Leased line

A leased line is a service contract between a provider and a customer, whereby the provider agrees to deliver a symmetric telecommunications line connecting two or more locations in exchange for a monthly rent (hence the term lease). It is sometimes known as a 'Private Circuit' or 'Data Line' in the UK or as CDN (Circuito Diretto Numerico) in Italy. Unlike traditional PSTN lines it does not have atelephone 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.

Typically, leased lines are used by businesses to connect geographically distant offices. Unlike dial-up connections, a leased line is always active. The fee for the connection is a fixed monthly rate. The primary factors affecting the monthly fee are distance between end points and the speed of the circuit. Because the connection doesn't carry anybody else's communications, the carrier can assure a given level of quality.

An internet leased line is a premium internet connectivity product, delivered over fiber normally, which is dedicated and provides uncontended, symmetrical speeds, Full Duplex. It is also known as an ethernet leased line, DIA line, data circuit or private circuit.

For example, a T-1 channel can be leased, and provides a maximum transmission speed of 1.544 Mbit/s. The user can divide the connection into different lines for multiplexing data and voice communication, or use the channel for one high speed data circuit. Increasingly, leased lines are being used by companies, and even individuals, for Internet access because they afford faster data transfer rates and are cost-effective for heavy users of the Internet.

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 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 TDM, Ethernet, or Layer 3 MPLS.

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 (specially when the savings on the telephone bill exceeded the fixed cost of the leased line).

As demand grew on data network telcos started to build more advanced network using packet switching on top of their infrastructure. Thus 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.

An IPLC is an International Private Leased Circuit that 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 the most common application for leased line is to connect a customer to its ISP Point of presence. With the changes that Internet brought in the networking world other technologies were developed to propose alternative 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). 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. An IPLC is an International Private Leased Circuit that 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.

Leased lines are more expensive than alternative connectivity services including (ADSL, SDSL, etc.) because they are reserved exclusively to the leaseholder. Some internet service providers have therefore developed alternative products that aim to deliver leased-line type services (Carrier Ethernet-based, zero contention, guaranteed availability), with more moderate bandwidth, over the standard UK national broadband network. While a leased line is full-duplex, most leased line alternatives provide only half-duplex or in many cases asymmetrical service.