A MODEM (MOdulator-DEModulator) is typically used to send digital data over a phone line. It is a DCE device and connects a computer ( or any other DTE device) to the voice channel (dial- up line). While transmitting the data, modem modulates the data into a signal that is compatible with the phone line, and while receiving the data, the modem demodulates the signal back into digital data.
There are primarily two types of modems. These are: i. Analog modem
ii. Digital modem
i. Analog modems:
diagram below shows an analog modem.
As seen from the above figure, you can use an external modem, internal modem that sits on the internal extension bus such as PCI bus, or and USB modem. For installing an analog modem, you will be needing modem driver software. This software will be different for different Operating Systems and you need to make sure that you have the right version of the driver software loaded on to the computer before attempting to connect to any external host. Usually, the software will also have self testing abilities that can be used for troubleshooting any problems with the installation.
The following features are configured frequently for communicating between hosts using a modem:
1. Speed : The speed at which the modem can send data in bps (bits per second). Typically modem speeds are: 300, 600, 1200, 2400, 4800, 9600, 14.4K, 19.2K, 28.8K bps
2. Auto Dial /Redial : Smart Modems can dial the phone number and & auto redial if a busy signal is received.
3. Auto Answer : Most modems can automatically answer the phone when an incoming call comes in. They have Ring Detect capability.
4. Self-Testing : New modems have self-testing features. They can test the digital connection to the terminal /computer and the analog connection to a remote modem. They can also check the modem's internal electronics.
5. Voice over Data : Voice over Data modems allow a voice conversation to take place while data is being transmitted. This requires both the source and destination modems to have this feature.
6. Synchronous or Asynchronous Transmission : Newer modems allow a choice of synchronous or asynchronous transmission of data. Normally, modem transmission is asynchronous. We send individual characters with just start and stop bits.
Synchronous transmission or packet transmission is used in specific applications. Analog Modems - AT command set and Modem scripts
Analog Modems are used for dial-up connections. Modem configurations vary by manufacturer. They use their own language for communication. The common alphabets of modem speak is given below:
a-z -->“alphabet” * -->“asterisk” ^ -->“carat” - -->“hyphen” $ -->“dollar sign” : -->“colon” % -->“percent sign”
@ -->“character command set” & -->“ampersand”
\ -->“backslash” )--> “parenthesis”
#--> “character command set” 1. AT Command Set
AT commands allows to control many of the functions of modem. AT commands can be used by typing them at the command line of any Terminal program.
See Appendix for AT command set. 2. Writing Modem Scripts
Scripts are those where-in we combine modem-speak commands. Ex: AT&FS0=1&C1&D3&K3&Q9&W
AT&F load factory defaults and settings S0=1 set modem to answer on first ring &C1&D3 set modem up for “action” (cd/dtr) &K3 set hardware flow control
&Q9 set compression
&W save configuration to modem Connecting to a telephone line
When the modem is turned on or after reset, the modem always goes to command state. It enters on-line state after successfully making a connection with a remote modem, either when answering or originating a call. When you return to command state from on-line state, the modem goes to local command state, which allows you to maintain the connection and enter commands.
AT refers to the command prefix (attention sequence) that precedes each command to the modem.
D Dial (Originate mode) : This command signals the modem that the numbers, letters, and commas that follow are all part of a telephone number that it should dial.
T Touch-Tone dialing / Enable tone (DTMF dialing) : When "T" is used as a subcommand of the "D" command (ATDT 12345) this causes all following numbers to be Touch-Tone dialed (default).
When it is used as ATT it sets the default dialing mode to Touch-Tone mode. Can be used in dial string (default). Ex: ATDT15551235 <Enter>
ATDT is the command for dial and enable tone i.e, connecting to a telephone line.
Disconnecting a telephone line
Hn Hook (hang up) When the modem is on-line, you must first enter the +++ escape sequence to take the modem back to command state before issuing the hang up command, ATH. 0 - Forces the modem on-hook. Hangs up the modems
connection to the telephone line and place modem in
command state. 1 - Forces the modem off-hook. After going off hook, operate auxiliary relay.
Ex: ATH 1 <Enter>
ii. Digital Modems :
A digital modem is faster than a analog modem, it does not have to convert between analog and digital signals. The types of digital modems are :
1. ISDN Modem : An ISDN (Integrated Services Digital Network) modem allows the user to connect to the Internet via a dedicated telephone line.
2. DSL Modem : DSL (Digital Subscriber Line) modem uses a copper wire transmission technology to access high speed broadband Internet over telephone lines. For more details on how to install and set up a DSL modem, click here.
3. Cable Modem : Cable modems use fiber optic technology to access the Internet. Cable modems are primarily used to deliver broadband Internet access in the form of cable Internet.
10. Additional Resources:
1. For configuring telnet and ftp on Linux workstation please refer to http://www.cae.wisc.edu/linconftcpip. 2. Modem AT Command Set and Router Configuration Guide:
http://www.cisco.com/en/US/docs/routers/access/modem/AT/Command/reference/atnextpt.html
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