BRUCE@ccavax.camb.com (Barton F. Bruce) (09/12/90)
With DDS II being quite widely available, and with DDS II rates often being exactly the same for 2.4 or 56kb, and with 56kb with secondary channel being common, and with 64kb clear channel being about to be tarriffed in many places, what one wants to order for CSU/DSUs, and from the phone company has changed. Most vendors don't yet support 64kb in their DSU/CSU, and most LEC don't offer it, BUT the 72kb line rate is IDENTICAL to what they provide to give you 56kb with secondary channel. I don't need the secondary channel, but DO object to losing 1/8th of each DS0 delivered on a 56kb class local circuit. If you can order 56kb with secondary channel under the new low priced DDS II tarriffs, and get yourself CSU/DSUs that have the option of going to 64kb (as well as traditional 2.4, 4.8, 9.6, 56, and the newer 19.2 and 38.4 all optionally with secondary channel), you have an EXCELENT chance of doing 64kb TODAY, and at very worst, you crank the knob back to 56 (with secondary - that you probably won't use) and wait until it is tarriffed. At least you have the 'right' CSU/DSU that won't be obsolete. (N.B. 64kb clear channel is WITHOUT any secondary low speed async channel.) Who make these? Well, I was totally dismayed to find VERY FEW companies selling CSU/DSUs that do 64kb. One company that does, and that does not seem to spend much time selling to end user datacomm types is ADTRAN. Their literature and documentation (all in BSP format) is clearly aimed at the TELCOs themselves. They make the cards that plug into the CO D4 channel banks to provision DDS. They make these cards and customer end DSU/CSUs that have enough receive sensitivity to pick off signals at -45db. Many competitive units stop at -34db. Telcos love them. You will too. The DSU/CSU in question is their # DSU II, and there is a version of it called the DSU II TST that is designed for installers to carry with them to test a new installed service. It has an LED that lights if the loop loss is > 34db. Since it costs exactly the same, this is the one to order for EVERYTHING. Additionally, with local digital loops going DOWN, and analog loops going UP there is another game to play. There are often times where you have to install an analog circuit just to cater to some old 14.4 modem that has a six way TDM built in. You can't simply replace it with a DDS ckt, because you would ALSO need a TDM, and probably there are foreseeable future plans that might obsolete ALL this. Of course you don't want to pay for analog now, and then for a reinstall later to switch to DDS. ADTRAN has this great little product clearly targeting TELCOs. It extends the A/D conversion out to your wall where they hand off the 4 w 3002 type analog circuit to you. You get a zero length analog loop. There are NO equalization issues at all! The loop to the CO is the same as the 64kb or 56kb w/secondary as above. The TELCO normally doesn't bother to tell you all this (and remember analog loops often cost more now). This unit is fully FCC blessed for YOU to install on the a DDS line where you would normally have put your DSU/CSU. Don't be put off by ADTRAN's pictures that show this unit BEFORE the analog DEMARK, they are marketing to TELCOs. YOU simply stick it AFTER the digital DEMARK. What I am saying is simply DON'T order more analog lines, get DDS II ones, and if you really must (e.g. TDM built into analog modem makes keeping it for a while practical) do the conversion to 3002 analog yourself. These little gems are $380 in 'Wescom 400' form factor so you can plug them into any spare slots in that type of shelf, or the same card boxed standalone is $485. You switch to full digital without involving the telco any time you want. ADTRAN is in Huntsville at 205.837.7800 - all good folks, but try for a customer service engineer named Bill Salmon - most helpfull! FWIW - I have no connection at all - I just like to find the 'right' stuff when no one else seems to bother making it!