DSL Technology (Digital Subscriber Line)
As mentioned in the previous entity for the use of broadband services, it is necessary to have high speed transmissions. The DSL Technology (Digital Subscribing Line) constitutes a point to point (point--point) connection between the user and the telecommunications company that allows the transport of data with high speeds, using the already existing telephone lines.
This possibility is very important, because with the exploitation of existing cupreous network that is used by all other countries for the cover of their telecommunications needs. DSL technology provides not only services of telephony and ISDN (services of narrow area) but also services of wide area zone.
The copper crosswired twisted pairs that were initially placed with a view to cover the existing telephone needs for decades they were used only for the transport of frequencies that corresponded to the voice.
After a lot of years went bye these cables were found almost in each house or office and constituted the basic trunke of transport of telephone communications. The volume of signal that was transported by appliance to appliance was very small and thus was not essential to complete exploitation of the biggest capacity that it can offer to the coper to the point a lot of frequencies remained unused.

When later the need was presented for the transmission of data between the users, it was realised that the creation of new telecommunications network for the exclusive transport of data was practically unfeasable . For this reason technologies were developed and appliances were used that achieved through the existing telephone network to transport also data. Initially for the transport of data simple Modems (Picture 2) were used, the biggest disadvantage of this type of connections (PSTN) being that the user cannot have services of telephony and Internet simultaneously whilst the speeds of transmission are very low.

These problems were overcome to a degree with connections B-ISDN, which allow the users to use their telephone appliance while, they are connected in the Internet and can have a speed of transmission 64 or 128 Kbps. However the technology that really allows the better exploitation of breadth of area of frequencies, that provide the copper cables,is the DSL.

With this technology, two special modems are placed at the both ends of the line and the simple telephone cable gets transformed to a channel of digital communication of big breadth of area that allows the transmission of voice and data simultaneously on the same line. Depending on the technique of configuration of signal and the type of modem that will be used a different speed is achieved while the transmission of data is realised entirely digitally.

While trying to explain simply how all this is achieved we should remember that the sound of human voice oscillates in a breadth of frequencies between 100Hz and 4.000Hz, without the transmission of all of this spectrum of frequencies being necessary in order for the voices conversing to become comprehensible. The DSL technology succeeds in the segregation of the signal in two channels of frequencies and uses a part of the breadth of area for the transport of proportional signal of (voice) and the rest for the transmission of data. That is to say the frequencies under the 4 kHz that are used for the transmission of voice and the frequencies above the 4 KHz that are used for the transmission of data, resulting to being able to use telephone and network services simultaneously.

In DSL technology various variants are presented mainly have to do with the speed of reception and transmission of data. In the case, where the speed of transmission and reception are both the same, we then have the symmetric variants, while asymmetrical are named those where the speed of transmission is different from the speed of reception. That is to say in the case of assymetric variants the speed the user receives the data is different from the speed with which it can be dispatched.

We mainly use the asymmetrical variants in cases where the users want to have fast access on the Internet and execute network applications with which they mailny transport data from the network in their PC (eg they usually download Video, programs, and they like hear music via the internet etc). The symmetric variants are better for the cases where the computers are used for the hospitality of web pages, in applications of videoconference, databases, IP telephony e.t.c.

Asymmetrical variants of technology DSL are: ADSL, G.lite, RADSL and VDSL while the symmetric variants of DSL include: SDSL, SHDSL, HDSL, hdsl-2 and IDSL (Table 1). The basic characteristics of xDSL technologies are presented in the next table, while the ¨x¨ is used so that we declare the existence of crowd of different variants where each one has different specifications and has been developed for the cover of different needs.

Table 1 (xDSL Technologies)

ADSL Technology: (Asymmetric Digital Subscriber Line)
The Asymmetrical Digital Subscribing Line of complete rythm of transmission that certain times we can meet with Greek characters as A.RY.S. (Assymetry Connection Rythm) is the most widespread xDSL variant. This technology is determined by the standards ITU-t Recommendation G.992.1 and the ANSI Standard T1.413-1998. Then and concretely in 2002 the International Union for Telecommunications (ITU) completed two new models of technology ADSL the G.992.3 and G.992.4, that are together named ADSL2 while in 2003 was developed the model G.992.5 that is more widely known as ADSL2plus or ADSL2 +.

One of the advantages of ADSL technology is the continuous access that the users have on the Internet, since their ADSL modem is exclusively connected to a corresponding modem on the side of supplier. More concretely on the local telephone centre of GREEK TELECOM (OTE) or in the local node of each supplier that offers access in network DSL technology, exists one unit that is named DSLAM (DSL Access Multiplexor) and there we can find the ADSL modems that are connected to the users. With the transformation of a line to ADSL is also committed simultaneously to an ADSL modem in the DSLAM. In this unit, that in practice constitutes of the multiplexer/demultiplexer of the digital subscribing lines,when the data of users from subscribing DSL lines reaches their destination there is a segregation of voice from the data and there is a promotion in the network of trunke of high speeds (usually ATM).

In Greece the particular technology is still not available for all and for that reason the user that wants to acquire Asymmetrical Digital Subscribing Line,will initially have to determine whether an technology ADSL is available in his region. For the Wider region of Municipality of Ialysos there is no problem of connection to an ADSL network, since this technology is available for a long time. The candidate user is therefore supposed to make an application for the transformation of his telephone line to ADSL, to get the special terminal equipment, and then to address to the Supplier of Internet Services (OTEnet, Forthnet, Tellas, HOL, |a|c|n|, Vivodi, SparkNet, Acis...) for the acquisition of ADSL connection.

Depending on the speed of access that he wants he must select the appropriate parcel of connection and in the case of ADSL the entire breadth of zone is exclusively used by him, the rythm of transmission that is achieved (Table 2)can then be is determined to a large extent by the Local Loop, the distance that is to say between the point that the user intends to install the connection of the nearest dslam supplier of telecommunications line.

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Relevant Links :

http://www.dslreports.com
http://www.dslforum.org
http://www.adslgr.com