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.