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3Com U.S.Robotics 56K FaxModem - line analisys

Ok, un altro articoletto nelllo stile di questo sito, molto andante senza troppi fronzoli.

Allora, avete un modem analogico (dinosauri !... come me) e non siete contenti della connessione, o è lenta, o salta o tante altre cose o siete curiosi.

Dunque, articolo non prorpio per tutti, cioè non sono giochi ma a chi interessa la telematica anche un paco o gli piace smanettare si fà così (ah tranqui, se fate errori il linea dei 5 nove non succcede nulla al vostro hardware, cosa sono la regola dei 5 nove ? allora, al 99.999% non succede nulla... si sono proprio 5 i nove, ecco tutto qui, demistificato, però dire regola dei 5 novi è bello che non 99.999% ... basta):

  • accendete il PC
  • accendete il modem e collegatelo alla linea del telefono
  • eseguite hiperterminal, il classico programmino di windows.

Date un nome alla connessione a caso, non componete nessun numero, arrivati alla riga di comando digitale

  • ATDT <numero di telefono del vostro ISP senza i segni di maggiore/minore>

Lasciatelo comporre e fare tutti i suoni strani che sentite (strani lo sono per voi ! razzisti !). Adesso vi arriva la richiesta dello Username o Login, premete tre volte il "+", cioè dovreste aver scritto "+++" sono i comandi di escape per avere modem connesso e modalità riga comandi, aspettate 1-2 secondi per il modem che processa i comandi, vi compare la scritta "OK".

Dunque scrivete:

  • ati6i11

E battete Enter.

Poi scrivete senza preoccuparvi subito dell'output:

  • aty11

Aspettate la fine dell'output, e spegnete il modem.

Adesso analizziamo i dati, incollateli su notepad o word fate vobis. Dovreste aver una cosa come questa:

atdt 0174211010
CONNECT 49333/ARQ/V90/LAPM/V42BIS


User Access Verification

Username: +++
OK
ati6i11
U.S. Robotics 56K FAX EXT Link Diagnostics...

Chars sent 3 Chars Received
Chars lost 0
Octets sent 3 Octets Received 45
Blocks sent 3 Blocks Received 4
Blocks resent 0

Retrains Requested 0 Retrains Granted 0
Line Reversals 0 Blers 1
Link Timeouts 0 Link Naks 0

Data Compression V42BIS 2048/32
Equalisation Long
Fallback Enabled
Protocol LAPM
Speed 49333/28800
V.90 Peak Speed 49333
Current Call 00:00:14

Online

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) 7/2
Recv/Xmit Level (-dBm) 19/16
Near Echo Loss (dB
Far Echo Loss (dB) 0
Carrier Offset (Hz) NONE
Round Trip Delay (msec) 12
Timing Offset (ppm) 2255
SNR (dB) 45.9
Speed Shifts Up/Down 0/0
Status : aa,5,12N,12.2,-0,0N,0,44.1,17.6
OK
aty11

Freq Level (dB)

150 24
300 23
450 23
600 22
750 22
900 22
1050 22
1200 23
1350 23
1500 23
1650 23
1800 23
1950 24
2100 24
2250 24
2400 24
2550 25
2700 25
2850 25
3000 25
3150 25
3300 26
3450 27
3600 30
3750 34
OK

E questo è il grafico fatto con la tabella dei valori:

Che rappresenta la risposta in frequenza della linea del telefono.

Al momento non ho molte paorle da dire o commenti da fare, la mia linea è abbastanza buona, si buona per avere 6 telefoni in parallelo... adesso incollo la spiegazione delle sigle, ripeto finchè non ho una mia versione/analisi (chiedo ai prof università se sanno qlc...) non metto nulla, poi farò paginetta.

Modulation - This field indicates the Modulation used to establish the connection. Several values can be seen here, the most likely being V.90, X2, or V.34 and the least likely being V.22bis, V.32/bis/terbo, VFC, or HST.


Carrier Freq - This field displays the carrier frequency of both the receiver and transmitter.


Symbol Rate - This field displays the symbol rate (the number of symbols or waveforms containing encoded bits) of both the receiver and transmitter. These values should not drop below 3200 for a good connection.


Trellis Code - This is the type of mathematical "anti-noise" operation that has been selected for transmitted data. This can vary and really has little bearing on determining noise, though it may be possible that a value of None/None could be problematic.


Nonlinear Encoding - This displays the state of the received signal and transmitted signal with respect to the status of Nonlinear Encoding activation upon handshake. This is an operation that the modem can perform on the transmitted signal to improve the received signal under certain conditions.


Precoding - This displays the state of the received signal and transmitted signal with respect to the status of Precoding activation upon handshake. This is an operation that the modem can perform on the transmitted signal to help in reducing the effects of noise increasing in the adaptive equalization process.


Shaping - This displays the state of the received signal and transmitted signal with respect to the status of Shaping activation upon handshake. This is an operation that the modem can perform on the transmitted signal to improve the received signal under certain conditions.


Preemphasis - This field indicates how much signal boosting the modem decided was enough to compensate for a poor signal. Keep in mind that there is often something here. I do not yet know what the values mean, but I do know that higher means more boosting had to be used.


Recv/Xmit Level - This field indicates the receive and transmit levels. Standard levels for the US are -10, but the values are adjusted by the modem during modulation. Ranges from -1 to -6 and -30 to -40 could indicate a poor local loop.


Near Echo Loss - This field indicates the transmit level that is being reflected at the telco hybrid. A normal value is around 30. If this value is in the 200's or higher there could a line problem.


Far Echo Loss - This field indicates the transmit level that is being reflected at the remote modem's hybrid circuit. This field will usually not be high if you are dialing into a digital server. A normal value is around 30. If this value is in the 200's or higher there could be an ISP or telco problem.


Carrier Offset - This field indicates the dc offset in the FM discriminator output. You should normally have "NONE" here, but sometimes there is an offset. If the offset is greater than 1000 you will most likely see v.34 connections, and if it is far greater than 1200 you may begin to see performance issues. This may indicated dc voltage issues on your telephone line.


Round Trip Delay
- This field indicates the amount of time that it takes for the modem to hear a reflection of it's own transmission. Since this field helps to indicate the physical distance between the modem and the server it is not very useful, though a number greater than 10-15 would often indicate that the modem is connecting to a "Virtual POP"/"Super POP", which is a normal practice by ISPs and should not really affect your connection.


Timing Offset - This field indicates the difference in synchronization between the local clock and the host clock. This field provides the local modem with a means of synchronizing its sampling clock to that of the host. This field will normally not indicate any problems, though there may be issues if the value is about 10,000 or lower than -10,000.


SNR - This field indicates the Signal to Noise Ratio. This is not always a true measure of the SNR (due to digital impairments), but if a low SNR is found, it is one of the best indications of noise issues. The following chart helps to show what SNR will often yield what stable speeds, but can vary.

SNR Range (Note: SNR = Signal to Noise Ratio) Expected "Stable" Speed
40+ 33,600 - 56,000
37-40 33,600-56,000 (a little unstable at high speeds)
34-37 31,200-33,600
31-34 28,800-31,200
29-31 26,400-28,800
28-29 24,000-26,400
25-28 16,800-24,000
23-25 14,400-16,800 (NOTE: If your SNR is around this level or lower the FCC may step in if your telco won't fix it, this level is not required to be supported, but often is by local teclos and must be upheld if in their charter.)
21-23 12,000-14,400
18-21 9600-12,000 (NOTE: If your SNR is around this level or lower the FCC will step in if your telco won't fix it, this level is REQUIRED, BARE MINIMUM support.)
14-18 7200-9600
13-14 4800-7200
6-13 2400-4800
1-6 300-2400 if you are lucky enough to connect.

Speed Shifts Up/Down - This field indicates the number of times your receive rate was bumped up/down due to your connection stability. If this number is above 10 you probably have a pretty unstable connection.


Status - This field lists information regarding the server and phone line that the modem has detected. The fields are as follows:
Item Number 1: This item indicates the type of Codec used, U-law or A-law. The US uses the U-law codec.
Item Number 2: This item indicates the number of sign bits and should be 5 under normal conditions.
Item Number 3: This item indicates the the maximum transmit level of the host. If you experience a value
of 6 or less, or 19 or higher there may be a signal or power problem. A Y in this field indicates that the server
is not capabale of compensating for digital pads (thus providing for a worse transmit level) and an N indicates
that the server is capable of this and the number indicated does not necessarily reflect the quality of your line
(though if it is a poor value, then the quality may be even worse than shown).
Item Number 4: This item indicates the sliding power window, which should be relative to item number 3.
A poor value here may also indicate poor signal or power issues.
Item Number 5: This item indicates the amount of padding used inbetween the client and host on the telephone
circuit. A value of 0 is the best, indicating no padding, a value of -7 indicating 3db of padding, and a value of -15
indicating 6db of padding which could cause potential problems.
Item Number 6: This item indicates the number of Robbed Bit Signal links in the telco circuit between client
and host. You can still achieve 56k type connect speeds with RBS links, however if you have a Y here, this
indicates a type B Codec which could mean that you are on a SLC96 (often referred to as a Slick). A slick is
used when there are not enough twisted pairs in the area to meet customer needs. When a slick is used you
are given extra Analog to Digital/Digital to Analog conversions, thus making v90 speeds impossible.
Item Number 7: This item usually indicates a value of 0. I do not have a clue as to what it is supposed to be for.
Item Number 8: This item indicates SNR using another type of training. Use the above chart to find the
meaning of your SNR value.
Item Number 9: This item indicates the Codec Distortion. A value here that is higher than 26 could indicate
troubles. (Note that item 9 works differently on v92 USRs.)
Item Number 10(only shown on v92 USRs): I do not yet have a clue as to what this item is supposed to be for.