【文章內(nèi)容簡介】 ? DATA SIGNAL SPREAD BY A PN CODE ? PROPERTIES OF PN CODE ? CHIP RATE ? DS PROCESSING GAIN ? PN CORRELATION AT RECEIVER ? PSK DATA MODULATION GP (dB) = 10LOG ( CHIP RATE DATA RATE ) AP96358 211 FREQUENCY (MHz) CW SIGNAL AMPLITUDE (dBm) SPREAD SIGNAL AMPLITUDE (dBm) 1 0 DATA BARKER CODE SPREAD DATA CHIP CLOCK 0 3 6 9 12 15 18 0 37 FHSS vs. DSSS in FHSS DSSS 頻寬 bandwidth 1M HZ MHZ( G Hz) 傳輸 transmission 1~2M bps 1~11M bps 距離 10~20公尺 20~150公尺 被干擾性 不易 易 成本 低 高 材料使用彈性 大 小 應用 38 藍芽使用 高斯頻率鍵控移位(gaussian frequency shift keying； GFSK) DSSS in ? 雖然在 (FHSS)、直序展頻 (DSSS)窄頻微波、紅外線等傳輸方式，但是在 頻 (DSSS)，也因此直序展頻成了目前所有廠商的標準。同時最高傳輸速率由 2Mbps提高到 11Mbps，使用的頻道在~ ? 同時為了向下相容早期 1~2Mbps的傳輸速率，因此 4種不同的傳輸速率。 傳輸速率 (Mbps) 調(diào)變方式 1 BPSK 2 QPSK CCK 11 Complementary Code Keying (cck) 資料來源： IEEE 39 DSSS in ? 無線電通訊系統(tǒng)是利用正弦波的三個特性： 振幅 (amplitude)、頻率 (frequency)和相位 (phase)。這三個特性代表的意義分別是：訊號有多大 (聲 )、訊號移動的有多快、它位於正弦波上哪一個位置。 ? 相位調(diào)變被廣泛地應用在數(shù)位通訊系統(tǒng)上，例如： 。相位鍵控移位 (PSK)的「鍵控」通訊協(xié)定所產(chǎn)生的序列(sequence)，就是用來決定調(diào)變訊號的相位變化，以傳輸數(shù)據(jù)。我們?？吹?BPSK(Binary PSK)、 QPSK(Quadrature PSK)、和 MPSK或 Mary PSK(M是符號狀態(tài)數(shù)目。若符號數(shù)目是 n，則 M=2n。 ? BPSK是二進位制相位鍵控移位，具有兩個符號狀態(tài) (symbol states)； QPSK是象限相位鍵控移位，具有四個符號狀態(tài)； MPSK是多階 (multilevel)相位鍵控移位，符號狀態(tài)數(shù)由 M值決定，M值越大通訊效果越佳。 40 IEEE Physical Layer： Frame format Control Duration Addr1 Addr2 Addr3 Addr4 Control Data CRC Distribution System AP1 AP2 AP3 A B C D E F Frame Type (RTS,CTS,…) ToDS FromDS Ultimate Destination (E) Immediate Sender (AP3) Intermediate Destination (AP1) Source (A) RTS: RequesttoSend CTS: CleartoSend 41 IEEE Physical Layer： Frame format (con’t) Header： 30Bytes including control information、 addressing、 sequence number、 duration Data ： 0~2312Bytes,changing with frame type Error control： 4Bytes,with CRC32 Frame control Duration /ID Addressing 1 Addressing 2 Addressing 3 Sequence control Addressing 4 Frame body CRC 42 IEEE Frame format (con’t) Protocol version Type Subtype To DS From DS More flag Retry Pwr mgt More Data WEP Order Frame control Duration /ID Addressing 1 Addressing 2 Addressing 3 Sequence control Addressing 4 Frame body CRC 43 MAC Layer： CSMA/CA ? Collision Resolution ? CSMA/CA ? Hidden Terminal effect ? How it works? Carrier Sense Multiple Access/Collision Avoidance 44 Collision Resolution ? Two senders might send RTS at the same time ? Collision will occur corrupting the data ? No CTS will follow ? Senders will timeout waiting for CTS and retry with exponential backoff RTS: RequesttoSend CTS: CleartoSend 45 transmission Protocol ? Sender A sends RequesttoSend (RTS) ? Receiver B sends CleartoSend (CTS) ? Nodes who hear CTS cannot transmit concurrently with A (red region) ? Nodes who hear RTS but not CTS can transmit (green region) ? Sender A sends data frame ? Receiver B sends ACK ? Nodes who hear the ACK can now transmit RTS CTS A B 46 Hidden Terminal effect (a) A and C cannot hear each other because of obstacles or signal attenuation。 so, their packets collide at B (b) goal: avoid collisions at B CSMA/CA: CSMA with Collision Avoidance 47 CSMA/CA (Collision Avoidance) sense channel idle for DISF sec (Distributed Inter Frame Space), send RTS receiver returns CTS after SIFS (Short Inter Frame Space) ? CTS “freezes” stations within range of receiver (but possibly hidden from transmitter)。 this prevents collisions by hidden station during data transmit data frame (no Collision Detection) receiver returns ACK after SIFS (Short Inter Frame Space) if channel sensed busy then binary backoff NAV: Network Allocation Vector (min time of deferral) (= min packet size in ) ? RTS and CTS are very short: collisions during data phase are thus very unlikely (the end result is similar to Collision Detection) 48 security features ? ESSID ? Network name, not encrypted ? Rudimentary because the ESS ID is broadcast in beacon frames ? Association ? Capability to register a station with a WLAN ? WEP (Wired Equivalent Privacy) ? encrypts data using RC4 with 40 to 128bit shared keys ? Some vendors do in software, others in hardware ? Symmetric Scheme – Same Key For Encrypt/Decrypt ? Intended For: ? Access Control (no WEP key, no access) ? Privacy (encrypt data stream) 49 Wired Equivalent Privacy ? Why Wired Equivalence Privacy? ? Wireless medium has no packet boundaries ? WEP control access to LAN via authentication ? Wireless is an open medium ? Provides linklevel security equivalent to a closed medium (note: no endtoend privacy) ? Two Types of Authentication ? Set on Client/Access Points (Same) ? Open (Default): ClearText Authentication ? No WEP key required for access ? SharedKey: ClearText Challenge (by AP) ? Must respond with the correct WEP key, or no access ? Broken due to bad use of the cipher [Walker, Berkeley Team, Arbaugh, Fluhrer] 50 WEP (cont.) ? RSA “FastPacket Keying” ? Fix Approved By IEEE Committee (2020) ? Generates Unique Encryption Keys For Data Packets ? Reduces Similarities Between Successive Packets ? Temporal Key Integrity Protocol (TKIP) ? Approved 2020/01/25, Optional Standard ? Helps Defeat Passive Packet Snooping ? Dynamic Keys Defeat Capture of Passive Keys (WEP Hole) ? Some Vendor