1. 3.7 DIGITAL SUBSCRIBER LINE After traditional modems reached their peak data rate, telephone companies developed another technology, DSL, to provide higher-speed access to the Internet. Digital subscriber line (DSL) technology is one of the most promising for supporting high-speed digital communication over the existing local loops. ADSL ADSL Lite together called xDSL HDSL SDSL VDSL Topics discussed in this section:

3. ADSL ADSL is an asymmetric( where download speed is greater than upload speed) communication technology designed for residential users; it is not suitable for businesses who need large BW in both directions. The existing local loops can handle bandwidths up to 1.1 MHz, but the filter installed at TELCO limits it to 4KHz( which is ok for voice) by using filters. If filter is removed,then the entire 1.1.MHz is available. But ADSL data rate is not fixed. ADSL is an adaptive technology. The system uses a data rate based on the condition of the local loop line such as distance,size of cable, signaling used.

4. Figure Bandwidth division in ADSL

5. Figure Discrete multitone technique

6. We require two modems  One at customer side called DSL modem  and one at telco called DSLAM

7. Local line coming from telco,is connected to splitter, which separates voice and data. The ADSL modem modulates ( upstream) or demodulates (downstream) data using DMT.Data line has to be installed by professional. Figure ADSL modem

8. At telco site, DSLAM ( digital subscriber line access mux) is installed, which again separates voice from data, and data is sent to ISP after it is packetized. Figure DSLAM

9. ADSL Lite  Also called universal / splitter less ADSL.  Installation of splitter at customer premises and new wiring for data can be expensive. So this version was developed in which splitter is not required.  In which ADSL modem is directly plugged into telephone jack and can be connected to PC.  Splitting is done at telephone company.  ADSL lite uses 8bit/signal instead of 15 bits as in ADSL.  Max download = 1.5 Mbps  Max upload =512Kbps

10. HDSL(high speed ) Uses T-1 lines Line coding 2B1Q Requires two twisted pairs for full duplex Max download = 2 Mbps Max upload =2 Mbps Distance = 1Km SDSL(symmetric speed ) One twisted pair version of HDSL. 768 KBPS in both directions. VDSL(Very high speed ) Uses co-ax, FOC, UTP for short distances Upstream =3.2 Mbps Downstream = 25 to 55 Mbps

11. 9.11 SUMMARY OF DSL TECHNOLOGIES