jst@cca.ucsf.edu (Joe Stong) (12/19/89)
I would like to hear of people's experiences with LADS (solid copper 4
wire or 2 wire circuits only for sites local to the same central
office). In particular, I'm curious about what you can get through
them in the way of frequency response.
My understanding at the moment is that T1 is delivered on a regular 4
wire connection to one's building. Are there equalization networks
along the "subscriber loop" of a T1 that make it "better" than a LADS
circuit?
Here's the scenario: I'm wondering if I/others can connect a LADS
circuit to an ARCnet board on a PC, running the Phil Karn (KA9Q)
software, with the ARCnet-TCP/IP driver in place (don't worry, we're a
non-commercial outfit) to get a TCP/IP connection between home and
work. The 1 megabaud (nominal?) rate from the ARCnet board should be
lower rate than a T1 at 1.544 megabaud; will it make it in terms of
frequency response? If this setup would work, it would let everybody
use relatively CHEAP interfaces and software to make a FAST
connection.
An old story: Back when 9600 baud modems were horribly expensive, we
arranged a remote terminal on a LADS circuit for a customer in Arcata,
CA. The "couplers" (not modems) that got purchased seemed to be
transformer coupled, and mostly seemed to convert to/from some RZ
coding scheme at a fixed baud rate (9600) onto the LADS line. The
whole setup was horribly sensitive to electrical noise (changing the
interior wiring that PacBell had done from silly "quad wire" (NOT
twisted pair) to twisted pairs helped some, but it was still subject
to garbage occasionally.
Would current-loop couplers have been better for this 6 mile circuit?
(Traditional sillyness, the offices were only 2 blocks apart, but the
phone company would only make the connection into the office on the
other side of town, and back out again) I notice that some other
high-speed devices use RS422 (like microwave transmitters), but I've
had better luck with current loop lines than differential or
single-ended lines.
At least in San Francisco, LADS circuits are extremely cheap, namely
$200 initial installation and $18 a month, when I checked a couple of
years ago.
Joe Stong jst@cca.ucsf.edusyd@dsinc.dsi.com (Syd Weinstein) (12/21/89)
jst@cca.ucsf.edu (Joe Stong) writes: >I would like to hear of people's experiences with LADS (solid copper 4 >wire or 2 wire circuits only for sites local to the same central >office). In particular, I'm curious about what you can get through >them in the way of frequency response. We had a LADS circuit of about 4 mile length for several years. I have plenty of experience. They are a 4 wire metalic connection with, if connected properly, sealing current on the line. Sealing current on LAD circuts is a relatively new item (couple of years) so not all have it, and if you don't get it converted. It cut our service calls from one per week to almost never. (And they did the conversion for free, just to save on their service calls) Ok, here is what it can do: Send a audio signal end to end, with about a 12Khz bandwidth. Why: transformers on both ends to trap the sealing current. In fact, they make special modems for LAD circuits. Those modems can do 19200 for about 1 mile, 9600 for about 3-4 and 1200 for about 12-20. In our case, we got 9600 to work pretty reliabably. There are strict standards as to what signals can be put on the line, to avoid cross talk with the other normal pairs in the cable plant. >My understanding at the moment is that T1 is delivered on a regular 4 >wire connection to one's building. Are there equalization networks >along the "subscriber loop" of a T1 that make it "better" than a LADS >circuit? T1 can be distributed on metallic, if you put regenerators ever so often, and its not that far apart at T1 rates. 56KB can go farther without regen, but even at 56KB, you probably will need one to go 6 miles. Getting ARCnet to run is not too likely. Remember to run T1, you must run CSU's, and they are fixed rate. ===================================================================== Sydney S. Weinstein, CDP, CCP Elm Coordinator Datacomp Systems, Inc. Voice: (215) 947-9900 syd@DSI.COM or {bpa,vu-vlsi}!dsinc!syd FAX: (215) 938-0235