Queuing Delay 1

Access Delay Some protocols require a sender to “gain access” to the channel –The channel is shared and some time is used trying to determine whose turn it is to send or waiting for that turn to begin That delay has been called Access Delay by some authors –From a user perspective the time between frames waiting for access effectively creates a lower data rate –The access delay is statistical in nature The number of competing senders and their traffic patterns determine whether our sender will find the channel busy and have to wait before sending a frame 2

More on Access Delay Notice that by making the atomic unit of transmission shorter (fewer bits in a frame) we decrease the transmit time for that frame – and thus decrease the time the network is busy with a frame that is blocking our access –But decreasing frame length increases the number of frames for a given file and therefore the number of times that the access mechanism and overhead must be used during the sending of that file 3

Access delay characteristics 4

Switching Most networks are switched –There is no direct physical channel between sender and receiver –Switching devices connect multiple physical channels together into a logical channel –The switching device 1) receives a frame or packet; 2) decides which output port is appropriate; 3) queues that frame or packet at the appropriate output port; and 4) when all of the previous frames in the queue have been sent then the frame is sent 5

More on Switching Today many switching devices use custom hardware to implement the switching decision –Early switches did this more slowly using software –Some authors call the time spent making this decision the switching delay (see the notes for this slide) –It is often just considered part of the queuing delay Some authors also consider time spent copying the data within the switch (from an input to output buffer for example) as part of the switching delay –Good switch design minimizes this delay 6

A switching device with no delay 7

Queuing Delay High traffic load increases the delay in the switching device (leading to congestion) –Intuitively, as the traffic load increases the probability that there is an earlier frame in the queue also increases –With certain assumptions about the statistical nature of the traffic the following result is obtained 8

Queuing delay continued Because the load is variable the queuing delay is variable –Variations in delay are called jitter –For some applications (e.g., You Tube) delay may be acceptable but significant jitter may not be acceptable Note that reducing the maximum size of a transmission unit affects the queuing delay in a manner similar to access delay Increasing the data rate on a heavily-loaded link reduces the queuing delay through that link 9

Cut-through with empty queue 10

Store & Forward with empty queue 11

Queue with competing frames 12

Higher output data rate 13

Summary The importance of access delay will depend on the protocol and the loading of the network –Most wired LANs use full-duplex Ethernet connections that do not have access delay –Wireless nets, WAN protocols often still have access- control overhead Queuing delay is primarily concerned with the delay caused while waiting to send –Previously received frames must be sent first –It is variable and it is unbounded As the load increases eventually frames are discarded That is essentially infinite delay!!! 14