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ADSL - FAQs
Q: What Is ADSL?
A: Asymmetric Digital Subscriber Line or ADSL has become a generic term to imply a high-speed data connection for Internet/Intranet Access or Video Broadcast over a conventional telephone line. In reality ADSL is but one incarnation of a whole family of xDSL standards and products designed to provide high data rate communication over the final `copper loop' of the PSTN network.
DSL is designed to provide a high-speed data connection for generic data access to your local exchange typically with data rates up to about 2Mbps but with some variants capable of data rates as high as 50Mbps or more. The end user can make use of this connection only when a service provider (not necessarily the xDSL `carrier' service provider) provides server equipment in your local exchange to which the end-user line can be connected (and obviously the hierarchy of high speed connections to get the data to and from the Internet or other wider accessible service).
To date there are many forms of DSL. A few of the forms that are currently in use or development around the world are listed here:
| Service | Downstream | Upstream | |
| ADSL | Asymmetric DSL | 2M to 384k | 256k to 128k |
| HDSL | High-bit-rate DSL | 1.5M | 1.5M |
| SDSL | Single-line DSL | 1.5M | 1.5M |
| VDSL | Very-High DSL | 13M to 52M | 1.5M to 2.3M |
| IDSL | ISDN DSL | 144k | 144k |
| RADSL | Rate Adaptive DSL | 512k | 278k |
| UDSL | Universal DSL | 1M to 384k | 384k to 128k |
Q: How does ADSL Works
A: All of the xDSL variants work in the same way, just the numbers are different!
A telephone line is actually a twisted pair, with reasonably well defined frequency characteristics, capable of carrying signals up to several MHz. The nature of the line means, however, that these signals will be distorted and attenuated in seemingly random ways dependent on the length, age, number of joins etc each individual subscriber line possesses. The basic subscriber line was originally only designed to take voice data and then only a limited frequency range of it deemed acceptable for relatively low grade speech quality. This it does reasonably well up to a few kHz, occasionally with the aid of passive correction circuits.
To make use of the potential bandwidth above 4kHz, special circuitry needs to be added. The frequency characteristics above 4kHz vary considerably with frequency, however they are reasonably stable within each segment of 4kHz from 4kHz itself all the way up to at least 2MHz. In practice it has been found usable up to 6MHz (cf 10baseT) though most systems don't go much above 2MHz for distances up to 4km.
Each 4kHz chunk of spectrum is treated as a separate carrier capable of conveying a signal independently of all the other carriers. This is called Discrete Multitone modulation or DMT and is currently the most widely used modulation used for xDSL (ref ANSI Standard T1.413.(1997))
The xDSL standard provides for the frequency range 0-26 kHz to be left free for conventional analogue telephone services (PSTN POTS). The 26-1130 kHz band can then accommodate 256 channels of 4.3125 kHz each. In principle these individual carriers can be used in either direction independently or simultaneously depending on the actual xDSL standard in use and line conditions. (The ISDN variant uses 224 channels from 140-1104 kHz; 0-140 kHz is kept clear for the ISDN carrier.)
The individual channels are Quadrature Amplitude Modulated (QAM) and carry between 2 bits and 15 bits per second per Hz depending on the individual channels' characteristics. The way in which these rates are allocated is adaptive and depends crucially on the channel characteristics. During the initialisation process the individual carriers are allocated `signal spaces' depending on the noise in each corresponding channel. Discrete channels losses up to 50db can be tolerated. The larger the signal to noise ratio, the larger the signal space and thus the number of bits that can be represented by each transmitted symbol in that channel. (For the technically curious the modulations are: 128-QAM, 64-QAM, 32-QAM, 16-QAM, 8-QAM, QPSK).
Each channel in the combined signal is capable of carrying up 64.7 kbits/s. This yields a maximum capacity in excess of 16 Mbits/s (for 256 channels). In practice the higher frequency channels are attenuated and noisy giving significantly less usable bandwidth.
The Standard suggests two possible methods for channel allocation, straight forward FDM (Frequency Division Multiplexing) or ECC (Echo Cancellation and Compensation). The former reserves the first 26 channels (ISDN, 32) for `upstream' data and the remaining 230 (ISDN, 192) for `downstream' data. In the ECC method (known as category 2 ADSL), the first 26 channels are reserved for `upstream' data as before, however `downstream' data uses all 256 channels. This is possible because the data directions are independent, though a special echo cancellation processor is needed to ensure stable operation.
The ADSL Coding Process
Each end of the `copper loop' has an ADSL modem to manage the conversion of digital traffic into the analogue carriers on each channel. Incoming data (w.r.t each modem) is applied to an encoder which allocates data to a specific channel depending upon its respective data carrying capability (established during setup). and adds Forward Error Correction (FEC) using a Reed Solomon code similar to that used in digital TV. The data streams are then individually encoded onto their respective carriers using an Inverse Fast Fourier Transform (IFFT) and framing data added. The data is then serialised and converted to an analogue form using a Digital to Analogue converter (ADC). The decoding process is the exact reverse of this.
The encoding/decoding process results in a transparent digital data channel without rigorous data protection, framing standards or control protocols. For this purpose the well tried ATM (Asynchronous Transfer Mode) transport layer is used, resulting in a connection that can then take `useful' data streams such as TCP/IP or real video data.
Q: Kilobits vs. kilobytes explained
A: In data communications, a kilobit is a thousand bits (1,000) bits. It's commonly used for measuring the amount of data that is transferred in a second between two points. Kilobits per second is usually shortened to Kbps (or kbps). For example, 53.333kbps is 53,333 bits per second.
- 1 kb/s = 1000 b/s
- 1 KB/s = 1024 B/s
- 1 KB/s = 8.192 kb/s
- 1 kb/s = .1221 KB/s
The lowercase b usually stands for bits while the uppercase B stands for bytes.
To find your theoretical download speed for your line, divide your advertised speed by 8.192:
53.333kbps / 8.192 = 6.51 KB/s
or by 10 to include overhead:
53.333kbps / 10 = 5.33 KB/s
Q: I think I have spyware on my system. How Can I Get Rid of It?
A: You can download a program called Spybot Search & Destroy here. It is a good search program. It has a major database of what to search for.
You can also download a program called Ad-aware here. Ad-Aware is the pionner of Spyware/Adware cleaning software but lost publicity after they did not update for 6 Months.
Q: Why would I need a Firewall?
A: A firewall allows you to closely control the traffic on your router. This allows you to prevent access to your computer from the outside world (eg. hackers) and also to prevent access to certain areas of the internet from your computer. A firewall is probably not necessary if you are running a standalone or very small network, but if you have a large network, then it is certainly advisable to setup a firewall.
Q: What is a microfilter?
A: A microfilter is a small device that sits between the phone socket and any phones/fax machines/modems you wish to connect through the ADSL line.
Q: What will happed if I have capped services and I reach my monthly limit?
Once the monthly limit is reached the service will stop. Additional capacity can be purchased by calling Rapid's sales department. Added capacity will be transferable if not used within the month to the next one.
Q: I want to share my internet among my two computers and my laptop at home and I want it to be wireless.
A: Just make sure you state the above to the sales person and they will quote you on the relevant equipment. You will not need an ADSL modem any more you will require an ADSL router. A router is a managed networking device where you can set user privileges and security restrictions. Due to the fact that configuring your router might be difficult for a non-IT skilled person, Rapid offers the option for the router to be preconfigured. All you have to do is to plug and play.