This technology is made possible by the placement of DSLAMs, or Digital Subscriber Loop Access Multiplexers, from such suppliers as Alcatel and Cisco, in the telco's Central Office, or sometimes a suitable remote location. DSLAMs come in various shapes and sizes, and are the one, single complex and costly component of a DSL connection. When a qualified phone line is connected to a modem at the user's end of the loop, a high speed digital connection is established, typically over ATM, or sometimes frame relay. The DSLAM splits the signal back into separate voice and data channels. The voice channel stays within the telco network, whereas the data is picked up by an ISP (typically).
Command-> show dslstatus --- Channel Info ATU-R ATU-C Current TX Rate - 384000 1500000 Previous TX Rate - 0 0 CRC Block Length - - - Interleave Delay - - - --- Physical Layer Info ATU-R ATU-C Current Attainable Rate - 448433 3890243 Current SNR Margin - 10.5 17.0 Current Attenuation - 54.5 31.5 Current Output Power - 3.0 16.0 Current Status: Defects detected - No No Loss of Framing - No Loss No Loss Loss of Signal - No Loss No Loss Loss of Power - No Loss No Loss Loss of Signal Quality - No Loss No Loss --- ATU-R Line Status Line Coding - DMT Line Type - Fast or Interleaved Command-> |
For Linux users, the modem is a very important consideration! There are many modems supplied by ISPs that are not Linux compatible. Your best bet is an external, ethernet interfaced modem (or modem/router combo) that connects via a standard ethernet NIC, since many other modem options (PCI, USB, onboard) will not work due to a lack of drivers at this time! All ethernet based modems will work fine. (See the Modems Section for an up-to-date list of compatible modems.) ISDN users will need a modem (NT) designed specifically for DSL over ISDN.
With ethernet modems, the only potential compatibility issue is the Network Card (NIC). (And really any compatible ethernet NIC should do just fine -- 100 Mbps is not even necessary.) You are probably better off anyway, since PCI and USB modems can be more problem prone. If your chosen provider does not offer a compatible modem as an option, then you either need to look elsewhere, or you will have to buy one outright from a third party.
As always, there are exceptions. Xpeed now has drivers for two PCI modems included with the kernel drivers (as of 2.2.18, not in 2.4 yet though AFAIK). These are the first open source Linux DSL modem drivers, and is welcomed news. Alcatel's ADSL SpeedTouch USB modem now has Linux drivers. And more recently, the Eci Hi Focus ADSL USB Modem has drivers (and some related chipsets are supported as well, see http://eciadsl.sourceforge.net/). IteX PCI ADSL modems, based on the Apollo chipset, have Linux drivers. (Modems using this chipset are sold under a number of various brand names.) Diamond also makes [made?] an internal PCI modem which has binary-only drivers, but it is not in widespread use, and seems to be discontinued at this point. It is also possible to make a direct ATM connection using a modem plus an ATM network card, though this delivery system is not used in the U.S. as far as I know, and should not be considered as a viable option. This would also require a 2.4 kernel.
The most common type of modem in use today is actually a combination "bridge" and modem device. The bridge is a simple device, typically with little configuration. Network traffic passes blindly across the ATM to ethernet bridge in either direction. Your point of exposure is the interface (typically a NIC) that is connected to the modem/bridge.
Some ISPs are also offering "routers". These are basically combination modem/routers that can handle NAT, and may have other feature enhancements such as port forwarding, a built in hub, etc. These are all external, so should work too. But probably not a big deal for Linux users, since Linux can do anything these do, and more. A locked down Linux box makes a most excellent firewall/gateway/proxy!
To confuse things even more, there are also all-in-one devices: combo bridge+router+modem, sometimes called "brouters". In this case, the modem can be configured for either bridged or routed modes -- but it can't be both at the same time.
All providers should make available a modem of some sort. Many ISPs will have more than one modem option. Some may give away the modem at no additional charge. Some may offer a free base model, and charge the difference for the better models with more features. Many of the modems that ISPs supply are not available through normal retail channels. Should you want to buy one yourself, this leaves used equipment outlets (e.g. ebay), or possibly buying a modem that your ISP may not support (i.e. a possibility of no tech support if you have a problem).
While some ISPs provide modems that are not readily available through normal retail channels, there are a number of manufacturers that are getting on the DSL modem bandwagon, and offering a good selection. Most have a number of enhancements. At this time Alcatel (now owned by Thomson), Intel, Zyxel, Cisco, 3Com, and Cayman have products available. Depending on model and feature set, prices range from a little over $100 US to $800 and up. Many of these handle their own authentication and encapsulation (DHCP, PPPoE, etc).
Are some modems better than others? Short, easy answer: no. Modems are not much of a factor in speed in most cases. But some do have enhanced features, such as diagnostics or the combo modem/routers. Ethernet modems are generally considered the most reliable. Fewer IRQ hassles, no buggy drivers, etc. So the fact that Linux users are mostly relegated to ethernet modems is a blessing in disguise really. Are any of these better than others? Hard to say since most of this is so new there is not enough of a track record to compare brands and models with any degree of assurance. In other words, any old external, ethernet modem should do -- provided it matches your provider's DSL, and is configured for that service. Remember, there can be differences here.
![]() | Make sure any third party modem or router you may purchase is compatible with your DSL provider. There are two major line encodings for ADSL (CAP and DMT a.k.a. Alcatel compatible), and several options for IP encapsulation. And different DSLs (SDSL, IDSL, etc) will require their own modems too, as will ISDN lines. Your provider should have a list of compatible options. It may well have to be configured for your ISP's service too. Don't expect it to work right out of the box either (unless it does indeed come from your provider). Many are accessible via telnet, or a web browser, where the configuration options are available. See the owner's manual for this. |