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WAN latency and what to do about it

Posted on 2008-09-30
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I have a group of users at multiple remote locations who utilize RDP to access our servers. Each of their local WAN connections from their ISPs experience periodic latecny on the network connections (ping responses jump from 58ms to 300+ms for upwards of a minute) Is this a factor that can be controlled/contained? Are we somehow causing this lag to occur? Is there even a point in contacting the ISPs support line?

A few notes: the bandwidth for some of these connections is low 383/786. We will be upping this shortly. Could this contribute? Could the routers play a factor in the latency issue? Some help please!
Thank you!
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Question by:JP_TechGroup
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by:harbor235
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The bandwidth numbers you identified are low by today's standards, however, it depends on what you are using. It could be the problem, Look at your edge device and find out how much bandwidth you are using.

if this is a cisco device perfroma show interface on the WAN connection ant please post, sanitize your IPs.

harbor235 ;}
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by:giltjr
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You can't control latency.  The minimum latency is based on how far the remote site is away from you, the number of hops, and to a point the maximum speed of the link between the two points.  There is a bit more to it than that, but lets start simple.

The maximum latency is based based on the minimum latency plus the wait/queue time.

Say you work at a fast food restaurant and think of latency is the amount of time it takes you to service a customer.  The timer starts when the customer gets in line.

Now, no matter what you do, there is a minimum amount of time that it takes you to service the customer.  Lets say that is 5 seconds.  So if there are no customers in line and somebody walks up to you, it will take you 5 seconds to service them.  That is you base, minimum, latency.

Now, lets say that all of a sudden there are 10 customers that come at once.  Will the 1st guy will get 5 seconds, but the second guy will get 10, the 3 guy will get 15, and so on until the last guy and he get 55 seconds.

Now to make it more complex, lets say that the service time is also based on what the customer orders (it take less time to transmit a packet that is 100 bytes than it does to transmit a packet of 1500 bytes).  So customer #1 order the simplest item and it take 5 seconds, but customer #2 orders something that take a bit longer, so it takes you 8 seconds. Well customer #2 latency is now 13 seconds (#1 5 + #2 8).

Basically, the more utilized your link is the higher the latency will go.  Either you reduce the volume of traffic, or  increase the amount of available bandwidth.
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by:JP_TechGroup
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I think I have it and this makes a kind of sense. Can latency be affected by taks being performed by the client? For example, on a terminal server connection, a user printing a large document off the server to a local printer...would this temporarily increase WAN latency at their location? Enough to make a ping return jump from 86ms to 300ms?
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by:harbor235
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Well, yes, but you can have latency but for your good customers you can have an express lane. Prioritizing traffic will can allow you to give preferential treatment to certain traffic types. This does work, however, if you do not own the network end to end this does not make sense. However, there are types of services (MPLS) that allow this special treatment, for a price.

You most likely need more bandwidth, looks like you may have a frame service, i would look into the new MPLS products, very robust.

harbor235 ;}

harbor235 ;}
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by:JP_TechGroup
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We do not own the network at both ends. :( Connections are run through T-1 at the server end to (typically) through 3 Mb Down / 512 K Up ADSL at the client ends (maximum transmission rate) We are trying to cut down on the periodic lags in the remote desktop sessions. Could I have some suggestions as to what to do about this?

I am not at all familiar with MPLS! I will research it here in a moment.
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by:giltjr
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I can affect latency.  If the traffic has to flow over the WAN link, then yes, it will increase the volume of traffic going over the WAN, which increases bandwidth utilization.  If it increases it to the point where traffic is queued up to go across the WAN, then latency increases, because latency includes queue (waiting in line) time.

MPLS is great for prioritizing traffic, but normally MPLS based networks are more WAN connection that appear to be private site to site connections.  Not a normal Internet type connection, which it appears you have.

You would also have to be able to identify the traffic: print from interactive.  IIRC when you print via term serv, the traffic is sent from the term server to the term client and then forwarded to the printer.  In that case the print traffic looks just like the interactive traffic and MPLS (or any other type of QOS setup) can't tell the difference between the interactive and print traffic.

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giltjr earned 100 total points
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First thing you need to do is identify what traffic is flowing over the Internet link.  The two biggest and broadest categories you need to look at first is work related traffic and non-work related traffic.

If you find you have a lot of non-work related traffic, then you need to see what actions you can (are allowed to do by company policy)  take to reduce it.  Installing a caching proxy server, implementing bandwidth limiting devices/software/policies to limit the amount of bandwidth non-work related traffic takes.

Then for the work related.  How much of it is between the two sites and how much of it is NOT term serv traffic?  For non-term serv traffic you might be able to look at "WAN acceleration" devices.  For the most part these are really just compression boxes, they actually do more, but this is their basic function.  For non-term serv traffic they can reduce the volume of traffic by anywhere from 20-90% depending on the type of traffic you have.  But most WAN acceleration devices don't buy you much of the majority of your traffic is term serv traffic.

After that you need to look at other things such as increasing bandwidth.  However, it could be less expensive to setup a term serv in the romote office, depending on the sizing requirements to support that office.

The other problem you may find you have is the fact you have ADSL.  Asymmetrical links introduce  more performance issues as link utilziation increases when compared to symmetrical links.  In a term serv session the volume of traffic is normally the same in both directions, so you perfomance is going to be limited by the slowest link, in your case the 384 Kbps upstream speed on the remote side.  Once that gets saturated, you will start seeing slow downs.
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by:JP_TechGroup
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Thank you guys! Looks like I have some research to do!
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