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Understanding Queueing

Unanswered Question
Oct 22nd, 2005
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Can somebody clear me of below matter: -


If my Router Link has capacity of 64Kbps, and traffic input to router is less than the output capacity, then will there be any packets waiting in the outputs queue of the interface to get out.


Also based on above clarification I want to know that why the PING response gives more delay if my traffic load is 70% than if traffic load is 50% of the output limit (64Kbps).


I feel delay should be same because my Link can withstand 64Kbps. And delay should only be there once my traffic input to router is above 64Kbps. So far the traffic input to router is less than the traffic output it can support, delay should always be constant as there will not be any output queuing of the traffic. And no delay should happen because of that.
































































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scottmac Sat, 10/22/2005 - 05:49
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Assuming that you are feeding the router with (at least) Ethernet, even at 10Mbps, you are going to buffer frames/packets.


Ethernet always runs at full speed (10, 100, 1000); if you are sending small packets/frames, they are still delivered at 10(100, 1000)Mbps.... a small package delivered very fast, relative to your 64k serial connection.


It must be buffered (enqueued) so that it can be serialized and re-packaged into the format of your serial link (HDLC, Frame-Relay, whatever).


So, again, even though your THROUGHPUT is below the rate of your serial link, the information is still delivered at full speed (10,100,1000) and requires conversion to your serial protocol ... both of which require buffering / queuing of some sort.


Good Luck


Scott


rohit_s Sat, 10/22/2005 - 06:29
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Hi Scott,


Thanks for the nice explanation. Correct me if I am wrong in understanding you as below: -


U mean to say, that packet input to Router is at 10/100/1000 Mbps and Output would be 64kbps, so that’s why queuing will happen. Also encapsulation of the data as per the Serial Link protocol requires queuing.


Also clarify me for the below point: -


1. The Output rate in the Show interface Serial command is the speed at which data is flowing over the link or is it the data which the router is sending over the transmission link in a second. I am confused in understanding that output.


2. Can Router send more than 64kb of data in a second??? And is it the clock, which limits the Router for the amount of data it can send per second??


3. Is there any delay in picking the data from the output queue and putting it over the link??



Thanks


Rohit



scottmac Sat, 10/22/2005 - 14:27
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There's rate and there's throughput. No matter how much data is flowing, it flows at the designated rate for the interface.


The actual amount of data passed is throughput.


You can have 64K of data pass from Host A to Host B - you have a throughput of 64K at a rate of 10Mbps (or whatever the interface speed is).


Data rates are expressed as quantity per time period i.e., "Bits Per Second," so the interface is displaying to you the rate that the data will flow. How much data actually passes would be described as the throughput (100 megabytes at a rate of 64kilobits per second).


The router interface cannot send faster then the set access rate (the clock rate).


There usually is a small delay from queue to the wire ... the "serialization delay."


Serialization happens several times from your host to the WAN - your PC (for example) handles information in parallel data paths (X bytes wide)- it send it to the NIC which send the bytes across the ethernet one bit at a time.


The receiving host de-serializes the bitstream back to bytes and sends it to the next step. In this case, it would be the WAN/Serial interface. The bytes are re-serialized and clocked out to the WAN according to the signaling protocol in-effect for that segment.


Every device that the data passes through wil add some delay, maybe a little, maybe a lot ... it depends on the device and the input/output rates.


Depending on the link, there may be other factors affecting the time it takes to traverse the link. For example, if you were using a satellite link, the latency from transmitter (through the satellite) to the receiver can be measured in seconds. The rate is still (say) 64Kbps, the throughput is still whatever it is, but the link itself is the source of significant delay (it takes time to go 22,500 miles up, then 22,500 miles back, plus any buffering in the satellite, the transmitting system, or the receiving system).


Hope this helps ...


Scott


rohit_s Mon, 10/24/2005 - 02:31
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Thanks for the update...


As per your quote..If the interface is displaying the rate the data will flow, then I hope it should be 64Kbps (which is the line speed).


I feel that input/output rate on Show interface output reflects the amount of data sent by the Router per second. Hope I am right here...


Also I feel that output queue will only have the stuck up data if the processor is sending data at more than 64Kbps which the serial cannot send.Otherwise the serial would be able to take the data from queue and send it across the link.(if the queue data is less than 64kbps/Sec)


Data will only be buffered in the input queue as receving speed is more because of 10/100 Mbps ethernet connection and Processor will process it slowly and will send it to output queue.


Does my above conclusion based out of the entire communication with you is OK. Or still I am mistaken.

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