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3Com/USR Diagnostic Screens

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US Robotics (USR)  modems include the ability to get statistics on the current or last connection with AT commands. This page shows screens from a USR/Sportster; the Courier's screens are only slightly different. The commands covered by this page are: [AT] i6 and i11, with reference to y11.

ATI6 - displays, among other things, the number of BLERS, as well as the receive and transmit carrier speeds at the time of disconnection (or, if online, the current connect rate):

U.S. Robotics 56K FAX EXT Link Diagnostics...

Chars sent              1393530      Chars Received         21501145
Chars lost                    0
Octets sent              878506      Octets Received        18922074
Blocks sent               55385      Blocks Received          182796
Blocks resent                78

Retrains Requested            0      Retrains Granted              0
Line Reversals                0      Blers                      3772
Link Timeouts                50      Link Naks                    20

Data Compression       V42BIS 2048/32
Equalization           Long
Fallback               Enabled
Protocol               LAPM/SREJ
Speed                  48000/28800
V.90 Peak Speed        52000
Last Call              08:34:48                      
Disconnect Reason is DTR dropped                                    

Note the above connection was 8˝hrs. The initial connect was reported at 49.3k. At the time the call ended, the rate was 48k receiving and 28.8k upstream. A V.90 peak speed of 52k is shown. There are no retrains, and 3772 BLERS.

What's a BLER?

A bler is an error in data and protocol blocks. It means that some data (or protocol blocks) had to be resent. A "perfect" connection wouldn't have any. However, how many a "good" connection can have may be misunderstood.

How Many BLERs is "normal"?

It's my understanding that 3Com modems are designed to operate with a bler rate approaching 4000 per hour. As you approach this high rate, the performance of the modem will be about that which you'd get if the receive carrier were throttled down a notch. At least that's what I'm told. I think there may be exceptions, though: when calling a server which doesn't support SREJ (selective reject), the retransmission is much less efficient. At this time, 3Com and Bay Networks servers support SREJ; Lucent and Ascend do not, and I'm not sure about Cisco, but I doubt they do. So, if you've got thousands of blers on a 1 or 2 hour call to your ISP, it may be OK to a 3Com or Bay server, but to the others, you might want to reduce your maximum rate a notch. See Limiting Your Connect Speed. You may also want to Check Your Throughput at both speeds to see which gives you better performance.

The i6 screen shown above was to a 3Com server - you can see selective reject in the Protocol line (SREJ).

What about the rest of the diagnostic information?

Retrains Requested & Granted - this is a time-consuming/no data-flow process, and you don't want any. 1 or 2 over a long connection may be acceptable, but if you're getting a lot of these, you may want to try and throttle back your connect speed.

Octets: Compressed data units. The difference between chars and octets is the result of the modem compressing data.

Blocks resent: Represents blocks the remote modem resent due to blers.

Link Timeouts: Protocol detection problems: communications were severed momentarily, and the modems probably recovered. Zero is a good number here.

Link Naks: Negative acknowledgments (one or more blocks) - the remote modem didn't acknowledge receipt of your data. Zero is a good number here.

Data Compression: Indicates the type of data compression negotiated for the call. A V42BIS response includes the size of the dictionary and the maximum string length used, for example 2048/32.

Fallback: Enabled/Disabled. indicates whether or not the modems negotiated online fallback

Protocol - Aside from SREJ discussed above, you want to see some error control protocol - either LAPM or MNP; if you don't see an error-correcting protocol, check your DUNS settings (use error control and should be required to connect).

Speed - Compare this speed to the CONNECT report you got; also note your upstream rate. (This field shows the receive data rate / transmit data rate)

Disconnect Reason - Some of the possible reasons the modem hung up are reported here as follows:

A Rootless Tree - Modem received an invalid v.42bis compression frame
Break Timeout - Incompatible processing of a break signal occurred
DISC: The remote modem sent a v.42 disconnect frame (the other end hung up on you)
DTR dropped: Your computer terminated the call
Escape code: The '+++' escape code was used
Extra Stepup: See rootless tree
GSTN Cleardown:
On non-error correcting connections, 1 end dropped DTR (hung up); or the disc frame was corrupted on error-correcting connections
Illegal Command Code: See rootless tree
Invalid Codeword:
See rootless tree
LD received:
The remote modem sent an MNP disconnect request
Loop loss disconnect: The modem detected a loss of current on the phone line; usually means remote end hung up; Could indicate the modem was unable to communicate with the remote at all before disconnection, telco or remote equipment failure.
Loss of carrier: Remote modem's carrier gone; usually means the remote end hung up on you
Retransmit Limit: The modems reached the maximum of 12 attempts to transfer a data frame without error.
SABME Timeout: The modems failed the v.42 Set Asynchronous Balance Mode Extended link negotiation.
Unable to retrain: After several attempts, the modems were unable to retrain and could no longer transmit or receive data.
XID Timeout: The modems failed to negotiate the v.42 detection (XID exchange) phase.

More Diagnostic Info....

3Com Modems give even more diagnostic information in the i11 and y11 screens. The y11 is covered in More Than 1 a/d Conversion?

The i11 screen is as follows:

U.S. Robotics 56K FAX EXT Link Diagnostics...

Modulation                  V.90
Carrier Freq     (Hz)       None/1920
Symbol Rate                 8000/3200
Trellis Code                None/64S-4D
Nonlinear Encoding          None/ON
Precoding                   None/ON
Shaping                     ON/ON
Preemphasis      (-dB)      8/4
Recv/Xmit Level  (-dBm)     22/12
Near Echo Loss   (dB)       8
Far Echo Loss    (dB)       0
Carrier Offset   (Hz)       NONE
Round Trip Delay (msec)     6
Timing Offset    (ppm)      -4260
SNR              (dB)       48.7
Speed Shifts Up/Down        5/6
Status :                    uu,5,12N,12.5,-7,1N,0,47.8,15.5  *

* - Note - with V.92 firmware the status line has an additional value:
Status :                    uu,5,12N,18.7,-7,1N,000,46.4,-51.3,52

Modulation - whether the connect is/was V.90, x2, or v.34

Recv/.Xmit Level - How "loud" the signals coming in and going out are in -dbm. In the US, FCC limits the level to -12dbm. If you know the level being sent by the server, subtracting it (usually 12 in the US) will give you an indication of the amount of loss in the analog portion of your telco facilities.

Near Echo Loss - A measure of how much of the modem's transmit signal is being reflected back at the telco hybrid

Far Echo Loss - Doesn't apply if the server modem has an all-digital connection to the network.

Round Trip Delay - The time it takes for a signal to go from your modem to the server modem and back. It can give you an idea how close to you the server modem actually is. The higher the number, the further away from you the server modem is; truly "local" numbers will rarely give you a number greater than 10msec; higher numbers may indicate that the number you are calling is a virtual POP connected to a distant location central site where the real modems are located.

SNR - Signal to noise ratio. The higher the better. A measure of the noise on the circuit. Note - this is from the modem's perspective, and often is not a "real" measure of the noise if the modem has been unable to identify and compensate for all digital impairments on the line.

Speed shifts Up/Down - How many times the receive rate went up and down. And indication of how stable your connection was - the lower the numbers, the better.

Status line:

This line reveals information the modem has discovered about the telephone line and server modem including the number of RBS links, the type of CODEC, padding, and server capabilities:

Status :   uu,5,12N,12.5,-7,1N,0,47.8,15.5

uu - Indicates u-law or a-law type codec; u-law is used in the US; a-law in some international systems; you might see a report of 'au' if you are calling from the US to an international server. The first letter indicates the server coding, the second, letter is the client-side codec type.

5 - number of sign bits. 5 is normal. See - What are sign bits?

12N - The maximum transmit level of the server modem in -dbm (ie, 12=-12dbm); the N indicates the server is capable of constructing a constellation that will result in the indicated transmit level at the client-side codec after compensating for any digital padding. (For example, if a 3dbm pad is detected, the server can construct a -9dbm constellation which will result in a -12dbm signal when it hits the codec. A 'Y' indicates the server modem is not capable of doing this.

12.5 - Sliding power window in -dbm. Will generally be close to the maximum transmit level; A large difference may be caused by procedures to eliminate inter-modulation distortion.

-7 - Indicates the padding in the telephone network to the client modem. 0=None; -7=3db; -15=6db; A value of -1 here indicates some minor padding, but less than the standard 3db.

1N - Indicates the number of RBS links (the numeric portion); a 'Y' here indicates a type-B codec, typically found in analog COs or in genuine SLC96. IMPORTANT- a type-B codec 'ignores' the LSB in the digital stream when converting the signal to your analog line; if you actually have a type-B codec, your 56k rates will be impaired; not all client modems' firmware work with a type-B codec, but 3Com clients should work. If you have an RBS-related problem, a 3Com client might sometimes erroneously indicate a type-B codec.

0 - unknown

47.8 - SNR ratio of 2 point training. See What is 2-point training?

15.5 - Codec distortion. Really good would be low teens; Really bad would be upper 20's. 20-ish is normal.

Note that the modem doesn't necessarily correctly identify pads, codecs, or RBS links - this is what the modem "thinks" it has identified.


More than 1 A/D Conversion?
What's a 56k-Compatible Line?
Troubleshooting Procedure
Limiting Your Connect Speed
Check Your Throughput

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