Bandwidth theory

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Jun 13th, 2009
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When we refer to bandwidth in terms , say 20 Mbps , it means both upload & download.

1.Does it indicate upload & download both are seperate logical channels with 20Mbps each?

2.Usage shown in bandwidth measurement tools mostly give download & upload as seperate lines.To determine level of congestion, do we add both of them to see the total usage of link or is it done someother way?

Thanks.

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Joseph W. Doherty Sat, 06/13/2009 - 04:38
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"When we refer to bandwidth in terms , say 20 Mbps , it means both upload & download. "


Usually, but not always. Sometimes used to denote sum of up and down.


"1.Does it indicate upload & download both are seperate logical channels with 20Mbps each? "


Again usually, but not always. Not all media supports duplex transmission. For example, 10 Mbps Ethernet, when "half-duplex", provides 10 Mbps that's "shared" up and down. 10 Mbps Ethernet, when "full-duplex", offers 10 Mbps up and down. Ten 10 Mbps Etherent, full-duplex, might also be called 20 Mbps.


For Ethernet, when half-duplex was common and full-duplex new, marketing would often refer to the bandwidth as 2x the half-duplex variant. Today, since full-duplex is often the norm, the bandwidth usually is up or down.


"2.Usage shown in bandwidth measurement tools mostly give download & upload as seperate lines.To determine level of congestion, do we add both of them to see the total usage of link or is it done someother way? "


Such tools show utilization, not congestion. To see congestion, you really need to see queuing. However, for typical "bursty" traffic, congestion is more likely as utilization increases.


Also, for duplex paths, each is considered separately.


suthomas1 Sat, 06/13/2009 - 06:20
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My understanding is as we see increased utilization & if it nears the subscribed bandwidth,our usage is touching peak.

If in full duplex links like Metroethernet & Serial, we do see downloads too high while uploads nominal.

What is the main command to see queuing and which factors to lookout to decide congestion.

Thanks.

Joseph W. Doherty Sat, 06/13/2009 - 10:46
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"What is the main command to see queuing and which factors to lookout to decide congestion. "


A command to see queue congestion depends on the device and its configuration.


Antoher method to decide if there's congestion, some type of monitoring that records ping times (SLA tests are best).

suthomas1 Sun, 06/14/2009 - 03:41
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Thanks, my doubts stem from a problem being faced with huge file transfer via ftp over mpls link. The capacity is 4Mbps & file size would be around 4GB. it is transferred by breaking it down into smaller segments of around 250 MB. its quite strange that even though the link usage is extremely low , at times the transfer doesnt complete and stops in the middle but sometimes it does complete.My guess, it may be due to way of tcp functioning.

Please correct me & Any suggestions to check on this.

Joseph W. Doherty Sun, 06/14/2009 - 03:56
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If bandwidth utilization is low and FTP won't complete; often indicative of a high drop rate.

suthomas1 Mon, 06/15/2009 - 03:35
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I have attached a view of usage on one of the links.My query is related to peaks circled on extreme left & extreme right.

This was one of the time when usage was high ( left) & file transfer got completed.

One on the right side is a smaller peak and during the file transfer couldnt complete.

1. For approximately the same duration(4hrs) on different days, the usage stays stable & moves smoothly without any lows or highs.What could be the possible reason for the flat top seen during both periods?

2. The rightmost trend seems to be constant at around 900Kbps, which puzzles me though there is still more bandwidth left to be used. How can that stream happen or be explained?


Is this do anything with tcp?

Thanks.



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Joseph W. Doherty Mon, 06/15/2009 - 04:41
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#1 "Flat top" normally would indicate full utilization at some barrier point. Could be physical, such as link bandwidth, could be logical, such as a shaper.


#2 Can it be explained? Usually yes, although might be very difficult to identify cause.


"Is this do anything with tcp? "


It's possible, alhough I would first suspect other causes.



Your postings have evolved from questions about duplex bandwidth and determination of congestion, to trying to determine why you're getting unexpected FTP performance results within your network.


By not making fully clear what your issue is on your origianal post, you deprive yourself of many readers expertise. You may want to repost. Don't ask general questions or interject your assumptions. Lay out what's the problem, as much inforamation as you have, ask for advice.


Unexpected poor network performance issues can be very difficult to resolve, especially when working across a provider's MPLS. These forums can't easily provide indepth analysis. If no simple solution is found, you might need to consider obtaining outside consulation.

suthomas1 Mon, 06/15/2009 - 23:21
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Thanks Joseph!I know i have mixed the posts up.Sorry for incovenience.

I will put it up as another post.


Thanks again



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