smb@ulysses.UUCP (Steven Bellovin) (08/12/86)
I'm looking for an introduction to the modulation techniques, etc., used on DDS lines, especially at 56Kb. For example, what does a CSU do, in distinction to a DSU? (I know that modern units integrate the two pieces.) What are the signals on a V.35 interface, and how are they actually used? (The V.35 spec will no doubt explain, in highly opaque language, how they're supposed be used, but that's a different matter...) I'm very familiar with ordinary modems and RS-232. --Steve Bellovin
qeds@mtuxo.UUCP (e.schulz) (08/14/86)
In response to Steve Bellovin's question: > I'm looking for an introduction to the modulation techniques, etc., used > on DDS lines, especially at 56Kb. For example, what does a CSU do, > in distinction to a DSU? (I know that modern units integrate the two > pieces.) What are the signals on a V.35 interface, and how are they > actually used? (The V.35 spec will no doubt explain, in highly opaque > language, how they're supposed be used, but that's a different matter...) > I'm very familiar with ordinary modems and RS-232. > > --Steve Bellovin The newest references I'm aware of are: PUB 62310, 9/83, DDS Channel Interface Specification (what's on the loop) PUB 41450, 11/81, DDS DSU Interface Specification (what's on V.35 or RS-232) Some useful info might be found in the "Dataphone II 2600-Series Data Service Units User's Manual," Document No. 999-100-196IS. These and other Technical References are available from the AT&T Customer Information Center 1-800-432-6600 (In Canada 1-800-255-1242, outside continental U.S. 1-317-352-8556). Very briefly... A CSU does the pulse shaping on transmit, and equalization and pulse slicing in the receive direction. There's no "modulation" as such; it's all baseband bipolar return-to-zero alternate-mark-inverted signaling on the loop. Basically, the CSU delivers a signal to the DSU that is squared off, with nice "digital" levels, but continuous in time. The DSU converts this signal to data and clock at the EIA or V.35 interface. This involves mostly timing recovery, to know when to sample the data and to give nice de-jittered clocks. The DTE interface signals are the normal leads for synchronous private line data (using RS-449 abbreviations just to confuse): SD, RD, RS, CS, DM, RR, ST, RT, TT, plus signal ground. The TT clock is only used when the DSU is used as a limited-distance modem (not connected to DDS; not all DSUs have this feature). Basically, RR follows RS at the other end -- "carrier off" is signaled on the loop with a special bipolar violation sequence, also known as control-mode idle. DM will go off if DDS sends another special BPV sequence that means the network knows it's sick. In V.35, all the control signals have RS-232 levels, while the data and clocks are each a pair of wires with nominally + or - 0.55 volts between the wires. There's plenty more, but those are the basics. -- Ed Schulz, AT&T Information Systems, Middletown, NJ (201)957-3899 {at&t}!solar!eds