Bandwidth bottleneck question.

Posted on 2007-07-19
Last Modified: 2013-12-12

We have a WAN network that connects 15 locations.  14 of the 15 locations are 1.5Mbps T1, and one is MetroEthernet 10Mbps.  The connections all physically come together at the phone company via NetVPN.  Here is the scenario we have:  One server at the MetroE site connected at 10Mbps, we'll call it Site A, is connected to a switch via gigabit.  The switch then connects to the MetroE link to go out over the WAN.  At one of the T1 1.5Mbps sites at the other end, we'll call it Site B, we have 15 servers and 30 PCs.  Most servers are on gigabit there.  If we have to backup 6GB of data from Site A to Site B, it is argued by a co-worker that the backup will finish sooner if the servers at each end are running at gigabit speed, and not 100Mb speed on the switches.  Another co-worker says that as long as there is at least one T1 1.5Mbps WAN link line involved between these two servers, it will not make any difference if they are gigabit or 100MB on each LAN because the 1.5Mbps part of the connection is the bottleneck irregardless.  In other words, data cannot travel any faster than the slowest bandwidth link involved between two computers.  Any thoughts?

Thanks for input from all.
Question by:miket71
    LVL 6

    Accepted Solution

    You second co-worker is  right. The slowest speed on the path will set the maximum speed.

    Just try to explain to yourself how you can fit a 1Gb/s flow into a 1.5Mb/s line ? or use water analogy, that works too.

    I hope that helps

    Author Comment

    The first co-worker seems to think that having a gigabit connection on a server will allow more connections to come in from the many WAN links compared to a 100MB connection, even when the 100MB connection was working fine all along.  He also thinks that end users will notice better performance when they are using an application at one end of a 1.5Mbps WAN link that is communicating with an application or database server at the other end if it is on a gigabit connection compared to 100MB.  Any thoughts on this analogy?
    LVL 6

    Expert Comment

    A line that is maxed out is a slow line :-)
    Do not forget to put some QoS or prioritization if you want your users at the other end of the T1 to suffer a minimum.
    I still think that your second co-worker is perfetcly right and , if it is a management problem, I would dare the first one to demonstrate his opinion in a lab :-)
    LVL 20

    Assisted Solution

    netnounours & the 2nd coworker are exactly right - the slowest link determines actual throughput speed.  
       Here's an analogy:
    A delivery truck traveling on a smooth-flowing freeway/superhighway (100 Mbps at server end), immediately exits onto a congested, slow-speed-limit road where traffic is running at a fraction of the speed (1.5 Mbps T1 connection)...  How fast your package is delivered is determined by the slowest path.

      Trying to spew traffic from the server at Gigabit speeds through a router w/ a 1.5 Mbps WAN connection isn't going to make things better.  You're still going to get a bunch of dropped packets while TCP adjusts the transmission rate until the receiving router quits dropping packets on the floor.  And sending from the branches out a T1 connection: the senders will be limited to T1 rates or fraction thereof.  Doesn't matter what's on the server end - once again, the slowest link is the limiting factor.
      Your 1st coworker needs to take Networking101 & maybe Logic101.  ;)

    LVL 57

    Assisted Solution

    As everybody has stated, the slowest link determines the through put between two sites.  

    You have 14 sites running at 1.54Mbps and 1 site at 10Mbps.  Assuming that the backup server is at the 10Mbps site and the servers being backup are at the 1.536 Mbps sites, then as long as the servers being backup are connected to a LAN and can get at least 1.536 Mbps then that is the fastest you can backup.

    However, if you are tring to backup multiple sites at once, you could get better throughput by increasing the speed at the central site.  A 10 Mbps link is just about equal to 6.5 T1s.  So if you are trying to backup 7 or more sites at once, the 10 Mbps link is slowing you down.

    If you wanted to backup all 14 sites at the same time, you would need a link at the central site of at least 22 Mbps (14 * 1.536 Mbps= 21.5 Mbps).

    To put it in the highway example.  You have a 14 lane highway and you are merging it down to 7, something got to give.

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