Introduction to Wireless Networking Muhammad Kamran Nishat These

Introduction to Wireless Networking Muhammad Kamran Nishat These slides contain material developed by Lili Qiu for CS 386 W at UT Austin and by J. F Kurose and K. W. Ross

Internet Protocol Stack r Application: supporting network applications m FTP, SMTP, HTTP r Transport: data transfer between processes m TCP, UDP r Network: routing of datagrams from source to destination m IP, routing protocols r Link: data transfer between neighboring network elements m application transport network link physical Ethernet, Wi. Fi r Physical: bits “on the wire” m Coaxial cable, optical fibers, radios 2

application transport Multiple Access Protocols network link physical

Multiple Access protocols r single shared broadcast channel r two or more simultaneous transmissions by nodes: interference m collision if node receives two or more signals at the same time multiple access protocol r distributed algorithm that determines how nodes share channel, i. e. , determine when node can transmit 4

Ideal Multiple Access Protocol Broadcast channel of rate R bps 1. when one node wants to transmit, it can send at rate R. 2. when M nodes want to transmit, each can send at average rate R/M 3. fully decentralized: m m no special node to coordinate transmissions no synchronization of clocks, slots 4. simple 5

MAC Protocols: a taxonomy Three broad classes: r Channel Partitioning m m divide channel into smaller “pieces” (time slots, frequency, code) allocate piece to node for exclusive use r Random Access m channel not divided, allow collisions m “recover” from collisions r “Taking turns” m nodes take turns, but nodes with more to send can take longer turns 6

Channel Partitioning MAC protocols: TDMA: time division multiple access r access to channel in "rounds" r each station gets fixed length slot (length = pkt trans time) in each round r unused slots go idle r example: 6 -station LAN, 1, 3, 4 have pkt, slots 2, 5, 6 idle 6 -slot frame 1 3 4 7

Channel Partitioning MAC protocols: FDMA: frequency division multiple access r channel spectrum divided into frequency bands r each station assigned fixed frequency band r unused transmission time in frequency bands go idle r example: 6 -station LAN, 1, 3, 4 have pkt, frequency bands 2, 5, 6 idle frequency bands FDM cable time 8

Random Access Protocols r When node has packet to send m transmit at full channel data rate. r Two or more transmitting nodes ➜ “collision”, r Random access MAC protocol specifies: m how to detect collisions m how to prevent collisions m how to recover from collisions (e. g. , via delayed retransmissions) r Examples of random access MAC protocols: m slotted ALOHA m CSMA, CSMA/CD, CSMA/CA 9

CSMA (Carrier Sense Multiple Access) CSMA: listen before transmit: If channel sensed idle: transmit entire frame r If channel sensed busy, defer transmission r human analogy: don’t interrupt others! 10

CSMA collisions spatial layout of nodes collisions can still occur: propagation delay means two nodes may not hear each other’s transmission collision: entire packet transmission time wasted note: role of distance & propagation delay in determining collision probability 11

CSMA/CD (Collision Detection) CSMA/CD: carrier sensing, deferral as in CSMA m collisions detected within short time m colliding transmissions aborted, reducing channel wastage r Collision detection: m easy in wired LANs: measure signal strengths, compare transmitted, received signals m difficult in wireless LANs: received signal strength overwhelmed by local transmission strength 12

application transport PHY Layer network link physical 13

Wireless Link Characteristics Wireless is a broadcast medium! Differences from wired link …. signal strength: radio signal attenuates as it propagates through matter (path loss) m interference from other sources: standardized wireless network frequencies (e. g. , 2. 4 GHz) shared by other devices (e. g. , phone); devices (motors, microwaves) interfere as well space m multipath propagation (fading): radio signal reflects off objects or ground, arriving at destination at slightly different times Signal Strength m decreased • Reflection • Diffraction • Scattering 14

Impact of Multipath Propagation Direct signal Reflected signal Received signal 15

Wireless Link Characteristics (2) – Fading • Channel characteristics change over time and location – e. g. , movement of sender, receiver and/or scatters • • quick changes in the power received (short term/fast fading) slow changes in the average power received (long term/slow fading) power short term fading long term fading t 16

Typical Picture Received Signal Power (d. Bm) path loss slow fading fast fading log (distance) 20

Signal Propagation Ranges • Transmission range – communication possible – low error rate • Detection range – detection of the signal possible – no communication possible • Interference range – signal may not be detected – signal adds to the background noise sender transmission distance detection interference 18

Signal, Noise, and Interference r Signal (S) r Noise (N) m Includes thermal noise and background radiation r Interference (I) m Signals from other transmitting sources r SINR = S/(N+I) (sometimes also denoted as SNR) r Large SINR = easier to extract signal from noise 19

d. B and Power conversion r d. B m Denote the difference between two power levels m (P 2/P 1)[d. B] = 10 * log 10 (P 2/P 1) 10 d. B: factor of 10 m P 2/P 1 = 10^(A/10) 3 d. B: factor of 2 m Example 1: P 2 = 10 P 1, P 2/P 1=10 d. B m Example 2: P 2/P 1 = 33 d. B, P 2 = 2000 P 1 r d. Bm and d. BW m Denote the power level relative to 1 m. W or 1 W m P[d. Bm] = 10*log 10(P/1 m. W) m P[d. BW] = 10*log 10(P/1 W) m Example: P = 0. 001 m. W = -30 d. Bm, P = 100 W = 20 d. BW 20

Wireless Link Characteristics (3) 10 -1 r SNR: signal-to-noise ratio m larger SNR – easier to extract signal from noise 10 -2 BER 10 -3 r SNR versus BER tradeoffs m given physical layer: increase power -> increase SNR -> decrease BER m given SNR: choose physical layer that meets BER requirement, giving highest throughput 10 -4 10 -5 10 -6 10 -7 10 20 30 40 SNR(d. B) QAM 256 (8 Mbps) QAM 16 (4 Mbps) BPSK (1 Mbps) 21

Rate Adaptation r base station, mobile 1. SNR decreases, BER increases as node moves away from base station 10 -2 10 -3 BER dynamically change transmission rate (physical layer modulation technique) as mobile moves, SNR varies 10 -1 10 -4 10 -5 10 -6 10 -7 10 20 30 SNR(d. B) 40 QAM 256 (8 Mbps) QAM 16 (4 Mbps) BPSK (1 Mbps) operating point 2. When BER becomes too high, switch to lower transmission rate but with lower BER 22

802. 11 LAN architecture v Internet v AP hub, switch or router BSS 1 AP BSS 2 wireless host communicates with base station § base station = access point (AP) Basic Service Set (BSS) (aka “cell”) in infrastructure mode contains: § wireless hosts § access point (AP): base station § ad hoc mode: hosts only 23

IEEE 802. 11 Wireless LANs 802. 11 b r 2. 4 -2. 5 GHz unlicensed spectrum r up to 11 Mbps 802. 11 a r 5 -6 GHz range r up to 54 Mbps 802. 11 g § 2. 4 -2. 5 GHz range § up to 54 Mbps 802. 11 n: multiple antennae § 2. 4 -2. 5 and 5 -6 GHz range § up to 600 Mbps § MIMO § Frame aggregation § Channel bonding 802. 11 ac § 5 -6 GHz range § up to 3. 4 Gbps § MU-MIMO v all use CSMA/CA for multiple access 24

802. 11: Channels r 802. 11 b/g: 2. 4 GHz-2. 485 GHz spectrum divided into 11 channels at different frequencies m AP admin chooses frequency for AP m interference possible: channel can be same as that chosen by neighboring AP! Only 3 non-overlapping channels! m 802. 11 a: 24 non-overlapping channels 25

802. 11: Association r host must associate with an AP m scans channels, listening for beacon frames containing AP’s name (SSID) and MAC address m selects AP to associate with m may perform authentication m will typically run DHCP to get IP address in AP’s subnet 26

802. 11: passive/active scanning BBS 1 AP 1 BBS 2 1 1 2 AP 2 BBS 1 AP 1 BBS 2 1 2 3 (1) beacon frames sent from APs (2) association Request frame sent: H 1 to selected AP (3) association Response frame sent: H 1 to selected AP AP 2 4 H 1 Passive Scanning: 2 3 Active Scanning: (1) Probe Request frame broadcast from H 1 (2) Probes response frame sent from APs (3) Association Request frame sent: H 1 to selected AP (4) Association Response frame sent: H 1 to selected AP 27

IEEE 802. 11 multiple access r 802. 11: CSMA - sense before transmitting r Differences from Ethernet m No collision detection (CD)! • Difficult to receive (sense collisions) when transmitting due to weak received signals (fading) • Can’t sense all collisions in any case: hidden terminal, fading • Goal: avoid collisions: CSMA/C(ollision)A(voidance) m Link layer ACKnowledgments/Retransmissions (ARQ) • High bit error rates A C A B B C C’s signal strength A’s signal strength space 28

IEEE 802. 11 MAC Protocol: CSMA/CA 802. 11 sender 1 if sense channel idle for DIFS then transmit entire frame (no CD) sender 2 if sense channel busy then 2. 1 start random backoff time DIFS timer counts down while channel idle, freezes when busy, resumes if idle for DIFS transmit when timer expires (why? ) 3 if ACK, wait for DIFS, then go to 2. 1 (why? ) 4 if no ACK, increase random backoff interval, goto 2. 1 (why? ) receiver data SIFS ACK 802. 11 receiver - if frame received OK return ACK after SIFS (ACK needed due to hidden terminal problem) 29

802. 11 Backoff Example 30

802. 11 Backoff Example After 15 slot countdown 31

802. 11 Backoff Example 32

802. 11 Backoff Example 33

802. 11 Backoff Example 34

802. 11 Backoff Example 35

Avoiding collisions (more) idea: allow sender to “reserve” channel rather than random access of data frames: avoid collisions of long data frames r sender first transmits small request-to-send (RTS) packets to BS using CSMA m RTSs may still collide with each other (but they’re short) r BS broadcasts clear-to-send CTS in response to RTS r CTS heard by all nodes m sender transmits data frame m other stations defer transmissions avoid data frame collisions completely using small reservation packets! 36

Collision Avoidance: RTS-CTS exchange A B AP RTS(B) RTS(A) reservation collision RTS(A) CTS(A) DATA (A) time ACK(A) defer ACK(A) 37

802. 11 Overhead DIFS Random Backoff Data Transmission SIFS ACK Transmission r Overhead: m DIFS m Random backoff m ACK/SIFS m Optional RTS/CTS handshake before transmission of data packet (often disabled due to its overhead) m Header overhead r 802. 11 has room for improvement. How? 38

802. 11 frame: addressing 2 2 6 6 6 frame address duration control 1 2 3 Address 1: MAC address of wireless host or AP to receive this frame 2 6 seq address 4 control 0 - 2312 4 payload CRC Address 4: used when a frame is sent from one AP to another AP Address 3: MAC address of router interface to which AP is attached Address 2: MAC address of wireless host or AP transmitting this frame 39

802. 11 Unicast vs. Broadcast 802. 11 unicast 802. 11 Broadcast r RTS/CTS r No RTS/CTS r ACK/Retx r No ACK/Retx r Exponential backoff r No exponential backoff r Autorate adaptation r Single (low) rate 40

Wireless, mobility: impact on higher layer protocols r logically, impact should be minimal … m best effort service model remains unchanged m TCP and UDP can (and do) run over wireless, mobile r … but performance-wise: m packet loss/delay due to bit-errors (discarded packets, delays for link-layer retransmissions), and handoff m TCP interprets loss as congestion, will decrease congestion window un-necessarily m delay impairments for real-time traffic m limited bandwidth of wireless links 41