**4. Case study**

346 Wireless Communications and Networks – Recent Advances

Unlike the behavior model where only single stimuli - response pair of a protocol entity is evaluated, the performance model introduces new entities which are needed for completing the communication scenario emulation, both on system and block level. On the system level besides multiple instances of the modified behavior model, it is crucial to introduce a simulation control block and a channel block. The control block manages the generation of block-instances and controls the simulation time, while the channel block ensures a proper emulation of exchanging packets (e.g. radio frequency packets) among the communicating entities. Each block instance is characterized by a unique process identifier (PID), which

As it was previously stated, in the foundation of the performance model lays the behavior model. It is necessarily modified (i.e. upgraded), in order to conduct its expected role of real communicating device emulation. Many new processes with an appropriate purpose should be introduced in this upgraded version of the behaviour model: controller of the primitive exchange process, procedures for queuing and prioritization of the signals, processes for segmentation and reassemble of the massages, manipulators of simulation time (timers and clocks), etc. All these processes are introduced according to the textual part of the protocol

After building the performance model using the SDL Graphical Representation (SDL-GR), abstract Data Types (ADTs) are added in order to introduce important functionalities (e.g.

reading and writing to file, different kinds of random number generators, etc.).

Fig. 3. Building a generic performance model.

specification.

enables differentiation and addressing of the identical entities.

This section contains an implementation case study of the previously proposed methodology. In particular, the case study extends the findings proposed by (Latkoski, 2010), and provides the needed validation of the analytical and numerical analysis contained in (Latkoski, 2010).

The targeted communication system is based on WiMAX technology, standardized by IEEE 802.16. It belongs to the group of wireless metropolitan area networks (WMANs), which are on the steady track of widespread deployment in many urban environments. This worldwide trend is facilitated by the ever growing demand for "last-kilometre" network connectivity in every part of those urban environments with guaranteed service quality. A significant portion of the WMAN installations are based on the IEEE 802.16 technology, which is mature enough for seamless and low-cost deployment.

The focus of the analysis provided here, is the protocol which is responsible for bandwidth allocation among the WiMAX users. The WiMAX channel access is controlled by one of the several available Medium Access Control (MAC) procedures. (Latkoski, 2010) studies the contention-based bandwidth request procedure based on original analytical model, facilitated by numerical analysis. In (Latkoski, 2010) the key parameters of the contention procedure are optimized in order to minimize the average bandwidth access delay, thus ensuring the highest possible quality of service (QoS) to the WiMAX users.

WiMAX supports several QoS classes: UGS – Unsolicited Grant Service (E1/T1), real-time Polling Service – rtPS (MPEG), non-real-time Polling Service – nrtPS (FTP) and Best Effort – BE (HTTP). Except for the UGS that uses dedicated uplink transmission slots, the remaining service classes use the bandwidth request procedures over the uplink to the base station (BS). Depending on the service class, the access scheme can be either contention-based or based on unicast polling. The vendor-specific implementation can offer two optional nonmandatory procedures: piggybacking and bandwidth stealing procedures. Here, we focus on the IEEE 802.16 contention-based bandwidth request access scheme, which supports the BE class of traffic, generated by most Internet applications (web surfing, FTP, etc.). Additionally we will compare this scheme with the round-robin polling scheme, as well as with the multicast-groping-based principal of bandwidth management. All three access schemes are briefly explained in the following subsections.
