On the Performance Behavior of IEEE Distributed Coordination Function M.K.Sidiropoulos, J.S.Vardakas and M.D.Logothetis Wire Communications Laboratory, Department of Electrical & Computer Engineering, University of Patras, Patras, Greece

Outline  Purpose of the paper.  DCF-An Example.  Mathematical Analysis ( Assumptions ).  DCF: Markov Chain Model and Steady State Analysis leading to a Saturation Throughput formula. leading to a Saturation Throughput formula.  Simulation results ( IEEE b network).  Conclusion.

Purpose of the paper  We propose a new Markov model for the DCF of IEEE based on Bianchi’s, Wu’s and Ziouva’s models. based on Bianchi’s, Wu’s and Ziouva’s models.  and derive an analytical formula for the Saturation Throughput for both Basic and RTS/CTS access schemes. for both Basic and RTS/CTS access schemes.  Simulation Study: Validation of our new Markov model based on throughput results by the NS-2.Validation of our new Markov model based on throughput results by the NS-2. Average end-to-end packet delay for both access schemes.Average end-to-end packet delay for both access schemes.

DCF-An Example DCF employs 2 mechanisms:  Basic access scheme: A 2-way handshaking technique. Note that:  After a DIFS time interval each station defers for an additional random backoff time.  The backoff counter is frozen if a transmission is detected on the channel BACKOFF SUSPENSION

DCF-An Example (cont.) DCF-An Example (cont.) RTS/CTS:  Request-To-Send / Clear-To- Send.  It is a 4-way handshaking technique.  Introduced to tackle the hidden terminal problem.  Improve throughput performance in case of long packets.

Mathematical Analysis Assumptions :  Ideal channel conditions ( error-free channel).  Finite number of stations, each of which has always a packet available for transmission. (saturation conditions) available for transmission. (saturation conditions)  Constant and independent collision probability p.  Probability independent of the backoff procedure.  Probability p b independent of the backoff procedure.

DCF: Markov Chain Model

Saturation Throughput model. Normalized Throughput: ( fraction of the channel time used for payload transmissions)  Ts: average time of a successful transmission  σ : duration of an empty slot time  Tc : average duration of a collision.  E[P]: average packet payload.  Ps : a successful transmission in a slot.  Ptr : at least one transmission in a slot.

Steady State Analysis Stationary Distribution of the chain ( Steady State ): From the chain we have:

Steady State Analysis (cont.) Normalization condition : Contention Window :

Steady State Analysis (cont.) Collision probability : Probability of channel being busy : Channel access probability :

Saturation Throughput model. Normalized Throughput: ( fraction of the channel time used for payload transmissions)  Ts: average time of a successful transmission  σ : duration of an empty slot time  Tc : average duration of a collision.  E[P]: average packet payload.  Ps : a successful transmission in a slot.  Ptr : at least one transmission in a slot.

Simulation Study  Performance metrics measured by simulation: by simulation: Saturation throughput.Saturation throughput. End-to-end average packet delay.End-to-end average packet delay.  Simulations in NS-2  IEEE b single-hop network. network.  Network Topology : No hidden stations, all have LOS.No hidden stations, all have LOS. CBR traffic over UDP links towards the AP.CBR traffic over UDP links towards the AP. No mobility.No mobility.

Model Validation: Simulation vs. Analysis: 1Mbps. Basic and RTS/CTS  Close match of analytical model and simulation results.  Our model is closer to simulation than Wu’s.  The RTS/CTS gives higher throughput than Basic due to the short RTS frames. ( Only exception for n =5).

Model Validation: Simulation vs. Analysis: 5.5 and 11 Mbps.  In both cases analysis and simulation are in satisfactory agreement.  Basic access scheme gives higher throughput than RTS/CTS when channel bit rate ↑. ( RTS, CTS packets are transmitted at 1Mbps).  Throughput ↓ as bit rate↑ ( DIFS,SIFS, Backoff delay remain unchanged)

Average Delay Simulation  As network size ↑ delay ↑ for both access schemes.  As channel bit rate ↑ delay ↓.  RTS/CTS delay is lower than Basic delay only for 1Mbps. Not efficient to use RTS/CTS for high data rates

Conclusion  We have developed an analytical model to enhance Bianchi’s and Wu’s analytical model for the saturation throughput of the DCF of the IEEE protocol.  Our model gives greater throughput results than Wu’s model for both access schemes, Basic and RTS/CTS.  Via numerous simulations with NS-2 we have shown that our model is close to simulation, for all network sizes.  As channel bit rate increases:  throughput decreases  Average delay decreases.  Basic vs. RTS/CTS :  In low rates RTS is better than Basic.  In higher rates Basic is preferable than RTS ( gives greater throughput and lower delay).  