Amazing #02 14 июля 1998

Модем

Miscellaneous - Modem: How to use the modem, implements the correction / compression MNP4/MNP5, v.42/v.42bis.

(C) 1998 by Evgrafov Glory

Music by Andrew Fer


                1. Modems


     1.1. The concept of the DCE and DTE


     From the standpoint of telecommunications equipment 
involved in the formation of channels and data transmission 
systems, are divided into: 

     - Terminal (DTE-Data Terminal
Equipment)

     - Communications (DCE - Data
Communication Equipment).

     In domestic terms DTE -
Ltd. (data terminal equipment) and DCE
- ADF (data communication equipment).

     To DTE include: computers, remote terminals, any device 
designed to receive or send data via serial interface or a 
serial transmission channel. 

     DCE - communication equipment, which allows to simplify 
serial communication, for example: a modem. 

     Due to the high integration Tues often DTE
and DCE combine. At these times it is
division of a liter of a Mr. Fr.

     When sending information allocates a certain class of 
signals, such as: signal TD and RD. 

     TD - data transmitted sequentially from the terminal 
equipment to a connected


     RD - data taken by the terminal
equipment connected to.

     The second pair of signals: RTS and CTS. This
handshake signals, they provide a link between the terminal and
communication equipment. Active Level
RTS signal terminal equipment indicates that it requires 
sending data to the connected equipment. Active STS level 
communication equipment indicates on willingness to accept data 
from the terminal equipment. 

     Data exchange carried out in accordance with a certain 
algorithm (a set of rules), which is called a protocol. One 
example of communication - communication via Interface RS232, 
containing the connectors for provide a connection in 
accordance with signals TD, RD, RTS, CTS. For this reason, 
communication via the RS232 interface is called communication 
protocol RTS / CTS. Connectors are also contain the conclusions 
of the relevant signals Synchronization: TC, RC.


     TC - sync signal transmitter or terminal equipment

     RC - sync signal receivers
(Communications equipment).


     1.2 Classification of modems.


     MODEM - as communication equipment communicates with the 
terminal equipment in accordance with all listed signals. 
Distinguish 

     komutiruemyh for networks with n e n and a
l b s n s (X.25);

     H a y e s - are compatible and there;

     to support with a p m o C n s e
n Sirota on to a l s, and no;

     to have additional e p a and c -
n s e function and no.

     in a t r a n s e n d and n m e n and th
modems;


     Work carried out modems on telephone lines, which are 
divided into Dialup (POTS) and non-switched (leased) line. 
Nonswitched line - it's a hard line connection, the connection 
is always there. In the general case line could be:


     1. Simplex (transfer goes into one
side).

     2. Half-duplex (one-variable transmission and the other 
side). 

     3. Duplex (simultaneous transmission
in different directions).


     The modem converts
digital signal to analog and vice versa, the frequency 
correlation of the signal, provides immunity signal (ie error 
correction). 

     In the modem can be implemented by various procedures 
transformation of discrete and analog signals. Modems also 
support different modes of exchange informatsieey with DTE. The 
modem is an intelligent device, which is controlled by means of 
special codes (submitted for User instruction set). As

international standard adopted a set of so-
called AT-commands, which was first
proposed by Hayes Mikrocomputer
Produkt. According to this, this set is now called H ayes - or 
AT commands - compliant. 


     Due to the intensification of certain
Commands the modem operates realizing the specified protocol 
V21, V22, ... As well as noise immunity and

compression protokoy type MNP (version
1-10), the standard MNP-5 and V.42bis.


     In the modem can be implemented
function of providing facsimile communications.
Facsimile implemented in accordance with the protocols:

                     V27 ter - 2400 baud;

                     V29 - 9600 baud.

     Facsimile equipment operates in
different standards defined groups:

     gr.1

     i. 2

     gr.3
Within the group can be realized the division into classes. To 
date, the standard is a group of G3 grade 1 or 2 (Class). 
Classes 1 and 2 are different communication protocols between 
the DTE and DCE. 

     Switching the modem in Fax mode
sold through AT-commands.

     Managing your modem in Fax-mode is carried out by a 
certain set of commands similar to AT-commands. Single

standard sets of commands for running Fax yet. Modems implement 
faxing in their code, usually contain symbols SR 96 24:


     Send-sends:

     Reseive-takes:

     96 of 9600:

     24 for 2400.

     Besides the functions mode Fax, modern
class of modems may implement the functions
communication with the scanner. To this end, the board modem is 
placed a separate unit. 

     Modems can be equipped with an attachment
Voice, which implements the functions of an answering machine. 
The voice message is converted into digital code and this video 
is stored on hard drive.


     Modems are equipped with the latest series
an automatic determinant
number (CLI) to allow
communicating only with different subscribers.

     One of the best to date
a modem company ZIXEL.


     Built-in - installed in a system slot (connector, located 
in the computer). External - connected to the serial port. 


     1.3. The structure of the modem.


     Generalized structure of a modem can
be represented by two blocks: a modulator
and demodulator. In accordance with the clock
pulses, Sr, formed by a clock generator TG, the input of the 
modem (on the block modulation of M) is applied a discrete 
signal T which performs carrier modulation

frequency generator formed by a low
frequency (LFO). Modulated signal passes through a bandpass 
filter (PF), ensuring consistency with the NC, and flows into 
the block demodulation. At the entrance to the demodulator PF

provides frequency coordination of signal equipment. In block 
DM signal is demodulated, is determined by its frequency and 
and time synchronization. In block C signal

synchronized.

     Formed sinhrnizatorom clock sequence is applied to
recording device P, which ensures the formation of pulses given
form.

     After the signal is fed to the demodulator
DTE. During the operation the modem
useful to monitor his condition. State is controlled by the 
main signal. For external modems, these states displayed on a 
special board. 


     1.4. Function and modem commands


     1) synchronization;

     2) Functional structure of states
modem;

     3) commands.


     1.4.1 Synchronization.


     Communications equipment should
be synchronized, because in
devices at one time should be the relevant events. Almost
synchronization boils down to that moment
start and end events, as well as the period
route must be defined. Basically, the transfer pulse sequences 
(IS) at the start is not known. Known only to the repetition 
period and pulse duration. For such a process PI 
synchronization is called a clock. 

     Events of the same name is called phase. Those two events 
occur simultaneously of the same name. 

     Events can be synchronous, but not
phase, and may not be in phase, but
not synchronous.

     Given that the flow of data between the information 
equipment can be: 

     - Own data (information
message);

     - Special characters (signals
timing).

     In any sample of the transmitted stream
should be focused information to identify the beginning of the 
transfer of ownership of data. One way is included in the data 
stream of special pulse clock Si.


     Depending of type of transmission method clock timing 
allocate: 

     1) The devices in which the signals Si
transmitted independently from information
characters by allocating a special channel synchronization. 
Such signals are called pilot signals. Dignity lies in the fact 
that Si is transferred outside information sequence. Drawback - 
the presence of an additional line. 

     2) The devices in which Si passed through the main 
channel. In this case, Si transmitted among information 
symbols. 

     3) devices, which is considered
Si that are transmitted through the main channel, with
The additional pulses are not included
an information sequence, and synchronization points are 
determined directly from the data by fixing in the demodulation 
of the beginning and end of transmission. 


     Depending on how you are Si on the code sequence 
information and how to implement their algorithm processing and 
synchronization of the receiver distinguish between 
asynchronous and synchronous transmission. 

     And with Mr. X and P O N N I paredacha more
Effectiveness in noisy channels. On this
they generally used. The main
sign of asynchronous communication is
that Si is an impulse, framing
each element of the code sequence.

     When C and H x p a n d n a transfer of information is 
transmitted block and the block is flanked by special 
start-stop characters. Synchronous transmission improves 
transfer rate for the same physical links. Synchronous modems 
are used in the construction of high-speed networks of the 
order of 64 - 128 kbps, in particular for build a network of 
X-25. 


     1.5. The structure of commands and registers.

     AT __ __ __ __

     A-Z - basic; 'A-'Z - extra;

     % A-% Z-helper;

     S0 - S20 (S0 - S99) - Registers


     A general description of the commands Hayes'a


     All teams must start the prefix
AT (or At, aT, at) and finish .
There are several exceptions that
will be described below. All input data
transmitted echo back to the computer (terminal). The modem 
automatically adjusts its data rate and format

sign to the parameters of the terminal. To change the baud 
rate, it is necessary to identify it command line  
completed at the terminal. The modem agrees to the imposed 
change and responds OK. Commands can be integrate into the 
command Stork, to improve clarity in the command line you can 
enter spaces, dashes, etc.: 

         AT DP 8W (044) 430-49-00, easier
read and understand than ATDP8W0444304900.

     The command line can contain
more than 40 characters, excluding spaces, and the prefix AT. 
If the string is longer than 40 characters, the modem returns 
an ERROR response and ignores the command line. Editing 
erroneous symbols command line by using keys

. For all entered correctly and
execution of a command modem responds OK. The modem can be 
configured to reply in the form of digital code (useful for 
software maintenance modem). Each answer is a one-or two-digit 
code. 

     After switching on the modem (usually)
configured to reply in character
form.



     The basic instruction set


     AT - the beginning (prefix) command line. After receiving 
this command, the modem automatically adjusts the transmission 
rate and format of the sign to the parameters of the terminal.



     The modem contains three types of memory: ROM,
RAM, NVRAM


     1.6. Working with AT-commands. (Dialing
ATD)


     1. Team ATD. Specifying the prefix
suffix: ATDT, ATDP.


     Ds - automatic dialing.

     Since the parameter T - tone dial, with
parameter P - digital set. After this
is the number to call. The composition
facilities may include special characters
These include: "" "" "!" "S" "R" "W".


     After receiving this command, the modem
dials the number and obtaining connection goes into transfer 
mode. Team consists of telephone numbers in the

which may include the following control options:

     s = P - pulsed digital set
indicates that the digits of phone number
walking after the symbol P, the modem must dial pulse (used 0-9
).

     s = T - set tone indicates
that the digits of phone number, after reaching
symbol T, the modem must dial frequency
(Razrasheno use characters 0-9, AD,
* And #).

     s =, - denotes a break before the next set of figures (2s).

     s =; - if used as the last character in the command line, 
the modem after dialing goes into command mode. 

     s = @ - modem waits 5 seconds of silence on the line if it 
does not appear during the 30s (the contents of register S7), 
modem disabled and meets NO ANSWER.


     s =! - If the sign! faces marks the dialing sequence, the 
modem goes into ON HOOK (hangs up) at 0,5 s, and then goes into 
a state of OFF HOOK (JIP). 

     s = S - modem dials the phone number recorded in the 
memory EEROM. 

     s = R - while writing as the last character in the command 
line, install the modem after dialing mode "answer", but only 
if the modem is ringing to the modem "originate".


     s = W - modem waits for a response station
(Dial tone) before further set
telephone number (for example, access to automatic 
long-distance call). 


     Example: ATDP 8W 2213343 - wait
dial tone after dialing 8

             ATDP 8W 035-3331122 ATDP 8W
0,95-333-11-23 - do puzu after 0

             ATDT 9, P2213343 - with 9
tone, pause and rest to gain a pulse, spaces and "-" ignored.

     "W" - before the next set of figures
waiting tone.

     "," - Pause, pause before dialing
following figures (duration 2 sec., register S8)

     "" - If this is the last character in the command line, 
after dialing the modem goes into command mode. 

     "!" - If standing in front of the number, then
modem first enters the "puts
up by 0.5 - 1 sec, and then begins
dialing. (ATDP! 2213344)

     "S" - a number from memory (AT Z2 = 2213344

                           ATDP S = 2).

     "R" - if it's the last character in
command line, after dialing
modem enters the "reply".



     1.7. Adjusting the modem.


     1.7.1. Configure Command V, X.

     AT V0 - the answer is given in digital form.

     AT V1 - the answer modem is given in symbolic form.

     1.7.2. Selecting the best values ​​for
register S6, S7, S8, S9, S10.


     The format for the installation of registers:

         AT S7 = 150 - written in the register;

         AT S7? - Read the display
sensitive.

     S0 - number of rings after which the modem picks up the 
phone, 

     S0 = 0 - call off the counter.

     S6 - this waiting time to a set (2
sec). Register sets the delay between
disconnecting the modem and the beginning of the next set of 
numbers (for X2 and X4, these values ignored).


     S7 - after dialing, the modem must
expect a time of occurrence, carrier
it is given in this case. If the modem
poorly defines the busy signal, while
expectations should be set lower. But we must
bear in mind that if the called modem
meets on the third call, and S7 = 5
sec., then the called modem answers the call, and
our modem will put her. On the other hand,
if the register set is very large
value (eg, 3 min.), the modem will wait 3 minutes after dialing
before you hang up. Optimal
considered:

     for intra - 20-40 sec.

     for long-distance - 40-70 sec.

     for the international - 40-180 sec.
__________________________________________
To be continued ...