What exactly are the differences between Cable and DSL Internet services?
First let's define our terms.
Bandwidth vs. Latency:
Bandwidth & Throughput
Digital bandwidth or throughput is a measurement of the amount of data that can be transferred over a digital line over a specific amount of time.
Latency
Latency is the time it takes to deliver a packet of data from one place to another. This includes the time it takes for an electrical or optical signal to travel a certain physical distance (propagation delay) and the time it takes to process these signals along the way (processing delay).
So bandwidth is a measure of frequency or a rate of service delivery while latency is a measure of time or signal delay.
Technology Comparision.
Cable service uses signals transmitted through the same coax cable that cable television uses. DSL uses normal phone lines to send digital signals to a DSL modem.
Coaxial cable has a far greater capacity than RJ-11 phone cable. But, the coaxial cable coming into your home is shared with many people outside your house and provides many services.
Cable modems have higher bandwidth though the connection is shared among cable users in your service area and even with your digital cable box. As a result the performance is quite similar to DSL.
Generally, cable modems can achieve better peak throughput, but this high bandwidth is not consistant and depends, again, on load, affected by time of day and other factors.
DSL service usually provides a more constant, consistant feed, with lower latency. The distance to your exchange or "switch" is the main factor that determines DSL quality of service. Look for the green (in Toronto) phone exchange box in your neighbourhood and note how far your home is from that box.
Service Comparison.
One of the advantages of DSL over cable modems is that the line to the exchange or "switch" is not shared. Though ISPs trying to sell DSL services will tout this as a major advantage, slow cable modem speeds have very little to do with this and have much more to do with the fact that cable service is shared among many users and services.
ISPs tend to enforce a bandwidth cap on both DSL and cable modem services.
The first cap is on data transferred per service period (usually per month). From my experience, DSL data transfer caps are more rigidly controlled than cable services. Your data cap may be as little as 1 gigabyte per month or, in the case of most cable modem services, unlimited.
Both cable and DSL services have a cap on bandwidth. Your service provider may promise you a certain downstream limit and a certain upstream limit. This means that you will generally be able to sustain a certain bit rate while downloading or receiving data and a different bit rate while uploading information.
In general, because of the far greater capacity of coaxial cable and the fact that only a certain number of lines exist connecting you to your switch, cable modems, if uncapped would provide advantages in bandwidth over DSL modems. In the real world, this is not the case. Both DSL and Cable companies limit bandwidths at both the modem level and at the service provider.
Shop Around.
If you are shopping around for a service or thinking of changing services, the best advice I have for you is to ask other users about their service. One of the best web sites I've seen for sharing this data is broadbandreports.com.
This site provides user-submitted reviews of their broadband service and charts day-to-day service fluctuations. Speed tests and other tools for optimizing your connection are available as well.
About the Author
Sid Choudhuri is an IT consultant in Toronto, Canada. Sid runs ScALR Network Solutions. http://www.scalr.com
