Specifications

26 Rockwell Automation Publication AG-SG001G-EN-P - April 2015
Chapter 2 Choosing a Telemetry Network
Private Leased Line (PLL)
PLL is a dedicated telephone line that is a permanent connection between two or
more locations and that is used for analog data transmission. The line is available
24 hours a day. In order for the line to be used for voice communication, a voice
option must be installed.
Advantages/Capabilities
• The media is cost-effective for applications that require large amounts of
data to be collected frequently from remote sites and/or applications that
require remote sites to have a constant connection to the master station.
• Regardless of how much you use the line, the phone company charges you
a flat, monthly fee based on the following:
– Distance between sites
– Area of the country
– Type of line conditioning
Leased lines have different levels of conditioning, or grades - the higher
the grade, the greater the modem data rate that can be supported by the
link, and the more the phone company charges for it.
• The standard, unconditioned line, supports speeds of up to 56 Kbps.
• Private leased lines provide a 4-wire connection. You can purchase modems
that operate the circuit in either half- or full-duplex mode. You can also
order a 4-wire multi-drop line.
Disadvantages
• The media cannot be used in areas that do not have access to the network,
such as an offshore oil or gas well.
• The lines can contain impairments that can cause modems to have error
rates of less than 1 error per 1,000,000 bits.
Equipment Required
Use standard Bell or CCITT modems. Contact the telephone company for
information about connecting to the network.
Phone Company
PLL PLL