A recent Wall Street Journal blog discussed the median Internet download speeds of various ISPs in the US, as measured by end users running Internet Speed Test. The results were interesting to investigate if there were any conclusions that can be drawn with respect to how cable modems perform vs DSL. The majority of the ISPs were over promising their speeds, at times by as much 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 modems and 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 the performance of cable modems 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. With cable modems, unlike DSL, 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 two 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 1.5 Mbps but in some rare cases with shorter loop lengths can get up to 20+ Mbps.
2) The higher peak rate of cable modems has a caveat, since that speed is shared among users on that cable wire in the neighborhood. If users start to simultaneously contend for that same bandwidth, the per-office bandwidth will fluctuate down. This behavior of the cable modems also created some marketing opportunities for cable ISPs, such as the “speed boost” feature whereby the cable provider can prioritize the lion’s 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 very fast on a cable modem, only to see it throttled down 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 modems vs DSL as well. Without getting into the technical details behind these differences, on average, cable modem connections will have a larger buffer size at the cable operator’s POP (Point of Presence) compared to a DSL operator’s POP. The larger buffer size of the cable operator’s 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 fluctuating latency in the network.
To conclude, for business applications that require high bandwidth and perhaps are less dependent on lower latency, cable modems 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, a DSL-based approach may provide a better option. Of course, being able to leverage the 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 two or more ISP lines and merge the high speeds of cable modems with the low and stable latency of DSL modems to provide a unique and optimal 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.