Modems guidelines and functions

AT: - Command line prefix. The ATtention command precedes all other commands listed below except for A/ and the escape code. For example, the command DT would be typed ATDT.

A/:- Repeat last command (not to be preceded by AT)

A: - The modem is set to answer mode and goes offhook immediately. Answer without waiting for ring.

Bn: - Bell mode - set 1200 bps protocol compatibility Selects V.22 or Bell 212A in the 1200bps mode. n=0, CCITT V.22 with 2100Hz Answer Back Tone. n=1, Bell 212A with 2225Hz Answer Back Tone.

Cn: - Carrier state: n=0, OFF n=1, ON Eg. C1 = carrier state on.

Dn: - Dial telephone number. Dial the number immediately following this command, where n is the number combined with other dial subcommands as shown below. Any combination of the subcommands listed below may be strung together to perform the desired dialing sequence. The number is dialed in the call progress or blind mode depending on register S11.
Dial Subcommand and Function
T: - Touch Tone Dialing
P: - Pulse dialing (some modems use by default to Pulse if T or P not used.
R: - Reverse mode. Puts modem in answer mode immediately after dialing. Use answer frequencies when originating a call.
S=n: - Dial a stored number. The numbers are stored in the registers of the microcontroller.
n=0, 1, 2, 3 - stored number that has been previously stored in non-volatile RAM, using &Zn command.

W: - Wait (for seconds dial or access tone). Wait for a continuous tone before dialing the next number. Useful for dialing MCI or Sprint. The modem waits S7 seconds (see Appendix B about registers) for the tone. If the tone is not heard, BUSY or NO DIALTONE result code is issued.

@:- Wait for a "quiet answer" - one or more rings followed by 5 seconds of silence - for the length of time defined.

, :- (comma) - Pause (a delay in dialing sequence). Pause the length of time specified by S8 (default time = 2s) each comma equals the pause length specified. You may use as many commas as you like. E.g. To disable call waiting in most areas use, *70,,312-666-9000 to connect to UIC .
! : - Flash (on-hook for 1/2 second). This character can be used to go temporarily on-hook in order to get a new dial tone.

;:- Return to command mode after dialing. This command can only be placed at the end of a dial command

0-9 # * ABCD: - Digits and characters that may be used in dialing. # * and ABCD may only be used when tone dialing.

know more about: leased line modem

Enthusiastic Leased Line modem

A fixed digital connection with the phone company
• Bidirectional Internet access at guaranteed speeds
• Speeds range from 56K to T1 (1.5M) on up
• Always on, unmet red
• ISP separate charge
• Fees 56K $80/month + ISP $300/m. 1.5M T1 $250/m + ISP $1500/m
Leased Line Modem Advantages
• The most reliable connection to the Internet
• Guaranteed bandwidth and service levels
• Great for hosting a web site or for managing your own security and firewalls
• 56K lines available anyplace there is a telephone
Leased Line Modem Disadvantages
• Expensive
• T1 and higher access confined to metro. Extremely expensive in rural areas
• Difficult to get installed
• Not easy to switch ISPs
• Requires dedicated hardware

Leased Line Modem and factors to be think while buying

Leased line modem are also called Base band modems or short-range modems. A leased line modem is a digital modem that may be used to inter-connect computers, terminals, controllers and similar digital equipment over distances of up to 16 kms (10 miles) for LAN interconnection, campus networking, or high-speed leased line internet links, over a single, unshielded twisted copper pair (two wires). These devices overcome distance limitations and noise problems by using special modulation and line equalization techniques and allow error-free communication over longer distances, at much higher data rates than conventional analog dial-up modems. Leased line modems are a cost effective and efficient solution for dedicated data-network access and in the "last mile" network access applications.
Factors to Be Considered While Buying a Leased Line Modem
While buying a leased line modem, the following factors need to be considered.
Full Duplex Operations Support
When a leased line modem operates in half duplex, it transmits or receives a signal in both directions but only one at a time, whereas in full duplex, it can transmit as well as receive simultaneously. Full duplex operation is normally achieved using an adaptive echo cancellation technique.
Synchronous/Asynchronous Mode
A leased line modem can support synchronous or asynchronous mode or both. While operating in the asynchronous mode, it transmits units of data, one character preceded by start bit and followed by stop bit at a time, whereas in synchronous mode, it transmits data bit at a fixed rate with the transmitter and receiver synchronized.
Modem Ranges For 2 Wire/4 Wire
Ranges for 2 wire/4 wire copper modems are measured over line simulators at 24AWG. Ranges for fiber optic modems are measured using 1550 mm laser diode interface.
Selectable Bit Rates
Normally leased line modems can operate to select data rates ranging from low speed (9.6 Kbps) to high speed T1 (1 Mbps) or E1 (2.048 Mbps). The higher the speed you select, the lower is the distance range over the twisted pair unconditional 24 AWG telephone line.
Interface
Digital interface includes V.24/RS -232, V.35, V.36, RS-530, built-in Ethernet bridge and G.703 (64 Kbps), whereas on analog line connector is RJ-45 and terminal block.
Modulation Technique
Conditional diphase modulation and 2BIQ are the most common modulation types, which provide immunity to background noise, eliminate nominal line distortion and enable efficient transmission over twisted pair.
Diagnostic Capabilities
It features diagnostic capabilities complying with the V.54 standard to perform local and remote loop backs. A built-in Bit Error Rate Tester (BERT) activated and monitored from front panel complies with V.52 standards to enable complete testing of both modems and lines.
Protection Circuit
Isolation transformers and a protection circuitry offer line protection. They protect leased line modems as well as the attached DTE from line hazards such as AC or DC over voltages.
Intelligent Modem
Some of the leased line modems are also available with a certain degree of intelligence. An intelligent modem comes with a remote control and a management channel that enables the user at a central site to configure the unit at the customer's premises by remote control, activate, diagnose and receive real-time alerts on system status.