LAB OBJECTIVES This lab addresses the Medium Access Control (MAC) sublayer of the IEEE 802.11 standard for the wireless local area network (WLAN). Various options of this standard are studied in this lab. The performance of these options is analyzed under multiple scenarios. OVERVIEW The IEEE 802.11 standard provides wireless connectivity to computerized stations that require rapid deployment, such as portable computers. The Medium Access Control (MAC) sublayer in the standard includes two fundamental access methods: distributed coordina-tion function (DCF) and point coordination function (PCF). DCF utilizes the carrier sense multiple access with collision avoidance (CSMA/CA) approach. DCF is implemented in all stations in the wireless local area network (WLAN). PCF is based on polling to determine the station that can transmit next. Stations in an infrastructure network optionally implement the PCF access method. In addition to the physical CSMA/CA, DCF and PCF utilize a virtual carrier-sense mecha-nism to determine the state of the medium. This virtual mechanism is implemented by means of the network allocation vector (NAV), which provides each station with a prediction of future traffi c on the medium. Each station uses NAV as an indicator of time periods during which transmission will not be initiated even if the station senses that the wireless medium is not busy. NAV gets the information about future traffi c from management frames and the header of regular frames being exchanged in the network. With DCF, every station senses the medium before transmitting. The transmitting station defers as long as the medium is busy. After deferral and while the medium is idle, the trans-mitting station has to wait for a random backoff interval. After the backoff interval and if the medium is still idle, the station initiates data transmission or optionally exchanges request to send (RTS) and clear to send (CTS) frames with the receiving station. The effect of RTS and CTS frames will be studied in the Mobile WLAN lab. With PCF, the access point (AP) in the network acts as a point coordinator (PC). The PC uses polling to determine which station can initiate data transmission. It is optional for the Wireless Local Area Network Medium Access Control for Wirelessly Connected Stations 2 1112 Network Simulation Experiments Manualstations in the network to participate in PCF and hence respond to polls received from the PC. Such stations are called CF-Pollable stations. The PCF requires the PC to gain control of the medium. To gain such control, the PC utilizes the Beacon management frames to set the NAV in the network stations. Because the mechanism used to set NAV is based on the DCF, all stations comply with the PC request to set their NAV, whether or not they are CF-Pollable. This way the PC can control frame transmissions in the network by generating contention-free periods (CFPs). The PC and the CF-Pollable stations do not use RTS/CTS in the CFP. The standard allows for fragmentation of the MAC data units into smaller frames. Fragmentation is favorable in case the wireless channel is not reliable enough to transmit longer frames. Only frames with a length greater than a fragmentation threshold will be fragmented. Each fragment will be sent independently and will be separately acknowl-edged. During a contention period, all fragments of a single frame will be sent as bursts with a single invocation of the DCF medium access procedure. In case of PCF and during a contention-free period, fragments are sent individually following the rules of the point coordinator (PC). PRE-LAB ACTIVITIES & Read Section 2.7 from Computer Networks: A Systems Approach, 5th Edition . : Go to www.net-seal.net and play the following animation: m Wireless Network and Multiple Access with Collision Avoidance PROCEDURE Create a New Project To create a new project for the Ethernet network: 1. Start OPNET IT Guru Academic Edition · Choose New from the File menu. 2. Select Project · Click OK · Name the project < your initials>_WirelessLAN , and name the scenario DCF · Click OK . 3. In the Startup Wizard: Initial Topology dialog box, make sure that Create Empty Scenario is selected · Click Next · Choose Offi ce from the Network Scale list and check Use Metric Units · Click Next twice · Click OK . Create and Confi gure the Network To create our wireless network: 1. The Object Palette dialog box should be now on the top of your project space. If it is not there, open it by clicking . Make sure that the wireless_lan is selected from the pull-down menu on the object palette. 2. Add to the project workspace the nine wlan_ station_adv (fi x) from the palette. a. To add an object from a palette, click its icon in the object palette · Move your mouse to the workspace · Left-click to place the object. Right-click when fi nished. 3. Close the Object Palette dialog box · Arrange the stations in the workspace as shown in the following fi gure · Save your project.13NodeNamenode_0node_1node_2node_3node_4node_5node_6node_7node_8Random58671342DestinationAddress Confi gure the Wireless Nodes 1. Repeat the following for each of the nine nodes: Right-click on the node · Edit Attributes · Assign to the Wireless LAN MAC Address attribute a value equals to the node number (e.g., address 1 is assigned to node_1) · Assign to the Destination Address attribute the corresponding value shown in the following table · Click OK . a. The following fi gure shows the values assigned to the Destination Address and Wireless LAN MAC Address attributes for node_1. LAB 2 Wireless Local Area Network14 Network Simulation Experiments Manual Traffi c Generation Parameters 1. Select all nodes in the network simultaneously except node_0 (click on all of them while holding the Shift key) · Right-click on any of the selected nodes · Edit Attributes · Check the Apply Changes to Selected Objects check box. 2. Expand the Traffi c Generation Parameters and the Packet Generation Arguments hierar-chies · Edit the attributes to match the following fi gure · Click OK . 3. Select all nodes in the network simultaneously, including node_0 · Right-click on any of the selected nodes · Edit Attributes · Check the Apply Changes to