A Practical Guide to ADSL

ADSL is currently one of the most common forms of broadband Internet access available to residential users. This article is intended as a guide for everyday people to help troubleshoot problems, and to get the most out of their broadband Internet connection.

What is ADSL?

ADSL stands for "Asymmetric Digital Subscriber Line." ADSL owes its popularity due to the fact that it operates over regular telephone lines (POTS). Regular telephones only use a small percentage of the available bandwidth on copper telephone lines. This leaves a huge swath of bandwidth out of the range of frequencies that humans can hear which until recently has been completely unused. ADSL takes advantage of this unused space by piggybacking a data signal over the pair (twisted pair cable) which runs from the telephone company's central office to your home, bridging the "last mile."

How Does ADSL Work?

If you have ADSL service, you will more than likely have an ADSL Modem. Although commonly referred to as a modem, a more correct term for it would be a bridge, as it joins two dissimilar networks (the telephone company's ADSL network and the Ethernet network in your home). The ADSL modem is the remote terminal component of the ADSL network, it is just one end of the connection. On the other end is a device called a DSLAM or DSL Access Multiplexer.

The DSLAM (also occasionally referred to as an ASAM or ADSL Subscriber Access Multiplexer) is generally located in your local central office. The DSLAM joins the "other ends" of the ADSL connections for everyone in your neighborhood and links them up with a big fat pipe called an ATM.

The ATM connects up to your ISP's router. The router may join several ATMs from several DSLAMs in different areas and connects to a large backbone connection to a larger ISP known as a GSP or Global Service Provider (i.e. Qwest or Genuity).

Most ADSL modems use a method known as DMT or Discrete Multi-Tone to transmit data across copper wires. DMT allows the modem to use many different frequencies to transmit data, thereby allowing for more efficient use of the bandwidth. Most of the time, your modem is not using all available frequencies to transmit the data, as it is not necessary.

The percentage of available frequencies currently allotted for transmitting data is known as the RCO or "Relative Capacity Occupance." Preferably, one's downstream RCO should be less than 50%, any higher may indicate that there is noise on the line.

Most ISPs provide devices known as microfilters along with the modem, to be installed on your telephones. The purpose of the microfilter is not only to prevent the DSL signal from "leaking" into your telephone conversations (creating an unpleasant staticky sound), but also to prevent noise generated by older/cheaply produced telephone equipment from encroaching on the swath of bandwidth allotted for ADSL and interfering with the ADSL signal. Due to the nature of these devices, they should never be placed on the modem (a common mistake), as they will actually block the signal coming to the modem. In addition, it is generally a bad idea to connect the modem through phone line surge supressors such as the ones found on power bars sold at radio shack, as they can cause intermittent service interruptions. On the other side of the coin, unfortunately, the most common reason a modem has to be replaced is due to damage from a power surge.

The measurement of noise on the line is known as the Noise Margin. Noise margin means specifically, the difference (in dB or decibels) in strength between the ADSL signal and the background noise on the line. Ideally, this number should be as high as possible. Rarely does it exceed 31 dB, and a number of less than 10 dB means there is hardly enough difference between the DSL signal and the background noise for the modem to correctly differentiate between the two.

Low noise margin can be caused by missing or defective microfilters, faulty telephone equipment in your home, a defective modem, or noise caused by outside sources such as cheap halogen lamps and stereo equipment, or problems with the telephone company's equipment. If the noise on the line is too high, the modem may drop out of "sync" (sync is when the modem has established a physical connection with the DSLAM). If your ISP provides "Rate Adaptive" DSL, noise on the line can cause the modem to drop to a lower bitrate, causing slow connection speeds. Before trying any of this, you should try disconnecting all the cables from the DSL modem for 60 seconds to see if this clears the issue, as sometimes modems will lose signal momentarily due to a power fluctuation and not reset completely, and 9 times out of 10, simply power cycling the modem will clear the issue.

Different ISPs will offer different connection speeds or bitrates. The "A" in ADSL stands for Asymmetric, which means that the "downstream" speed (the speed at which you recieve data at) is always much higher than the "upstream" speed (the speed at which you transmit data at). This works out fine since most end users recieve much more information than they send out.

The upstream bitrate provided by most ADSL providers is 128 kilobits per second, but there is a myriad of different downstream bitrates offered. The most common downstream speeds are 320 Kbits/sec (not very common, found mostly on SDSL), 768 Kbits/sec (most common, although many providers are now upgrading their 768 customers), 1.5 Mbits/sec (also more common, the speed package which most providers are currently pushing), and 3 Mbits/sec (not quite as common, sold mostly to "power users" and businesses, not offered by all providers).

Common Questions about ADSL

Why is it taking so long for you guys to turn on my DSL? Don't you just flip a switch on your end? I don't understand why there is such a holdup. Provisioning your phone line for ADSL can be a lengthy process. First it must be determined if your line can support ADSL, and what speed it can be trained at. Next, a field technician must be dispatched to your local central office to connect you to the DSLAM. The field technician may at this point connect test equipment on the line to determine if it is suitable to carry ADSL. This process may take several days, and may take longer if all the field technicians are busy, if they are waiting for a port to become free on the DSLAM, or if equipment is in the process of being upgraded. Lastly, your ISP will either ship you your modem and microfilters or dispatch a technician to your home to complete the installation.

What do you mean I'm too far from the central office? If your ISP has told you that you can't get ADSL, the most common reason is that your loop length is too large. The loop length is the measurement in feet of the pair of copper wires running from your home to the central office. Currently the practical limitations for loop length are 13,000 feet for 1.5 Mbits/sec and 18,000 feet for 768 Kbits/sec for most providers. Beyond this distance, the signal loss is too great to guarantee any kind of reliable service, so most providers will refuse to offer ADSL to customers beyond this limit.

Which is better? Cable Broadband or ADSL? Ah, the eternal question...everyone I speak to seems to be very strongly in favour of one or the other. My suggestion is to shop around. Quality of service seems to differ not only from provider to provider, but also from area to area. For example, cable subscribers in one area where the network segment has a lot of people on it may get poor performance, and it may be a better choice to go with ADSL. In another area, you may be very far from the central office and may only be able to get the lowest speed package from an ADSL provider, but may be able to get a faster speed package from a cable provider. Ask your neighbors who have broadband, check out some broadband websites such as http://www.broadbandreports.com or http://www.rbua.org (in canada) for comparisons and reviews.

Why do I need microfilters? My cousin Joe has DSL, and he doesn't have microfilters on any of his phones and it works fine. Although your service may work without filters, my advice is to install them anyways, as mentioned above, noise can interfere with your DSL signal, reducing performance. Even though it works, it will work better with microfilters correctly installed.

If I bring my modem over to my friend's house, can I use it there? Only if your friend has DSL as well. ADSL must be activated on your phone line before you can use the modem.

I have two computers, can I get a second modem and hook it up to my other computer to get on the Internet? No. ADSL is point-to-point, which means that on your line there can only be one terminal unit at each end. At one end is your ADSL modem, and at the other the DSLAM. Try purchasing a router, such as those offered by Linksys, D-Link, Netgear, and SMC. It will allow you to share a connection between many computers, you could even bring your friend's computer over and hook it up to your network and have a LAN Party.

Note: If you find any errors/omissions, feel free to msg me and I will correct them. Most of this info applies to Verizon Online, although it should carry over to most other US/Canadian ADSL providers.