Date of Publication by HGI: May, 2016 ------------------------------------------------------------------------ Testing the impact of IEEE 802.11 interference on IEEE 802.15.4/Zigbee Networks 1. Activities Description 1.1 Reference Scenario We will consider a point-to-point IEEE 802.15.4/Zigbee network, composed of a Zigbee Coordinator (ZC) and a Zigbee end-device (ZED). The two devices will implement the Home Automation application profile. Application implemented: The Zigbee Coordinator will send a query (write attribute data packet) every 1 s to the ZED, which will send back a reply (to notify the result of the write). A payload of 10 bytes will be set. For what concerns the IEEE 802.11 Network, we will consider two cases (A=2): 1. AP1 and AP2, using 802.11.n with 2 antennas and using the 40MHz band. The two APs will contemporaneously generate a 10 Mbps video traffic, each, toward two different clients. 2. AP1 and AP2 as (1) above, and also AP3 set to.11b with video traffic at maximum possible rate toward another client. The AP3 access point will be set to use a PHY rate of 2Mbps in order to maximize length of packets. The two antennas at AP1 and AP2 will be vertically oriented. The configuration related to case 1 is reported in the following figure. Two APs will be located at a distance of 3 m from the ZC and will send data to two clients, 3 meters far from the APs, themselves. Four different positions will be considered (B=4) for the ZED: four different distances between the ZC and the ZED (see the figure below). 1
The configuration related to case 2 is reported in the following figure. Three APs will be located at a distance of 3 m from the ZC and will send data to three clients, 3 meters far from the APs, themselves. Four different positions for the ZED will be considered. The above experiments will be repeated by locating the APs at 10 cm from the ZC (C=2). 2
1.2 Channels Case 1 (see figure above): The two APs Wi-Fi, using.n with 40 MHz band, will both use channel 6, while for the Zigbee network the following two cases will be considered (D=2): 1. Non overlapped channel: we will use channel 25 (centered at 2.475 GHz) 2. Overlapped channel: we will use channel 17 (centered at 2.435 GHz). Case 2 (see figure above): The two APs working with.n will use channel 6, and the AP working on.b (22 MHz band) will use channel 6 too. The same channels identified above will be used for the Zigbee network. With reference to the Zigbee network we will let the Zigbee network to select the channel to be used according to the PAN formation procedure defined in the standard, to check if the selected channel is overlapped or not. No transmission measurements will be made; it is simply a test that automatic channel selection is appropriate. 1.3 Transmit Powers We will set the defaults transmit power at the APs, and then we will check the level of interference generated by the two/three APs on the Zigbee devices. To the latter aim, we will switch on the APs and we will let them implement the traffics defined above and we will measure the level of power received by each ZED and by the ZC device in each of the 16 channels and the total power received by each device (ZC and ZEDs) from the two or three APs. The latter will result in a certification of the interference environment considered in the experiment (see below). For what concerns the transmit power used by the IEEE 802.15.4 devices we will set (E=2): Pt=0 dbm and Pt=-5 dbm. 3
1.4 Experiments Certification In order to certify the experimental environment, channel conditions and interference conditions by providing for each experiment: 1. 802.15.4 RSSI Matrix in the absence of interference: It will be a 5 x 5 Matrix (where 5 is the number of Zigbee nodes in the network, including the ZC). Each Matrix element, RSSI_ij, will be the average (averaged over 10.000 transmissions) level of RSSI received by node i when node j is transmitting. The latter matrix will be provided for the two channels used by the Zigbee network, channels 17 and 25. 2. RSSI Matrix in the presence of Interference: It will be a 5 (nodes) x 16 (channels) Matrix including in each row the RSSI received by each node when the APs are transmitting. The different columns refer to the 16 available channels. The latter measure will be performed for the 2 traffics identified above (2 APs and 3 APs). Being nodes in fixed positions into boxes, we will perform the measurements above only once for the different nodes locations, transmit powers, etc.. Measure 1: Each experiment will last in 15 minutes and the experiments will be repeated two times for the two transmit powers. Resulting in 30 minutes of experiments. Measure 2: the measure will take 16 minutes per device (1 minute per channel) and per traffic case, that is a total of: 16 minutes x 5 x 2 = 160 minutes, almost 2 and half hours (75 minutes each). 1.5 Performance Metrics In each experiment we will evaluate: - The number of replies (from the ZED to the ZC) that are not received by the ZC, that is the Packet Loss Rate (PLR). Note that ACK at MAC level will be used and up to three retransmission per link (from ZC to ZED and viceversa) will be considered. - The average Round Trip Time During each experiment, we will also set up and run: 1. Spectrum analyzer checking the occupancy (in frequency and time) of the whole 2.4 GHz band (see figure below). 2. Wi-Fi sniffer: gathering all the Wi-Fi packets sent on channel 6 3. Zigbee sniffer: gathering all the Zigbee packets sent on channels 17 or 25 (depending on the experiment). 4
1.6 Conclusions on Parameters setting at the IEEE 802.15.4/Zigbee Network Parameter Settings Transmit Power -5 dbm, 0 dbm Profile HA ACK Used at MAC level Retransmissions Up to 3 Payload 10 bytes Channel ED to check the used channel, Channel 17, Channel 25 2. PMs Requested Tests Durations We expect to have low packet loss rate in the case of non-overlapped channel, while large PLR for the case of overlapped channel. For this reason we will transmit: - 1.000 packets for the case of channel 17, that we could report a minimum PLR of 10% - 16 minutes each experiment. - 10.000 packets for the case of channel 25, that is we can show a PLR up to 1% - 3 hours each experiment. Tests Summary Preliminary Tests (1) Setup 5 Zigbee nodes and measure the RSSI Matrix (30 minutes) (2) Setup Wi-Fi APs according to case 1 and measure the RSSI matrix of the level on generated interference (75 minutes) (3) Setup Wi-Fi APs according to case 2 and measure the RSSI matrix of the level on generated interference (75 minutes) Tests The complete set of tests is summarized below. Wi-Fi (A=2) 2 APs.n 2 APs.n and Wi-Fi (A=2) 2 APs.n Zigbee locations ZED Pos1 ZED Pos2 ZED Pos3 ZED Pos4 (B=4) Wi-Fi locations 3 m from the ZC 10 cm from the ZC (C=2) Channels (D=2) Channel 17 Channel 25 5
Transmit (E=2) Power Pt=0 dbm Pt=-5 dbm The latter brings to: (A x B x C x E) tests of 16 minutes each plus (A x B x C x E) tests of 3 hours each. The worst case scenario will be measured first, in order to check that the transmission conditions are sufficient to impair Zigbee operations, i.e. AP1, AP2 and AP3 all in operation at 10cm from the ZC, operating at channel 17 at weak -5dBm transmit power. If for some reason there is less than 30% packet-loss for the ZED Zigbee transmissions, an attempt will be made to increase the bit-rate of AP1 and AP2 until approximately 30% or more losses are seen for the most-strongly-affected ZED. That higher bit rate will then be used instead of the proposed 10Mbps for all the measurements in all the experiments. Total tests time: 30 minutes + 75 minutes + 75 minutes + 32 x 16 minutes + 32 x 3 hours = 107 hours, that is 13 days (8 h per day). To the latter we should add 1 h per test needed for the set up: 1 h x (3 + 64) = 67 hours, that is 8 days (8 h per day). Other tasks to be considered - Software development and parameters setting - 1 week - Initial worst case test as described above - Data Processing and reporting - 1 week The latter results in 1.5 PM 6