Packet Dispersion in IEEE Wireless Networks Mingzhe Li, Mark Claypool and Bob Kinicki WPI Computer Science Department Worcester, MA Second International Workshop on Performance and Management of Wireless Local Area Networks (P2MNet) Tampa, Florida, November 14, 2006

P2MNet Workshop November 14, Outline Introduction Packet Dispersion in WLANs WLAN Packet Dispersion Model Model Validation Model Analysis Conclusions

P2MNet Workshop November 14, Introduction Bandwidth estimation techniques focus on network capacity or available bandwidth. Most bandwidth estimation has involved only wired networks. This paper focuses on packet dispersion in wireless LANs (WLANs).

P2MNet Workshop November 14, Outline Introduction Packet Dispersion in WLANs WLAN Packet Dispersion Model Model Validation Model Analysis Conclusions

P2MNet Workshop November 14, Packet Dispersion Techniques Packet pair dispersion –two equal-sized packets are sent back- to-back through the network. Packet train dispersion –multiple back-to-back probe packets are sent through the network.

P2MNet Workshop November 14, Packet Pair Dispersion narrow link

P2MNet Workshop November 14, NS-2 Additions to Model WLANs Receiver Based Auto Rate (RBAR) module was re-implemented in NS 2.27 to model dynamic rate adaptation [ Sadeghi et al.] An NS-2 extension to model Rayleigh fading was implemented [Ratish et al.]

P2MNet Workshop November 14, Wireless Rate Adaptation under Rayleigh Fading NS-2 simulation results

P2MNet Workshop November 14, Probing traffic –Packet pairs or trains sent downstream through the AP to the wireless client. Crossing traffic –Downstream traffic going through the AP to other wireless clients. Contending traffic –Upstream traffic that accesses the shard wireless channel and competes with probe packets on the estimated path. Wireless Traffic Classification

P2MNet Workshop November 14, Wireless Traffic Classification

P2MNet Workshop November 14, WLAN Packet Dispersion Issues Wireless frame retries – increase the packet delay variance that produce packet dispersion inconsistencies. Dynamic Rate Adaptation Fading channel Noisy wireless channel - BER Contending Traffic

P2MNet Workshop November 14, Simulated Wireless Effects on Bandwidth Estimation CDF represents 1000 packet-pair estimates 1000-byte probe packets and CBR contending packets Contending traffic = 1Mbps upstream flow

P2MNet Workshop November 14, Outline Introduction Packet Dispersion in WLANs WLAN Packet Dispersion Model Model Validation Model Analysis Conclusions

P2MNet Workshop November 14, WLAN Packet Dispersion Model Assumptions Bottleneck link is the WLAN last hop. No crossing traffic. Dispersion, T, between two packets in a packet pair is characterized in terms of E[T] and V[T] for a WLAN that includes: packet size, link data rate, BER and access methods (base or RTS/CTS). While wireless channel conditions can be characterized by received signal strength indicator (RSSI), SNR and BER, our simplified model assumes these other factors impact BER. –The impact of channel conditions on bandwidth estimation is evaluated via V[T], the packet dispersion variance.

P2MNet Workshop November 14, WLAN Packet Dispersion Model From previous models, we build a new model for wireless packet dispersion where dispersion is defined as the delay between the arrival times of the first and second packets in the packet pair. Thus, the model must include the delay before the transmission of the second packet, E[D], and the time to transmit it, T s, and dispersion is: E[T] = E[D] + T s

P2MNet Workshop November 14, WLAN Packet Dispersion Model Since E[D] depends on the wireless link rate, C l, the average packet size, L and the number of nodes in the contention domain, n, we have E[D] = d (C l, L, n) Similarly, for the transmission time, T s = t s (C l, L)

P2MNet Workshop November 14, Bandwidth Estimate Defining, C est, as the bandwidth estimate based on the wireless packet pair dispersion model, our model is defined by:

P2MNet Workshop November 14, Outline Introduction Packet Dispersion in WLANs WLAN Packet Dispersion Model Model Validation Model Analysis Conclusions

P2MNet Workshop November 14, Model Validation Validation results include an ideal WLAN and a scenario with contention and BER. Ideal validation includes NS-2 simulations, analytic model and actual measurements.

P2MNet Workshop November 14, Ideal WLAN Scenario The ideal scenario consists of an AP with a single wireless client for both basic (BAS) and RTS/CTS access methods. Simulations: 500 pairs Measurements: 100 pairs

P2MNet Workshop November 14, Model Validation Random Model Simulation Parameters Topology

P2MNet Workshop November 14, Models with Contention and Wireless Transmission Errors RTS/CTSBasicRTS/CTSBasic Mean Error 8.05%4.90%9.40%7.67% Standard Deviation 6.72%4.28%5.30%3.82% Errors in the Bandwidth Estimation Model Compared with Simulations Error Free BER = 10 -5

P2MNet Workshop November 14, Outline Introduction Packet Dispersion in WLANs WLAN Packet Dispersion Model Model Validation Model Analysis Conclusions

P2MNet Workshop November 14, Model Analysis In a non-saturated WLAN with low BER, low contention, packet-pair dispersion estimates the maximal channel capacity, effective capacity, C e. With considerable contending traffic and/or BER, the dispersion metric is referred to as the achievable throughput, A t, for the current level of contending traffic.

P2MNet Workshop November 14, Model Analysis Effective Capacity Achievable Bandwidth Relationship with Available Bandwidth

P2MNet Workshop November 14, Achievable Throughput

P2MNet Workshop November 14, Impact of Channel Rate on Effective Capacity

P2MNet Workshop November 14, Impact of BER on Achievable Throughput Five node, BAS wireless network with 1500-byte packets no achievable throughput

P2MNet Workshop November 14, Impact of RTS/CTS on Achievable Throughput Model Results for Wireless Networks with 1500 Byte Packets

P2MNet Workshop November 14, Standard Deviation of Achievable Throughput

P2MNet Workshop November 14, Packet Size Effect on Standard Deviation of Bandwidth Estimations Five node, BAS wireless network with 1500-byte packets

P2MNet Workshop November 14, Impact of Channel Rate on Standard Deviation of Bandwidth Estimations

P2MNet Workshop November 14, Impact of BER on Standard Deviation of Bandwidth Estimations

P2MNet Workshop November 14, Outline Introduction Packet Dispersion in WLANs WLAN Packet Dispersion Model Model Validation Model Analysis Conclusions

P2MNet Workshop November 14, Conclusions Packet dispersion measures the effective capacity and the achievable throughput of a wireless network instead of the capacity as in a wired network. Wireless channel conditions, such as BER and RTS/CTS access method impact the bandwidth estimation results and the variance of the results.

Packet Dispersion in IEEE Wireless Networks Mingzhe Li, Mark Claypool and Bob Kinicki WPI Computer Science Department Worcester, MA Second International Workshop on Performance and Management of Wireless Local Area Networks (P2MNet) Tampa, Florida, November 14, 2006 Thank You !!

P2MNet Workshop November 14,