Chapter 1
Routing and Scheduling for WiMAX Mesh Networks Jianhua He† , Xiaoming Fu‡ , Jie Xiang§ , Yan Zhang§ , Zuoyin Tang† †
Institute of Advanced Telecommunications, Swansea University, UK.
Email:
[email protected]. ‡
University of Goettingen, Germany.
§
Simula Research Laboratory, Norway.
1.1
Introduction
In today’s telecommunications networking and services are changing in a rapid way to support next generation Internet (NGI) user environment. Wireless networks will play an important role in NGI. Wireless broadband networks are being increasingly deployed and used in the last mile for extending or enhancing Internet connectivity for fixed and/or mobile clients located on the edge of the wired network [1]. With high data rate, large network coverage, strong QoS capabilities and cheap network 1
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CHAPTER 1. ROUTING AND SCHEDULING FOR WIMAX MESH NETWORKS
deployment and maintenance costs, WiMAX is regarded as a disruptive wireless technology and has many potential applications [1] [2]. It is expected to support business applications, for which QoS support will be a necessity. Depending on the applications and network investment, WiMAX network can be configured to work in different modes, point-to-multipoint (PMP) or Mesh mode. For example, the network can have a simple base station (BS) working in PMP mode and serving multiple subscriber stations (SS) if the potential SSs can be covered by the BS. Mesh topology is an optional configuration for WiMAX networks. In the Mesh mode, traffic demands are aggregated at a set of Subscriber Station (SS) nodes which are equipped with 802.16 interfaces. Subsequently, the traffic demands at SS nodes are delivered to a set of Base Stations (BS) nodes which functions in the PMP mode. These BS stations can be connected by a backhaul and connected to Internet Access Point (IAP) nodes. An amendment to the IEEE 802.16 specifications (where WiMAX is based) is IEEE 802.16j, Multihop Relay Specification. IEEE 802.16j is being developed by IEEE 802.16’s Relay Task Group. It is expected to extend reach/coverage through relaying [3]. In this chapter, we will focus on the Mesh mode. Wireless mesh network offers increased reliability, coverage and reduced network costs [4] [5]. There are extensive research, standardization and commercial development activities on mesh networks [4] [5] [6] [7]. For example, several IEEE special task groups have been established to define the requirements for mesh networking in wireless personal area networks (WPANs), wireless local area networks (WLANs) and wireless metropolitan area networks (WMANs). A brief description of the standardization activities can be found in [4]. Wireless mesh networks can be a prospective solution for broadband wireless Internet access in a flexible and cost-effective manner. However wireless mesh networks also raises a number of research challenges, e.g., network routing, scheduling, QoS support, network management, etc [4] [8]. Those challenges are faced by WiMAX mesh networks without exception. WiMAX Mesh mode is defined with OFDM for frequency between 2 and 11 GHz and time division multiple access (TDMA) is used in the MAC layer to support multiple users. Unlike the single hop wireless networks, routing algorithms are required to determine routes for the connections between a subscriber station (SS) and a base station (BS). As WiMAX networks operate synchronously in a time slotted mode, it is also necessary to allocate time slots without collision over the network to achieve assigned bandwidth for each connection. More
1.1. INTRODUCTION
3 Internet
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Access Point 3G Base Station
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(a) WiMAX PMP network Internet
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WiMAX Mesh BS
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(b) WiMAX mesh network
Figure 1.1: WiMAX PMP network and mesh network architectures. challenging is that the routing and scheduling for WiMAX networks are tightly coupled. The routing and scheduling problem for WiMAX networks is different from 802.11 based mesh networks. In the 802.11 based mesh networks, the MAC layer is contention based; routing algorithms and MAC layer protocols can be designed and operated separately. Although IEEE 802.16 standards specify several QoS schemes and related message formats, the problems of scheduling algorithms for both PMP and Mesh mode are left unsolved. Routing algorithms for WiMAX networks are outside the scope of the standard work as well. In this chapter, we will investigate the issues of routing and scheduling in WiMAX mesh networks. Both distributed and centralized routing algorithms will be studied, and their effectiveness on alleviating potential network congestion will be compared. The scheduling
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CHAPTER 1. ROUTING AND SCHEDULING FOR WIMAX MESH NETWORKS
problem will also be mathematically modelled by taking into account the interference constraints. Solutions are developed to maximize the utilization of network capacity subject to fairness constraints on allocation of scarce wireless resource among the subscriber stations. The Chapter is organized as follows. The WiMAX mechanisms defined for Mesh mode is overviewed in Section I. Existing research on scheduling and routing for WiMAX mesh networks is presented in Section II. Both distributed and centralized routing algorithms will be presented in Section III. A scheduling problem for WiMAX mesh networks is mathematically modeled and solved in Section IV. Typical numerical results are presented in Section V. Open research issues are discussed in Section VI. Finally we conclude the Chapter.
1.2
Overview of 802.16 Mechanisms for Mesh mode
Unlike the PMP mode that only allows communication between the BS and SS, each station is able to create direct communication links to a number of other stations in the network instead of communicating only with a BS. However, in typical network deployments, there will still be certain nodes that provide the BS function of connecting the Mesh network to the backbone networks. When using Mesh centralized scheduling to be describe below, these BS nodes perform much of the same basic functions as the BSs do in PMP mode. Communication in all these links in the network are controlled by a centralized algorithm (either by the BS or decentralized by all nodes periodically), scheduled in a distributed manner within each node’s extended neighborhood, or scheduled using a combination of these. The stations that have direct links are called neighbors and forms a neighborhood. A node’ s neighbors are considered to be one hop away from the node. A two-hop extended neighborhood contains, additionally, all the neighbors of the neighborhood. In this section, we will briefly introduce the frame structure, network entry procedures, bandwidth request and grant mechanisms defined for WiMAX Mesh mode, which will be the base requirement for the design of routing and scheduling algorithms to be presented later.
1.2. OVERVIEW OF 802.16 MECHANISMS FOR MESH MODE 1.2.1
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Identifications
There are several types of identifications that an SS will use for different purposes. Each SS has a 48-bit universal MAC address, which uniquely defines the SS from other SSs. The MAC address is used during the network entry process and as part of the authorization process by which the candidate SS and the network verify the identity of each other. After authorized to the network, a candidate SS will receive a 16-bit node identifier (Node ID) upon a request to the Mesh BS. Node ID is the basis for identifying SSs during normal operation. The Node ID is transferred in the Mesh subheader, which follows the generic MAC header, in both unicast and broadcast messages [2]. To facilitate communications with local neighboring SSs, an SS will use a 8-bit link identifiers (Link IDs). Each SS shall assign an ID for each link it has established to its neighbors. The Link IDs are communicated during the Link Establishment process as neighboring SSs establish new links. The Link ID is transmitted as part of the Connection ID in the generic MAC header in unicast messages. The Link IDs are used in distributed scheduling to identify resource requests and grants. Since these messages are broadcast, the receiver nodes can determine the schedule using the transmitter Node ID in the Mesh subheader, and the Link ID in the payload of the MSH-DSCH (Mesh mode Schedule with Distributed Scheduling) message. The MSH-DSCH message will be introduce later.
1.2.2
Frame structure
Unlike in PMP mode, there are no clearly separate downlink and uplink subframes in Mesh mode. A Mesh frame consists of a control and a data subframe [2]. There are two types of control subframes, which serves different functions, which will be introduced below. All transmissions in the control subframe are sent using QPSK-1/2 with the mandatory coding scheme. The data subframe is divided into minislots [2]. A scheduled allocation consists of one or more minislots.
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CHAPTER 1. ROUTING AND SCHEDULING FOR WIMAX MESH NETWORKS time Frame n-1
Frame n
Frame n+1
Network Control subframe Network Entry
Network Configure
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Network Entry Long Preamble
MAC PDU w/ Guard Guard MSH-NENT Symbol Symbol
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Guard Symbol
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Figure 1.2: WiMAX Mesh Network Control subframe.
Network Control Subframe
The first type of control subframe is termed Network Control subframe, used to create and maintain cohesion between the different systems. The Network Control subframe is illustrated in Fig.1.2. Frames with a Network Control subframe occur periodically. The period of occurrence is indicated in the Network Descriptor. The length of the control subframe is fixed and of length OFDM symbols, which is also indicated in the Network Descriptor. During a network control subframe, the first seven symbols are allocated for network entry, followed by sets of seven symbols for network configuration with MSH-NCFG messages. MSH-NCFG messages provide a basic level of communication between nodes in different nearby networks. All the nodes (BS and SS) in the Mesh network will transmit MSHNCFG [2]. The MSH-NCFG message format is shown in Fig.1.3. Through the MSH-NCFG message, a BS or an SS will report a number of its neighbors. The number of neighbors reported on may be a fraction of the whole set of neighbors known to this SS. A node will also report the Mesh BSs that its neighbors report and report the distances in hops to the BSs. The Embedded Packet Flag is used to indicate if an embedded data information element (IE) is included in the message. Network Descriptor is one of the 5 defined embedded data IE that can be included in MSH-NCFG. Transmit Antenna indicate the logical antenna
1.2. OVERVIEW OF 802.16 MECHANISMS FOR MESH MODE
Management Message Type=39 (8 bits) NumNbrEntries (5 bits) NumBSEntries (2 bits) Embedded Packet Flag (1 bits) Transmit Power (4 bits) Transmit Antenna (3 bits) NetEntry MAC Address Flag (1 bit) Network base channel (4 bits) reserved (4 bits) NetworkConfig Count (4 bits) Timestampe Frame Number (12 bits) NetControl Slot Num in Frame (4 bits) Synchronization Hop Count (8 bits)
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NetConfig schedule info Next Xmt Mx (3 bits) Xmt Holdoff Exponent (5 bits) If (NetEntry MAC Address Flag) NetEntry MAC Address (48 bits) for (i=0; i
