Data over Voice Lines

URL: http://www2.rad.com/networks/1994/modems/modem.htm
http://www.techtutorials.info/hdmodems.html
http://www.rad.com/RADCnt/MediaServer/3656_ldv-2.pdf
http://www.vocal.com/data_sheets/v925.html

Voice-grade telephone connections (standard 3 kHz voice circuits) are made either by dialing by the user or by being wired in the telephone company's central office and remain connected until the service is discontinued. The latter kind is called a private line, leased line or dedicated line. The user usually pays a fixed fee every month for this service. The leased line is a point-to-point circuit, with both end points identified to the telephone company. The user has no knowledge of where or how the circuit is routed between the two end-points, unless the telephone company is specifically requested to provide diversity for security and availability purposes.

Point-to-point circuits are cost effective for high-speed communication between two devices, but are more expensive when compared to dial-up applications where the circuit is connected and charged only for the duration of the connection. Low speed point-to-point voice-grade circuits are known in the industry as 3002 circuits. Voice-grade circuits can carry uncompressed data from the slowest speed up to 19.2 kbps. Higher speeds (up to 56 kbps) can be carried with data compression.

Digital circuits or Digital Data Services (DDS) can carry data at 2.4, 4.8, 9.6, 19.2, 56, and 64Kbps. Digital circuits are not technically voice-grade, but they can be used to carry either voice or data. Slow speed leased lines have been used extensively by utilities to provide quick connections to various facilities and devices that cannot otherwise be reached in a structured network or telecommunications scheme.

Main Features of Technology

Technology	3 kHz bandwidth telephone voice circuit
Operation	Low speed, limited by bandwidth and circuit quality
Capacity	Usually dedicated to one modem per line for most applications, but multi-drop connections to multiple RTUs are common
Coverage	As far as phone company phones lines can reach
Data rate	Up to 64kbps with special line conditioning
Power	Terminal equipment to provide
Regulatory	None required

Key Advantages

Coverage	Telephone company circuits reach most locations within the United States and can be installed rapidly
Maintenance	Telephone company provides all maintenance
Interference	Phone company usually guarantees the performance of the line
Availability	Phone company usually guarantees the availability of the line along with time to repair
Security	Each line is dedicated and not shared with others, but eavesdropping is relatively easy
Cost	Phone company is responsible for the line, so no installation and maintenance costs

Key Disadvantages

Reliability/availability	User has no control over line reliability and availability. Situation has worsened since de-regulation because a single line may have multiple carriers, which makes it difficult to trouble-shoot problems and hold one carrier responsible
Data rate	Low data rates
Cost	Recurring charges, high installation cost if remote site does not have nearby phone facility

Keywords: Physical layer

Digital Subscriber Line (DSL) Technologies

URL: http://www.dslforum.org/
http://www.t1.org/t1e1/t1e1.htm

ADSL was first standardized in 1995 by the American National Standards Institute as T1.413, and then by the ITU in 1999 as G.992.1. ADSL can transmit data at speeds up to 8 megabits per second ("Mbps") downstream and up to 640 Kbps upstream. In 1999, the ITU also standardized a lower speed version of ADSL, known as G.Lite or G.992.2. G.Lite can transmit data at speeds up to 1.5 Mbps downstream and up to 512 Kbps upstream without using special filtering equipment required by full-rate ADSL.

In 2002, the ITU standardized a new family of ADSL standards known as ADSL2 or G.992.3 and G.992.4. These standards provide numerous improvements over previous ADSL standards, including line diagnostics, power management, power cutback, reduced framing, and on-line reconfiguration. In January 2003, the ITU standardized an extension of ADSL2 known as ADSL2+ or G.992.5. ADSL2+ builds upon the ADSL2 standard by increasing achievable data rates to speeds of up to 25 Mbps upstream on phone lines as long as 3,000 feet (20 Mbps out to 5,000 feet). Reach-Extended ADSL2 (RE-ADSL2) - the new ADSL2 Annex L standard - was ratified by the ITU in October 2003. Annex L proposes new power spectral density (PSD) masks that can result in a significant increase in ADSL's reach.

The DSL Forum is a consortium of networking, computing, and service provider companies that promote the development and worldwide acceptance of the Digital Subscriber Line family of technologies.

Committee T1 is sponsored by the Alliance for Telecommunications Industry Solutions (ATIS) and accredited by the American National Standards Institute (ANSI) to create network interconnections and interoperability standards for the United States. Within T1, T1E1 is concerned with Interfaces, Power, and Protection of networks. The T1E1.4 working group (DSL Transmission) addresses high-speed bi-directional digital transport via metallic facilities. The work of this group focuses on the physical layer transceiver functionality.