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Understanding Congestion in IEEE 802.11b Wireless Network. Amit P.jardosh Krihna Romashandran Kevin C.Almeroth Elizabith M.Royer University of California Santa Barbara. Introduction The occurrence of high density nodes with a single collision domain in W.N result in congestion . problems:
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Understanding Congestion in IEEE 802.11b Wireless Network Amit P.jardosh Krihna Romashandran Kevin C.Almeroth Elizabith M.Royer University of California Santa Barbara
Introduction • The occurrence of high density nodes with a single collision domain in W.N result in congestion . • problems: Dropping in throughput . Unacceptable packet delay
We aim to answer: 1. what does congestion mean in W.N ? 2. how do we identify it ? 3. what are the challenges un the congestion analysis ?
Some factors to explain the behavior of MAC layer : 1. Channel busy time. 2. RTS-CTS mechanism. 3. Frame transmission. 4. Acceptance delay
Overview for Distribution Coordination Function DCF: • Designed to reduce contention. • Use CSMA/CA. • Use RTS-CTS
CSMA/CA • Designed to reduce contention. the node sense link if ( busy ) wait (back off interval) else transmit
How to reduce contention in DCF? • By using RTS-CTS RTS: contains information about size of data and channel time needed. other node record these information and wait for a back off interval (BO) by assign a counter. sensing busy stop the counter.
Multi Rate Adaptation protocol • Adapt the rate of which frames are transmitted , HOW? by using Auto Rate Fallback (ARF) Dropping packets decreasing rate Successful packets increasing rate
The Data Collection Framework Vicinity sniffing:a method used to collest data from the MAC layer. capturing data within two session: • Day session . • Plenary session. • Day session in an early time divided it into sniffers. • Plenary session in an late time dividing it into sniffers.
Sniffer: amount of time about 2.5 hours. • Implementing by distributing the APs in different location and collecting the needed data for the MAC layer thus we can study the traffic of the flowing data between terminals and nodes. • The data contains the control messages and data packets transmitted through each session
The traffic in the plenary session is lower than it in the day session. • And so by this way we can monitor the congestion domain between the N.W terminal so we can implement the multi rate adaptation.
Vicinty sniffing challenges • Choice of sniffer locations. • Unrecorded frames. 1. frame dropped due ro bit errors. 2. hardware limitation (such as buffers) 3. hidden terminal problem.
Solution for these challenges • Estimation for the unrecorded data: 1. Data frames: Every data frame need an ACK and SIFS time. 2. RTS frames: RTS need CTS after SIFS delay. 3. CTS frames: The node which send RTS and receive CTS will send a data frame.
But if the data, CTS and RTS are missing? The algorithm support a percentage equation for this problem as follows: Unrecorded% = unrec-frames unrec-frames + captured frames
Defining congestion • Congestion: is the state which the channel is closed to being completely utilized by the transmission of bytes. • Can be measured by the mean of channel busy-time.
Channel Utilization • Channel utilization= transmission time for all data and control frames + delay time (SIFS + DIFS)
When the channel utilization increases? 1. The channel utilization increases for larger data frame (according to the time spent) 2. Decreasing the rate of transmission.
Data frame : CBT= DDIFS +DDATA(s, r) • RTS frame : CBT = Drts • CTS frame : CBT= DSIFS + DSIFS • ACK frame : CBT = DSIFS + DACK • Beacon frame : CBT = DDIFS + DBEACON
Throughput and Goodput • Throughput : total number of recorded bits per second. • Goodput :total number of recorded control and successfully acknowledged data frame per second.
Classifying Congestion • We classify N.W state into: • Uncongested state (30% utilized). • Modrately congested state(30-84% utilized). • Highly congested state (above 84% utilized).
Acceptance Delay: is the time for the data to be acknowledged (the time between the transmission of data and the time when the ACK was recorded)
conclusion • The N.W controlled by monitoring the traffic of the congestion domain by using protocol like vicinity sniffing and its parameters. • After monitoring we can control it by using Auto Rate Fallback (ARF) protocol.