Ask LH: Why Is My Cable Internet Performance So Unpredictable?

Hi Lifehacker, My internet (Optus Cable) is quite strange and I’m wanting to know how to fix these problems. Firstly, for some downloads I get 60Kb/s, and for some I can get up to 2,000Kb/s. Is this huge difference just to do with server speed and location of the download? It’s quite frustrating having to wait 10 minutes for a 30MB file, when I have downloaded 1GB+ movies in that same time.

Secondly, my upload speeds. According to I am getting 0.49Mb/s, compared to 18Mb/s download. Why is there such a big gap? I understand a lower upload speed, but 17.51Mb/s…what!!

Thirdly, if I change my server location on to Adelaide, I suddenly get 5Mb/s, as opposed to 18Mb/s. Is this because the Melbourne server (where I am) is hosted by Optus, and somehow they are rigging the speeds?

Lastly, could you explain the difference between Mb/s and Kb/s?

Thanks, Frustrated At My Internet

Dear Frustrated,

While we can understand your frustration, there’s nothing obviously wrong with your Internet connection: indeed, this is pretty much how we’d expect it to behave. Some of your issues are specific to how cable connections work, but the most important point — that download speeds will vary all the time, no matter what you do — applies to everyone.

Whenever potential speeds are quoted for a connection, you’re looking at a best-case scenario. Factors including the number of other people trying to access a file or site, and the connection speed at which the server on which a web site is hosted, will mean that performance is always variable, and hardly ever reaches the theoretical maximum. HTTP, the mechanism used in browsers, is also not particularly efficient as a means of transmitting data: for large files, using alternative approaches such as FTP or BitTorrent can be quicker. But even then, you’re somewhat at the mercy of the connection speeds for those services.

In the case of cable Internet from Telstra and Optus, there’s a fairly specific issue at play: contention ratios. You’re competing for downloads with anyone else in your neighbourhood who is also using cable for their Internet service. The total bandwidth available on the cable has to be divided amongst the number of users, which means the speeds you get can vary dramatically depending on who else is also using their service at the time. (The scenario is somewhat different for ADSL, as the phone line which comes into your house does come uniquely into your house.)

Because of this, it’s not uncommon for cable users to notice a degradation over time: if you’re an early adopter, then as new customers come along in your area, the speed will inevitably decrease. I can remember testing Optus cable way back in the late 1990s when it was first introduced, and getting much higher speeds simply because I was taking part in a press preview and no-one else in the neighbourhood could use it. Once it became generally available, speeds fell rather quickly. (Experience also suggests that your download speeds will drop dramatically around 4pm when teenagers get home from school.)

On to uploads. Virtually all consumer Internet services in Australia, whether cable, ADSL or wireless, are asymmetric: the upload and download speeds aren’t equivalent. On top of that, in the case of cable, the need to compete with everyone else on your neighbourhood cable again creates an issue. Typically, on a cable Internet connection you’re assigned a series of timeslots for uploading files: you get to send a certain amount of data upstream, then the next person on the network does, and so on. You can’t actually see this happening from your PC, but you can see the effects in a (relatively) low upload speed.

A related issue is that commonly accessed download files may well be cached by your ISP, meaning that they’re much faster to access because a copy is stored locally on the ISP’s network, rather than having to be pulled in from overseas. This improves download speeds, but can make your upload speeds feel even slower by comparison.

An underlying principle here is that distance matters when it comes to Internet speeds. If you deliberately set to your location on to another state, it’s not surprising the results vary. As a speed testing option, that site is often subject to criticism, but if you are going to use it, leaving it with the defaults will give you the most broadly comparable results. It’s also worth bearing in mind that since it quotes back the highest speed over a given connection, not an average speed, it’s again tending towards a best-case scenario rather than a measured analysis of throughput.

On the last question, there’s 1,000 kilobits in a megabit, so 1Mb/s is equivalent to 1,000Kb/s. Back when connection speeds were slower overall, Kb/s was commonly used, but with higher speeds, Mb/s are easier to process for most people. It doesn’t matter which one you use, but it helps to only use one for consistency.

Cheers Lifehacker

Lifehacker 101 is a weekly feature covering fundamental techniques that Lifehacker constantly refers to, explaining them step-by-step. Hey, we were all newbies once, right?


  • I was going to comment about the whole kilobits vs kilobytes vs kibibits vs kibibytes topic, but I’m sure that there will be other readers who can do it a lot more proficiently than I can.

    I will condense the important part of it down though:

    While it is *technically* correct that there is 1000 bits to a (metric) kilobit, we incorrectly use the “kilo”, “mega”, “giga” & etc prefix in Australia to refer to a 1024 base system.

    Also worth noting that ISP’s will always advertise their connection speed in BITS (b) not BYTES (B). Download managers and other various software on the other hand will report their download speeds in bytes. As most of us know, there are 8 bites to the byte. So worth remembering that an ADSL2+ connection that syncs up at 16Mb/s (megabits per second) will only yield a theoretical maximum download speed of 2MB/s (megabytes per second).

    Gus – suggestion for a future article might be one that puts all these differences into layman’s terms for those interested who aren’t familiar in the differences.

    • I nearly left that part of the question out, because it’s a large and complex issue and not directly related to the rest of the questions! But I will revisit the bits/bytes theme in a future 101.

      • Unfortunately, it’s a complicated answer brought about by a simple question – raised due to various parts of the IT&T industry deciding they want to advertise their products using different measurements.

        I’ve long fielded questions from friends and family since back in the old days of dial up, as to why they only downloaded at “4-5 kilobytes per second on a 56k modem?”.

        Similarly with HDD manufacterer’s advertising their products’ capacity in metric bytes; I’ve had plenty of questions along the lines of “Why is my HDD size only 460GB in Windows when the box said it has 500GB”.

  • I do agree with this article….however….
    The speeds the user is experiencing is actually an error on Optus’s part!
    Optus is experiencing international congestion which started just over a month ago.
    Read this the Whirlpool thread over here:

    Basically it means local sites are fine however international sites and downloads may be much slower than usual.
    I connected to Optus cable late last year. At first speeds were fantastic however the past month they have been quite dismal.
    So yes there is something wrong with your internet connection, just something you can’t fix yet.
    Your upload speeds are correct as this article states however
    Frustrated, you are not alone…

  • Bottom line on Cable internet is it is based on 10base2 tech.

    HFC uses a Carrier-Sense Multiple Access/Collision Detection (CSMA/CD) protocol. The more connections on the current run the less access to the network you get because of collisions.

  • As an optus cable user i feel your pain.

    Your right there is something wrong, this article explains the situation accurately but the bigger picture is in Caspez’s link.

    Over the past month i have noticed a dramatic drop in my optus cable speeds and extremely variable ones at that. Some mornings its fine, others im back to near dial up speeds.

    This is not normal cable behavior.

  • Its is true that the HFC cable into your home has shared bandwidth and your connection is capped at 8, 30 or 100 mbit.

    It is true that the ADSL cable into your home is a dedicated line from the exchange.

    BUT going upstream after the exchange its all shared bandwidth. An analogy is the road system and your driveway at home. If you live in a block of units you share your driveway. If you live in a house you have a private driveway. But there is more to your journey time than the performance of your driveway.

    The effect of contention on HFC is exaggerated as it only comes into play when the cable segment is operating near saturation. Whether your cable segment can even reach saturation capacity will also depend on if all the upstream network can supply data to it at that rate. The algorithms for managing contention and QOS are very efficient. There are many bottle necks all over the internet, you connection is only one of them.

    The bottom line, the internet is all shared bandwidth, as the road system is shared lanes and roads.

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