Downloading large file half duplex

[Sswweennyy]

Hello, if I have a half duplex 10 mbs connection, is the max speed to download a large file 5 mbs or would the download tend to hog the channel direction?

 

[PhilFrisbie]

It depends on the protocol being used. If this is homework that is all you get. . .

 

[Sswweennyy]

It depends on the protocol being used. If this is homework that is all you get. . .

No not homework - a difference of opinion with my network guy who wants me to spend some money. He is modern and I am an old mainframe performance guy so I am applying some ancient logic to new stuff that frankly I have a reasonable working knowledge of but I am not an expert (nor I fear is he - he claims queuing theory does not apply to networks which was my bread and butter).

I run a resort across 300 acres. There is a backbone layer 2 network that basically goes from the gateway wirelessly to a tower & feeds two access points. One AP is for the guests to access the internet and the second to send a PXP signal on to the next tower where the same configuration applies - ie another AP for clients and a PxP link to the next tower. We have a 10 meg down/1.5 meg up internet link which is running slowly.

He claims it is because the switches at the towers are running in 1/2 duplex mode and therefore cannot run over 5 meg/s and need to be replaced. My argument is that 1) the wireless links pretty much by derfinition must run 1/2 duplex and therefore, if he is right about the 1/2 speed, we would have to re-jig the wireless links as well. 2) 1/2 duplex means it runs and full speed in one direction at a time only. ie a big file is coming 'in' it will occupy the line most of the time and should run pretty close to 10 meg during this operation. If the traffic were 50/50 then yes, I would expect to see half the the throughput in each direction.

 

[PhilFrisbie]

Most traffic is TCP; which is generally asymmetrical. In other words, a half duplex link would likely be more like 90% download for the data, and 10% upload for the acknowledgements.

 

[Sswweennyy]

Most traffic is TCP; which is generally asymmetrical. In other words, a half duplex link would likely be more like 90% download for the data, and 10% upload for the acknowledgements.

Yes I agree but I don't think that has anything to do with a line being full or half duplex - ie it is 90/10 for full duplex as well. The question is,when it is half duplex and the data is coming down, I believe that it will 1) prevent any data up whilst transmitting down and 2) it will run at 'full' speed in either direction while that direction has control of the wire?

 

[Hawkeye22]

You didn't mention how many users you have or how they are using the internet. It may simply be a case of you flooding out your 1.5Mbit uplink.

I believe you are correct in saying that you should get full speed in either direction, but only in one direction at a time.

 

[PhilFrisbie]

Yes I agree but I don't think that has anything to do with a line being full or half duplex - ie it is 90/10 for full duplex as well.

That would be true if TCP waited for each acknowledgment (ack) before sending the next message, but it does not. TCP will send a number of messages while waiting for acks, so it can saturate one direction of a full duplex link.

 

[Sswweennyy]

That would be true if TCP waited for each acknowledgment (ack) before sending the next message, but it does not. TCP will send a number of messages while waiting for acks, so it can saturate one direction of a full duplex link.

Yes I know that but this is not my question. I'll start again and incorporate your last answer. If I'm alone, downloading a large file on a high quality, 1/2 duplex link, where, say, 50 packets are shipped to one ACK, would you think it (the file) would come down at closer to 10 mbs, 5mbs, 0.5 mbs or at some other speed? As noted, I have 10 down and only 1.5 up so it seems to me that making the end-to-end service full duplex is a waste of money as I would expect that the 10/1.5 service offering pretty much reflects the kind of performance I would expect to get in a 1/2 duplex situation

As a supplemental, is it not the case that the line always transmits at 10 mbs (or at least at a constant speed) and it is more about the amount of time it can sustain the speed that dictates the throughput of the connection. It seems to me that it may be a like CPU busy - there is no such thing as 30% CPU - a CPU is always either 100% or 0% busy. It is just that when measured over a period of time, the CPU is found to be busy in 30% of the samples & not that the CPU goes at different speeds. So, if I get 3 mbs down does that not mean that 70% of the time the line is either idle or uploading and 30% of the time it is downloading at 10 meg?

 

[Sswweennyy]

Best answer selected by SSwweennyy.

 

[Sswweennyy]

You didn't mention how many users you have or how they are using the internet. It may simply be a case of you flooding out your 1.5Mbit uplink.

I believe you are correct in saying that you should get full speed in either direction, but only in one direction at a time.

Yes, this could be the problem in which case an investment in making the service full duplex whilst maintaining the current 10/1.5 circuit would be a waste of money - do you agree? I think it would make more sense in upgrading to a 10/10 service? Usually about 10 concurrent , 20 max users doing general surfing, eMail etc - typical 'hotel' user. We lock an individual house down to 768k after a 2 megabyte download burst and 256k/512 for upload. The issue is that we are really out in the wild and it is difficult to get the links at any reasonable cost so I'm trying to determine where to put the dough. The main infrastructure expense is that the links are wireless which are or would be 1/2 duplex links between full duplex switches so if the problem is duplexing I have to replace the whole backbone..