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«Performance Analysis for a New Medium Access Control Protocol in Wireless LANs»

By

YOUNGGOO KWON

and

YUGUANG FANG

Presentation by Ampatzis Efthimios

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Presentation Scheduling

1. Introduction and Backrounds

2. Fast Collision Resolution Algorithm

3. Performance Evaluation of FCR

4. Conclusions

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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?

• IP of the network layer is prevalently cooperating with TCP and UDP in order to support:

1. Transparency in data transfer2. Flow and congestion control3. Ordering and receiving data4. ACK signals

*note TCP , UDP run above MAC Layer

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Deficiency in WLANs

1. Packet Collisions due to the increasement of active users.

2. Deferring Stations are likely to collide again in the future.

3. 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) *aSlottTimeThe 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-to-end 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 collisionsE[Bc]: avg num of idle slots ts : time slot duration

/m : avg packet lengthSIFS: short interframe spaceDIFS: distributed coordination interframe spaceack :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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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 transmis-

sion limit to keep good fairness.

Fast Collision Resolution Algorithm

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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 StateCW= 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 EvaluationThe 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>=1Thus 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.

2. Preserves simplicity in implementation.3. Supports TCP and UDP protocols of transport layer by

evaluating the degree of fairness .4. Also improves the performance at higher levels due to

efficient collision resolution mechanisms.5. Incorporates innovative ideas about CW sizes that are

reflected in BT periods which in turn change the transmission probabilities to avoid future collisions.