Editing Leased line (section)

==Applications==
[[File:EPN Leased Line and dial-up Network.svg|thumb|upright=1.5]]
[[File:EPN Frame-Relay and Dial-up Network.svg|thumb|upright=1.5]]
Leased lines are used to build up private networks, private telephone networks (by interconnecting [[private branch exchange|PBX]]s) or access the internet or a partner network ([[extranet]]).

Here is a review of the leased-line applications in [[network design]]s over time:

===Site to site data connectivity===
Terminating a leased line with two routers can extend network capabilities across sites. Leased lines were first used in the 1970s by enterprise with proprietary protocols such as IBM [[System Network Architecture]] and Digital Equipment DECnet, and with [[TCP/IP]] in University and Research networks before the Internet became widely available. Note that other [[Network Layer|Layer 3]] protocols were used such as Novell IPX on enterprise networks until TCP/IP became ubiquitous in the 2000s. Today, point to point data circuits are typically provisioned as either [[Time-Division Multiplexing|TDM]], Ethernet, or Layer 3 MPLS.

===Site to site PBX connectivity===
Terminating a leased line with two PBX allowed customers to by-pass PSTN for inter-site telephony. This allowed the customers to manage their own [[dial plan]] (and to use short extensions for internal telephone number) as well as to make significant savings if enough voice traffic was carried across the line (especially when the savings on the telephone bill exceeded the fixed cost of the leased line).

===Site to network connectivity===
As demand grew on data network [[Telephone company|telcos]] started to build more advanced networks using [[packet switching]] on top of their infrastructure. Thus, a number of telecommunication companies added ATM, Frame-relay or ISDN offerings to their services portfolio. Leased lines were used to connect the customer site to the telco network access point.

===International private leased circuit===
An international private leased circuit (IPLC) functions as a point-to-point private line. 
IPLCs are usually [[time-division multiplexing]] (TDM) circuits that utilize the same circuit amongst many customers. The nature of TDM requires the use of a [[CSU/DSU]] and a router. Usually the router will include the CSU/DSU.

Then came the Internet (in the mid-1990s) and since then the most common application for leased line is to connect a customer to its ISP point of presence. With the changes that the Internet brought in the networking world other technologies were developed to propose alternatives to frame-relay or ATM networks such as [[VPN]]s (hardware and software) and MPLS networks (that are in effect an upgrade to TCP/IP of existing ATM/frame-relay infrastructures).