ADSL (Asymmetric Digital Subscriber Line)
E ‘characterized by asymmetric bandwidth ie the download bandwidth is different from the upload bandwidth and is a multiplexing technology that divides the bandwidth of the twisted-pair ADSL between multiple users.
Accesses internet ADSL or equivalent are considered “broadband” (or broadband), and have now almost completely supplanted the traditional type of modem dial-up , enabling speeds of up to 56 kb / s download and 48 kb / s upload speed (V.92) , and the lines ISDN of up to 128 kb / s (using dual-channel 64-kb / s) symmetrical.
ADSL is used in the so-called “last mile” or the length of telephone cable between the home user and the central phone. This stretch of the pair is normally dedicated to a single user, even though for most is in multi-pair cables with other cable pairs. One of the strengths of ADSL technologies is the ability to use them without having to change your existing telephone cables.
This result is obtained at the cost of a certain technological complexity: the physical capabilities of the transmission line are exploited to the limit, and well beyond the use for which the lines were designed. Consequently, the achievable performance depends strongly on the distance from the center, the quality of the cables, by the presence of electromagnetic disturbances along the line.
Twisted pair copper telephone was designed, and is traditionally used for voice communication, which uses frequencies up to 4000 Hertz, but actually has a bandwidth of a few MHz to exploit the available bandwidth, techniques are used frequency division multiplexing to separate the voice signal (below 4 kHz) from the traffic data (above 25,875 kHz ), and upload traffic from that download.
In the early seventies the same principle was used for the radio, the signal of which is sent using frequencies up to 450 kHz.
(NB in the case in which there is the music, remote alarms or telesoccorsi you must inform the provider with which you want to enter into a contract ADSL)
The ADSL connection is terminated by an ADSL modem in the user’s home (often incorporated into a router), and aDSLAM in the telephone exchange. separation between the speech signal and the data is done by using filters called “splitters” located at the home user and the central office. The low-frequency part of the signal is sent respectively to phones and existing telephone switches, preserving the telephone service, the high-frequency to theDSLAM. Another advantage of ADSL technology is the separation of the bands dedicated to downloading and uploading, to unlike for example the connection using V.90 protocol that being very close to the theoretical limit of a telephone channel does not allow a duplex system at maximum speed.
- On the connection between the modem and the DSLAM, the DSLAM and to the access server, you create a virtual circuit ATM .
– To select the protocol carried in ATM, you must use a method of encapsulation, VC-MUX or LLC .
– In the virtual circuit is transported PPPoA or PPPoE
– In the circuit PPP , is finally transported IP
At present, the speed of the adsl arrive in Italy to a maximum of 24Mbps while in some European countries such as France and Germany through technology VDSL2 we arrive at a symmetrical speed of 50 megabits / sec in both download and upload.
SHDSL (Single-pair High-speed Digital Subscriber Line): for those who have a need to dowload and upload the same (usually offered with business contracts) is a technology that uses a method of signal transmission over twisted copper wire type pair and allows symmetrical data transfer in and out.