Download «Performance Analysis for a New Medium Access Control Protocol I

«Performance Analysis for a New Medium
Access Control Protocol in Wireless LANs»
By
YOUNGGOO KWON
and
YUGUANG FANG
Presentation by Ampatzis Efthimios
1
Presentation Scheduling
1.
Introduction and Backrounds
2.
Fast Collision Resolution Algorithm
3.
Performance Evaluation of FCR
4.
Conclusions
2
Why choosing a Contention Based MAC
Protocol for WLANs?
• Combination of efficient sharing to limited
spectrum resources and simplicity in
implementation.
• Incorporating Carrier Sense Multiple Access as well
as handshaking mechanisms for collision
avoidance (CA)
• Provides high throughput, low latency and fairness
in performance, using an FCR algorithm.
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Notes to be taken in consideration
• Wireless Medium/Channel causes severe
degradation on the performance of Transport Layer
due to low bandwidth and high error rate.
Degradation due to the characteristics incorporated
by the physical layer
• The encapsulation of packets , in MAC Layer may
cause many retransmissions of segments. MAC
Layer overheads usually cause additional collisions
and delays. During RTO (retransmission time out)
intervals TCP operations (ack signals) cannot be
carried out and performance degradates.
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What this scheme include/incorporate?
•
1.
2.
3.
4.
IP of the network layer is prevalently cooperating
with TCP and UDP in order to support:
Transparency in data transfer
Flow and congestion control
Ordering and receiving data
ACK signals
*note TCP , UDP run above MAC Layer
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Deficiency in WLANs
1.
2.
3.
Packet Collisions due to the increasement of
active users.
Deferring Stations are likely to collide again in
the future.
Wasted idle slots due to back offs in each
contention cycle.
*deferring stations are those that attempting to transmit, have
faced a collision and backed off
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IEEE 802.11 MAC protocol
• A packet transmission is considered to be
successful if only destination station answers back
with an «ack» signal.
• A carrier sense mechanism is used to check the
medium status. If the medium is idle, transmission
may proceed.
• If the medium is busy the station will defer until a
number of idle time slots is being detected (DIFS).
• Then, the station takes a back off time period,
based on the current Contention Window size.
Backoff Time (BT)=Random( uniform distribution) *aSlottTime
The choice is over the interval [0,CW-1].
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• The back off time period will be decreased by a
time slot, if only there is an idle time slot detected.
• When this period reach zero, transmission begins.
• After a SIFS time if an ack signal arrive then
transmission is complete.
• Then the contention window size has it’s minimum
value.
• In case that there is no ack signal the CW size will
be increased in order to obtain a higher probability
to transmit successfully the segment.
• The increment of CW size from it’s min value to its
max, will take place in a BEB way.
*BEB stands for binary exponential back off
*note MAC protocol is based on CSMA/CA mechanism
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Transport Layer
•
•
•
Provides end-to-end communication services
Flow control, error recovery, ack signals,
mult/demultiplexing sessions.
Incorporates two protocols:
TCP (transmission control protocol)
UDP (user datagram protocol)
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Transmission Control Protocol
• Provides reliability for each end host.
• Retransmits the segments that had no ack signal
response within RTO interval.
• It controls new packets’ transmission, taking into
consideration the amount of data, receiver’s
buffering capabilities, network congestion and
packet loss.
• Including mechanisms that allow multiple
transmission before the ack signals arrive back to
the sender.
• In case that a packet gets lost, mechanisms of
packet retrieval are activated.
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Transmission Control Protocol
• There exist a mechanism of slow start when a
packet loss occurs, fact that damage the end-toend throughput. Fast retransmit and fast recovery
have been proposed to speed up the recovery of the
connection.
• Indicative ack responses have been adopted
in order to point to the next packet expected to be
sent (monitor duplicated ack signals).
• There is an upper limit of lost packets that don’t
force the sender change in slow start mode. Thus
Fast Recovery Mechanisms perform instead of slow
start mode which ends up in data congestion.
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User Data Protocol
• UDP is transaction oriented and not connection
oriented as TCP.
• Connectionless.
• Minimizes overheads of the protocol mechanisms
• Delivery and duplicate protection are not assured.
• «Send and Pray» Protocol.
• UDP encapsulates raw IP datagrams and sending
them in a «connectionless» way.
• Suitable for one request and one response.
• UDP can be considered to be a bit closer to the
segment «format» and application process because
of its clear functionality onto the segment of data.
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Fast Collision Resolution Scheme
• Throughput Performance per Transmission Cycle
E[Nc]: avg num of collisions
E[Bc]: avg num of idle slots
ts : time slot duration
/m : avg packet length
SIFS: short interframe space
DIFS: distributed coordination interframe space
ack :acknowledgment signal duration
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MAC protocol design
• Best case scenario: «A successful packet
transmission must be followed by another packet
transmission without overheads».
• In this case E[Nc]=0 , E[Bc]=0 which ends up to:
• Back off time Bi assignment , should be zero for
transmission station and infinite for all other
stations. This is the idea for perfect scheduling.
• The probability of packet transmission for station i
would be:
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• The station i keeps it’s CW size (min value)
and transmits again with ptrans =1.
• But, dominance of the medium by one single
node must be prevented.
• In this case we reduce the amount of idle
back off slots for the deferred stations and
keep up fairness in the usage of the medium
by inserting a fast back off timer reduction
algorithm into the FCR.
• Generally ptrans = 1/(Bi+1)
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Operational Characteristics of a MAC protocol
with high throughput and good fairness
• Small random back off timer for the station that
just completes a successful transmission in a
contention cycle.
• Large random back off timer for the stations that
are deferred in the specific contention cycle.
• Adaptive assignment of back off timers according
to stations current states. Transmitting or
deferring.
• All deferred stations will give more time to the
transmitting station to finish with back logged
packets (net effect).
• Upper limit of successive transmissions by any
node.
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Fast Collision Resolution Algorithm
•
•
•
•
•
Special Topics in proposed “FCR”
CW size for deferring changes whenever an idle slot
is detected.
Additional back off time regeneration for
transmitting stations to avoid future collisions.
Use much smaller minimum initial CW size and
much larger maximum CW size than IEEE 802.11
MAC.
Reduce the back off timers exponentially when a
prefixed number of idle slots are detected.
When back of timers increase, the probability of
collisions decrease.
There is a maximum successive packet transmission limit to keep good fairness.
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Detailed FCR algorithm
Back off procedure
• Initial Decrement of Back off Time:
BTnew = BTold- aSlotTime
• If (minCW+1)*2-1 idle time slots being detected
BTnew = BTold-BTold/2
Transmission Failure (packet collision)
• If a station receive no ack signal the CW size will be
increased and a new random back off time will be
chosen:
CW=min (max CW, CW*2)
BT=uniform(0,CW-1)*aSlotTime
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Successful packet transmission
• If a station receive an ack signal new CW,BT values
will be assigned.
CW=min (CW)
BT=uniform(0,CW-1)*aSlotTime
• If the station reach the upper limit of successive
transmissions then:
CW=max (CW)
BT=uniform(0,CW-1)*aSlotTime
Deferring State
CW= min (maxCW,CW*2)
BT=uniform(0,CW-1)*aSlotTime
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• After a successful transmission, a station has the
minimum CW size and smaller back off timer,
hence higher probability to gain access of the
medium.
• When a station reach the limit of successful
transmission another station may win the
contention by increasing it’s probability to gain
medium access.
• If a good channel is being detected the modulation
rate can be change in order to transmit more
packets in the specific time period which is secured
by Network Allocation Vector.
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Performance Evaluation
The parameters used in simulations based on the
IEEE 802.11 network configurations
• The packet lengths are geometrically distributed (best effort
data trials) hence the probability:
P [PacketLength] = qi-1*(1-q) , i>=1
Thus the avg transmission time for a packet length
Mean (m) = ts/1-q (μs)
ts= aSlotTime
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•For 50 and 100 stations the IEEE 802.11 MAC algorithm
shows very poor throughput performance because the number
of collisions becomes higher as the number of active stations
becomes larger.
In the proposed FCR, future collisions can be prevented due to
the increment of CW size (makes the probability of transmission
lower of all the deferred stations by choosing properly CW sizes
which is reflecting on BT period).
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• The proposed MAC protocol works well at saturated situation
load.
• It also performs efficiently under light load conditions while
providing high throughput as network loads increases.
• The number of stations hardly affects the performance of
FCR.
• Adaptive characteristics of FCR in the medium access and
medium status.
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• FCR resolves collisions faster than IEEE 802.11
• The delay to the proposed FCR is the time period from the
time that the packet arrives from the higher layer to the MAC
layer until it is successfully transmitted. No queuing delay
because of no other packet generation till the complete
transmission of the current one.
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FCR improves the throughput of FTP connections
about 15-35% and also fairness index.
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Conclusions
•
FCR algorithm:
1.
Improves the performance (throughput, fairness, packet
delivery ratio) of the transport layer by reducing the
average number of wasted idle slots.
Preserves simplicity in implementation.
Supports TCP and UDP protocols of transport layer by
evaluating the degree of fairness .
Also improves the performance at higher levels due to
efficient collision resolution mechanisms.
Incorporates innovative ideas about CW sizes that are
reflected in BT periods which in turn change the
transmission probabilities to avoid future collisions.
2.
3.
4.
5.
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