For those wanting to analyse a wireless LAN in OPNET IT Guru there are many different (often confusing) options to choose from. Here are some simple steps to get started.
There are two sets of wireless LAN objects: wireless_lan and wireless_lan_adv. For new users wireless_lan is sufficient in most cases.
Within the wireless_lan set of objects there are many node models to choose from. You have two options for analysing a wireless LAN:
There is a substantial difference in how traffic is modelled for the above two cases. For the case of the real network with clients, access points and servers, realistic applications and user profiles must be selected. That is, you must at least add and configure an Application Config and Profile Config object. There is a demo that illustrates what is needed for configuring these objects.
For the case of the generic network using only stations, you do not need the Application Config or Profile Config objects but instead configuring the traffic directly from the nodes. As shown in the Getting Started with IT Guru demo, when you edit attributes of each node you have the options to specify the Traffic Generation Parameters.
Which of the two cases should you choose? It depends on what you are interested in. If you want to know the performance of a network when different specific applications or types of users are in use, or you are analysing a network containing both wired and wireless portions, then the first case of the 'real' network should be used. However if you are only interested in performance characteristics of a wireless LAN (especially as parameters such as traffic load and packet size change), then the second case of the generic network can be used.
When you edit the attributes of any wireless LAN node (station, client, server, access point) you will eventually see a set of Wireless LAN Parameters. A brief explanation and some suggested (or default) values for a IEEE 802.11b network are:
| Parameter | Value | Description |
|---|---|---|
| WLAN MAC Address | Auto Assigned | Let OPNET automatically choose appropriate MAC addresses |
| RTS Threshold | None | If the data to send is greater than the RTS Threshold than RTS/CTS will be used; otherwise Basic Access will be used for that frame. |
| Fragmentation Threshold | None | If the data to send is greater than the Fragmentation Threshold than the data will be fragmented and sent as multiple frames. Note the comment about maximum size (read the Help by clicking on ?) |
| Data Rate | 11Mb/s | Physical layer data rate. IEEE 802.11b has a maximum of 11Mb/s. |
| Physical Characteristics | Direct Sequence | Physical layer used. Direct Sequence is common in IEEE 802.11b. |
| Packet Reception - Power Threshold | 7.33 E-14 | The minimum power at which the receiver can successfully receive. The suggested value (with the default transmit power) gives a range of approximately 300m. |
| Short Retry Limit | 7 | Number of retransmission attempts for a frame using Basic Access before the MAC gives up and drops the frame |
| Long Retry Limit | 4 | Number of retransmission attempts for a frame using RTS/CTS before the MAC gives up and drops the frame |
| Access Point Functionality | Disabled | Whether this node acts as an AP or not |
| Channel Settings | (...) | Allows you to set the Bandwidth and Minimum Frequency of the channel (defaults to 1000 and BSS Based) |
| Buffer Size | 1024000 | Amount of data received from higher layer that can buffered before it is dropped. If the application (or traffic generator) sends too fast, then the buffer may fill up leading to drops. |
| Max Receive Lifetime | 0.5 | Time to wait to reassemble a frame if fragmentation is used |
| Large Packet Processing | Drop | If Drop then all packets larger than 2304Bytes are dropped (read the help; it interacts with Fragmentation) |
| BSS Identifier | Not Used | Needed if you have more than 1 BSS |
| PCF Parameters | Disabled | If enabled, PCF is used instead of DCF |
The main global wireless statistics which record performance for the entire network are:
| Statistic | Description |
|---|---|
| Data Dropped | Rate at which data is dropped either due to the transmit buffer being full or because there were too many retransmissions. |
| Delay | Time it takes to successfully deliver data (including buffer delay) |
| Load | Rate at which data is sent by the wireless LAN source |
| Media Access Delay | Time it takes to successfully deliver a frame |
| Throughput | Rate at which data is received by the wireless LAN destination |
There are also detalied statistics available for individual nodes.
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