DSL Variations: ADSL, SDSL, VDSL, And More!
By Chris Bryant, CCIE #12933
One step up from the cable modem, we have Digital Subscriber Line, or DSL. DSL uses a preexisting phone line for broadband delivery. DSL comes in quite a few flavors, though, which you should be aware of when purchasing DSL service.
Asymmetrical DSL works under the assumption that the user will download more information than they send, and for the average Internet user, that's a safe assumption. The connection speed from the provider to the user is going to be 3 - 4 times faster than the speed from the user to the provider. A typical ADSL connection of 512 kbps will give the user 384 KBPS download capabilities, but only 128 KBPS uploading capability.
ADSL uses several different modulation methods, but the most well-known is G.lite (also known as G.922.2), which requires no splitter at the customer location. The customer simply hooks up a G.lite modem in the same way an old-fashioned analog modem would be installed. G.lite's limitation is speed - where standard ADSL can offer 8 MBPS download speed and 1.5 upload speed, G.lite's maximum capability is 1.5 MBPS downloading and 512 KBPS uploading. The key is that while G.lite is slower than true ADSL, it's still a lot faster than the dialup options available to today's home users.
Part of the joy in working with ADSL is that there are two separate standards for ADSL, which are totally incompatible. The first implementation was CAP, which divided the phone line into three separate channels - one for voice, one for upstream traffic, one for downstream traffic. CAP has been replaced in most of today's ADSL hardware by DMT (Discreet MultiTone), which uses a whopping 250 channels to carry data.
The distance limitation of ADSL must be taken into account as well. Officially, there's an 18,000-foot limitation on ADSL services, but most ISPs put a lower limit on ADSL to avoid poor quality service for those near the end of the cable. Of course, that limitation is for data transmission, not voice.
If we have asymmetric DSL, it makes sense that we'd have symmetric DSL (SDSL) as well. The term "symmetric" refers to the fact that the sending and receiving speed are the same. The drawback is that the phone cannot be used while SDSL is in use.
Very High Bit-Rate DSL (VDSL) has the capability to deliver speed up to 52 MBPS. That's am amazing speed to deliver over copper wire, but there's a drawback - VDSL over copper has a maximum distance of 4000 feet. However, as more and more fiber-optic cable is installed by the telephone companies, VDSL is becoming available in more communities as the distance issue is resolved by the use of fiber.
You may remember a technology called Long Range Ethernet from your BCMSN studies. LRE is built upon VDSL.
Rate-Adaptive DSL (RADSL) is just what it sounds like - the software calculates the maximum download and upload speeds on the customer's preexisting phone line and dynamically adjusts those rates.
Anyone who lives or has lived in a rural area knows the challenge of trying to get a broadband connection. Satellite Broadband certainly sounds like a technology that could meet that challenge, and theoretically it does just that. Theoretically, that is. First, satellite broadband speeds just aren't comparable to other broadband delivery methods. One major ISP advertises DSL speeds up to 3 MBPS, but their satellite service downloads at 500 KBPS and uploads at a paltry 50 KBPS.
You may think "that's not the best speed in the world, but it's better than nothing". The bad part is that those speeds aren't even guaranteed - any ISP that sells you satellite broadband is going to hit you with a lot of waivers as to how weather, wind, the phone company, or Kryptonite is going to adversely affect your connection speed and status.
Okay, maybe not Kryptonite. But the bottom line is that satellite broadband is inherently unreliable. Since satellite broadband also requires extra equipment on the customer side in the form of a satellite dish, it's better to avoid this option if at all possible.
To your success,
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