Wireless Networks Background q wireless mobile phone subscribers

Wireless Networks Background: q # wireless (mobile) phone subscribers now exceeds # wired phone subscribers! q computer nets: laptops, palmtops, PDAs, Internet-enabled phone promise anytime untethered Internet access q two important (but different) challenges o o communication over wireless link handling mobile user who changes point of attachment to network 6: Wireless and Mobile Networks 1

Elements of a wireless network wireless hosts q laptop, PDA, IP phone q run applications q may be stationary (non- network infrastructure mobile) or mobile o wireless does not always mean mobility 6: Wireless and Mobile Networks 2

Elements of a wireless network base station q typically connected to network infrastructure wired network q relay - responsible for sending packets between wired network and wireless host(s) in its “area” o e. g. , cell towers 802. 11 access points 6: Wireless and Mobile Networks 3

Elements of a wireless network wireless link q typically used to connect network infrastructure mobile(s) to base station q also used as backbone link q multiple access protocol coordinates link access q various data rates, transmission distance 6: Wireless and Mobile Networks 4

Characteristics of selected wireless link standards 54 Mbps 5 -11 Mbps 802. 11{a, g} 802. 11 b . 11 p-to-p link 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 5

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

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

Wireless Link Characteristics Differences from wired link …. o o o 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 ad destination at slightly different times …. make communication across (even a point to point) wireless link much more “difficult” 6: Wireless and Mobile Networks 8

Wireless network characteristics Multiple wireless senders and receivers create additional problems (beyond multiple access): C A B Hidden terminal problem C C’s signal strength A’s signal strength space q B, A hear each other Signal fading: q A, C can not hear each other q B, C hear each other q B, A hear each other means A, C unaware of their interference at B q A, C can not hear each other interferring at B 6: Wireless and Mobile Networks 9

Code Division Multiple Access (CDMA) q used in several wireless broadcast channels q q q (cellular, satellite, etc) standards unique “code” assigned to each user; i. e. , code set partitioning all users share same frequency, but each user has own “chipping” sequence (i. e. , code) to encode data encoded signal = (original data) X (chipping sequence) decoding: inner-product of encoded signal and chipping sequence allows multiple users to “coexist” and transmit simultaneously with minimal interference (if codes are “orthogonal”) 6: Wireless and Mobile Networks 10

CDMA Encode/Decode sender data bits code Zi, m= di. cm d 0 = 1 -1 -1 -1 1 1 1 -1 -1 -1 slot 1 channel output 1 -1 1 1 1 d 1 = -1 1 channel output Zi, m -1 -1 -1 slot 0 channel output M Di = S Zi, m. cm m=1 received input code receiver 1 1 1 1 -1 -1 -1 1 -1 -1 -1 slot 1 M 1 1 -1 -1 slot 0 d 0 = 1 d 1 = -1 slot 1 channel output slot 0 channel output 6: Wireless and Mobile Networks 11

CDMA: two-sender interference 6: Wireless and Mobile Networks 12

IEEE 802. 11 Wireless LAN q 802. 11 b o 2. 4 -5 GHz unlicensed radio spectrum o up to 11 Mbps o direct sequence spread spectrum (DSSS) in physical layer • all hosts use same chipping code o widely deployed, using base stations q 802. 11 a o 5 -6 GHz range o up to 54 Mbps q 802. 11 g o 2. 4 -5 GHz range o up to 54 Mbps q All use CSMA/CA for multiple access q All have base-station and ad-hoc network versions 6: Wireless and Mobile Networks 13

802. 11 LAN architecture q wireless host communicates Internet AP hub, switch or router BSS 1 AP BSS 2 with base station o base station = access point (AP) q Basic Service Set (BSS) (aka “cell”) in infrastructure mode contains: o wireless hosts o access point (AP): base station o ad hoc mode: hosts only 6: Wireless and Mobile Networks 14

802. 11: Channels, association q 802. 11 b: 2. 4 GHz-2. 485 GHz spectrum divided into 11 channels at different frequencies o AP admin chooses frequency for AP o interference possible: channel can be same as that chosen by neighboring AP! q host: must associate with an AP o scans channels, listening for beacon frames containing AP’s name (SSID) and MAC address o selects AP to associate with o may perform authentication o will typically run DHCP to get IP address in AP’s subnet 6: Wireless and Mobile Networks 15

IEEE 802. 11: multiple access q avoid collisions: 2+ nodes transmitting at same time q 802. 11: CSMA - sense before transmitting o don’t collide with ongoing transmission by other node q 802. 11: no collision detection! o difficult to receive (sense collisions) when transmitting due to weak received signals (fading) o can’t sense all collisions in any case: hidden terminal, fading o 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 16

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, increase random backoff interval, repeat 2 802. 11 receiver - if frame received OK sender receiver DIFS data SIFS ACK return ACK after SIFS (ACK needed due to hidden terminal problem) 6: Wireless and Mobile Networks 17

Avoiding collisions (more) idea: allow sender to “reserve” channel rather than random access of data frames: avoid collisions of long data frames q sender first transmits small request-to-send (RTS) packets to BS using CSMA o RTSs may still collide with each other (but they’re short) q BS broadcasts clear-to-send CTS in response to RTS q RTS heard by all nodes o sender transmits data frame o other stations defer transmissions Avoid data frame collisions using small reservation packets 6: Wireless and Mobile Networks 18

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) 6: Wireless and Mobile Networks 19

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 20

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 21

802. 11 frame: more frame seq # (for reliable ARQ) duration of reserved transmission time (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 22

802. 11: mobility within same subnet q H 1 remains in same IP subnet: IP address can remain same q switch: which AP is associated with H 1? o self-learning: switch will see frame from H 1 and “remember” which switch port can be used to reach H 1 router hub or switch BBS 1 AP 2 H 1 BBS 2 6: Wireless and Mobile Networks 23

802. 15: personal area network q less than 10 m diameter q replacement for cables (mouse, keyboard, headphones) q ad hoc: no infrastructure q master/slaves: o o slaves request permission to send (to master) master grants requests q 802. 15: evolved from Bluetooth specification o o 2. 4 -2. 5 GHz radio band up to 721 kbps P S P radius of coverage M S P M Master device S Slave device P Parked device (inactive) 6: Wireless and Mobile Networks 24