6: Wireless and Mobile Networks6-1 Chapter 6: Wireless and Mobile Networks Background: r # wireless (mobile) phone subscribers now exceeds # wired phone subscribers! r computer nets: laptops, palmtops, PDAs, Internet-enabled phone promise anytime untethered Internet access r two important (but different) challenges m communication over wireless link m handling mobile user who changes point of attachment to network

6: Wireless and Mobile Networks6-2 Chapter 6 outline 6.1 Introduction Wireless r 6.2 Wireless links, characteristics m CDMA r 6.3 IEEE wireless LANs (“wi-fi”)

6: Wireless and Mobile Networks6-3 Elements of a wireless network network infrastructure wireless hosts r laptop, PDA, IP phone r run applications r may be stationary (non-mobile) or mobile m wireless does not always mean mobility

6: Wireless and Mobile Networks6-4 Elements of a wireless network 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 e.g., cell towers access points

6: Wireless and Mobile Networks6-5 Elements of a wireless network 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 Networks6-6 Elements of a wireless network network infrastructure 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 Networks6-7 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

6: Wireless and Mobile Networks6-8 Wireless Link Characteristics Differences from wired link …. m decreased 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) interfere as well m 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 Networks6-9 Wireless network characteristics Multiple wireless senders and receivers create additional problems (beyond multiple access): A B C Hidden terminal problem r B, A hear each other r B, C hear each other r A, C can not hear each other means A, C unaware of their interference at B A B C A’s signal strength space C’s signal strength Signal fading: r B, A hear each other r B, C hear each other r A, C can not hear each other interferring at B

6: Wireless and Mobile Networks6-10 Code Division Multiple Access (CDMA) r used in several wireless broadcast channels (cellular, satellite, etc) standards r unique “code” assigned to each user; i.e., code set partitioning r all users share same frequency, but each user has own “chipping” sequence (i.e., code) to encode data r encoded signal = (original data) X (chipping sequence) r decoding: inner-product of encoded signal and chipping sequence r allows multiple users to “coexist” and transmit simultaneously with minimal interference (if codes are “orthogonal”)

6: Wireless and Mobile Networks6-11 CDMA Encode/Decode slot 1 slot 0 d 1 = Z i,m = d i. c m d 0 = slot 0 channel output slot 1 channel output channel output Z i,m sender code data bits slot 1 slot 0 d 1 = -1 d 0 = slot 0 channel output slot 1 channel output receiver code received input D i =  Z i,m. c m m=1 M M

6: Wireless and Mobile Networks6-12 CDMA: two-sender interference

6: Wireless and Mobile Networks6-13 Chapter 6 outline 6.1 Introduction Wireless r 6.2 Wireless links, characteristics m CDMA r 6.3 IEEE wireless LANs (“wi-fi”)

6: Wireless and Mobile Networks6-14 IEEE Wireless LAN r b m 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 a m 5-6 GHz range m up to 54 Mbps r g m GHz range m up to 54 Mbps r All use CSMA/CA for multiple access r All have base-station and ad-hoc network versions

6: Wireless and Mobile Networks LAN architecture r wireless host communicates 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 BSS 1 BSS 2 Internet hub, switch or router AP

6: Wireless and Mobile Networks : Channels, association r b: 2.4GHz-2.485GHz 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! 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 Networks6-17 IEEE : multiple access r avoid collisions: 2 + nodes transmitting at same time r : CSMA - sense before transmitting m don’t collide with ongoing transmission by other node r : 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) A B C A B C A’s signal strength space C’s signal strength

6: Wireless and Mobile Networks6-18 IEEE MAC Protocol: CSMA/CA sender 1 if sense channel idle for DIFS then transmit entire frame (no CD) 2 if sense channel busy then start random backoff time timer counts down while channel idle transmit when timer expires if no ACK, increase random backoff interval, repeat receiver - if frame received OK return ACK after SIFS (ACK needed due to hidden terminal problem) sender receiver DIFS data SIFS ACK

6: Wireless and Mobile Networks6-19 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 data frame collisions completely using small reservation packets!

6: Wireless and Mobile Networks6-20 Collision Avoidance: RTS-CTS exchange AP A B time RTS(A) RTS(B) RTS(A) CTS(A) DATA (A) ACK(A) reservation collision defer

6: Wireless and Mobile Networks6-21 frame control duration address 1 address 2 address 4 address 3 payloadCRC seq control frame: addressing Address 2: MAC address of wireless host or AP transmitting this frame Address 1: MAC address of wireless host or AP to receive this frame Address 3: MAC address of router interface to which AP is attached Address 3: used only in ad hoc mode

6: Wireless and Mobile Networks6-22 Internet router AP H1 R1 AP MAC addr H1 MAC addr R1 MAC addr address 1 address 2 address frame R1 MAC addr AP MAC addr dest. address source address frame frame: addressing

6: Wireless and Mobile Networks6-23 hub or switch AP 2 AP 1 H1 BBS 2 BBS : mobility within same subnet router r H1 remains in same IP subnet: IP address can remain same r switch: which AP is associated with H1? m self-learning (Ch. 5): switch will see frame from H1 and “remember” which switch port can be used to reach H1