Leased line

Leased line

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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 a telephone number
Telephone number
A telephone number or phone number is a sequence of digits used to call from one telephone line to another in a public switched telephone network. When telephone numbers were invented, they were short — as few as one, two or three digits — and were given orally to a switchboard operator...

, each side of the line being permanently connected to the other. Leased lines can be used for telephone
Telephone
The telephone , colloquially referred to as a phone, is a telecommunications device that transmits and receives sounds, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other...

, data or Internet
Internet
The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...

 services. Some are ringdown
Ringdown
-Operator signaling:In telephony, ringdown is a method of signaling an operator in which telephone ringing current is sent over the line to operate a lamp or cause the operation of a self-locking relay known as a drop...

 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
Digital Signal 1
Digital signal 1 is a T-carrier signaling scheme devised by Bell Labs. DS1 is a widely used standard in telecommunications in North America and Japan to transmit voice and data between devices. E1 is used in place of T1 outside North America, Japan, and South Korea...

 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
Multiplexing
The multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred...

 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
Internet access
Many technologies and service plans for Internet access allow customers to connect to the Internet.Consumer use first became popular through dial-up connections in the 20th century....

 because they afford faster data transfer rates and are cost-effective for heavy users of the Internet.

History


Leased lines services (or private line services) became digital in the 1970s with the conversion of the Bell backbone network
Backbone network
A backbone network or network backbone is a part of computer network infrastructure that interconnects various pieces of network, providing a path for the exchange of information between different LANs or subnetworks. A backbone can tie together diverse networks in the same building, in different...

 from analog to digital circuits. This conversion allowed AT&T to offer Dataphone Digital Services (later re-branded digital data services) that started the deployment of ISDN and T1
Digital Signal 1
Digital signal 1 is a T-carrier signaling scheme devised by Bell Labs. DS1 is a widely used standard in telecommunications in North America and Japan to transmit voice and data between devices. E1 is used in place of T1 outside North America, Japan, and South Korea...

 lines to customer premises to connect.

Leased lines were used to connect mainframe computer
Mainframe computer
Mainframes are powerful computers used primarily by corporate and governmental organizations for critical applications, bulk data processing such as census, industry and consumer statistics, enterprise resource planning, and financial transaction processing.The term originally referred to the...

s with terminals and remote sites, via IBM Systems Network Architecture (created in 1974) or DECnet
DECnet
DECnet is a suite of network protocols created by Digital Equipment Corporation, originally released in 1975 in order to connect two PDP-11 minicomputers. It evolved into one of the first peer-to-peer network architectures, thus transforming DEC into a networking powerhouse in the 1980s...

 (created in 1975).

With the extension of digital services in the 1980s leased lines were used to connect customer premises to Frame Relay
Frame relay
Frame Relay is a standardized wide area network technology that specifies the physical and logical link layers of digital telecommunications channels using a packet switching methodology...

 or ATM
Asynchronous Transfer Mode
Asynchronous Transfer Mode is a standard switching technique designed to unify telecommunication and computer networks. It uses asynchronous time-division multiplexing, and it encodes data into small, fixed-sized cells. This differs from approaches such as the Internet Protocol or Ethernet that...

 networks. Access data rates increased from the original T1 option up to T3 circuits.

In the 1990s with the advances of the Internet, leased lines were also used to connect customer premises to ISP
Internet service provider
An Internet service provider is a company that provides access to the Internet. Access ISPs directly connect customers to the Internet using copper wires, wireless or fiber-optic connections. Hosting ISPs lease server space for smaller businesses and host other people servers...

 Point of Presence
Point of presence
A point of presence is an artificial demarcation point or interface point between communications entities. It may include a meet-me-room.In the US, this term became important during the court-ordered breakup of the Bell Telephone system...

's whilst the following decade saw a convergence of the aforementioned services (frame relay, ATM, Internet for businesses) with the MPLS
Multiprotocol Label Switching
Multiprotocol Label Switching is a mechanism in high-performance telecommunications networks that directs data from one network node to the next based on short path labels rather than long network addresses, avoiding complex lookups in a routing table. The labels identify virtual links between...

 integrated offerings.

Access data rates also evolved dramatically to speeds of up to 10Gb/s in the early 21st century with the Internet boom and increased offering in long-haul optical networks or Metropolitan Area Network
Metropolitan area network
A metropolitan area network is a computer network that usually spans a city or a large campus. A MAN usually interconnects a number of local area networks using a high-capacity backbone technology, such as fiber-optical links, and provides up-link services to wide area networks and the...

s.

Applications


Leased lines are used to build up private networks, private telephone networks (by interconnecting PBX's) or access the internet or a partner network (extranet
Extranet
An extranet is a computer network that allows controlled access from the outside, for specific business or educational purposes. An extranet can be viewed as an extension of a company's intranet that is extended to users outside the company, usually partners, vendors, and suppliers...

).

Here's is a review of the leased line applications in Network designs 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
DECnet
DECnet is a suite of network protocols created by Digital Equipment Corporation, originally released in 1975 in order to connect two PDP-11 minicomputers. It evolved into one of the first peer-to-peer network architectures, thus transforming DEC into a networking powerhouse in the 1980s...

, and with TCP/IP in University and Research networks before the Internet became widely available. Note that other Layer 3
Network Layer
The network layer is layer 3 of the seven-layer OSI model of computer networking.The network layer is responsible for packet forwarding including routing through intermediate routers, whereas the data link layer is responsible for media access control, flow control and error checking.The network...

 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
Time-division multiplexing
Time-division multiplexing is a type of digital multiplexing in which two or more bit streams or signals are transferred apparently simultaneously as sub-channels in one communication channel, but are physically taking turns on the channel. The time domain is divided into several recurrent...

, 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 (specially 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 telcos
Telephone company
A telephone company is a service provider of telecommunications services such as telephony and data communications access. Many were at one time nationalized or state-regulated monopolies...

 started to build more advanced network using packet switching
Packet switching
Packet switching is a digital networking communications method that groups all transmitted data – regardless of content, type, or structure – into suitably sized blocks, called packets. Packet switching features delivery of variable-bit-rate data streams over a shared network...

 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.

International Private Lease Circuit


An International Private Leased Circuit (IPLC) functions as a point-to-point private line. IPLCs are usually Time-division multiplexing
Time-division multiplexing
Time-division multiplexing is a type of digital multiplexing in which two or more bit streams or signals are transferred apparently simultaneously as sub-channels in one communication channel, but are physically taking turns on the channel. The time domain is divided into several recurrent...

 (TDM) circuits that utilize the same circuit amongst many customers. The nature of TDM requires the use of a CSU/DSU
CSU/DSU
A CSU/DSU is a digital-interface device used to connect a Data Terminal Equipment device or DTE, such as a router, to a digital circuit, such as a T1 line. The CSU/DSU implements two different functions...

 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
Point of presence
A point of presence is an artificial demarcation point or interface point between communications entities. It may include a meet-me-room.In the US, this term became important during the court-ordered breakup of the Bell Telephone system...

. 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).

In the United Kingdom


In the U.K.
Telecommunications in the United Kingdom
Until 1982, the main civil telecommunications system in the UK was a state monopoly known as Post Office Telecommunications. Broadcasting of radio and television was a duopoly of the BBC and Independent Broadcasting Authority : these two organisations controlled all broadcast services, and...

, leased lines are available at speeds from 64 kbit/s increasing in 64 kbit/s increments to 2.048 Mbit/s over a channelised E1
E-carrier
In digital telecommunications, where a single physical wire pair can be used to carry many simultaneous voice conversations by time-division multiplexing, worldwide standards have been created and deployed...

 tail circuit and at speeds between 2.048 Mbit/s to 34.368 Mbit/s via channelised E3
E-carrier
In digital telecommunications, where a single physical wire pair can be used to carry many simultaneous voice conversations by time-division multiplexing, worldwide standards have been created and deployed...

 tail circuits. The NTE
Network termination
A network termination is a device that connects the customer's data or telephone equipment to a carrier's line that comes into a building or an office...

 will terminate the circuit and provide the requested presentation most frequently X.21
X.21
X.21 is an interface specification for differential communications introduced in the mid 1970s by the ITU-T. X.21 was first introduced as a means to provide a digital signaling interface for telecommunications between carriers and customers' equipment...

 however higher speed interfaces are available such as G.703
G.703
G.703 is an ITU-T standard for transmitting voice or data over digital carriers such as T1 and E1. G.703 provides specifications for pulse code modulation .G.703 also specifies E0...

 or 10baseT. Some ISPs however use the term more loosely, defining a leased line as “any dedicated bandwidth service delivered over a leased fibre connection".

In the United States


In the U.S.
Communications in the United States
The primary regulator of communications in the United States is the Federal Communications Commission. It closely regulates all of the industries mentioned below with the exception of newspapers and the Internet service provider industry.- Press :...

, 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
Digital Signal 1
Digital signal 1 is a T-carrier signaling scheme devised by Bell Labs. DS1 is a widely used standard in telecommunications in North America and Japan to transmit voice and data between devices. E1 is used in place of T1 outside North America, Japan, and South Korea...

bearer circuit with 1 to 24, 56k or 64k timeslots
Time division multiple access
Time division multiple access is a channel access method for shared medium networks. It allows several users to share the same frequency channel by dividing the signal into different time slots. The users transmit in rapid succession, one after the other, each using its own time slot. This...

. Customers must manage their own network termination equipment—Channel Service Unit and Data Service Unit (CSU/DSU
CSU/DSU
A CSU/DSU is a digital-interface device used to connect a Data Terminal Equipment device or DTE, such as a router, to a digital circuit, such as a T1 line. The CSU/DSU implements two different functions...

).

In Hong Kong


In Hong Kong
Communications in Hong Kong
A wide-ranging and sophisticated communications network has developed in Hong Kong, reflecting its thriving commerce and international importance....

, leased lines are usually available at speeds of 64k, 128k, 256k, 512k, T1 (channelized
Channelized
Channelized in a telecommunications environment means that the line that communications have been transmitted over contains more than one message thread, separated in some fashion....

 or not) or E1 (less common). Whatever the speed, telcos usually provide the CSU/DSU and present to the customer on V.35 interface.

In India


In India
Communications in India
The Republic of India possesses a diversified communications system that links all parts of the country by Internet, telephone, telegraph, radio, and television...

, leased lines are available at speeds of 64 kbit/s, 128 kbit/s, 256 kbit/s, 512 kbit/s, 1 Mbit/s, 2 Mbit/s, 4 Mbit/s, 8 Mbit/s, 16 Mbit/s T1(1.544 Mbit/s) or E1(2.048 Mbit/s). Customers are connected either through OFC, telephone lines ADSL, or through Wifi. Customers would have to manage their own network termination equipment, namely the Channel service unit
Channel service unit
In telecommunications, a channel service unit is a line bridging device for use with T-carrier that:*is used to perform loopback testing,*may perform bit stuffing,*may also provide a framing and formatting pattern compatible with the network,...

 and Data service unit
Data service unit
A data service unit, sometimes called a digital service unit, is a piece of telecommunications circuit terminating equipment that transforms digital data between telephone company lines and local equipment. The device converts bipolar digital signals coming ultimately from a digital circuit and...

.
Most service providers give 99% uptime guarantee.

In Italy


In Italy
Communications in Italy
Telephones - main lines in use:20.031 million Telephones - mobile cellular: 88.58 million Telephone system: modern, well-developed, fast; fully automated telephone, telex, and data servicesdomestic:...

, leased lines are available at speeds of 64 kbit/s (terminated by DCE2 or DCE2plus modem) or multiple of 64 kbit/s from 128 kbit/s up to framed or unframed E1 (DCE3 modem) in digital form (PDH service, known as CDN, Circuito Diretto Numerico). Local TELCOs also may provide CDA (Circuito Diretto Analogico), that are plain copper dry pair between two buildings, without any line termination: in the past (pre-2002) a full analog base band was provided, giving an option to customer to deploy xDSL technology between sites: nowadays everything is limited at 4 kHz of bearer channel, so the service is just a POTS
Plain old telephone service
Plain old telephone service is the voice-grade telephone service that remains the basic form of residential and small business service connection to the telephone network in many parts of the world....

 connection without any setup channel.
For many purposes, leased lines are gradually being replaced by DSL and metro Ethernet
Metro Ethernet
A Metro Ethernet is a computer network that covers a metropolitan area and that is based on the Ethernet standard. It is commonly used as a metropolitan access network to connect subscribers and businesses to a larger service network or the Internet...

.

Leased line alternatives


Leased lines are more expensive than alternative connectivity services including (ADSL, SDSL
Symmetric Digital Subscriber Line
Symmetric digital subscriber line can have two meanings:* In the wider sense it is a collection of Internet access technologies based on DSL that offer symmetric bandwidth upstream and downstream...

, 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
Carrier Ethernet
Carrier Ethernet is a marketing term for extensions to Ethernet to enable telecommunications network providers to provide Ethernet services to customers and to utilize Ethernet technology in their networks....

-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.

See also


  • Circuit ID
    Circuit id
    A circuit ID is a company-specific identifier assigned to a data or voice network between two locations. This circuit, for example a tie line, is then leased to a customer by that ID...

  • Dedicated line
    Dedicated line
    In computer networks and telecommunications, 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....

  • Dry pair
  • Private line
    Private line
    In wired telephony, a private line or tie line is a service that involves dedicated circuits, private switching arrangements, and/or predefined transmission paths, whether virtual or physical, which provide communications between specific locations. Most private lines connect only two locations...

  • Tie line
    Tie line
    A tie line, also known as a tie trunk, is a communication connection between extensions of a private telephone system, typically two PBXes.-References:...