Elements of a wireless network infrastructure wireless hosts

Elements of a wireless network infrastructure wireless hosts r laptop, PDA, IP phone r run applications r may be stationary (nonmobile) or mobile m wireless does not always mean mobility 6: Wireless and Mobile Networks 1

Elements of a wireless network infrastructure base station r typically connected to wired network r relay - responsible for sending packets between wired network and wireless host(s) in its “area” m cell towers m 802. 11 access points 6: Wireless and Mobile Networks 2

Elements of a wireless network infrastructure wireless link r typically used to connect mobile(s) to base station r also used as backbone link r multiple access protocol coordinates link access r various data rates, transmission distance 6: Wireless and Mobile Networks 3

Characteristics of selected wireless link standards mesh 54 Mbps 5 -11 Mbps 802. 11{a, g} 802. 16 e. 11 or p-to-p Mobile link. Wi. MAX 802. 11 b 802. 15 3 G UMTS/WCDMA, CDMA 2000 384 Kbps 2 G IS-95 CDMA, GSM 56 Kbps Indoor Outdoor Mid range outdoor Long range outdoor 10 – 30 m 50 – 200 m – 4 Km 5 Km – 20 Km 6: Wireless and Mobile Networks 4

Wi. Max Vs. Wireless Mesh r Wi. Max m Similar to cellular network infrastructure m Use licensed spectrum m 10 Mbit/s at 10 km in good environment m Is under development by many companies r Wireless Mesh m Extension of 802. 11 Wireless LAN m Use unlicensed public spectrum m 802. 11’s access routers interconnect together • Ad Hoc (usually non-mobile) networking and routing m Currently used in some places • Town & small city’s government agents (firefighter, police) – More popular in Europe than in US • Challenges: complex routing, high error rate, Qo. S 6: Wireless and Mobile Networks 5

Elements of a wireless network infrastructure mode r base station connects mobiles into wired network r handoff: mobile changes base station providing connection into wired network 6: Wireless and Mobile Networks 6

Elements of a wireless network Ad hoc mode r no base stations r nodes can only transmit to other nodes within link coverage r nodes organize themselves into a network: route among themselves Wireless active research area: Ad hoc network Sensor network 6: Wireless and Mobile Networks 7

Ad Hoc Vs. Sensor Networks r Ad Hoc network m Challenge Mobility of nodes m Good features: Plenty of power, computation resource m Applications • Mostly mobile laptops or PDAs • Vehicular network r Sensor network m Challenge limited power, computing resource m Good features: • Usually stationary, dense network m Applications • Military battlefield, civil engineering, environmental monitoring 6: Wireless and Mobile Networks 8

Wireless Link Characteristics Differences from wired link …. m m m decreased signal strength: radio signal attenuates as it propagates through matter (path loss) interference from other sources: standardized wireless network frequencies (e. g. , 2. 4 GHz) shared by other devices (e. g. , phone); devices (motors) interfere as well multipath propagation: radio signal reflects off objects ground, arriving at destination at slightly different times …. make communication across (even a point to point) wireless link much more “difficult” 6: Wireless and Mobile Networks 9

IEEE 802. 11 Wireless LAN r 802. 11 b m 2. 4 -2. 485 GHz unlicensed radio spectrum m up to 11 Mbps m direct sequence spread spectrum (DSSS) in physical layer • all hosts use same chipping code m widely deployed, using base stations r 802. 11 a m 5. 1 -5. 8 GHz range m up to 54 Mbps r 802. 11 g m 2. 4 -2. 485 GHz range m up to 54 Mbps m Use OFDM in physical layer r All use CSMA/CA for multiple access r All have base-station and ad-hoc network versions 6: Wireless and Mobile Networks 10

802. 11 LAN architecture r wireless host communicates Internet AP hub, switch or router BSS 1 AP BSS 2 with base station m base station = access point (AP) r Basic Service Set (BSS) (aka “cell”) in infrastructure mode contains: m wireless hosts m access point (AP): base station m ad hoc mode: hosts only 6: Wireless and Mobile Networks 11

802. 11: Channels, association r 802. 11 b: 2. 4 GHz-2. 485 GHz spectrum divided into 11 channels at different frequencies m m m 11 channels are partial overlapping (1, 6, 11 non-overlapping) AP admin chooses frequency for AP interference possible: channel can be same as that chosen by neighboring AP! 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 [Chapter 8] m will typically run DHCP to get IP address in AP’s subnet 6: Wireless and Mobile Networks 12

IEEE 802. 11: multiple access r 802. 11: CSMA - sense before transmitting m don’t collide with ongoing transmission by other node r 802. 11: no collision detection! m difficult to receive (sense collisions) when transmitting due to weak received signals (fading) m can’t sense all collisions in any case: hidden terminal, fading m goal: avoid collisions: CSMA/C(ollision)A(voidance) C A B C C’s signal strength A’s signal strength space 6: Wireless and Mobile Networks 13

IEEE 802. 11 MAC Protocol: CSMA/CA 802. 11 sender 1 if sense channel idle for DIFS then transmit entire frame (no CD) 2 if sense channel busy then start random backoff timer counts down while channel idle transmit when timer expires if (no ACK) sender receiver DIFS increase random backoff interval, repeat 2 data else /* received ack */ return back to 2 (why? ) to transmit next frame 802. 11 receiver - if frame received OK SIFS ACK return ACK after SIFS (ACK needed due to hidden terminal problem) (no ack in ethernet!!) DIFS: distributed inter-frame spacing, SIFS: short inter-frame spacing 6: Wireless and Mobile Networks 14

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 RTS heard by all nodes m sender transmits data frame m other stations defer transmissions Avoid long data frame collisions using small reservation packets! 6: Wireless and Mobile Networks 15

Collision Avoidance: RTS-CTS exchange A B AP DIFS RTS(B) RTS(A) reservation collision RTS(A) CIFS CTS(A) DATA (A) time defer CIFS ACK(A) 6: Wireless and Mobile Networks 16

RTS/CTS in Practice r RTS/CTS introduces delay, consume channel resource. m Benefit when the data frame is much larger than RTS/CTS. r APs set threshold of data frame length in order to use RTS/CTS m If > threshold, use RTS/CTS r Many APs skip RTS/CTS by using a threshold larger than the Max frame length 6: Wireless and Mobile Networks 17

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 only in ad hoc mode Address 3: MAC address of router interface to which AP is attached Address 2: MAC address of wireless host or AP transmitting this frame 6: Wireless and Mobile Networks 18

802. 11 frame: addressing R 1 router H 1 Internet AP R 1 MAC addr AP MAC addr dest. address source address 802. 3 frame AP MAC addr H 1 MAC addr R 1 MAC address 1 address 2 address 3 802. 11 frame 6: Wireless and Mobile Networks 19

802. 11 frame: more duration of reserved transmission time (data, RTS/CTS) 2 2 6 6 6 frame address duration control 1 2 3 2 Protocol version 2 4 1 Type Subtype To AP 6 2 1 seq address 4 control 1 From More AP frag 1 Retry 1 0 - 2312 4 payload CRC 1 Power More mgt data 1 1 WEP Rsvd frame type (RTS, CTS, ACK, data) 6: Wireless and Mobile Networks 20

802. 11: mobility within same subnet r H 1 remains in same IP subnet: IP address can remain same r switch: which AP is associated with H 1? m self-learning (Ch. 5): switch will see frame from H 1 and “remember” which switch port can be used to reach H 1 m AP 2 broadcast H 1’s MAC to switch router hub or switch BBS 1 AP 2 H 1 BBS 2 6: Wireless and Mobile Networks 21

802. 15 MAC and Bluetooth r 802. 11 MAC m 11 Mbps – 54 Mbps m Up to 100 meters range r 802. 15 MAC m Wireless personal area network (WPAN) m < 10 meters range m Simple (cheap) device, low power assumption m Cable, wire replacement • E. g. , mouse, keyboard, headphone m Example: Bluetooth 6: Wireless and Mobile Networks 22

Bluetooth r Physical layer properties: m 2. 4 GHz unlicensed spectrum m Frequency-hopping spread spectrum • 79 channels with different frequencies • TDM transmit: jump among channels with preset sequences (coding) m Up to 721 bps (802. 11 is 11 Mbps to 54 Mbps) 6: Wireless and Mobile Networks 23

Bluetooth r Ad hoc network structure r One master, <=7 slaves m m Odd time slot: master Even time: slaves r Parked: inactive devices r Problem: slow speed can be achieved by RF device m Much cheaper, simpler 6: Wireless and Mobile Networks 24

CDMA Principle (6. 2. 1) r Code Division Multiple Access m Wide spectrum technique m All users use the full spectrum m Users with different codings not interfere r Each bit is encoded by much high rate signal (code) m Receiver can recover the bit with the corresponding code 6: Wireless and Mobile Networks 25

CDMA example 6: Wireless and Mobile Networks 26

Working with multiple users r How to extract data when multiple users transmit at the same time? r Assumptions: m Interfering signals are additive m Signal 1+1+1+(-1) = 2 r New signals in the air (N senders): Same decoding formula! 6: Wireless and Mobile Networks 27

Why extract correctly By each user? A: user codes are orthogonal 6: Wireless and Mobile Networks 28