DSL Filter, SDSL, ADSL, Advantages, ADSL Router

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What is a DSL Filter and How DSL Filters Work

A DSL filter is an analog device that improves a DSL connection by minimizing telephone signal interference with the DSL system. When installed, it acts as a barrier to prevent low frequency telephone signals to interfere with the high frequency ADSL system and vice versa.

How DSL Filters Work

Telephone signals usually range from 300 to 3400 hertz, while ADSL systems use frequencies between 25 KHz and 1.1 MHz to carry fast data traffic. Both signals are present and they pass through the same copper wire. With both signals working at the same time in the same line, interference is possible causing problems for ADSL (low speed) and telephone (line noise) connections.

A DSL filter is usually just a small plastic box with a plug which goes to your phone socket. It has two outputs, one for your DSL modem and another for the telephone. By filtering and sending the right frequencies to the appropriate sockets, a DSL filter reduces interference problems. It also maximizes the 1.1 MHz frequency capacity of the copper line by attenuating the tendencies of the two signals interfering with each other.

Installing DSL filters does not require a technician's help. More often than not, most companies send the micro-filters with the DSL modem directly to the end user. To install a DSL filter, you have to take the steps listed below.

1. Plug your phone and modem into the RJ11 jack.

2. Switch on the DSL modem and see if your computer will connect to the Internet.

3. Check your phone for a dial tone.

The methods for installing more than one DSL filter are listed below:

1. Unplug other phones in your house before installing the DSL filters.

2. Immediately check for a dial tone on each phone after installing the filters.

What is Symmetric DSL

Symmetric DSL, more commonly known as SDSL, is one kind of Digital Subscriber Line (DSL).

SDSL provides bandwidth between 72kb/s and 2,320kb/s.

Symmetric DSL is called "symmetrical" because the upstream and downstream connections have the same bandwidth.

SDSL requires only one pair of wired copper line to support a connection. These wires may reach as long as 3 kilometers.

Unlike Asymmetric DSL, Symmetric DSL uses the entire available bandwidth of the two copper wires. This means that regular analog telephone service through the PSTN is not possible over the same wires.

Symmetric DSL is a type of DSL technology which focuses on rate adaptation.

SDSL is usually more expensive than ADSL, but less expensive than a T1/E1 leased line. SDSL outperforms ADSL for dedicated data connections where upstream and downstream traffic is similar.

Symmetric DSL is slowly being replaced with G.SHDSL (Single-Pair High-speed Digital Subscriber Line).

What is ADSL

ADSL is the acronym for Asymmetric Digital Subscriber Line. ADSL is a loose set of protocols that allows for high speed internet access over normal copper telephone lines or what is more commonly known as POTS (Plain Old Telephone Service). This is possible because the signals are sent digitally instead of through analog waves. ADSL is known as asymmetric because the download and upload speeds are not symmetrical with download speeds being averagely faster than upload speeds. Upstream data speeds are lower because requests for web pages normally do not require a lot of bandwidth.

ADSL provides internet access that is constantly on unlike dial up phone access. That way, your computer remains connected to the internet when it is powered on unless the cable is manually disconnected. ADSL allows for the simultaneous use of normal telephone services (voice), Integrated Service Digital Network (ISDN) and high speed data transmission such as video. ADSL provides much higher bandwidth than traditional dial up connections. Speeds can range from 512Kbps to 9 Mbps.

Advantages of ADSL

1. Unlike traditional dial up connection where one session could only be used by one user, ADSL allows several users to share one account meaning the cost can be split between multiple users.

2. ADSL uses standard telephone lines for digital transmission thus setting the analog transmission signals apart from the digital. This allows normal usage of the telephone facility even as you are browsing the internet.

3. Allows for high speed internet access without the cost of ISDN.

4. ADSL 2 and ADSL 2+ have the advantage that they offer several improvements to ADSL. Some of these improvements include faster data transmission of up to 20 Mbps, dynamic data rate adaptation, standby power saver, better resistance to noise etc.

5. ADSL transfers data digitally and digital data has a higher noise tolerance than analog data.

How ADSL works

ADSL makes use of your existing telephone line. It splits the line into two distinct channels, one for data connection and the other for voice. The ADSL signal requires 2 special modems. One is used at your end while the other is used in the telephone exchange. Each modem is geared to a different frequency. Your telephone line is prepared for ADSL by opening up the copper pair to high frequencies and then routing the digital data in these frequencies to a Digital Subscriber Line Access Multiplexer (DSLAM) which converts this signal to ATM packets. The signal is then sent back to the servers and the server then forwards the request and assigns an IP address to the client.

ADSL modulation exists in two schemes. One is Carrierless Amplitude (CAP) and the other is Discrete Multi-Tone (DMT). These modulation schemes allow data transmission over high frequencies thus increasing internet access speed. The two schemes are used for both upstream and downstream data transmission. However, most modern installations are based on the DMT modulation scheme. The idea behind DMT is to split the bandwidth into a large number of sub channels and allocate data so that the throughput of every single sub channel is maximized.

What is an ADSL Router

Definition

An ADSL router is also known as a DSL modem. The router is used to connect the computer to the DSL phone line for using the ADSL service. Some countries also use the term NTBBA, which stands for Network Termination BroadBand Access. There are some ADSL routers that are also capable of sharing a single Internet connection with a group of computers on a network. This system is also known as the residential gateway.

The ATU-R

Every ADSL router has a functional block called ADSL Terminal Unit-Remote, or the ATU-R (transceiver). The ATU-R is responsible for functions like demodulation, modulation, and framing. There are other functional blocks as well that perform specific functions like IP routing and bridging. The interfaces for the ADSL router are either Ethernet or USB. The ADSL modem might have been assigned an IP address from the beginning for management purposes, though an ADSL router that works as a bridge does not need an IP address.

Router Placement

The ADSL router in most cases is not placed inside the computer. It is connected to the computer's port, like the USB port or Ethernet. Voiceband modems, on the other hand, are placed inside the computer. The Windows operating systems, as well as other operating systems, do not recognize the ADSL router. There is no property sheet or an internal method to manage them. The reason is that the computer and the transceiver are separate nodes in the LAN. The transceiver (ADSL modem) is not controlled by the computer, unlike the keyboard and mouse.

Configuration

The ADSL router can be configured manually by opening a Web page in the browser. Some routers require no configuration because they are incorporated into the physical layer of the network. The frequencies of the router range between 25 kHz and 1 MHz. Hence, it does not interfere with the voice service, whose bandwidth ranges between 0 and 4 KHz. Hence, you can talk on the phone even if you have powered on the router and are using the Internet. Voiceband modems, on the other hand, work on the same frequency as the telephone; hence, they might interfere with the voice service.

Speed

The speed of the ADSL router varies and depends on the plan you have purchased from your ISP. Speeds vary from hundreds of kilobits per second to megabits per second. The ADSL router exchanges data with the wired DSLAM that is connected to the Internet. The router is configured for particular protocols only and might not work even on another line in the same house or company.

Hardware Components

The hardware components of the ADSL router are a transformer and a data connection such as Ethernet or USB. Also required are the digital data pump, a line driver, a filter, and a micro controller.