A recent Wall Street Journal blog discusses the median Internet download speeds of various ISPs in USA, measured by Internet speed test by end users. The results were interesting to investigate to see if there were any conclusions that can be drawn out of the study with respect to how cable modem vs DSL performance is. The majority of the ISPs were over promising the speeds, at times as high as 41%. Remember, these are the median numbers, which means individual performances can be much lower than the list indicates.
There are various differences between cable modem vs DSL Internet connectivity that relates to download speed, upload speed, latency, jitter, reliability and consistency of performance. Let’s start with the fundamentals to dissect cable modem vs DSL.
DSL uses the dedicated twisted pair copper wires that carries plain old telephone service. These wires go from the end-user office location to the DSLAM concentrator that the Telco operates. Usually from the DSLAM to the Telco network, there is a fast transport that carries the traffic to the Internet. Unlike DSL, with cable modems, the end user offices are on the same cable wires (coaxial cable) for the neighborhood and are not dedicated. In other words, the last mile wire capacity is shared among the end-users in the neighborhood.
This has 2 implications:
1) Cable modems are able to provide higher peak rates both for download as well as upload compared to DSL modems. Coaxial cables are better suited to carry larger number of bits per second and therefore can achieve higher bit rates on the cable, such as 10,15,20 or even up to 100+ Mbps depending on the cable ISP. DSL on the other hand has to work with the older and in most cases less than perfect twisted copper wires. The rates on DSL can be as low as 1.5Mbps and in some rare cases with shorter loop lengths can get up to 20+ Mbps.
2) The higher peak rate of cable modem has a caveat, since that speed is shared among users on that cable wire in the neighborhood, if users start to simultaneously content for that same bandwidth, the per office bandwidth will fluctuate down. This behavior of the cable modems also created some marketing opportunities for cable ISP, such as the “speed boost” feature whereby, the cable provide can prioritize the lion share of the bandwidth to a connection at the beginning of the session (including speed test) and therefore portray a much faster speed than what can be sustained over the lifetime of that session. In other words, it is common to see a file transfer start real fast on a cable modem, only to see it go down speed-wise a few seconds later. DSL on the other hand, because of its dedicated nature is less vulnerable to speed fluctuations.
There are some other differences in the core network provisioning for cable modem vs DSL as well. Without getting into the technical details behind these differences: on average, cable modem connections will have a higher buffer size at the cable operator’s POP (Point of Presence) compared to a DSL operators POP. The higher buffer size of the cable POP will allow for TCP to ramp up to higher speeds (by avoiding becoming the bottleneck buffer in the network), however this will at times also cause higher latency for the connection. So for real-time business applications, a controlled latency environment is better than a fluctuating latency in the network.
To conclude: for business applications that require high bandwidth and perhaps are less dependent on lower latency, cable modem might be a better option compared to DSL. However, if you are relying on real-time traffic over the broadband lines, such as VOIP, video, UC, or office-to-office VPN, DSL based approach may provide a better option. Of course being able to leverage best of both worlds is the ideal scenario whenever possible. If you are lucky enough to be in the coverage area of both cable modem service and DSL service, consider utilizing a Broadband Bonding appliance that can bond 2 or more ISP lines and merge the high speeds of cable modem and low and stable latency of DSL modem to provide a unique connectivity option for your business.
Cahit Akin, CEO, Mushroom Networks, Inc.
Mushroom Networks is the provider of SD-WAN (Software Defined WAN) and NFV solutions capable of Broadband Bonding that enables self-healing WAN networks that route around network problems such as latency, jitter and packet loss.