds of 2400, 1200, or 300. These modems, if I remember right, are Hayes >command set compatible, with extensions for their own capabilities. The modem in question is the Telebit Trailblazer, marketed under an OEM agreement by Digital Communications Assoc. (makers of IRMA PC-3270 card) as the 'DCA Fastlink'. The only difference is the faceplate. The modem uses an on-board 68K plus the TI signal processing chip plus lotsa RAM. It breaks up the bandwidth of the phone line into something like 512 subchannels, then moves data through them at roughly 18kbps. On the receiving end, the subchannels are recombined, CRC checks are performed, and the data stream is presented at 9600 baud. The major advantage of this modem over the CCITT V.32 modems now reaching the market is that under less-than-ideal line conditions (i.e. most of the time), the CPU will drop only those subchannels showing poor signal-to-noise, allowing falloff in increments of < 100 bps. The V.32 modems fall back by halving (9600 to 4800 to ...). It emulates 103, 212 and V.22bis (300, 1200 and 2400 respectively) modems and uses the Hayes command set. Friends have used them and love them for bulk data transfer. In a 'worst-case' test they did, they 'looped back' between two Fastlinks in their office via MCI, SPRINT, and ATT and still got > 7Kb throughput. Bob Halloran, Consultant ========================================================================= UUCP: topaz!caip!unirot!halloran DDD: (201)251-7514 USPS: 19 Culver Ct, Old Bridge NJ 08857 ATTmail: RHALLORAN Disclaimer: I speak for myself. Quote: "No matter where you go, there you are." - Buckaroo Banzai
rb@cci632.UUCP (07/13/86)
In article <1207@mhuxo.UUCP> gjphw@mhuxo.UUCP (WYANT) writes: > > While considering the purchase of a 1200 baud modem, I began wondering >if there is some optimum data rate for modem operations over the U.S. >telephone lines. I have seen a few advertisements for 2400 baud modems, >and read mention of 9600 bps modems becoming available. Where does >the retransmission rate due to lost bits negate the higher data >transmission rate? >What might serve to be the most efficient data rate >using ordinary (unconditioned) telephone lines? Well let's first consider the theoretical maximum. The yellow book indicates that the analog value of the line will be "polled" 9600 times/second, and a dynamic range of 64 db can be represented in 1 db steps (7 binary bits, but only six are "reliable"). So in theory, you could jam a d/a converter and get nearly 23 Kbaud. Of course this isn't quite reliable for real communications, and the phone company might have fits. This technique has been used for several "illicit" perposes, most commonly fooling the phone company into thinking you got a wrong number, when in fact, during the few seconds you were connected, you had sent a 3 page report. :-). Now, to be legal, the fastest legal standard is the V.33 or "Trellis Coding" standard. Most modem makers are combining this with mnp protocol, and a relatively standard version of compression. In addition, the mnp error checking can be shut off for situations where external error checking is needed (long modem->computer lines, multiplexors,...). Vanilla V.33 give a generic full duplex 9600 baud rate, with reasonable error rates for "terminal mode" types of transmission, where a lost bit doesn't cost too much. The compression nearly doubles the data rates in many situations, but multiplies the effects of errors. The MNP protocol allows error detection/correction. Most of these modems also "double buffer" the line, allowing synch or asynch in, and synch or asynch out. This is necessary because after compression and error checking/correction the rate can vary from 19200 baud in "perfect" conditions to as low as 10Kb in "very noisy conditions" (including data that won't compress nicely). >Would the long distance >digital telephone trunks be a more serious limiting factor than local or >short haul analog lines? Actually just the opposite. The distance to the pcm converter provides the greatest possiblity for LC filtering, standing waves, ringing, and several other problems. The digital lines are very easy to get good repeaters for, and the loss of a millivolt can be recovered. Not so on the analog lines. Even in trellis coding, a millivolt or an extra pf on the line can mean a 300 baud drop in speed. Let's hope that telephone companies can get ISDN (digital/analog) services available at reasonable rates. Then we can talk in terms of 56 Kb/line, over multiple lines if desired. It would be nice if a parallel modem interface, rather than serial was a valid option as well. > > Any suggestions? I'm writing this at home, so there may be some errors, so, Any corrections? > Patrick Wyant > AT&T Bell Laboratories ^^^^ Why do I feel this is a sucker question? :-).
henry@utzoo.UUCP (Henry Spencer) (07/17/86)
> Let's hope that telephone companies can get ISDN (digital/analog) > services available at reasonable rates... Hold not thy breath over the "reasonable rates" part. Most residential users don't need ISDN, and the phone companies tend to be under a fair bit of pressure to make as much money as possible off other services to keep the residential rates low. IDSN will be a "premium", "business" service, and will be priced accordingly. -- Henry Spencer @ U of Toronto Zoology {allegra,ihnp4,decvax,pyramid}!utzoo!henry