in Figure 3-1. In the context of protocol structure, we will frequently use the terms "service" and "protocol." It must be noted that services and protocols are distinct concepts. A service is a set of primitives or operations that a layer provides to the layer(s) with which it is interfaced [2]. The service defines what operations a layer performs without specifying how the operations are implemented. It is further related to the interface between two adjacent layers. A protocol, in contrast, is a set of rules presiding over the format and interpretation of the information/messages that are exchanged by peer entities within a layer. The entities use protocols to implement their service definitions. Thus, a protocol is related to the implementation of a service. Mobile WiMAX. DOI: 10.1016/B978-0-12-374964-2.10003-7 Copyright © 2011 Elsevier Inc. All rights reserved. CHAPTER 3 IEEE 802.16m Reference Model and Protocol Structure OSI Seven-Layer Network Model Application Layer Presentation Layer Session Layer Scope of IEEE 802.16 Standards Protocols Transport Layer Service Specific Network Layer Convergence Sublayer Data-Link Layer Medium Access Control Common Part Sublayer Physical Layer Security Sublayer Physical Layer FIGURE 3-1 The mapping of IEEE 802.16 protocol layers to an OSI seven-layer network model Layer N+1 Layer N+1 Layer N Layer N Layer N Services or Functions Protocol Services or Functions Layer N-1 Layer N-1 Source Destination FIGURE 3-2 An illustration of service, protocol, PDU, and SDU concepts [2] As shown in Figure 3-2, a Protocol Data Unit (PDU) is a packet exchange between peer entities of the same protocol layer located at the source and destination. On the downward direction, the PDU is the data unit generated for the next lower layer. On the upward direction, it is the data unit received from the previous lower layer. A Service Data Unit (SDU), on the other hand, is a data unit exchanged between two adjacent protocol layers. On the downward direction, the SDU is the data unit received from t