QoS provisioning
Our architecture aims at achieving service differentiation and performance isolation (at layer 2.5) in IEEE 802.11-based WMNs is proposed. It is our intention to provide strict QoS performance bounds (which is still an open issue in IEEE 802.11 WLANs), instead we want to propose a lightweight and backward compatible DiffServ architecture that aims at enhancing the perceived quality of experience by combining traffic prioritization and packet aggregation in IEEE 802.11-based WMNs. It is worth stressing that the proposed technique does not require any modification to the IEEE 802.11 MAC and can be readily implemented over existing hardware.
Our prioritization scheme exploits the DiffServ framework in order to allow classification and differentiated treatment. DiffServ QoS provision is a coarse-grained, class-based mechanism as opposed to the fine-grained, flow-based mechanism that characterize IntServ. The DiffServ framework defines a set of mechanism to classify and mark packets belonging to a specific class. Forwarding properties associated with a traffic class are implemented as Per-Hop Behaviors (PHBs). Different PHBs may be defined to provide, for example, low-loss, low-latency forwarding properties or best-effort forwarding properties. The PHB is indicated by encoding a 6-bit value, called the Differentiated Services Code Point (DSCP), into the 8-bit Differentiated Services (DS) field of the IP packet header (the former Type of Service, ToS field).
The overall architecture of the traffic prioritization scheme is sketched in the next figure.
Network traffic entering a mesh router is classified by DSCP code and then fed to a suitable queue. Traffic differentiating is provided by means of a Deficit Weighted Round Robin (DWRR) scheduler which pulls packets from buffers, according to some input weights. The aggregation buffer has been modified in order to exploit the A-DRR scheduling policy.
Our DiffServ framework is implemented on top of Roofnet, an experimental IEEE 802.11-based WMN deployed at Cambridge, Massachusetts (USA) by the Massachusetts Institute of Technology (MIT). Roofnet routes packets using a modified version of DSR called SrcRR exploiting ETX as routing metric. Routing is implemented using the Click modular router. We have extended the default Roofnet configuration by implementing the additional elements responsible for packet aggregation and scheduling.
Scheduling elements:
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ADWRRSched. Schedules packets according to the airtime deficit weighted round robin policy. The inputs usually come from Queues or other pull schedulers.
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DWRRSched. Schedules packets according to the deficit weighted round robin policy. The inputs usually come from Queues or other pull schedulers.
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WFQSched. Performs simple packet-based weighted round robin scheduling (implements weighted fair queuing spread).
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WRRSched. Performs simple packet-based weighted round robin scheduling (implements statistical weighted round robin).
Buffer elements:
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Aggregator. Concatenates multiple MAC PDUs. Buffers can be polled in using either the ADRR or the DRR disciplines.
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DeAggregator. Expects aggregated packets as input. Splits incoming packets.
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FairBuffer. Incoming packets are classified according to their Ethernet destination address and fed to a buffer. Buffers can be polled in using either the ADRR or the DRR disciplines.
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
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 \ in Proc. of Milcom 2007, Orlando, Florida, USA.