Chapter 3: Protocol Layers and Topologies

In this chapter, the protocol layer architecture of WiMAX/802.16 is introduced. The main objectives of each sublayer are given as well as the global functions that they realise. Links are provided to the chapters of this book where each of these sublayers or procedures are described in much more detail.

3.1 The Protocol Layers of WiMAX

The IEEE 802.16 BWA network standard applies the so-called Open Systems Interconnection (OSI) network reference seven-layer model, also called the OSI seven-layer model. This model is very often used to describe the different aspects of a network technology. It starts from the Application Layer, or Layer 7, on the top and ends with the PHYsical (PHY) Layer, or Layer 1, on the bottom (see Figure 3.1).

Figure 3.1: The seven-layer OSI model for networks. In WiMAX/802.16, only the two first layers are defined

The OSI model separates the functions of different protocols into a series of layers, each layer using only the functions of the layer below and exporting data to the layer above. For example, the IP (Internet Protocol) is in Layer 3, or the Routing Layer. Typically. only the lower layers are implemented in hardware while the higher layers are implemented in software.

The two lowest layers are then the Physical (PHY) Layer, or Layer 1, and the Data Link Layer, or Layer 2. IEEE 802 splits the OSI Data Link Layer into two sublayers named Logical Link Control (LLC) and Media Access Control (MAC). The PHY layer creates the physical connection between the two communicating entities (the peer entities), while the MAC layer is responsible for the establishment and maintenance of the connection (multiple access, scheduling, etc.).

The IEEE 802.16 standard specifies the air interface of a fixed BWA system supporting multimedia services. The Medium Access Control (MAC) Layer supports a primarily point to-multipoint (PMP) architecture, with an optional mesh topology (see Section 3.7). The MAC Layer is structured to support many physical layers (PHY) specified in the same standard. In fact, only two of them are used in WiMAX.

The protocol layers architecture defined in WiMAX/802.16 is shown in Figure 3.2. It can be seen that the 802.16 standard defines only the two lowest layers, the PHYsical Layer and the MAC Layer, which is the main part of the Data Link Layer, with the LLC layer very often applying the IEEE 802.2 standard. The MAC layer is itself made of three sublayers, the CS (Convergence Sublayer), the CPS (Common Part Sublayer) and the Security Sublayer.

Figure 3.2: Protocol layers of the 802.16 BWA standard. (From IEEE Std. 802.16-2004 ^[1]. Copyright IEEE 2004, IEEE. All rights reserved.)

The dialogue between corresponding protocol layers or entities is made as follows. A Layer X addresses an XPDU (Layer X Protocol Data Unit) to a corresponding Layer X (Layer X of the peer entity). This XPDU is received as an (X-1)SDU (Layer X-1 Service Data Unit) by Layer X-1 of the considered equipment. For example, when the MAC Layer of an equipment sends an MPDU (MAC PDU) to a corresponding equipment, this MPDU is received as a PSDU (PHYsical SDU) by the PHYsical Layer (see Figure 3.2).

In this chapter, the different layers are introduced. Each of these layers or sublayers and many of their functions are described in the following sections.

^[1]IEEE 802.16-2004, IEEE Standard for Local and Metropolitan Area Networks, Air Interface for Fixed Broadband Wireless Access Systems, October 2004.