Re: [Discuss-gnuradio] IEEE 802.11 receive

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Re: [Discuss-gnuradio] IEEE 802.11 receive

From:	Vivek Raghunathan
Subject:	Re: [Discuss-gnuradio] IEEE 802.11 receive
Date:	Mon, 07 Aug 2006 11:25:21 -0400
User-agent:	Mail/News 1.5 (X11/20060404)

All,

As I had mentioned in the last email, we have a working implementationof IEEE 802.11 receive on GNU Radio, and have been able to successfullydecode and inject 802.11 frames into a NetBSD kernel. I forgot toinclude a link to our GNU Radio sources in that email. All of our GNURadio development is on our public development server athttp://acert.ir.bbn.com/cvs/?group=adroitgrdevel


The IEEE 802.11 receive stuff is in adroitgrdevel/gr-bbn/src/examples/.

A readme file in the directory describes how to run the receiver. Thereare currently two python scripts that take the same arguments that canbe used for receiving 802.11 packets. One script just prints outinformation about received packets (The other one currently sendspackets to the Linux tap driver, which does not support 802.11 mode. Donot use this script.)

In order to inject 802.11 frames into the kernel, you'll need to useNetBSD and our modified NetBSD tap(4) kernel interface. We are currentlyin the process of cleaning up this code to integrate it back intoNetBSD-current. The current "bleeding edge" version of the code is onour public server athttp://acert.ir.bbn.com/cvs/?group=netbsd and can be found innetbsd/src/sys/net/if_tap.{c,h}.


-Vivek


Vivek Raghunathan wrote:

Hello all,
As part of the ADROIT project at BBN Technologies, we have been workingon getting GNU Radio to be able to receive and decode IEEE 802.11 framesand inject these frames into the NetBSD kernel networking stack.
As of now, our GNU Radio extensions are successfully receiving anddecoding IEEE 802.11 frames at the base rate (1 Mbps). The 802.11module is capable of demodulating 1 and 2 Mbps packets when the 1 MbpsPLCP header is used (sometimes referred to as "long preamble" mode). Thecode seems to max out a 1.6 GHz Mobile Pentium. To receive the packets,the USRP is programmed to produce 8-bit samples (8 bits for I and 8 bitsfor Q) at 8 mega samples per second. This means we are looking at abouta 4 MHz (baseband) bandwidth. Although the 802.11 signals are 11(baseband) MHz wide, there is apparently enough energy in the 4 MHz weare looking at to receive some 1 Mbps packets, and once in a whilereceive a 2 Mbps packet. To save processor resources, the receiver doesnot do any phase or frequency synchronization. The receiver is composedof five blocks - an FIR filter, a time synchronizer block (which I callslicer), a demodulator block, a de-scrambler block, and a "plcp" block.The FIR filter attempts to de-spread the barker. The processor spendsmore time on this block than any other block. The slicer tries to findthe peaks of the filter output and produces samples at the symbol rate(1 mega symbols/sec). The demodulator simultaneously demodulates at 1Mbps (differential BPSK) and 2 Mbps (differential QPSK). There no timeor frequency synchronization. The de-scrambler reverses the 802.11scrambling operation. And, finally, the "plcp" block runs the receivestate machine. It looks for a preamble, verifies the physical layerheader, and verifies the data frame checksum. If everything checks out,a small header is prepended to the packet, the frame checksum isstripped off, and the packet is forwarded up. The RSSI is calculated inthe demodulator block. It averages the energy from 128 successivesamples (output from the slicer), and converts to dB.
To support the injection of IEEE 802.11 frames from user-space intokernel-space, we have extended the tap(4) interface to support IEEE802.11 frames in addition to the standard Ethernet mode. It is possibleto use BPF on this tap(4) interface using three different link types:
1. DLT_EN10MB: Ethernet header
2. DLT_IEEE802_11: IEEE 802.11 header
3. DLT_IEEE802_11_RADIO: IEEE 802.11 header + radiotap header formeta-data using one of three DLT types
IEEE 802.11 frames received and decoded by GNURadio are written to afile. A separate user-space program reads these frames from the file andinjects them into the NetBSD kernel using the new tap(4) driver. Receivemeta-data is also pushed into kernel-space in the form of a radio-tapheader in DLT_IEEE802_11_RADIO format.
I am attaching a sample capture of 802.11 received frames using tcpdumpon the extended tap(4) interface with link type set toDLT_IEEE802_11_RADIO. The first 28 bytes of every packet in the dumpfile is the radiotap header, followed by the IEEE 802.11 header, and thepayload. The radiotap header is of the form:
struct tap_rx_radiotap_header {
struct ieee80211_radiotap_header wr_ihdr;
u_int64_t    wr_tsf;  // currently bogus,= rx timestamp from GNURadio
u_int8_t    wr_flags; // 0
u_int8_t    wr_rate; // from GnuRadio
u_int16_t    wr_chan_freq; // from GnuRadio
u_int16_t    wr_chan_flags; // 0
u_int8_t    wr_antenna; // 0
u_int8_t    wr_antsignal; // from GnuRadio
u_int8_t    wr_antnoise; // -100
};
Note that our GNU Radio IEEE 802.11 receive code currently reportssignal strength in dB with respect to an arbitrary baseline. We haven'tyet calibrated this to dBm. tcpdump expects signal strength and noise tobe 1 byte unsigned numbers - thus, 234dB reported by tcpdump is really-20dB from the GNURadio IEEE 802.11 receive code.
-Daniel Sumorok (address@hidden)
-Vivek Raghunathan (address@hidden, address@hidden)
-Greg Troxel (address@hidden)


------------------------------------------------------------------------

22:57:22.380382 27093352us tsft 1.0 Mb/s 2437 MHz antenna 0 234dB signal 156dB 
noise Beacon (GnuRadio) [1.0* 2.0* 5.5* 11.0* Mbit] ESS CH: 6
        0x0000:  009e 1c00 0f38 0000 6869 9d01 0000 0000  .....8..hi......
        0x0010:  0002 8509 0000 00ea 9cea bfbf 8000 0000  ................
        0x0020:  ffff ffff ffff 0016 b647 0d10 0016 b647  .........G.....G
        0x0030:  0d10 007a 8791 0924 b901 0000 6400 0100  ...z...$....d...
        0x0040:  0008 476e 7552 6164 696f 0104 8284 8b96  ..GnuRadio......
        0x0050:  0301 0605 0400 0100 00dd 0600 1018 0201  ................
        0x0060:  f4                                       .
22:57:22.380608 27195784us tsft 1.0 Mb/s 2437 MHz antenna 0 238dB signal 156dB 
noise Beacon (GnuRadio) [1.0* 2.0* 5.5* 11.0* Mbit] ESS CH: 6
        0x0000:  009e 1c00 0f38 0000 88f9 9e01 0000 0000  .....8..........
        0x0010:  0002 8509 0000 00ee 9cea bfbf 8000 0000  ................
        0x0020:  ffff ffff ffff 0016 b647 0d10 0016 b647  .........G.....G
        0x0030:  0d10 107a 8321 0b24 b901 0000 6400 0100  ...z.!.$....d...
        0x0040:  0008 476e 7552 6164 696f 0104 8284 8b96  ..GnuRadio......
        0x0050:  0301 0605 0400 0100 00dd 0600 1018 0201  ................
        0x0060:  f4                                       .
22:57:22.381206 27707816us tsft 1.0 Mb/s 2437 MHz antenna 0 235dB signal 156dB 
noise Beacon (GnuRadio) [1.0* 2.0* 5.5* 11.0* Mbit] ESS CH: 6
        0x0000:  009e 1c00 0f38 0000 a8c9 a601 0000 0000  .....8..........
        0x0010:  0002 8509 0000 00eb 9cea bfbf 8000 0000  ................
        0x0020:  ffff ffff ffff 0016 b647 0d10 0016 b647  .........G.....G
        0x0030:  0d10 707a 8bf1 1224 b901 0000 6400 0100  ..pz...$....d...
        0x0040:  0008 476e 7552 6164 696f 0104 8284 8b96  ..GnuRadio......
        0x0050:  0301 0605 0400 0100 00dd 0600 1018 0201  ................
        0x0060:  f4                                       .
22:57:22.381376 28629352us tsft 1.0 Mb/s 2437 MHz antenna 0 234dB signal 156dB 
noise Beacon (GnuRadio) [1.0* 2.0* 5.5* 11.0* Mbit] ESS CH: 6
        0x0000:  009e 1c00 0f38 0000 68d9 b401 0000 0000  .....8..h.......
        0x0010:  0002 8509 0000 00ea 9cea bfbf 8000 0000  ................
        0x0020:  ffff ffff ffff 0016 b647 0d10 0016 b647  .........G.....G
        0x0030:  0d10 007b 8401 2124 b901 0000 6400 0100  ...{..!$....d...
        0x0040:  0008 476e 7552 6164 696f 0104 8284 8b96  ..GnuRadio......
        0x0050:  0301 0605 0400 0100 00dd 0600 1018 0201  ................
        0x0060:  f4                                       .
22:57:22.381543 28731800us tsft 1.0 Mb/s 2437 MHz antenna 0 234dB signal 156dB 
noise Beacon (GnuRadio) [1.0* 2.0* 5.5* 11.0* Mbit] ESS CH: 6
        0x0000:  009e 1c00 0f38 0000 9869 b601 0000 0000  .....8...i......
        0x0010:  0002 8509 0000 00ea 9cea bfbf 8000 0000  ................
        0x0020:  ffff ffff ffff 0016 b647 0d10 0016 b647  .........G.....G
        0x0030:  0d10 207b 8b91 2224 b901 0000 6400 0100  ...{.."$....d...
        0x0040:  0008 476e 7552 6164 696f 0104 8284 8b96  ..GnuRadio......
        0x0050:  0301 0605 0400 0100 00dd 0600 1018 0201  ................
        0x0060:  f4                                       .
22:57:22.381711 29038992us tsft 1.0 Mb/s 2437 MHz antenna 0 238dB signal 156dB 
noise Beacon (GnuRadio) [1.0* 2.0* 5.5* 11.0* Mbit] ESS CH: 6
        0x0000:  009e 1c00 0f38 0000 9019 bb01 0000 0000  .....8..........
        0x0010:  0002 8509 0000 00ee 9cea bfbf 8000 0000  ................
        0x0020:  ffff ffff ffff 0016 b647 0d10 0016 b647  .........G.....G
        0x0030:  0d10 507b 8441 2724 b901 0000 6400 0100  ..P{.A'$....d...
        0x0040:  0008 476e 7552 6164 696f 0104 8284 8b96  ..GnuRadio......
        0x0050:  0301 0605 0400 0100 00dd 0600 1018 0201  ................
        0x0060:  f4                                       .
22:57:22.381880 29653432us tsft 1.0 Mb/s 2437 MHz antenna 0 235dB signal 156dB 
noise Beacon (GnuRadio) [1.0* 2.0* 5.5* 11.0* Mbit] ESS CH: 6
        0x0000:  009e 1c00 0f38 0000 b879 c401 0000 0000  .....8...y......
        0x0010:  0002 8509 0000 00eb 9cea bfbf 8000 0000  ................
        0x0020:  ffff ffff ffff 0016 b647 0d10 0016 b647  .........G.....G
        0x0030:  0d10 b07b 86a1 3024 b901 0000 6400 0100  ...{..0$....d...
        0x0040:  0008 476e 7552 6164 696f 0104 8284 8b96  ..GnuRadio......
        0x0050:  0301 0605 0400 0100 00dd 0600 1018 0201  ................
        0x0060:  f4                                       .
22:57:22.382335 36718872us tsft 1.0 Mb/s 2437 MHz antenna 0 234dB signal 156dB 
noise Beacon (GnuRadio) [1.0* 2.0* 5.5* 11.0* Mbit] ESS CH: 6
        0x0000:  009e 1c00 0f38 0000 1849 3002 0000 0000  .....8...I0.....
        0x0010:  0002 8509 0000 00ea 9cea bfbf 8000 0000  ................
        0x0020:  ffff ffff ffff 0016 b647 0d10 0016 b647  .........G.....G
        0x0030:  0d10 0082 8771 9c24 b901 0000 6400 0100  .....q.$....d...
        0x0040:  0008 476e 7552 6164 696f 0104 8284 8b96  ..GnuRadio......
        0x0050:  0301 0605 0400 0100 00dd 0600 1018 0201  ................
        0x0060:  f4                                       .
22:57:22.382504 39483712us tsft 1.0 Mb/s 2437 MHz antenna 0 233dB signal 156dB 
noise Beacon (GnuRadio) [1.0* 2.0* 5.5* 11.0* Mbit] ESS CH: 6
        0x0000:  009e 1c00 0f38 0000 4079 5a02 0000 0000  address@hidden
        0x0010:  0002 8509 0000 00e9 9cea bfbf 8000 0000  ................
        0x0020:  ffff ffff ffff 0016 b647 0d10 0016 b647  .........G.....G
        0x0030:  0d10 d083 84a1 c624 b901 0000 6400 0100  .......$....d...
        0x0040:  0008 476e 7552 6164 696f 0104 8284 8b96  ..GnuRadio......
        0x0050:  0301 0605 0400 0100 00dd 0600 1018 0201  ................
        0x0060:  f4                                       .


------------------------------------------------------------------------

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