Transmit Delay

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The significant delay in the LAN• Propagation delay is not the only type of

delay– On a LAN the propagation delay is very small

because distances are very short– Yet we have all waited for a file transfer to

complete• On an uncongested LAN the transmit time

or transmit delay is often the most significant delay– Transmit delay is the time required for bits to be

transmitted or equivalently to pass by any point on the channel or link

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bitsseconds

bits/second_bT data rate

Implications of transmit time• Transmit time is proportional to the number

of bits and inversely proportional to the data rate of the channel– For a given channel (a fixed data rate): more bits

require more time to transmit– For a given number of bits: a higher data rate

means less transmit time • Propagation delay is difficult to change but

we can “throw money” at the transmit delay to reduce it– Buy better network equipment with a higher data

rate3

Duration is related to the data rate• The channel design determines the data rate

– The channel has some atomic group of bits that cannot be interrupted once their transmission begins• This might be an entire Layer-2 frame or some subset

with a name like a slot, or a mini-slot, etc.• The signals that represent those bits have a certain

duration• The duration of the signals divided by the number of bits

they represent is the inverse of the data rate• An individual bit may not be distinguishable in the real

channel• We use a simple channel with the same data

rate as the real channel so that we can explore the transmit delay concept more easily

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Relationship to propagation• The transmit delay is not affected by the

propagation velocity!!!– This seems counterintuitive because we are used

to physical objects whose size is fixed rather that logical objects whose duration is fixed

– For real objects – we know their size; we know their propagation velocity; we divide size by velocity to determine their duration (how long it takes to pass by)• For example a train going by

– For bits – we know their duration from the data rate before we even consider propagation• If we want to determine their physical extent we can

multiply their duration by their propagation velocity5

Our simple example again• For very simple channels we can illustrate

these concepts with a space-time diagram– We had either a red=0 or a green=1 light on for

½ second and then no light for ½ second – That is a data rate of 1 bit/s

• During each second we are signaling either a 0 or a 1– Invert the data rate to obtain 1 second/bit, i.e.,

the duration of a bit is one second• The time allocated on the channel for the signal

associated with that bit is 1 second even though half of the time there is no light on

• Our encoding scheme is very inefficient on this simple channel

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Sending 4 bits in our example

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Half the propagation velocity

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Half the data rate instead

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Practical considerations• Normally we are concerned with the transmit

time for the atomic unit of transmission (e.g., a frame) rather than an individual bit– A frame’s transmit time indicates how long the

channel is busy once that frame starts• The transmit time for a file is often

computed by dividing the bits in the file by the data rate– This is often not precise but may be good enough– There may be time lost between frames for

“housekeeping” (sometimes called an InterFrame Gap)

– There may be time lost between frames for access control on a shared network 10