Ask
The Right Question
A
leased line is a fixed, dedicated, digital, point
to point line for data transfer. The term "leased
line" is a general description of a point to
point circuit from a data carrier supplier. These
circuits can be supplied at various speeds and be
presented at your site with various interfaces.
A leased line is not connected to a telephone exchange
and does not provide DC power, dial tone, busy tone
or ring signal.
Transmission
speeds of leased lines vary from 64kbps up to several
megabytes, but the costs increase dramatically beyond
128kbps. For most small companies or home users,
ISDN is quite suitable, as it is capable of speeds
of up to 128kbps. Unlike ISDN, there is no usage
charge for a leased line, only fixed annual costs,
which is at a higher rate than ISDN. Due to usage
charges, it is generally recommended that if you
are using the ISDN line for more than four hours
a day (total on-line time), then it may be more
cost effective to move to a fixed circuit.
An
added complication to working out the cost effectiveness
of a leased line, is that the majority of circuit
suppliers work out the annual cost, based on the
distance between the two sites to be connected.
Leased
Line Modem
Leased line modems are also called Baseband 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.
Half
Duplex/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 Or Fiber Modem
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 loopbacks.
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 overvoltages.
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.
Diagnostic
LED
LEDs on the front panel are TD, RD, RTS, DCD, Test,
Error and Power, which are useful for visual diagnostics.
Available
Models
Leased line modems are either available as standalone
units or as modem cards to be fitted in the rack
(rack modem).
So,
keep these points in mind while buying a leased
line modem, and keep track of the other products
that will be featured in the coming months.
Source:
LanBit India Pvt. Ltd.
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