LAB 2 Wireless Local Area Network Medium Access Control for Wirelessly Connected Stations 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 coordination 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 mechanism 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 traffic 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 traffic 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 transmitting 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 11 Network Simulation Experiments Manual stations 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 acknowledged 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 12 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 Office from the Network Scale list and check Use Metric Units Click Next twice Click OK Create and Configure 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 fix 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 finished 3 Close the Object Palette dialog box Arrange the stations in the workspace as shown in the following figure Save your project LAB 2 Wireless Local Area Network Configure 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 Node Name Destination Address node 0 Random node 1 5 node 2 8 node 3 6 node 4 7 node 5 1 node 6 3 node 7 4 node 8 2 13 a The following figure shows the values assigned to the Destination Address and Wireless LAN MAC Address attributes for node 1 Network Simulation Experiments Manual Traffic 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 Traffic Generation Parameters and the Packet Generation Arguments hierarchies Edit the attributes to match the following figure Click OK Buffer Size specifies the maximum size of the higher layer data buffer in bits Once the buffer limit is reached the data packets arriving from the higher layer will be discarded until some packets are removed from the buffer so that the buffer has some free space to store these new packets 14 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 Selected Objects check box 4 Expand the hierarchy of the Wireless LAN Parameters attribute Assign the value 4608000 to the Buffer Size bits