Asymmetric Digital Subscriber Line

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Asymmetric Digital Subscriber Line


INTRO

If I say "Asymmetric Digital Subscriber Line " ; most of us wonder what it is, but if I say "ADSL" almost everyone of us know it. That's it, we will discuss what ADSL is what are the features which has.

  • Broadband is new generation high speed access to the internet.



Early days it used to be accessed by using dial up access with a 56k modem. But by the end of 20th century those modems were tended to drop by the customers because of its low bit rate (download up to 56kbps). Then the technology of DSL (Digital Subscriber Line) came to the picture. Although the speed of DSL is higher, the speed basically depends on 2 facts; technology available at the time & the applications used by the country. But the specialty of 56k modem & other later technologies use data over PSTN. It means data & voice come up with in the same copper line used in PSTN, & also both data & voice can occur simultaneously.


| DSL (Digital Subscriber Line) |

DSL is a family of technologies for digital data transmission which uses the existing telephone cables. Then you may wonder how this voice & data transmission occur simultaneously. The solution is simple, DSL uses the bandwidth apart from voice (which is usually 0-4kHz)

DSL families

Family up-link speed(max) down-link speed(max)
ADSL (Asymmetric DSL) 1 Mbps 8 Mbps
S Lite 512 kbps 1.4 Mbps
RDSL (Rate Adaptive DSL) 784 kbps 4 Mbps
SHDSL(Symetric speed DSL) 2 Mbps 2 Mbps
VDSL (Very high speed DSL) 1.5 Mbps 52 Mbps


WHY ADSL?

Most customers demand for web browsing,audio & video on demand & download related facts , not much in up link based applications. So with the use of the technology needed we go for ADSL. It assures that it is more than enough for the ordinary users, not in the co-operate level users.

  • LET'S GET IN......

First we should look up the word ADSL.

Asymmetric: usually downstream is faster than upstream
Digital: data is digital & only at the end modulated to be carried over the line.
Subscriber Line: data is carried over a single pair copper loop (existing telephone line)


  • Why ADSL became popular?
1. Voice & data services can be achieved at the same time.(But in previous dial-up modems at a time one service can be obtained.)

2.High speed with compared to dial-up modems(max of 56kbps) & CDMA(max of 156 kbps)

3. Connectivity to more than single computer.

4.No need to share the bandwidth, it is dedicated. (For example consider a 3G service provider a service of 3 Mbps & there are 50 users in a BTS. So every user has to share the present bandwidth which means single user will have a speed of 3000/50= 60 kbps)

5. Uninterrupted service (always on).


  • How it is possible to deal with voice & data at the same time?
Usually a bandwidth of 0-4kHz bandwidth is used for voice.(Human audible range is 20Hz- 20kHz ). So we need to use the bandwidth apart from this range, exactly that is used in ADSL.

0-4 kHz :for PSTN,voice(theoretically its zero but here indicates 30 Hz)
4-25 kHz : "Guard Band" to avoid interfere of voice to data & vise versa. 25kHz is the minimum frequency to ensure no interference.
25-138 kHz : for upstream.
138-1104 kHz: for downstream.

There the word asymmetric comes to the picture.


  • Dependent factors of ADSL speed
  1. Distance of the subscriber from the central office.
  2. Guage of the wire & its condition.
  3. Modulation technique used by DSL modulator. & so on.
- line attenuation
- SNR(Signal to Noise Ratio)

SNR= 10 log (Ps/PN)
Ps : Signal Power
PN : Noise Power

* If attenuation increases Ps decreases & also if noise increases PN increases. both will reduce the SNR. But good SNR is chosen to transmit ADSL signal. Typically for 32dB attenuation & 28dB SNR, maximum of 8 Mbps download speed can be obtained. We almost gain 512 kbps maximum download speed, for that 60dB attenuation & SNR of 10dB will be there.
  • ADSL customer premises equipments
- ADSL modem
- Computer
- Telephone
- Splitter
- Ethernet/USB cable
- Telephone cable(RJ 11)
- Micro filter (opt.)

Modem
modem= mo + dem
mo : modulator
dem : demodulator
it does both functions.

Splitter
- Splitter consists of 3 ports.
- Input port is telephone line which is coming from the rosset.
- It consists low pass filter, center frequency at 4kHz, which is used to filter voice signals.
- Some consists of a high pass filter , center frequency at 4kHz to split out ADSL signals or some does just pass all of the incoming signals throught the next output port.


  • ADSL Router and ADSL Modem?
- Modems perform both modulation and demodulation while router performs roting process alone with the processes performed by modem.
- Modems provide USB connectivity while router provides Ethernet(&/or USB) connectivity.
- Single PC can be connected to modem while router can be used to connect more than one PC.
-W-LAN/LAN can be established using a router while modems cannot perform that.

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