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School of 3.1. Introduction to WLAN IEEE 802.11 Engineering WCOM, WLAN, 1 References [1] J. Schiller, „Mobile Communications“, 2nd Ed., Pearson, 2003. [2] Martin Sauter, "From GSM to LTE", chapter 6, Wiley, 2011. [3] wiki to WLAN: http://de.wikipedia.org/wiki/IEEE_802.11 [4] wiki, https://de.wikipedia.org/wiki/IEEE_802.11a [5] List of WLAN channels: http://en.wikipedia.org/wiki/List_of_WLAN_channels [6] 802.11 OFDM Overview: http://rfmw.em.keysight.com/wireless/helpfiles/89600B/WebHelp/subsystems/wlan-ofdm/Content/ofdm_80211-overview.htm School of Wireless LAN IEEE 802.11 Engineering WCOM, WLAN, 2 http://www.keysight.com/main/application.jspx?nid=-34915.0.00&cc=CH&lc=ger https://www.neratec.com/de/wlan-produkte/uebersicht ODIN-W1 from uBlox CC3200 from Texas Instruments Host-based multiradio module series SimpleLink™ Wi-Fi® and Internet- with Wi-Fi 802.11a/b/g/n of-Things-Solution, a Single-Chip Wireless MCU (and Bluetooth v4.0 Dual Mode) e.g. integrated into a coffee machine School of Introduction Engineering WCOM, WLAN, 3 IEEE standard 802.11 is the most famous WLAN standard belongs to the 802.x LAN standards specifies PHY and MAC layer adapted to special requirements of wireless LANs standardization is ongoing, some examples 802.11b: 1999, 2.4 GHz, 22 MHz, 11 Mbps 802.11a: 1999, 5 GHz, 20 MHz, 54 Mbps, OFDM 802.11g: 2003, 2.4 GHz, 20 MHz, 54 Mbps, OFDM 802.11n: 2009, 2.4/5 GHz, 20/40 MHz, 600 Mbps, OFDM, MIMO 802.11ac: 2013, 5 GHz, 80/160 MHz, >1 Gbps, OFDM, MIMO 802.11ad: 2016, 60 GHz, 1760 MHz, <7 Gbps, OFDM, MIMO 802.11ah: 2016, 868/900 MHz, 1-16 MHz, >0.65 Mbps, longer distance, IoT 802.11p: vehicle2vehicle and vehicle2roadside School of System Architecture Engineering [2], chapter 6.3.1 WCOM, WLAN, 4 1 AP and some STAs in the same radio coverage form a BSS. AP periodically broadcasts the STA1 SSID in beacon frames. station Radio range is 30 – 300 m. BSS2 Access Point Portal Distribution System 802.X LAN BSS1 Internet basic service set ESS extended service set The Distribution System connects several BSS to form a single network (ESS) with extended coverage. School of System Architecture Engineering [2], chapter 6.3.1 WCOM, WLAN, 5 Conditions for problem-free intra-ESS roaming • the APs must belong to the same IP-subnet size of an ESS is limited (e.g. to the size of a building) • all AP have the same BSS-ID (SSID) • APs transmit on different frequencies • the APs of an ESS should come from the same supplier IEEE has not specified the distribution system yet (cf. IEEE 802.11f), but specified distribution system services. • overlapping radio coverage of the APs frequency gap of at AP AP AP AP least 5x5 = 25 MHz! channel 6 channel 11 channel 1 channel 6 School of System Architecture Engineering [2], chapter 6.3.1 WCOM, WLAN, 6 STAs have to agree on some parameters: SSID, channel, key, IP addresses ad-hoc mode is not used often independent because of «complex» configuration BSS no routing no relay! STA5 can communicate directly with STA4, but not with STA3 School of Protocol Architecture Engineering WCOM, WLAN, 7 IEEE 802.11 fits seamlessly into other 802.x standards for wired LANs [1], Figure 7.5, p. 210 bridge Application should not notice „anything“ WLAN behaves like a «slow» wired LAN School of IEEE 802.11 Standards Engineering [2], chapter 6.2 WCOM, WLAN, 8 5 / 6- 1- 1- (6-600 Mbps) MIMO, OFDM OFDM OFDM CCK DPSK/DQPSK DSSS School of Management Operations Engineering [2], chapter 6.4 WCOM, WLAN, 9 Scanning and Beacon Frames AP periodically broadcasts (e.g. every 100 ms) beacon frames with SSID, capability information, supported data rates, … STAs perform passive scanning or active scanning with probe requests Authentication and Association or Shared Key Authentication with challenge-response-procedure open or WEP protected or better, WPA / WPA2 protected School of Management Operations Engineering [2], chapter 6.4 WCOM, WLAN, 10 Reassociation and Roaming in an ESS STA can change AP (mobility or better radio reception) School of Management Operations Engineering [2], chapter 6.4 WCOM, WLAN, 11 Power Saving (PS) Mode AP stores frames and broadcasts Traffic Indication Map (TIM) STAs listen periodically (e.g. every 300 ms) to TIM in beacon frames and sends PS-Poll frames to get data frames School of MAC Coordination Functions Engineering WCOM, WLAN, 12 Distributed Coordination Function (DCF) mandatory, for asynchronous data service packet exchange on best effort (no delay bounds can be given) based on a version of CSMA/CA SIFS short interframe space (highest priority for ACK, CTS, …) PIFS PCF interframe space (medium priority) DIFS DCF interframe space (lowest priority for asynchronous data) Point Coordination Function (PCF) optional, for time-bounded services, contention free polling method School of MAC - CSMA Engineering WCOM, WLAN, 13 Carrier Sense Multiple Access = 20 us School of MAC – CSMA Engineering [2], chapter 6.5.1 WCOM, WLAN, 14 Carrier Sense Multiple Access on the air interface (physical carrier sensing, via RSSI) RSSI: Received Signal Strength Indicator on the MAC-layer (virtual carrier sensing, via NAV-timer) NAV: Network Allocation Vector (virtual reservation scheme) Collision Avoidance (CA) with random backoff-procedure 802.11b and g first Tx attempt: random slot <= CWmin = 31 slots further Tx attempts: random slot <= 63, 127, … CWmax CWmax = 1023 slots (20 ms), then the frame is discarded 802.11n CWmin = 15 slots (0.3 ms) School of MAC - CSMA Engineering WCOM, WLAN, 15 Packet data transmission without RTS/CTS DIFS Source Data SIFS Destination ACK DIFS CW Other NAV defer access backoff after defer DIFS DCF Interframe Space SIFS Short Interframe Space NAV Network Allocation Vector CW Contention Window School of MAC - CSMA Engineering WCOM, WLAN, 16 Packet data transmission with RTS/CTS (optional) CA for long packets avoiding the hidden terminal problem DIFS SIFS Source RTS Data SIFS SIFS Destination CTS ACK DIFS NAV (RTS) CW Other NAV (CTS) NAV (Data) defer access backoff started RTS Request To Send CTS Clear To Send School of MAC – HCF (IEEE 802.11e) Engineering [2], chapter 6.8 WCOM, WLAN, 17 Hybrid Coordination Function (HCF) DCF (QoS) extension in IEEE 802.11e WiFi Multi-Media (WMM specification) different CWmin and CWmax for 4 different QoS-classes School of MAC frames Engineering WCOM, WLAN, 18 IP frames do not usually exceed 1500 bytes [1] user data, management or control frame Interpretation of 48 bit MAC addresses SA: Source Address, DA: Destination Address School of IEEE 802.11b Frame Structure Engineering WCOM, WLAN, 19 IEEE 802.11b PHY packet formats, please cf. [1], Figure 7.22, p. 232 School of PHY – Operating Channels for 802.11 Engineering WCOM, WLAN, 20 only only ETSI/Japan Pmax = 100 mW (20 dBm) ETSI for 802.11b/g at least 5 channels spacing bandwidth 802.11b (bandwidth 802.11g = 16.6-20 MHz) School of PHY – Operating Channels for 802.11 Engineering WCOM, WLAN, 21 [5] US (FCC) US (FCC) channels 1 + 5 channels 9 + 13 US (FCC) School of PHY – Operating Channels for 802.11 Engineering WCOM, WLAN, 22 EU / USA EU USA [4] ETSI / EU DFS: Dynamic Frequency Selection, TPC: Transmitter Power Control School of PHY – 802.11b Engineering WCOM, WLAN, 23 ** * * *** * symbol spread with 11 Chip Barker Code (DSSS) ** header always with 1 Mbps *** 11 Mbit/s can only be achieved over short distances of a few meters School of PHY – 802.11b Engineering WCOM, WLAN, 24 Energy Spread Sequence by spreading with 11 Mchip/s (robustness) 802.11b channel bandwidth 22 MHz School of PHY – 802.11g Engineering WCOM, WLAN, 25 Extended-Rate PHY (ERP) OFDM with 52 subchannels (4 pilot channels and 48 data channels) symbol rate = 250 kSps (symbol period = 4us) total bandwidth 16-20 MHz (same channel use as 802.11b) Data Rates = 48 channel · 6 bit / channel · 3/4 (conv. code rate) / 4 us (symbol time) School of PHY – 802.11g Engineering WCOM, WLAN, 26 802.11a and g have the same PHY-parameters = fs = (52+1)·312.5 kHz Symbol Interval Time TSYM = 4 µs (= TGI + TFFT) Guard Interval TGI = 0.8 µs FFT Period TFFT = 3.2 µs (= 1 / 312.5 kHz) [6] School of PHY – 802.11g Engineering WCOM, WLAN, 27 Backward compatible with 802.11b protection measures CTS before packet transmission to set NAV-timer of 802.11b terminal PHY header with 1 Mbps 40% performance loss G-only option to avoid 802.11b/g interworking overhead Speed Comparison 802.11g (optimal condition): 20 Mbps (terminal-terminal 10 Mbps) 802.11b (optimal condition): 5 Mb/s (terminal-terminal 2.5 Mbps) 802.11a PHY almost identical with 802.11g, but allocated in 5 GHz band but no backward compatibility required (higher throughput) School of PHY – 802.11n Engineering WCOM, WLAN, 28 High Throughput (HT) PHY 40 MHz channels frame aggregation 400 ns instead of 800 ns OFDM guard interval 5/6 FEC (convolutional coding) MIMO OFDM-Parameters School of PHY – 802.11n Engineering WCOM, WLAN, 29 Frame Aggregation 1 ACK for many packets 2x2 MIMO School of PHY – 802.11n Engineering WCOM, WLAN, 30 Data Rate Comparison School of Interesting WiFi- / IoT-solution Engineering WCOM, WLAN, 31 Station- or Access-Point-mode 802.11 b/g/n WLAN / Internet wireless access via browser School of Interesting WiFi- / IoT-solution Engineering WCOM, WLAN, 32 some more CC3200- (RF-) Parameters – Wi-Fi CERTIFIED™ Chip – TX Power: 18.0 dBm @ 1 DSSS, 14.5 dBm @ 54 OFDM – RX Sensitivity: –95.7 dBm @ 1 DSSS, –74.0 dBm @ 54 OFDM – Application Throughput: UDP: 16 Mbps, TCP: 13 Mbps – RX-current (MCU Active): 59 mA @ 54 OFDM – TX-current (MCU Active): 229 mA @ 54 OFDM, Maximum Power – 0.5-mm Pitch, 64-Pin, 9-mm × 9-mm QFN – Ambient Temperature Range: –40°C to 85°C School of Some Scanners, Protocol Analyzers, … Engineering WCOM, WLAN, 33 inSSIDer wireshark Matlab WLAN System Toolbox to simulate, analyze, and test the PHY of WLAN http://ch.mathworks.com/products/wlan-system/ .
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  • 6.3 Wireless Traffic Capture and Analysis 219

    6.3 Wireless Traffic Capture and Analysis 219

    i “Davidhoff” — 2012/5/17 — 19:59 — page 219 — #21 i i i 6.3 Wireless Traffic Capture and Analysis 219 • History of client signal strength (can help identify geographic location) • Routing tables • Stored packets before they are forwarded • Packet counts and statistics • ARP table (MAC address to IP address mappings) • DHCP lease assignments • Access control lists • I/O memory • Running configuration • Processor memory • Flow data and related statistics 6.2.3.2 Persistent Again, like wired routers and switches, WAPs are not designed to include much local persis- tent storage space. The WAP operating system and startup configuration files are maintained in persistent storage by necessity. Persistent evidence you may find on a WAP includes: • Operating system image • Boot loader • Startup configuration files 6.2.3.3 Off-System Wireless access points can be configured to send event logs to remote systems for off-site ag- gregation and storage. Syslog and SNMP are commonly supported. Enterprise-class devices may include other options, often proprietary. Check the documentation for the model you are investigating and review local configuration to locate devices that may contain off-system WAP logs. 6.3 Wireless Traffic Capture and Analysis Capturing and analyzing wireless traffic often provides valuable evidence in an investiga- tion, for the same reasons we discussed in Chapter 3. However, there are some additional complexities involved in capturing wireless traffic, as opposed to sniffing traffic on the wire. In this section, we review some important notes for capturing and analyzing wireless traffic. For further discussion of passive evidence acquisition and analysis, please see Chapter 3, “Evidence Acquisition.” i i i i i “Davidhoff” — 2012/5/17 — 19:59 — page 220 — #22 i i i 220 Chapter 6 Wireless: Network Forensics Unplugged 6.3.1 Spectrum Analysis There are, literally, an infinite number of frequencies over which data can be transmitted through the air.