In a response to Rene Debets dated 97-03-09, Duane Marcroft writes: << I do not like the local loop simulators supplied by TAS or PTT. In my opinion they are a poor representation of real world local loops. They are sourced at 600 ohms and should be sourced at 900 ohms. I don't use them unless a client insists they must be used. In my opinion TSB-37A testing makes poor and good modems look good. >>
Duane and Rene: I've been following (a bit delayed) the dialog that you and Rene have been having regarding return loss and trans-hybrid loss. While it is true that the TAS simulator uses a 600 ohm termination, I'm not sure that it makes as much of a difference as you may think. On short loops (0 kft to 3 kft), switching the termination at the central office between 600 ohms and 900 ohms does make a noticeable difference in the impedance seen by the TE. However, on longer loops, the impedance of the loop dominates what the TE sees, and changing the termination at the central office has very little effect. At least, this has been my observation in laboratory tests on trans-hybrid loss. So, unless the differences that you see between "real world" performance and performance on a TAS are on very short loops (e.g., loop 1 in TSB 37A), I suspect that the 600 ohm termination in the TAS is not the culprit. In my experience, most modems tend to have more trouble on the longer loops in TSB-37A (loop 4 and loop 5). On these loops, the receive signal level is lower, there are larger loop-related impairments, and the impedance mis-match is usually greater. When these loops are used, the termination at the central office end (be it 600 ohms or 900 ohms) has very little effect on what the modem sees. So much for my opinions on TAS vs. the "real world." After reviewing the correspondence that has taken place on the issue of modem performance, I can summarize my views as follows: 1) There is no compelling case for modems to have high return loss, whether it be referenced to 600 ohms, 900 ohms, or a complex reference. 2) Trans-hybrid loss is much more likely to affect modem performance than return loss. While return loss and trans-hybrid loss are related, they are NOT measures of the same parameter. 3) As Duane has pointed out, the main reason that return loss becomes an issue for modem performance is that inexperienced DAA designers sometimes make arbitrary changes to improve return loss (a parameter that is subject to regulatory requirements), without checking to see what effect these changes have on trans-hybrid loss (a parameter that is not subject to regulatory requirements). Sometimes, the effect is catastrophic. 4) As both Rene and I have noted, a good near echo canceler in the modem can make up for poor trans-hybrid loss. In theory, with a perfect D/A, perfect A/D, and infinite precision in the near canceler, it is not necessary for the DAA to provide any trans-hybrid cancelation at all. The extent to which cancelation is required in the DAA relates to how non-ideal these resources are. 5) My impression is that in many commercial chip sets, these resources have improved in recent years. I am familiar with one commercial chip set that seems to perform well as long as the trans-hybrid loss (THL) is greater than 6 dB. However, the "THL tolerance" will always be implementation-dependent, so greater THL may be required for certain vendors or certain implementations (Duane: can you venture an opinion about the required THL for Rockwell modems?). Joe Randolph Telecom Design Consultant Randolph Telecom, Inc.
