DSL (Digital Subscriber Line) is an always-on internet connection that normally terminates in a socket on your wall, one that looks much like a phone socket. In the US, the socket is exactly a phone socket, and, for the popular residential DSL, (ADSL), the same housewiring does indeed carry both phone and data.
DSL is billed on a monthly basis, usually for a fixed price, and for the majority of providers it includes unlimited usage. In other words, whether you use it for email once a day, or you are a net addict and use it constantly, your bill is always the same.
Once you have a DSL line, you can use all the resources of the internet in the same way as you did from a regular modem and a dial-up account. The difference is now you can use them 24 hours a day with no connection delay, and usually (although not always) without a ‘username’ and ‘password’. You need not worry about busy signals or any connection/disconnection process.
The key advantage of DSL over a dial-up modem is speed. DSL is from several to dozens of times faster than a modem connection. A complex web page that could take up to a minute to finish loading at 56K can appear in just seconds over DSL.
Connection speed, reliability, and the ‘always-on’ nature of DSL are the main reasons it is so popular. For small businesses, DSL is also a great way to save money compared to pay per minute ISDN service, or expensive T1 lines.
DSL vs CABLE?
This is a question that is asked everywhere you look. Which do I want … DSL or Cable?
DSL service shares bandwidth amongst ALL users connected to the same DSLAM. Cable shares bandwidth amongst ALL users connected to the same CMTS.
DSL’s advantage?
The dedicated circuit prevents other users from affecting your connection to any significant degree. (In most cases.)
Cable’s advantage?
Generally cable can support higher bandwidth rates, and can usually provide service to a larger area than 18,000 wire-feet, DSL’s limit.
Cable modems are typically faster for downloads than most if not all DSL lines, when the cable infrastructure is new or well maintained. One of the most common complaints seen in our cable forums is that of increased latency and other problems as more subscribers in a given area come on line. Additionally, cable has a few other disadvantages when compared to DSL.
The first disadvantage is that cable is an RF network — this means that it is vulnerable to transient problems “within the network” from RF interference. Since cable is a shared media, there is a possibility that performance may degrade over time as additional households plug in, connect additional devices (videos, game machines etc.) to the TV lines.
A cable company may react slowly to decreases in performance if it reacts at all, as they never sell access by speed, or promise consistent speed or latency.
Another of the disadvantages of cable over DSL is the upstream (return path). Cable companies are using a very narrow band for return signalling, and this is positioned below all the space allocated for TV channels. This band is prone to RF interference and is very limited in capacity. Upstream transmissions may therefore compete with others in the area, get delayed (suffer high latency) due to noise fighting techniques, and cable TOS (Terms Of Service) typically prohibit any kind of constant upstream use. Internet use is shifting away from central servers broadcasting to many individuals and some interesting peer to peer applications are appearing (games, voice and video applications, communal libraries). These applications need a strong upstream channel.
In summary, cable modems are currently good value and strong competition for residential casual use, often available more cheaply and far faster than their ADSL competition. However, DSL is probably the more future-proof system, offering digital direct from the internet infrastructure. If your DSL ISP is on the ball, your performance in either direction will not be different from peak hour to early morning, and DSL lines are available for a wide variety of purposes, both business and residential.
What is the fastest speed that DSL connections are capable of?
There are a lot of variables to consider with answering this question.
Typical residential offerings usually have a maximum of 1.5Mbps (1.5 megabits per second), but special connections for home and office can be obtained to go well above that. Keep in mind that you will not normally see 1.5 megabits in a speed test … due to overhead the more commonly seen speed with this type of connection is in the neighborhood of 1200-1250.
There are two limiting factors that are in place for most connections: the modem and the wiring used to transfer the signal from the modem to the computer. Most home use DSL modems are limited to 10Mbps on the user (LAN) side. The wiring from the modem to the computers is normally Category 5 wiring, or thin ethernet, and that is limited to 100Mbps speed.
Many DSL users are now connected at speeds equal or less than 3000/768, whether they are on ADSL or SDSL.
Can I FAX over DSL?
You cannot FAX over a DSL connection, but you can still send and receive faxes over your regular telephone lines.
Upgrading to DSL may add a DSL modem of some kind, but if you keep your old modem, you can still use software fax products like winfaxpro. You simply use your telephone line as before, by plugging a telephone line into your dial-up modem as before. This holds true whether or not you get ADSL over your home phone or SDSL on another line.
You may also try the internet solution, and subscribe to a service like jfax.com or efax.com which provides your own dedicated fax number, and you may send faxes using a utility, and receive fax messages (and voice messages!) in your mail inbox. You might also check out faxbeep.com for a list of internet fax providers.
Latency versus Bandwidth – What is it?
One of the most commonly misunderstood concepts in networking is speed and capacity. Most people believe that capacity and speed are the same thing. For example, it’s common to hear “How fast is your connection?” Invariably, the answer will be “640K”, “1.5M” or something similar. These answers are actually referring to the bandwidth or capacity of the service, not speed.
Speed (latency) and capacity (bandwidth) are two very separate things. The combination of latency and bandwidth gives users the perception of how quickly a webpage loads or a file is transferred. It doesn’t help that broadband providers keep saying “get high speed access” when they probably should be saying “get high capacity access”. Notice the term “Broadband” – it refers to how wide the pipe is, not how fast.
Latency:
Here’s the most common example to compare latency and bandwidth:
Imagine water running through a pipe. The pressure is latency, the width of the pipe is bandwidth. If you have a wide pipe but low pressure, you can move more water through the pipe but at a slower rate. If you have a narrow pipe but high pressure, you can move less water but at a faster rate.
Another example that is sometimes given:
Imagine people in an aircraft. In this example, people are the data packets, the size of the aircraft is the bandwidth, and the speed of the aircraft is the latency. A 747 can carry about 400 people but a 707 can carry only 200 people. Both fly at about 500 knots. If both leave New York at the same time, they will arrive in Los Angeles at the same time. But notice that although the 747 has more capacity (or bandwidth) it is the same speed (latency) as the 707.
Latency is normally expressed in milliseconds. One of the most common methods to measure latency is the utility ping. A small packet of data, typically 32 bytes, is sent to a host and the RTT (round-trip time, time it takes for the packet to leave the source host, travel to the destination host and return back to the source host) is measured.
The following are typical latencies as reported by others of popular circuits type to the first hop. Please remember however that latency on the Internet is also effected by routing that an ISP may perform (ie, if your data packet has to travel further, latencies increase).
Ethernet .3ms Analog Modem 100-200ms ISDN 15-30ms DSL/Cable 10-20ms Stationary Satellite >500ms, mostly due to high orbital elevation DS1/T1 2-5ms
Bandwidth:
Bandwidth is normally expressed in bits per second. It’s the amount of data that can be transferred during a second.
Solving bandwidth is easier than solving latency. To solve bandwidth, more pipes are added. For example, in early analog modems it was possible to increase bandwidth by bonding two or more modems. In fact, ISDN achieves 128K of bandwidth by bonding two 64K channels using a datalink protocol called multilink-ppp.
Bandwidth and latency are connected. If the bandwidth is saturated then congestion occurs and latency is increased. However, if the bandwidth of a circuit is not at peak, the latency will not decrease. Bandwidth can always be increased but latency cannot be decreased. Latency is the function of the electrical characteristics of the circuit.
Why is distance important with DSL?
If you have heard even something about DSL, you will have heard about distance. Your phone lines normally terminate at a telco office, usually nearby. This distance, (the length of your line between your location and the telco office), is a very important factor in whether or not you can get DSL, and what speed you can get.
Here are some rules of thumb for distance ranges. Please be aware that especially with non Telco ADSL lines, distance limits for speeds can vary widely from company to company. There are cases where it is policy for “residential” DSL lines not to be offered as far out as functionally identical “business” products!
The subject of distance and DSL is so important, we’ve added a special page for it, check this for detailed distance information per DSL network : Distance charts.
| Distance In Feet |
comment |
|---|---|
| less than 5000 | You will have little trouble getting all speeds of DSL |
| 5000-10600 | You may have trouble getting the highest speeds on offer |
| 10600-15000 | The danger zone for DSL from national CLECs like Covad and NorthPoint. Speeds on offer are pinned back steadily until they reach 192k for 15,000 feet. If your line is longer than around 15000 feet, they may not accept an order. |
| 15000-18000 | In this range, Telco ADSL is normally still available, although it may be restricted to 300-500k speeds |
| 18000-22000 | Telco ADSL is not available, although in a few areas, RADSL may be a product you can get. RADSL is speed-variable. Some smaller DSL specialist CLECs may have solutions for you. |
| 22000-28000 | Using less commonly used DSL equipment, it is still possible to use lines of this length. |
| 18000-28000 | IDSL is an alternative or possibly the only alternative. IDSL is 144k/sec, about four-six times modem speed. |
| 28000-38000 | IDSL is the only alternative |
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Amit Chawla
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