The IEEE 802.16 standard series represents the state-of-the-art in technology for metropolitan area broadband wireless access networks. The point-to-multipoint (PMP) mode of IEEE 802.16 has been designed to enable quality of service (QoS) in operator-controlled networks and, thus, is foreseen to complement existing third-generation cellular networks. In contrast, the optional mesh (MESH) mode of operation in IEEE 802.16 enables the setup of self-organizing wireless multi-hop mesh networks. A distinguishing characteristic of the IEEE 802.16 standard series is its support for QoS at the Medium Access Control (MAC) layer. However, the QoS specifications and mechanisms for the PMP and the MESH mode are not consistent. This article presents a novel QoS architecture as a key enhancement to the IEEE 802.16 MESH mode of operation. The architecture is based on the QoS mechanisms outlined for the PMP mode and, thus, enables a seamless coexistence of the PMP and the MESH mode. In particular, we look at the various options the standard provides and the trade offs involved when implementing QoS support in the 802.16 MESH mode, with a focus on the efficient management of the available bandwidth resources. This article is meant to provide researchers and implementers crucial anchor points for further research.
The demand for ubiquitous connectivity is the driving force for innovation in the field of wireless networks. To satisfy the differing demands of users a huge variety of wireless network platforms has developed over the years. Figure 1 shows some of the contemporary wireless access technologies.
From the figure, one can see that the IEEE 802.16 standard as published in intends to support metropolitan area networks, rural networks, or enterprisewide networks. Initially these networks are expected to support only static nodes (subscriber stations). The standard IEEE 802.16-2004 specifies two modes of operation as shown in Figure 2. In the point-to-multipoint mode (PMP) all the Subscriber Stations (SSs) are required to be in direct range of the Base Station (BS). The SSs can directly communicate only with the BS. Direct communication between two SSs is not supported in the PMP mode. On the other hand, when operating in the MESH mode, the SSs are allowed to establish communication links with neighboring nodes and are able to communicate with each other directly. In addition, they are also able to send traffic to and receive traffic from the BS (a MESH BS is a SS, which provides backhaul services to the mesh network). The MESH mode of 802.16 allows for flexible growth in the coverage of the mesh network and also increases the robustness of the network due to the provision of multiple alternate paths for communication between nodes. An overview of the 802.16 standard is provided in. In addition, current efforts in the 802.16e task group have led to the publication of the IEEE 802.16e specification. The latter mainly offers enhancements to the IEEE 802.16-2004 to support mobility.
Figure 1: (a) Overview of contemporary wireless access network technologies showing the geographical scale of the wireless technologies. (b) Point-to-multipoint (PMP) mode of operation in 802.16. (c) Mesh mode (MESH) of operation in 802.16.
A distinguishing feature of the IEEE 802.16 standard is the extensive support for QoS at the MAC layer. In addition, the IEEE 802.16 standard outlines a set of physical layer (PHY) specifications which can be used with a common MAC layer. This flexibility allows the network to operate in different frequency bands based on the users' needs and the corresponding regulations. The QoS support and flexibility at the PHY layer in the 802.16 standard make it an optimal base to support multi-service networks. Thus, 802.16 networks are expected to play a significant role in next-generation broadband wireless access (BWA) networks. Such networks cater to the demand for the so-called "Triple Play" networks, that is, a single network supporting broadband Internet access, telephony, and television services. They are thus expected to replace conventional Digital Subscriber Line (DSL)-based access networks. The needs of each of these application categories are however varying. For example, applications such as interactive video conferencing, telephony, etc. require predictable response time and a static amount of bandwidth continuously available for the life-time of the connection. On the other hand, traffic like variable rate compressed video streams (e.g., to support television services) relies on accurate timing between the traffic source and destination but does not require a static amount of bandwidth over the duration of the connection. Some other applications such as data transfer using File Transfer Protocol (FTP) have no inherent reliance on time synchronization between the traffic source and destination. However, these applications benefit when the network attempts to provide a guaranteed bandwidth or latency. Some other services may not be very important from the providers' point of view, and traffic belonging to this class may be serviced on space-available basis. This type of traffic has usually no reliance on time synchronization between the traffic source and destination. An example of application generating the latter type of traffic is web surfing. The 802.16 standard defines different data scheduling services to support these types of traffic; thus, providing tools for network operators to support multi-service networks.
We provide an overview of the QoS specification for the MESH mode as specified by the 802.16 standard. In particular, we introduce the mechanisms available in the PMP mode and the MESH mode. In future, IEEE 802.16-based networks are expected to support seamless interworking of nodes operating in the PMP and the MESH modes. The QoS specifications for the two modes however are not consistent. We provide an overview of our proposed QoS architecture for management of the bandwidth in the MESH mode to enable support of the data scheduling services similar to those available in the PMP mode.
Finally, we provide an insight into the benefits and effects of deploying our proposed QoS architecture which we have been able to observe via an intensive simulation study. The simulation study was carried out using a MESH mode simulator we built into the JiST/SWANS environment. We will here also highlight some promising areas for further investigation and outline areas for research which can build up on and extend the QoS architecture described by us.
