Eliminating Handoff latencies in 802.11 WLANs using Multiple Radios: Applications, Experience, and Evaluation Vladimir Brik, Arunesh Mishra, Suman Banerjee Presented by: Ibrahim Ben Mustafa For Wireless Newtorked Systems Class (Spring 2013)
Outline Problem Definition Motivation Contribution Introduction Background MultiScan Evaluation Related Work Conclusion Page:2
Outline Problem Definition Motivation Contribution Introduction Background MultiScan Evaluation Related Work Conclusion Page:3
The Problem Deployment of real-time streaming apps are limited in WLAN. Handoff latencies for mobile users are high It may result in connectivity gap. Effect the quality of service for real-time streaming applications such as VoIP. prior solutions are unpractical Page:4
Motivation Improves the performance of the real-time streaming applications for mobile users in WLAN. Removing the limitation of developing the Realtime streaming application in 802.11 WLAN. Page:5
Contribution Multi-Scan technique: Multiple radios. Implemented entirely on the client side. Compatible with any wireless card. Operates completely at link layer. Completely eliminating the Handoff latency. Page:6
Outline Problem Definition Motivation Contribution Introduction Background MultiScan Evaluation Conclusion Page:7
Introduction IEEE 802.11 WLAN enable the users to move and stay connected. The communication range of 802.11 devices is limited. Multiple APs are required. Handoff operations can occur very often. Page:8
Introduction contd... Handoff: takes between 60-400ms. Adequate for discrete scenarios. Inadquate for continues scenarios Poor handoff performance is a major obstacle to deployment of VoIP. Solution: eliminate the handoff latency. Page:9
Introduction contd... Using multiple Radios Practical and timely. May degrade the performance TCP based application did not perform well due to re-ordering effects in multi-radio mesh network. Page:10
Introduction contd... Experiment (1) One radio interface Experiment (2) Two radio interfaces. Each one associated to separate Aps. Non-interfering Channels 1 and 11. An interface performs ping flood to its AP. Packets assumed lost if no response in 100ms. Page:11
Introduction contd... Page:12
Introduction contd... The experiment with two interfaces experienced higher interference (higher RTTs) Can not be used for simultaneous data transfer. MultiScan uses: One radio interface as the primary data transfer interface. The other interface is used to facilitate the a fast make-before-break handoff. Page:13
Outline Problem Definition Motivation Contribution Introduction Background MultiScan Evaluation Related Work Conclusion Page:14
Background WLAN consists of a number of APs. Neighboring APs operate on independent channels Different 802.11 standard have different number of non-interfering channels available. 802.11b has 3 such channels 1,3,11. A client needs to change its association with different Aps (handoff). Page:15
Background contd... Handoff process stages: 1.Scanning 2.Authentication 3.Association %90 of the time is spend on scanning. The interface engaged in scanning can not be used for communication. Page:16
Outline Problem Definition Motivation Contribution Introduction Background MultiScan Evaluation Related Work Conclusion Page:17
Handoff in Multiscan Multi-radio scenario: Primary interface associated with AP (old) and used for communication. Secondary interface associated with AP(new) and used for association related task Page:18
Handoff in Multiscan (A) naïve approach: Secondary interface performs the scanning stage. The primary interface continues to communicate. Once the secondary interface determines the AP(new), the primary card starts the handoff. It skips the scanning stage (20ms) The node does not do any unexpected operation for the APs. Page:19
Handoff in Multiscan (B) Aggressive approach: Secondary interface proceeds to associate with AP(new). Once it finishes its associations, the roles of the two interfaces are swapped. Page:20
Handoff in Multiscan Detailed approach: 1. Normal operation: - Primary interface for communication. - Secondary interface for scanning channels. 2. Re-association - if necessary, the secondary interface starts an association with new AP. 3. Interface switch: - All the outgoing traffic is sent via secondary interface. 4. Completion: - the two interfaces completely switch roles. Page:21
Handoff in Multiscan Address Management Requirement: no change in the APs infrastructure Both interfaces uses the same IP and MAC addresses Standard utilities allow for setting MAC address. AP does not know about the two radios Page:22
Outline Problem Definition Motivation Contribution Introduction Background MultiScan Evaluation Related Work Conclusion Page:23
Experimental Evaluation Experiment Setup: Client: Single laptop running linux It has Multiscan installed. Two independent APs: Set up on the same host using independent wireless cards with two widely separated antenna. Operate on channel 1 and 11. Page:24
Experimental Evaluation ICMP ping floods and handoffs Echo request packets. 10 handoffs within one minute. Page:25
Experimental Evaluation ICMP ping floods and handoffs Page:26
Experimental Evaluation Skype and handoffs 10 handoffs in one minute. Two metrics: End-to-end latencies: between 10 and 30ms. Quality of audio, measured by cross-correlation: Page:27
Experimental Evaluation Skype and handoffs Page:28
Experimental Evaluation Skype and handoffs Page:29
Outline Problem Definition Motivation Contribution Introduction Background MultiScan Evaluation Related Work Conclusion Page:30
Related Work Multi-Radios for different applications: Reducing the energy consumption. Improving web performance in cellular networks. Improving performance for wireless mesh networks. Improving handoffs performance: Neighbor graph: extra functionality is implemented in both clients and APs. SyncScan: requires time synchronization. Page:31
Related Work Page:32
Conclusion Using two radio interfaces is feasible and is heighly recommended in eliminating the latency in WLANs. using both interfaces for data transfer simultaneously may degrade the performance. Utilizing multiple radios does not impose any additional on the wireless spectrum resources. MultiScan is developed as a public and it will shortly be available. The cost of using two radios solution is modest. Page:33
Questions?