Wireless Mesh Networks (WMNs) have matured in recent years and the visibility of WMN deployments has attracted commercial operators to investigate this technology for applicability in their networks. Having their roots in the Mobile Adhoc Network (MANET) world and rather cheap off-the-shelf single-radio WLAN routers, WMN routing protocols were not designed for applicability in carrier-grade back-haul networks. For example, protocols such as OLSR or B.A.T.M.A.N. can not address the QoS-requirements of a modern operator back-haul network with its increasing demand for triple-play content. Although numerous solutions have been proposed to introduce QoS-awareness at the protocol or the technology level, traditional WMNs fail to meet commercial operator requirements in terms of reliability, traffic engineering and QoS guarantees. This chapter proposes a novel approach combining an IEEE 802.21-based control plane and an MPLS-based data plane. To provide support for ubiquitous high-bandwidth multi-media services, it seamlessly integrates unidirectional broadcast technologies such as DVB into the heterogeneous multi-radio WiBACK architecture.
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The visibility of successful WMN deployments has raised interest regarding a commercial exploitation of this technology. Compared to traditional wireless operator back-haul networks, WMNs potentially offer easier deployment and flexible self-configuration at lower costs (Chieng, 2010). For a WMN-type architecture to be considered as an alternative for an operator back-haul network, it must meet strict requirements such as guaranteed Quality of Service (QoS), high availability and predictable behavior in high load situations. QoS support for WMNs has received a lot of attention from the research community (Kone, 2007)(Abu-Ali, 2006)(Zhou, 2009), but has yet to show its applicability in operator deployments.
Compared to OLSR(Clausen, 2003) or B.A.T.M.A.N. based community mesh networks, such as FREIFUNK, which rely on low-cost off-the-shelf hardware, a carrier- grade Wireless Back-Haul (WiBACK) network may assume more reliable and higher performing multi-radio hardware potentially yielding more stable and orthogonal links. Thus, a WiBACK network, as developed by the EU FP7 CARrier grade wireless MEsh Network (CARMEN) project (Azcorra, 2009)(Banchs, 2008), can be compared to a typical operator back-haul network with two major limiting factors: the severe capacity constraints due to scarce wireless spectrum resources and temporarily varying link conditions. A modern operator back-haul network is required to sup- port triple-play services (web, video, voice) which emphases the need to provide QoS guarantees while efficiently utilizing the resources in a capacity-constrained network. Wireless cell resource utilization can be optimized by using the most suitable technology for a given payload. For example, the delivery of live multimedia content is facilitated by only one transmitter, hence the overhead of MAC protocols can be avoided. Unidirectional technologies such as Digital Video Broadcast - Terrestrial (DVB-T) offer a very efficient distribution medium for 1-to-N traffic and usually span across multiple mesh hops.
Typically, WMNs rely on the bi-directionality of links and can therefore not utilize Unidirectional Links (UDLs). For example, Ad hoc On-Demand Distance Vector Routing Protocol (AODV) (Perkins, 2003) black-lists UDLs since they break the distance vector algorithm. What's more, most MANETs and WMN protocols cannot differentiate between, for example, a faulty IEEE 802.11 link and an inherently unidirectional link provided by a unidirectional technology. In this chapter we discuss our IEEE 802.21-based approach (Kretschmer, 2009) to seamlessly integrate Unidirectional Technologies (UDTs) into a heterogeneous WiBACKs network so that higher layer protocols and services can transparently utilize them when beneficial for a given payload or receiver distribution.