sfi

NDP

ENTERPRISE IRELAND

NCNRC

Communications Network Research Institute

Handoff on WLAN Mesh Networks

Yin Chen

WLAN Mesh Networks (WMNs) are a type of wireless network based on the IEEE802.11 standards that provides an innovative method to building large-scale, high bandwidth, multi-hop networks which dynamically establish connections between neighboring nodes on demand. As an example of a next generation network, a WMN has the following advantages: ease of deployment, low cost, reliability and scalability.

WMNs are continuing to grow in popularity - with new public and private deployments announced almost daily. The need to add voice applications requires the network to increase its overall performance in order to handle real time applications. Voice users are far more mobile that data users and will require a handoff capability that can transfer a call from one node to another in less than 50ms. WMNs pose a challenging problem to support VoIP-type delay sensitive applications, because the standard handoff procedure implemented in many current 802.11 products occurs with a delay that is unacceptable to voice users. The time required for handoff arises from a number of the mechanisms associated with the processes of association, authentication, and encryption defined in the IEEE802.11 WLAN standard.

This project will investigate the handoff process in WLAN mesh networks and will seek to identify the mechanisms that limit the performance. The problem arises when an 802.11 device moves between nodes - it only has about 50 milliseconds to make the change, i.e. it has only 50 ms to find a new node, complete its association and authentication, and set up quality-of-service (QoS) parameters for VoIP application. To solve this problem in a client can request that the QoS parameters be set up before committing to the move, which makes the transition smoother. This requires that a client-side application be developed that provides mechanisms to determine the best available node and to be able to work with associated nodes to pre-establish a connection to the next node. This application should be fully compliant with the emerging 802.11r and 802.11k standards.

Research to date (June 2008)

During the past few months, a series of simulations and modeling have been conducted to investigate the importance of beacon frames to the support of handoff.

Background

Under the IEEE 802.11 WLAN standard, there are three steps involved in the handoff: Discovery, Authentication and Re-association. Based on the nature of these steps, they can be divided into two phases, the scan phase and the execution phase .

Figure 1.0: Handoff Procedure

Delays in the two phases

By comparing the delay in two phases, it emerges that 90% of the handoff delay is incurred during the scan phase.

Beacon Frames

One of the key factors in fast handoff is when handoff should occur. The beacon frames [1] are the key to this problem. By changing the beacon frame interval, a client node can identify the nearby nodes quickly to provide fast handoff when handoff is required.

Beacon Frame Modeling

In order to study the bandwidth usage for different beacon frame intervals, the following equation is introduced:

Beacon Frame Modelling Equation
Fig 2.0: MNs BW used for beacon transmission

When operating under the 802.11b settings, approximately 4% of the capacity is used by the beacon frames. Figure 2 indicates that the beacon interval can be reduced to 70ms without significantly increasing the bandwidth used by beacon frames.


Fig 3.0: BW used for beacon transmission versus payload

Figure 3 shows that the beacon payload does not significantly impact on the bandwidth used by beacon which allows for the addition of more control data in the beacon frame. This will be utilized by future work in this area.

Simulation Setup

The network simulator (NS2) was used in the simulations. The simulations scenario involved two mesh nodes (MN) and a client node (CN): MN0, MN1 and CN0 respectively. Each MN has a 20 meters coverage radius and the two coverage areas do not overlap with each other. The MNs were configured to send beacon frames at the specified beacon interval. CN0 was moving back and forth between the two MNs at speed of 10m/s for 30 times in each simulation.


Fig 4.0: NS2 Simulation scenario

Different beacon frame intervals were used in the different simulation. No back traffic was produced during the simulation runs.

The results for the simulation were focused on the delay of the execution phase which includes the Authentication and Re-association delays.

Table 1: Summary of simulator Results

Figure 5: Delay versus Beacon Interval in Execution Phase

Table 1 shows the average delay time for the different beacon intervals. From Table 1 and Figure 5, it can be seen that the beacon interval can be deceased to 80ms without introducing a significant increase in the overall delay.

Conclusions

  1. The beacon interval can be reduced to 70ms without significantly increasing the bandwidth used by the beacon frames.
  2. When the beacon interval is reduced below 80ms significant increases in the delay time of the execution phase occur.
  3. One needs to establish the beacon interval threshold required to achieve a balance between the scan and execution delays.

Future Work

  1. Setup a testbed for testing the active scanning and passive scanning costs in terms of the size of the beacon interval.
  2. Develop a client-side application to manage handoff, i.e. to make the decision when handoff occurs and to decide which node to associate with.
  3. Modify the beacon frame to improve the performance of the client-side application.
  4. Configure or add extra functions to provide for fast handoff at the network-side.

References

[1] Beacon Frames are frames that have control information and are transmitted in each of the 11 channels and help a wireless station to identify nearby wireless access points (AP) in passive scanning mode. They tell the stations in the Basic Service Set (BSS) about the existence of the network. They can also be transmitted by the AP for polling purposes. The Beacon Frame sent by the AP contains control information and can be used by Mobile stations to locate an AP if it is on active scanning mode.