Private Line Service Technical Review
Private Line Service
Private lines can be compared to dedicated lines, private circuit, data
line and leased line. Private line services involves
dedicated circuits, private switching arrangements, and/or predefined
transmission paths, whether virtual or physical, which provide communications
between specific locations. Private line services is often used to describe
a one part switched telephone line as opposed to a party line.
Dedicated Line Service
A dedicated line is a communications cable or other facility dedicated
to a specific application, in contrast with a shared resource such as
the telephone network or the Internet.
In practice, such services may not be provided by a single, discrete, end-to-end cable, but they do provide guarantees of constant bandwidth availability and near-constant latency, properties that cannot be guaranteed for more public systems. Such properties add a considerable premium to the price charged.
As more general-purpose systems have improved, dedicated lines have been steadily replaced by intranets and the public Internet, but they are still useful for time-critical, high-bandwidth applications such as video transmission.
Leased Line
A leased line is a symmetric telecommunications line connecting two locations.
It is sometimes known as a private circuit or data line. Unlike traditional
PSTN lines it does not have a telephone number, each side of the line
being permanently connected to the other. Leased lines can be used for
telephone, data or Internet services. Some are ringdown services, and
some connect two PBXes.
In the U.K., leased lines are usually available at speeds of 64k, 128k, 256k, 512k, 2Mb/s and provided to the customer on X.21 presentation. Higher speeds are available on alternative interfaces.
In the U.S., low-speed leased lines (56 kbit/s and below) are usually provided using analog modems. Higher-speed leased lines are usually presented using FT1 (Fractional T1): a T1 bearer circuit with 1 to 24 56k or 64k timeslots. Customers must manage their own network termination equipment—Channel Service Unit or Data Service Unit (CSU/DSU).
In Hong Kong, leased lines are usually available at speeds of 64k, 128k, 256k, 512k, T1 (channelized or not) or E1 (less common). Whatever the speed, telcos usually provide the CSU/DSU and present to the customer on V.35 interface.
For many purposes, leased lines are gradually being replaced by Metro Ethernet.