Adaptive traffic aggregation

Our architecture performs traffic aggregation on top of the MAC layer, allowing us to reduce the overhead due to both protocol headers and the contention mechanism regulating the IEEE 802.11 standard. The novelty of the proposed approach lies on the adaptive aggregation scheme that leverages the channel probing functionalities of mesh routers: such information are exploited in order to compute the optimal saturation burst length. A linear scaling is then applied in order to re-modulate the burst length to the unsaturated operation point. The proposed scheme is tested over an IEEE 802.11-based WMN testbed in the specific case of VoIP traffic, and showed a very large gain in the voice capacity attained, even in presence of background traffic.

Our aggregation scheme concatenates several MAC Service Data Units (MSDUs) to form the data payload of a large MAC Protocol Data Unit (MPDU). The PHY header and the MAC header together with the Frame Check Sequence (FCS) are then appended in order to build the Physical Service Data Unit (PSDU). The frame format for an Aggregated MSDU (A-MSDU) is the following:

The building blocks of the Aggregation Buffer and their relationships are sketched in the next figure.

Incoming MAC frames are first classified according to their destination address and then fed to a different queue. Each Aggregation Buffer maintains a pool of unused queues and an hash table that associates the MAC destination addresses with the corresponding queue. Unused queues are moved from the hash table to the pool, this is done in order to alleviate the need for repeated memory allocation as neighbors come and go. For each queue, an A-MSDU is generated when either an aggregation timer is expired or a burst of optimal length can be generated.

According with our architecture, packets aggregation and de-aggregation is performed at each hop. Albeit such an approach could lead to increasing delays as the number of hops increases, we postulate that, at intermediate nodes, medium access delay is sufficient to collect enough packets so that burst generation is triggered by the optimal frame length without incurring in any aggregation delay. The pseudo code for the hop-by-hop burst generation process is the following:

1: if size(queue) ≥ LOpt then
2:   if size(queue) ≤ BM ax then
3:     generate a burst no longer than LOpt bytes
4:   else
5:     generate a burst no longer than BM ax bytes
6:   end if
7: else if Aggregation timer is expired then
8:   aggregate all the packets in the queue
9: end if

The adaptive traffic aggregation scheme presented in this section is implemented as part of our QoS Provisioning framework.

R. Riggio, Francesco De Pellegrini, Daniele Miorandi, Imrich Chlamtac
A traffic aggregation and differentiation scheme for enhanced QoS in IEEE 802.11-based Wireless Mesh Networks (comcom2008.pdf)
Elsevier's Computer Communications Journal, Volume 31, Issue 7, May 2008, Pages 1290-1300

R. Riggio, Francesco De Pellegrini, N. Scalabrino, Pan Li, Yuguang Fang, and I. Chlamtac
Performance of a Novel Adaptive Traffic Aggregation Scheme for Wireless Mesh Networks (milcom2007.pdf)
in Proc. of Milcom 2007, Orlando, Florida, USA.