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1IN2P3 octobre 2002 Jean-Pierre Thomesse
Chapter 2
Data transmission
2IN2P3 octobre 2002 Jean-Pierre Thomesse
Data transmission
Summary
lBasic delaysl Space-time representationl Delays related to the MAC
3IN2P3 octobre 2002 Jean-Pierre Thomesse
Propagation delay
l The propagation delay is the time for a bit to be transported along the channel
l d : the channel length in m,l V : the propagation speed in m/s,l d/V : the propagation delay or propagation time
Tp=d/VV usually is 2c/3, suppose 2 *10 8 m / s
4IN2P3 octobre 2002 Jean-Pierre Thomesse
Transmission time
l The time transmission is the delay to transmit or to receive a frame
l L the frame length in bits, l B the data rate in bit/s, l the transmission time or delay
Tt = L/B
5IN2P3 octobre 2002 Jean-Pierre Thomesse
Channel length
l Lc the length of the channel in bitsl Lc is the number of bits present along the channell Lc is the length of a frame such that
– the first bit is received when– the last bit is sent
Lc=B*d/VLc=B*Tp
6IN2P3 octobre 2002 Jean-Pierre Thomesse
Throughput
l The throughput is the real data rate taking into account the frame length and the propagation delay
Th=L/(Tp+Tt) in bit/s
l Tp+Tt is sometimes called “transfer delay”
7IN2P3 octobre 2002 Jean-Pierre Thomesse
Channel use Ratio
l The channel use ratio is the proportion betwwenthe throughput and the data rate
U = Th/B
l Let a = Lc/L, a = Tp/Tt
U = 1 /(1+ a)
8IN2P3 octobre 2002 Jean-Pierre Thomesse
Station load
l The load of a station represents the information quantity transmitted by time unit.
l The load is a function of the station activity l The load is dued to :
– periodic traffic– aperiodic traffic
l The load is expressed in :– frames/second– bits/s
9IN2P3 octobre 2002 Jean-Pierre Thomesse
Network load
l The load of a channel is the total of all the station loads.
l It depends on the protocols used in the profile
10IN2P3 octobre 2002 Jean-Pierre Thomesse
Data transmission
Summary
l Basic definitionsl Basic delays
lSpace-time representationl Delays related to the MAC
11IN2P3 octobre 2002 Jean-Pierre Thomesse
Frame propagation-1
l Delays
TtTp
12IN2P3 octobre 2002 Jean-Pierre Thomesse
Frame propagation-2
l Exercise
l Bus topologyl Ring topologyl Draw the positions of frames in both of the
topology cases, according to the a values :a<1, a=1, a>1
13IN2P3 octobre 2002 Jean-Pierre Thomesse
Frame propagation-3
l Bus topology
a = 1
a > 1
a < 1
14IN2P3 octobre 2002 Jean-Pierre Thomesse
Frame propagation-4
l Draw the frame propagation according to the value of “a” on a space-time diagram
Tp
Tta>1
Tp
Tta=1
Tp
Tta<1frame L>Lc
frame L<Lc
frame L=Lc
15IN2P3 octobre 2002 Jean-Pierre Thomesse
Data transmission
Summary
l Basic definitionsl Basic delaysl Space-time representation
lDelays related to the MAC
16IN2P3 octobre 2002 Jean-Pierre Thomesse
Turnaround Time
l The turnaround time is the delay necessary to switch from the receiving state to the transmitting one.
l It is always present in MAC based on a controlled access.
l Its value depends on the IC performances, but usually is equal to some tens of microseconds.
17IN2P3 octobre 2002 Jean-Pierre Thomesse
Transit time
l The transit time is the time when a frame is delayed in an equipment for any treatment, as example, for routing.
l This time is usually variable, depending on the load of the equipment and of the channels.
l It will be considered as constant.
18IN2P3 octobre 2002 Jean-Pierre Thomesse
Transit time
Repeaterl A repeater has to regenerate the signal to increase
the maximum distance between the stations.l It is often considered that the transit delay is equal
to 1 bit.l Exercise
– Compute the transit time in s.
19IN2P3 octobre 2002 Jean-Pierre Thomesse
Transit time
Routerl A router is an equipment which switches the PDU
from an arrival (input) channel towards an exit (output) channel.
l The principle is also called “Store and Forward”.
20IN2P3 octobre 2002 Jean-Pierre Thomesse
Transit time
Commutationl A switch is an equipment which provides a type of
routing, but without completely storing the PDU.l Only the first bits are stored until finding the
output channel associated to the destination address.
l The transit time depends on the number of bits to be stored and on the load of the switch.
21IN2P3 octobre 2002 Jean-Pierre Thomesse
Conclusion
l Definitions of elementary conceptsl Definitions of elementary delays
– useful for dimensioning and choice of network– other delays issued from upper layers– stochastic models necessary for
l random access and l for aperiodic traffic, l sporadic is a simplification