Contention - in real terms?

Posted on 2005-04-26
Last Modified: 2013-12-14
I have just migrated from an ISP that gave me only a 512k connection but guarenteed no contention.

For the same price per month, my new ISP allows me 2mb connection (altough I can only use 1Mb on my phone line) for the same price as my old ISP.  However, there is a 1:30 contention on the line.

What does this mean to me in real-terms?  How will I be impacted?  Does the extra bandwidth more than compensate for the amount of contention on the line?   Which is the better deal (bearing in mind they are the same price per month)?

Thanks very much.
Question by:bgcm12
    LVL 7

    Expert Comment

    So, what does 1:30 contention mean - does it mean there are 30 other potential contenders?

    Were you using the entire 512K bandwidth before? How much of the 1Mb are you using?

    If you were using a lot of bandwidth a lot of the time, and there are 30 others possibly contending for the line, sounds like you better hope they don't use it much.

    But I must admit, I don't really have that expperience.

    Author Comment

    yes I believe that 30 people share that resource.  But I think that only applies the ISP connection to the backbone - the BT backbone is set at a contention of 50:1 anyway apparently.  I realy am not sure how much contention is an issue if someone could clarify for me.

    I was getting connection speeds of 470k on my old ISP - I get similar results with the new ISP as it hasn't yet been upgraded to 1mb (you have to migrate from one ISP to another at the same connection speed)- yet things seem slower.  This could be psycological tho...
    LVL 7

    Accepted Solution

    So then, the effect will depend on two things:
    1. the spped of the ISP connection to the backbone (that really shouldn't be a problem there - it out to be multiple Gbps, but you could ask anyway)
    2. what the other people who share your connection are doing.

    One rule of thumb that I have heard somewhere is that during contention only 60 to 70 % of the bandwidth can be used.  That percentage can go down the more contenders there are simultaneously.  If that is true then let's say there are two of you using the line simultaneously at all times.  If the line is a 100Mbps line and each of you is doing 1Mbps then there is plenty left over.
    If 50 are doing 2Mbps simultaneously then the line is overly saturated and performance will be poor.  But it is very unlikely that 50 people will send 2Mbps each al the time.

    If the bottleneck is only at the ISP connection to the net, you should not have a problem. But you should clarify this with your ISP.
    LVL 7

    Expert Comment

    1:30 contention means that they have oversold the capacity.  This isn't bad, it's quite normal.

    Say they have a 2 Mbps connection from their site to "the internet".

    if they sell ONE customer the entire 2 Mbps, well, 99.9% of the time it's going to sit idle.  Right?  You'll be surfing along, grab a few KB or MB for a page and then read it for a few seconds, click some where, grab another MB, etc......

    Instead they sell 30 customers that 2 Mbps.  For normal surfing, you'll never notice it.  Occasionally there is a tiny (unnoticable) delay when somebody else is grabbing their MB page using 'your' bandwidth.  The technical term here is 'jitter'.  It always takes 72ms to get the reply from the remote site, except every few seconds it takes 92ms.

    BUT: The whole thing is based upon the assumption of that bursty traffic pattern.  So that on average (over a period of 1-2 seconds), the bandwidth gets evenly shared and nobody notices the delay.

    The kicker is that today's access patterns don't always work like that.  Streaming music and VoIP services require low jitter.  That next packet HAS to arrive on time or there is nothing to play on the speaker.   So streaming players and VoIP phones are programmed to recover - various ways, irrelevant here - and mostly you won't notice.  Could be a little 'static' or a pop, but people have amazing tollerance for these little glitches.

    Still, as more and more of your 30 close friends (ie bandwidth sharers) use these kinds of time critical services, the worse things get.  Until one day...  ("Sorry, the internet is busy.  Try again later").



    Author Comment

    Hmmm - thanks for that Burton.  I now have worrying visions of an engaged tone when i click on my browser icon!!!

    One last point to clear up please...

    The BT line that my ISP uses has a 1:50 contention.  The ISP sells the bandwidth on a 1:30 contention.  How do these figures interact?

    Thanks again.
    LVL 7

    Assisted Solution

    It's not additive.  You have to know the network layout and whether there are caches present, etc.

    The point is that only a network engineer really cares (and should care).  It's about like saying your car has 16 stitches per inch in it's upholstery.  Without understanding industry standards, whether 17 or 15 is better and why, it's a meaningless number.

    As a customer, you care about the CIR and BIR.  Committed Information Rate and Burst Information Rate.

    CIR is what you can stuff into the pipe 24/7 - GUARANTEED.  BIR is what you can burst up to (maybe without penalty, maybe with).

    So your old ISP had CIR=BIR=512KBps

    Your new ISP is using the oversubscription rates to hide that they haven't quoted CIR/BIR.  

    It could be CIR=2Mbps BIR=whatever they're buying from BT (Probably an E1 (2.048 Mbps) rate on an 100BaseT Ethernet connection (2.048 * 50 = 102)).  But it's more likely that CIR=0 (or 68Kbpse. 2.048/30) and BIR=2Mbps.

    Regardless it's all meaningless mumble.  What matters is whether it supports your style/patterns of access to the internet.  Or not.

    LVL 7

    Expert Comment

    I agree with Burton that you have swapped a guaranteed 512Mbps for for an unguaranteed but sometimes possible 2Mbps (1Mbps for yo since your phone line limits you)

    Like I said in that previous post
    "So then, the effect will depend on...
    2. what the other people who share your connection are doing."

    Write Comment

    Please enter a first name

    Please enter a last name

    We will never share this with anyone.

    Featured Post

    Enabling OSINT in Activity Based Intelligence

    Activity based intelligence (ABI) requires access to all available sources of data. Recorded Future allows analysts to observe structured data on the open, deep, and dark web.

        Over the past few years, small business and home owners have become so dependent on internet that a need for redundancy has arisen.    What happens when your small business or home / home office loses its internet connection?  The results c…
    This solves the problem of diagnosing why an internet connection is no longer working. It also helps identify the likely cause of the lost connection if the procedure fails to re-establish your internet connection. It helps to pinpoint the likely co…
    Hi everyone! This is Experts Exchange customer support.  This quick video will show you how to change your primary email address.  If you have any questions, then please Write a Comment below!
    Sending a Secure fax is easy with eFax Corporate ( First, Just open a new email message.  In the To field, type your recipient's fax number You can even send a secure international fax — just include t…

    779 members asked questions and received personalized solutions in the past 7 days.

    Join the community of 500,000 technology professionals and ask your questions.

    Join & Ask a Question

    Need Help in Real-Time?

    Connect with top rated Experts

    15 Experts available now in Live!

    Get 1:1 Help Now