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Emulab Tutorial - Using Wireless Networking

Emulab Tutorial - Using Wireless Networking

Some Emulab nodes contain 802.11 a/b/g wifi interfaces (Atheros 5212 chipset), and are scattered around at various locations in a large building. To find out where they are located and what their node IDs are, see the wireless floormaps page. When you click on one of the colored dots, you will be taken to a page describing the node and what type of interfaces the node has installed in it.

In addition to the nodes deployed throughout the building, we've also concentrated a subset of them in our machine room. 36 of the pc600s now have 802.11 a/b/g interfaces, also with the Atheros 5212 chipset. Each has an external antenna deployed on the backs of the racks containing the pc600s. To find out which nodes are where, look at the floormaps for this sub-cluster. For connectivity, antenna positions, and related information, see the Environmental Information subsection of this document.

Before using a wireless network interface in an experiment, please read the ''Wireless Acceptable Use Policy'' regarding wireless interfaces.

Wireless Acceptable Use Policy

By using our wireless nodes, you agree to be bound by this AUP.

  • Receiving: You may listen to and monitor anything you like. You may not disclose, on purpose or accidently, the IP addresses or DNS names of connections used by others. If contemplating disclosing the applications used by others, or similarly private information, you must first get explicit approval from testbed-ops. In case of any doubt about disclosure, contact testbed-ops. If you happen to observe any other information that a user would expect to be private, such as plaintext Web passwords or account names, you will not exploit that information, and you will take care not to let it leak out publicly, e.g., in log files.
  • Transmitting (1): Do not transmit on channels that another experiment on the testbed is using, unless it's your own. You can find out which channels are currently in use by looking at the top of the wireless floormap.
    Where possible, choose channels that have the least frequency overlap with other experiments. (channel/freq table coming.)
  • Transmitting (2): Do not flood a wireless network with non-responsive traffic for any significant period of time. The following channels are "production networks" used by others at this location, so are more restricted. You may not send "large" amounts of traffic on them, and may send only low rates of non-responsive traffic.

Protocol Channels
A 36, 38, 45, 52, 56, 60, 64
B/G 1, 3, 5, 6, 7, 9, 11
  • Question and Exceptions: send to Testbed Ops if you're not sure, or want an exception.

Creating an Experiment with Wireless-Capable Nodes

To use a wireless network in an experiment, you must provide a few Emulab specific NS directives in your NS file, which are illustrated in the following small example:

	source tb_compat.tcl
	set ns [new Simulator]

	# Allocate the nodes.  Their "wifi-ness" is determined later,
	# by the type of networks you request to be set up on them.
	set nodew1 [$ns node]
	set nodew2 [$ns node]
	set nodew3 [$ns node]
	set node4  [$ns node]

	# A wireless lan connecting the first three nodes.
	set lan0 [$ns make-lan "$nodew1 $nodew2 $nodew3" 54Mb 0ms]

	# A regular duplex link from a wireless-capable node to a plain node.
	set link0 [$ns duplex-link $nodew1 $node4 100Mb 0ms DropTail]

	# Choose the wireless lan protocol.
	tb-set-lan-protocol $lan0 "80211g"

	# Set an access point.  This node becomes the access point;
	# others in the LAN become stations of it.  You can also set other
	# modes for your LAN, such as Adhoc mode.
	tb-set-lan-accesspoint $lan0 $nodew1

	# Choose some other settings.
	tb-set-lan-setting $lan0 "channel" 2
	tb-set-node-lan-setting $lan0 $nodew1 "txpower" "auto"

	# Select a wireless-capable image (i.e., FC4-WIRELESS)
	# Let the other node default to RHL-STD (currently 9.0).
	tb-set-node-os $nodew1 FC4-WIRELESS
	tb-set-node-os $nodew2 FC4-WIRELESS
	tb-set-node-os $nodew3 FC4-WIRELESS

	# Turn on static routing.
	$ns rtproto Static
	$ns run

As mentioned above, there are two different node types with wireless interfaces. The `pc3000w` nodes are deployed in a wide range around our building. We have also added a single wireless interface to 36 of the `pc600`s. If you wish to include only pc3000w nodes in your experiment, for each node, set its hardware type:

       set nodew1 [$ns node]
       tb-set-hardware $nodew1 pc3000w
       set nodew2 [$ns node]
       tb-set-hardware $nodew2 pc3000w
       set nodew3 [$ns node]
       tb-set-hardware $nodew3 pc3000w

If you wish to include only pc600 nodes in your experiment, for each node, add a "desire" to the node:

       set nodew1 [$ns node]
       $nodew1 add-desire pc600wifi 1.0
       set nodew2 [$ns node]
       $nodew2 add-desire pc600wifi 1.0
       set nodew3 [$ns node]
       $nodew3 add-desire pc600wifi 1.0

These are simply alternative ways of instructing Emulab's resource allocator, assign, to select nodes of specific type or feature when you wish.

In this example, we created a simple infrastructure mode LAN with a single access point. You can also place LANs in Adhoc mode, for instance. If you wish to do this, do not specify an access point with the tb-set-lan-accesspoint command. Instead, set the mode for the LAN explicitly:

	tb-set-lan-setting $lan0 "mode" "adhoc"

A few additional points should be noted:

  • lan0 is created with standard make-lan directive, but with a bandwidth that reflects the typical speed of a wireless link (54Mb for 80211a and 80211g, 11Mb for 80211b). Note that you may not use duplex links of wireless interfaces; just LANs. We assign each LAN a unique ssid.
  • After you create the LAN, choose the protocol for the LAN with the tb-set-lan-protocol directive. Currently, you can set the protocol to one of 80211a, 80211b, or 80211g.. If you fail to set the protocol, the LAN will consist of wired links, most likely with delay nodes inserted!
  • link0 is a plain duplex link. You may create plain links as long as the nodes have enough wired interfaces. If you try to create an experiment that uses more links (wired or wireless) on a node than are available in Emulab, the experiment will fail to map. Of the current 54 wifi nodes, the 18 pc3000w wifi nodes (pcwf1-18) contain two wifi cards and one wired experimental interface and the 36 pc600 (all pc600s except pc8,pc11,pc30,pc38) nodes contain one wifi card and four experimental interfaces.
  • In the example above, we let Emulab decide what wireless nodes to use for the LAN. This is not an ideal approach, as Emulab does not currently track the connectivity of nodes, and so might pick a set of nodes (randomly) that cannot communicate with each other. See the section below on choosing your nodes for more information on how to overcome this problem.
  • You will want to put your LAN or individual nodes in a specific operating mode. In the example above, we used the tb-set-lan-accesspoint command to specify that nodew1 be the access point, and all other nodes become stations connected to that node. Rather than using dedicated access points, Emulab's implementation of wireless LANs uses the interface's capability to become an access point for a LAN. The node that is chosen to be the access point should obviously be within range of all of the nodes in the LAN. There is currently no automated mechanism to pick the access point for you, but one is planned for the future. If you wish to put your LAN into another operating mode (i.e., Adhoc), you must specify this via the tb-set-lan-setting command. In the above example, you could add:
        tb-set-lan-setting $lan0 "mode" "adhoc"
    

For more information on wireless modes, see the section of this document entitled Wireless Link Configuration Settings.

  • Wireless LANs allow a number of configuration parameters to be specified, either for the LAN as a whole, or for individual members of a LAN. There are two Emulab specific directives that you can use; tb-set-lan-setting sets a configuration parameter for the entire LAN. In the example above, we set the channel to be used for the LAN to channel five. Per-node settings can be specified with the tb-set-node-lan-setting directive. In the example, we set the transmit power for nodew1 to auto; all other nodes will default to an interface specific setting.
  • You must use Fedora Core 4 or RedHat 9.0 to take advantage of wireless interfaces. Be sure to set the OSID for all of your wireless nodes to FC4-WIRELESS or RHL90-WIRELESS. FC4-WIRELESS contains a 2.6.x kernel and very recent madwifi drivers; RHL90-WIRELESS contains a 2.4.x kernel. We strongly suggest you use FC4-WIRELESS unless you specifically require a 2.4.x Linux kernel.

Wireless Link Configuration Settings

Numerous interface settings are possible using tb-set-lan-setting and tb-set-node-lan-setting. These mostly correspond to options that are available using the iwconfig, iwpriv, and wlanconfig commands on Fedora Core 4 or Redhat 9.0. For iwconfig-specific options, the format of the value you provide for the option must be acceptable to iwconfig. At some point in the future we hope to make this more explicit so that know exactly what options are available each type of card, and what their legal values are. For now you will want to be familiar with iwconfig and the madwifi drivers for Atheros chipsets. Here is a guide to what you can currently specify:

  • mode: Set the mode for the LAN or interface. The mode argument may be one of Master, Managed, Adhoc, or Monitor. Master makes the interface become an access point (note that you probably do not want to set an entire LAN in Master mode). Managed makes the LAN or interface become a station which connects to an access point, if one exists. Adhoc puts the interface or LAN into ad-hoc mode. Monitor puts the interface or LAN into monitor mode, which may be useful for network sniffing. Note that the Adhoc and Monitor values can be given when calling tb-set-lan-setting, but you probably only want to pass Master or Managed values to tb-set-node-lan-setting. In fact, you can more easily set nodes to be access points by calling tb-set-lan-accesspoint for a wireless LAN; in this way, the node you specify will be the access point and all others in the LAN will become stations of it.
  • freq: Set the channel for the LAN to either a channel number or a frequency. If you set the value to an integer less than 1000, you are setting a channel number; if you set the value to an integer greater than 1000, or to a floating point number with a unit (i.e., 2.437GHz), you are setting a frequency directly. If you do not set the channel one will be choosen for you, and that's probably just as bad as setting it yourself without knowing what other people are using!
  • rate: Some interfaces support setting a bit rate different then the default. The default is to tell the interface to use "auto" mode (varies the rate according to RF conditions), but you can specify a specific rate for either the entire LAN or for just one node. The value should be in bits per second with no units, or a floating point value with a unit (i.e., 11M, 54M). See the iwconfig man page for more details on the format of this option.
  • txpower: Some interfaces support setting the transmit power to something different then the default. The default is to tell the interface to use "auto" mode. See the iwconfig man page for more details on the format of this option. Not all interfaces support this option.
  • sens: Some interfaces support setting the sens parameter (i.e., sensitivity) to something different then the default. The default is to tell the interface to use "auto" mode. See the iwconfig man page for more details on the format of this option. Not all interfaces support this option.
  • rts: This parameter can be set to enable RTS/CTS. From the iwconfig man page: "This parameter sets the size of the smallest packet for which the node sends RTS; a value equal to the maximum packet size disables the mechanism." You can also set the value to "auto" or "off"; note that the default is "off" on our hardware. Setting this parameter may not have the intended result on all hardware.
  • frag: Any IP packets larger than the value of this parameter will be split into multiple packets. The value should be specified in bytes. You can also set it to "auto" or "off".

After your experiment is swapped in, the above settings (including the accesspoint) can be changed on the fly, using either the link_config script on users.emulab.net, or with the XMLRPC interface. For example, if you want to change the accesspoint of a LAN, first determine the MAC address (dotted or undotted notation is fine) of the interface you want to be the accesspoint, and then use link_config:

	link_config myproj myexp lan0 accesspoint=00:09:5B:94:26:AF

Or you can use the XMLRPC interface from your desktop or from users.

	sshxmlrpc_client.py link_config proj=myproj exp=myexp
		link=lan0 "params={'accesspoint': '00:09:5B:94:26:AF'}"

If you want to change your LAN to adhoc mode, you can simply run link_config from users.emulab.net:

        link_config myproj myexp lan0 mode=adhoc

Or you can use the XMLRPC client on users.emulab.net:

        sshxmlrpc_client.py link_config proj=myproj exp=myexp
                link=lan0 "params={'mode': 'adhoc'}"

You may also change the settings for an individual node in a wireless LAN (although in some cases this could make the LAN unusable if you were to change a setting on just a single node). To do this, use the -s option to link_config or the src argument to the XMLRPC interface:

	link_config -s nodew1 myproj myexp lan0 txpower=50

	sshxmlrpc_client.py link_config proj=myproj exp=myexp
		src=nodew1 link=lan0 "params={'txpower': 50}"

To enable and disable the interface for individual nodes on the LAN:

	link_config -s nodew1 myproj myexp lan0 enable=no
	link_config -s nodew1 myproj myexp lan0 enable=yes

	sshxmlrpc_client.py link_config proj=myproj exp=myexp
		src=nodew1 link=lan0 "params={'enable': 'no'}"

Note this currently operates by bringing the interface up/down with the ifconfig command.

Advanced Dynamic Link Configuration

The goal of this section is to provide extra functionality via link_config so that you can work around issues encountered with the combination of Linux wireless support and the madwifi drivers.

One "problem" that arises for wireless interfaces with Atheros chipsets due to the combination of the generic Linux wireless tools and the madwifi drivers, is that to change modes (i.e., from Managed to Adhoc), you must actually remove the athX device, change the mode with wlanconfig, and finally re-create the athX device. This effectively removes all current iwconfig settings of the device and resets them to defaults. Consequently, if you want to keep your settings without having to respecify them as parameters to the command line, you can pass a flag (set the "remember" parameter to "1") to link_config that will do this for you. When the "remember" flag is set, the current state of the device to be reconfigured is captured via iwconfig. Any parameters you specify along with "remember" override the current settings. If the device is currently down and we cannot obtain current settings via iwconfig, the original Emulab configuration from your NS file is used. Important Note: Using the "remember" flag can be dangerous. For instance, if your device is currently operating in 80211b on channel 4, and you try to switch it to 80211a without specifying a new channel in the 5GHz spectrum, your kernel may panic (this has been observed on our RHL90-WIRELESS image with the latest 2.4.x kernel and madwifi drivers). So, be careful!

In other situations, the Atheros/wireless tools combination will simply not execute your commands. For instance, sometimes you cannot switch from one protocol to another (i.e., 80211g to 80211b); iwpriv will fail. One solution to this problem is to remove the Atheros and wireless kernel modules, re-insert them, and then run the configuration commands. If you find that some of your dynamic configuration commands fail, try setting the "resetkmod" parameter to "1" when calling link_config. A less drastic approach that sometimes works is simply using wlanconfig to destroy the athX device and re-create it with the new settings. You can try this by setting the "resetwlan" parameter to "1" when calling link_config. Important Note: Removing and re-inserting kernel modules can have unintended side effects. For instance, if you have multiple Atheros interfaces enabled on your node, removing the modules will remove any other athX devices you may have configured (and it might even panic your kernel). Unfortunately, at this time, link_config cannot work around this possibility.

Choosing Physical Nodes

As mentioned above, Emulab's default mapping of nodes in your virtual topology, to physical nodes with wireless interfaces, does not currently take into account physical connectivity (walls, floors, electrical conduits, etc. all conspire to make it possible that two wireless nodes 50 feet apart from each other will not actually be able to communicate with each other). We are planning to add this capability in the future, but in the meantime it is mostly up to you to choose nodes that make sense for your topology. You might end up having to make some trial and error runs, trying to find the right set of nodes (including setting the accesspoint to different nodes in a LAN) before you get a working set.

To assist in this chore there are two Emulab specific NS extensions you can use in your NS file. The first approach is to use the tb-use-physnaming extension.

	tb-use-physnaming 1

	set pc222 [$ns node]
	set pc223 [$ns node]
	set pc224 [$ns node]

which says that whenever a node is named by an existing physical node in the testbed, do an implicit fix-node. This saves a little bit of typing, and in some cases might be easier to manage then the second approach, which is to use tb-fix-node for all (or some) of the nodes in your LAN. Using the wireless floormaps choose which nodes you want, and then in your NS file:

	set nodew1 [$ns node]
	set nodew2 [$ns node]
	set nodew3 [$ns node]

	tb-fix-node $nodew1 pc222
	tb-fix-node $nodew2 pc223
	tb-fix-node $nodew3 pc224

Keep in mind that the physical nodes you choose must be free when you swap in your experiment!

Hardware Details

The `pc3000w` Netgear WAG311 cards, which use the Atheros 5212 802.11a/b/g chipset. Each `pc600` node contains a single D-Link DWL-AG530, which also use the Atheros 5212 chipset. This chipset is quite flexible since most of the 802.11 MAC layer is handled in software. Emulab provides the standard madwifi Atheros driver by default, but there are alternatives such as the madwifi stripped driver from the MIT roofnet project and SoftMAC from the Universtiy of Colorado at Boulder. Here are some useful links:

Environment and Deployment Information

Due to the dense nature of the pc600 mini-cluster, we describe some of its properties and deployment characteristics in detail in this section. You can view a map of the deployment here.

The external antennas for each wireless interface in the pc600s are deployed in a 6 x 6 grid on the back side of the racks housing the pc600s and pc850s. Each antenna is currently pointed straight down towards the floor. The grid is 300cm x 224cm. All nodes in the grid are easily able to communicate with each another; the interesting nature of this grid becomes more evident when many nodes attempt to send packets at the same time. Because the nodes are deployed in our machine room, the environment is hostile, with many other racks, machines, conduits, and other items, conspiring to provide various types of signal interference.

To give you an idea of what you might expect when many nodes in proximity are sending packets at the same time, we ran some simple tests. In this experiment, all nodes are placed in ad-hoc mode in a single LAN, and send packets all at once to the broadcast address in their subnet. Each node sends 1000 1024-byte packets at a rate of 10pps. We then do this iteratively over all channels in each of a, b, and g modes. The results are presented in terms of the percentage of broadcast packets received at each node. The Aggregate Statistics table simply aggregates all statistics gathered from each 802.11 mode across all receivers.

Mode min mean max stddev var
802.11 a 0.10 65.21 99.40 15.69 246.27
802.11 b 2.10 25.19 91.10 15.01 225.25
802.11 g 0.30 26.04 83.10 15.84 250.99

Finally, here are some pictures of the pc600 mini-cluster. On the back of the racks, you can see the antennas in the 6 x 6, 36-node grid. Some antennas are outlined to make them easier to see; however, once you find the corners of the grid, it becomes easier to spot them all.

pc600wifi_back_thumb.jpg pc600wifi_back_marked_thumb.jpg pc600wifi_back_detail_thumb.jpg

pc600wifi_front_left_thumb.jpg pc600wifi_front_right_thumb.jpg