Brands
3Com
Alcatel-Lucent
Allied-Telesis
Avaya
Brocade
Cisco
D-Link
Dell
Emulex
Enterasys
Extreme
Force10
Foundry
H3C
HP
Huawei
Intel
Juniper
Linksys
Marconi
McAfee
Netgear
Nortel
Planet
Qlogic
Redback
SMC
Sun
TRENDnet
Vixel
ZTE
ZyXEL

Theory behind DSL

Theory behind DSL, like many other forms of communication, can be traced back to Claude Shannon's seminal 1948 paper: A Mathematical Theory of Communication. An early patent was filed in 1987 for the use of wires for both voice phones and as a local area network.

The motivation of digital subscriber line technology was the Integrated Services Digital Network (ISDN) specification proposed in 1984 by the CCITT (now ITU-T) as part of Recommendation I.120, later reused as ISDN Digital Subscriber Line (IDSL). Employees at Bellcore (now Telcordia Technologies) developed Asymmetric Digital Subscriber Line (ADSL) and filed a patent in 1988 by placing wide-band digital signals above the existing baseband analog voice signal carried between telephone company telephone exchanges and customers on conventional twisted pair cabling facilities.

Consumer-oriented ADSL was designed to operate on existing lines already conditioned for BRI ISDN services, which itself is a switched digital service (non-IP), though most incumbent local exchange carriers (ILECs) provision Rate-Adaptive Digital Subscriber Line (RADSL) to work on virtually any available copper pair facility—whether conditioned for BRI or not. Engineers developed higher-speed DSL facilities such as High bit rate Digital Subscriber Line (HDSL) and Symmetric Digital Subscriber Line (SDSL) to provision traditional Digital Signal 1 (DS1) services over standard copper pair facilities.

A DSL circuit provides digital service. The underlying technology of transport across DSL facilities uses high-frequency sinusoidal carrier wave modulation, which is an analog signal transmission. A DSL circuit terminates at each end in a modem which modulates patterns of bits into certain high-frequency impulses for transmission to the opposing modem. Signals received from the far-end modem are demodulated to yield a corresponding bit pattern that the modem retransmits, in digital form, to its interfaced equipment, such as a computer, router, switch, etc.

Unlike traditional dial-up modems, which modulate bits into signals in the 300–3400 Hz baseband (voice service), DSL modems modulate frequencies from 4000 Hz to as high as 4 MHz. This frequency band separation enables DSL service and plain old telephone service (POTS) to coexist on the same copper pair facility. Generally, higher bit rate transmissions require a wider frequency band, though the ratio of bit rate to bandwidth are not linear due to significant innovations in digital signal processing and digital modulation methods.

December 14, 2011
Bestsellers
10GBASE-SR SFP+ 850nm 300m
SFP-10G-SR
5 out of 5 Stars! $175.00
5 out of 5 Stars!
1000BASE-SX SFP 850nm 550m
GLC-SX-MM
5 out of 5 Stars! $25.00
5 out of 5 Stars!
1000BASE-T SFP RJ45 100m
GLC-T
0 out of 5 Stars! $45.00
0 out of 5 Stars!
10GBASE-LR SFP+ 1310nm 10km
SFP-10G-LR
0 out of 5 Stars! $399.00
0 out of 5 Stars!