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Monthly fees for digital subscriber lines are coming down, and, as they do, the service is becoming increasingly popular for private use. In this month's Connectivity, Rawn Shah discusses his first-hand experience having DSL installed in his home office, and the troubleshooting that was necessary to get the operation up and running. (2,600 words)
|By Rawn Shah|
When I first learned of DSL services years ago, I was more than anxious to replace the lethargic 56 Kbps analog modem line I was then using. I got even more excited when I learned that my home town, Tucson, AZ, was to be one of the test sites. However, we in Tucson ended up having to wait well over a year after the announcement of DSL before the service was actually available. So much for test markets.
What is DSL?
Digital subscriber line services use the high-end frequencies on a regular phone line to provide data transfer services up to 8 Mbps. There are many different types of DSL services available, and each is implemented in a different way (see my January and February 1998 Connectivity columns for more details on how DSL technology works). These services connect your computer or network to the telephone company or DSL physical line provider, then to your Internet service provider (ISP) or corporate network, and finally out to the Internet.
The physical line provider -- usually the local telephone company or a competitive local exchange carrier -- is responsible for providing the physical connection between you and your ISP. The ISP is responsible for passing your data on to the Internet. Thus, in most cases you will end up paying two bills: one for the physical line and another for Internet service.
The most popular DSL service to date is asymmetric DSL (ADSL), which offers two different transfer rates, depending upon which direction data is traveling. ADSL can transmit information downstream (from your ISP to you) at speeds of up to 8 Mbps, and upstream (from you to your ISP) at 1 Mbps. This transfer speed capability decreases as you move further away from the telephone company or DSL physical line provider.
There are two subtypes of ADSL that use different transmission technologies and are incompatible with each other: discrete multitone (DMT) and carrierless amplitude-phase (CAP) modulation. DMT is the superior service, providing up to a full 8 Mbps at distances of up to 15,000 feet from the phone company's central office (CO). To meet legal requirements of decent communications quality, most cities have multiple COs spread throughout, so you usually won't be much further from a CO than this. CAP doesn't fare as well; in fact, after 12,000 feet the rates it can support rapidly decline. At 15,000 feet with CAP ADSL, you will probably get 4 Mbps at best.
Because the technology is cheaper and carriers are able to more densely cram together CAP DSL modems in their COs, the US West territory unfortunately uses CAP ADSL. DMT modems tend to generate a lot more heat and have to be spaced further apart, leading to less dense modem racks. In a CO, every inch of space counts and greater density means reduced floor space and smaller, more crowded rooms.
Plain old telephone service runs at low frequencies, while DSL uses much higher frequencies, so a user is able to use both simultaneously with the same line. They need to be kept apart, however, and any phone lines that come before the DSL should have a microfilter installed to prevent the frequencies from interfering with one another. This also means that you can use an analog modem and a DSL modem on the same line. Some vendors even have both technologies within the same modem unit, thus providing a secondary path in case the DSL service unexpectedly fails.
Opting for a speed of 768 Kbps, I signed up for US West's MegaBit DSL services through its online Web site. US West provides several levels of data services, primarily differentiated by speed. Basic service provides 256 Kbps upstream and downstream, while the top of the line package goes up to 4 Mbps downstream, 1 Mbps upstream. The pricing, of course, follows incrementally, starting at $40 per month and going all the way up to $480 per month.
I got a better deal by choosing to go with my existing ISP, RTD Systems and Networking in Tucson (which also happened to be the company I once owned) rather than sign on with US West's USWest.net.
Any ISP that signs up to accept DSL connections has to install the proper hardware at its end-point. My local phone provider, US West, and most DSL physical line service providers use asynchronous transfer mode (ATM) to create a virtual circuit between the customer and the ISP. With ATM, all traffic going between the customer and the ISP is, for all intents and purposes, private to that circuit and does not interact with the traffic from any other DSL customer or ISP also connected to the physical line service provider. Just think of the ATM virtual circuit as a continuous line directly between the customer and the ISP.
The ISP then has to connect the signal to the Internet, and, in the most basic scenario, each computer has only one internal DSL modem connected to it. On a Windows or Macintosh system this internal modem assembles packets to send over to the ISP. Unlike your 56 Kbps analog modem, it does not convert digital bits and bytes into an analog signal, but instead passes them straight through. That's what the digital in DSL is all about.
The internal modem does, however, have to assign Internet Protocol address information to the packets. This can be a static address (e.g., 192.168.0.22) permanently assigned to that user's computer, or it can be dynamically allocated from a block of addresses to each customer as he or she connects. Fortunately, the ISP will usually provide this address to the customer. Dynamic addressing comes in handy when an ISP needs to share the block of addresses across more users, but it's most effective when users regularly disconnect from their ISP.
US West offers Selective 256, which is a nonpermanent connection to the USWest.net ISP. The service is similar to analog modems in that the user is disconnected when he or she is no longer using the network. Understandably, this is a cheaper service than one that is permanently connected at all times.
Because I have multiple computers in my home office, as well as two LANs, I wanted to give some of my computers access to the Internet directly, which meant I had to get an external DSL modem/router (a Cisco 675) to connect to one of the LANs.
US West will offer an external DSL modem if it can't support your operating system platform (as was the case with my Linux box), or if you prefer not to use an internal modem. The external modem connects to an Ethernet network interface card (NIC) -- either 3Com's Etherlink III ISA or the Etherlink XL PCI card -- that goes into your machine. Because it is much easier to get Ethernet device drivers than drivers for something as specialized as a DSL modem, the external modem makes it easier to interface with other nonsupported platforms. What's more, the 3Com Etherlink cards are probably the most popular name-brand cards available, and getting driver support for them is therefore much more likely than it is with some generic $10 Ethernet card. As a piece of advice, if you have both ISA and PCI slots, go for the PCI Ethernet card. Although it is still 10 Mbps, the Etherlink XL (3C900B) US West provides has an overall better performance than the earlier generation Etherlink III.
The package I got from US West did not include the Etherlink XL as it was supposed to, so I had to call the company up and request one. Luckily, my computer came preinstalled with an SMC Ethernet card that works just as well. I still wanted the NIC from US West, though; it's a great card, I paid for it, and if I am to expect any future support from US West, I will likely need that specific card.
By using a crossover Ethernet cable, the DSL modem connects directly to the NIC on your computer. Such cables are used when you do not have an Ethernet hub between your devices. I had to replace it with a regular Ethernet (i.e., noncrossover) cable of my own and hook the modem up to my 10/100 Mbps Ethernet hub. The computers then connect to the hub.
Finally, everything was connected properly. The DSL line went from the wall to the modem; the Ethernet connected the hub to the modem; more Ethernet ran from the hub to the computers; a serial line for modem management was connected from one Windows PC directly to the modem; and a phone line was connected from the modem to my portable phone. Elsewhere in the house, the other phones on that same line had a microfilter installed between them and the wall jack, so as to reduce any interference caused by the DSL service.
I then connected to the DSL modem over the serial line in order to configure it for my service. Typically you would connect to your ISP using the Point-to-Point Protocol (PPP), which is the same service used with analog modems. With PPP you can ensure that users are authenticated before they access the service. To do so, your ISP issues you an account and password. I programmed this account information, along with other needed parameters, into the DSL modem in accordance with the instructions provided by US West. Next, I installed the supplied Windows software to connect, disconnect, and monitor the service.
With everything in place I made my first attempt to connect to the service. The modem began looking for its remote counterpart at the ISP -- and continued to simply sit there, searching. I called a friend over at my ISP (hats off to you, Mike!) and checked to see if everything over there was configured properly. After a little deliberation, we determined that my ISP was all set, but there was still no go on my side.
I made a tech-support call to US West and a helpful customer service representative and I went through the entire connection and debugging process. Still nothing. He then told me that he could not send out a person to check the condition of the line until the day after the service was first connected, as per standard procedure. I attempted every now and then to connect that night, with no luck. I even switched the line going into the DSL modem from the secondary phone line (where it was supposed to be) to my main line -- I have two phone lines like most households these days -- again with no luck. I decided to wait until the next morning, as advised by US West. Apparently, its services can require up to 24 hours for full installation
I tried connecting again first thing the next morning -- still no luck. I called my ISP's help personnel to check again, but nothing had changed; even with the debugging mode set on the ISP side, it still didn't see any traffic. Then I had an idea. I called US West and tried to find out which line the service had been installed on. The person at the help desk replied that it was on the main line.
US West had installed the DSL service on the wrong line, a mistake that wasn't entirely its fault. I had asked for the service to go into my second line, but wanted it billed to the same account as my primary line. US West's technicians had assumed that, because I wanted the billing to go onto the same statement as my primary line, they should install the DSL service on that line as well. Once we cleared that up, I simply resigned myself to using the primary line rather than trying to get US West to correct it. Switching over the lines, the service finally connected to the modem on the ISP end. Apparently, when I had tried it on the primary line the day before, the service had yet to be installed.
Directing traffic properly
Then came the second problem: although the modems connected, the router refused to authenticate my account and password. I called my ISP and ran through the problem with its help personnel, but we couldn't get the PPP connection working properly. We decided on the less pleasant method of setting up the DSL modem as a bridge. A bridge simply forwards all packets from a local network to a remote one. It does not check where the traffic is supposed to go, and expects some other device to handle routing. It is inherently insecure and generates more traffic than necessary, because all the traffic from my LAN would be automatically bridged to the other end. I did not like the situation one bit.
When I configured the machines on my end to use the remote router at the ISP, the service finally worked. I could actually start getting pages from the Internet at 768 Kbps. Not satisfied with the bridging situation, however, I attempted to switch it back to a PPP connection. No luck. The Cisco 3810 at the ISP either could not provide a PPP connect service, or was not at that moment configured to do so.
At least the ISP's Cisco device was smart enough to limit my bridged traffic to my particular circuit. The device is capable of creating a virtual LAN that contains all my traffic within its fold, and acts as a router to the rest of the ISP's network. This means I won't be seeing any unwanted traffic from other machines or networks on the ISP, and vice versa.
DSL at last
So, I am finally connected to the Internet at the full service rate of 768 Kbps. Actually, it goes even faster than that -- all the way up to 816 Kbps, which I attribute to the fact that my house is only a few blocks away from the US West central office.
Even after the 1996 Telecommunications Act, which deregulated the national telephone system and allowed other providers to compete on a par with US West, PacBell, Ameritech, SouthWest Bell, and all the other Baby Bells, most lines are still in their hands. Many of these lines and phone switches were installed years ago and have to be tested to determine whether they support new services like DSL. Companies are finding that many need to be upgraded or replaced, and this can be a very expensive proposition with added costs running into the billions of dollars.
As a result, the Baby Bells have been slow to implement DSL services. Some, like PacBell, are more progressive, while others who serve more rural areas are less so. Even when faced with serious competition from cable companies and cable-based Internet services, phone companies have periodically lagged behind. Don't look for this trend to continue. Now even PC vendors like Dell and Compaq are beginning to include advanced modem technologies with their products.
The future network for consumers and small offices will probably be an amalgamation of cable modem services, DSL services, and advanced wireless services. And pricing is starting to arrive at levels most people can afford. In fact, companies may soon begin supporting remote users with these technologies. This would reduce the amount of time spent waiting for downloads, although it may increase the amount of download activity and network traffic. A good look at your routers and remote access equipment is warranted.
Furthermore, you will want to examine the load on your outgoing Internet lines. With higher speeds, people will spend more time accessing the Web and the Internet, and it may not help to get a 1 Mbps line if the outgoing Internet connection is only a single, heavily utilized T-1. It will simply bog your system down to the same speed as a 56 Kbps analog modem, or slower.
As for me, this is only my second week of DSL service, but so far I
have no complaints. As I said, I'm quite interested in your experiences
with DSL services in other locations, so please do share them.
About the author
Rawn Shah is an independent consultant based in Tucson, AZ. He has written for years on the topic of Unix-to-PC connectivity and has watched many of today's existing systems come into being. He has worked as a system and network administrator in heterogeneous computing environments since 1990.
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