Introduction

This is a wireless medium simulation tool for Linux, based on the netlink API implemented in the mac80211_hwsim kernel driver. Unlike the default in-kernel forwarding mode of mac80211_hwsim, wmediumd allows simulating frame loss and delay.

This version is forked from an earlier version, hosted here:

https://github.com/cozybit/wmediumd

Prerequisites

First, you need a recent Linux kernel with the mac80211_hwsim module available. If you do not have this module, you may be able to build it using the backports project.

Wmediumd requires libnl3.0.

Building

cd wmediumd && make

Using Wmediumd

Starting wmediumd with an appropriate config file is enough to make frames pass through wmediumd:

sudo modprobe mac80211_hwsim radios=2
sudo ./wmediumd/wmediumd -c tests/2node.cfg &
# run some hwsim test

However, please see the next section on some potential pitfalls.

A complete example using network namespaces is given at the end of this document.

Configuration

Wmediumd supports multiple ways of configuring the wireless medium.

Perfect medium

With this configuration, all traffic flows between the configured interfaces, identified by their mac address:

ifaces :
{
	ids = [
		"02:00:00:00:00:00",
		"02:00:00:00:01:00",
		"02:00:00:00:02:00",
		"02:00:00:00:03:00"
	];
};

Per-link loss probability model

You can simulate a slightly more realistic channel by assigning fixed error probabilities to each link.

ifaces :
{
	ids = [
		"02:00:00:00:00:00",
		"02:00:00:00:01:00",
		"02:00:00:00:02:00",
		"02:00:00:00:03:00"
	];
};

model:
{
	type = "prob";

	default_prob = 1.0;
	links = (
		(0, 2, 0.000000),
		(2, 3, 0.000000)
	);
};

The above configuration would assign 0% loss probability (perfect medium) to all frames flowing between nodes 0 and 2, and 100% loss probability to all other links. Unless both directions of a link are configured, the loss probability will be symmetric.

This is a very simplistic model that does not take into account that losses depend on transmission rates and signal-to-noise ratio. For that, keep reading.

Per-link signal-to-noise ratio (SNR) model

You can model different signal-to-noise ratios for each link by including a list of link tuples in the form of (sta1, sta2, snr).

ifaces :
{
	ids = [
		"02:00:00:00:00:00",
		"02:00:00:00:01:00",
		"02:00:00:00:02:00",
		"02:00:00:00:03:00"
	];

	links = (
		(0, 1, 0),
		(0, 2, 0),
		(2, 0, 10),
		(0, 3, 0),
		(1, 2, 30),
		(1, 3, 10),
		(2, 3, 20)
	);
};

The snr will affect the maximum data rates that are successfully transmitted over the link.

If only one direction of a link is configured, then the link will be symmetric. For asymmetric links, configure both directions, as in the above example where the path between 0 and 2 is usable in only one direction.

The packet loss error probabilities are derived from this snr. See function get_error_prob_from_snr(). Or you can provide a packet-error-rate table like the one in tests/signal_table_ieee80211ax

Path loss model

The path loss model derives signal-to-noise and probabilities from the coordinates of each node. This is an example configuration file for it.

ifaces : {...};
model :
{
	type = "path_loss";
	positions = (
		(-50.0,   0.0),
		(  0.0,  40.0),
		(  0.0, -70.0),
		( 50.0,   0.0)
	);
	directions = (
		(  0.0,   0.0),
		(  0.0,  10.0),
		(  0.0,  10.0),
		(  0.0,   0.0)
	);
	tx_powers = (15.0, 15.0, 15.0, 15.0);

	model_name = "log_distance";
	path_loss_exp = 3.5;
	xg = 0.0;
};

Gotchas

Allowable MAC addresses

The kernel only allows wmediumd to work on the second available hardware address, which has bit 6 set in the most significant octet (i.e. 42:00:00:xx:xx:xx, not 02:00:00:xx:xx:xx). Set this appropriately using ‘ip link set address’.

This issue was fixed in commit cd37a90b2a417e5882414e19954eeed174aa4d29 in Linux, released in kernel 4.1.0.

Rates

wmediumd's rate table is currently hardcoded to 802.11a OFDM rates. Therefore, either operate wmediumd networks in 5 GHz channels, or supply a rateset for the BSS with no CCK rates.

Send-to-self

By default, traffic between local devices in Linux will not go over the wire / wireless medium. This is true of vanilla hwsim as well. In order to make this happen, you need to either run the hwsim interfaces in separate network namespaces, or you need to set up routing rules with the hwsim devices at a higher priority than local forwarding.

tests/test-001.sh contains an example of the latter setup.

Example session

The following sequence of commands establishes a two-node mesh using network namespaces.

sudo modprobe -r mac80211_hwsim
sudo modprobe mac80211_hwsim
sudo ./wmediumd/wmediumd -c ./tests/2node.cfg

# in window 2
sudo lxc-unshare -s NETWORK bash
ps | grep bash  # note pid

# in window 1
sudo iw phy phy2 set netns $pid

sudo ip link set wlan1 down
sudo iw dev wlan1 set type mp
sudo ip link set addr 42:00:00:00:00:00 dev wlan1
sudo ip link set wlan1 up
sudo ip addr add 10.10.10.1/24 dev wlan1
sudo iw dev wlan1 set channel 149
sudo iw dev wlan1 mesh join meshabc

# in window 2
ip link set lo

sudo ip link set wlan2 down
sudo iw dev wlan2 set type mp
sudo ip link set addr 42:00:00:00:01:00 dev wlan2
sudo ip link set wlan2 up
sudo ip addr add 10.10.10.2/24 dev wlan2
sudo iw dev wlan2 set channel 149
sudo iw dev wlan2 mesh join meshabc

iperf -u -s -i 10 -B 10.10.10.2

# in window 1
iperf -u -c 10.10.10.2 -b 100M -i 10 -t 120