6: Wireless and Mobile Networks6-1 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 Sections 6.1, 6.3 In text

6: Wireless and Mobile Networks6-2 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 cell towers m access points

6: Wireless and Mobile Networks6-3 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 Networks6-4 Characteristics of selected wireless link standards 384 Kbps 56 Kbps 54 Mbps 5-11 Mbps 1 Mbps b {a,g} IS-95 CDMA, GSM UMTS/WCDMA, CDMA p-to-p link 2G 3G Indoor 10 – 30m Outdoor 50 – 200m Mid range outdoor 200m – 4Km Long range outdoor 5Km – 20Km e or Mobile WiMAX

6: Wireless and Mobile Networks6-5 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 Networks6-6 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 Networks6-7 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 at destination at slightly different times …. make communication across (even a point to point) wireless link much more “difficult”

6: Wireless and Mobile Networks6-8 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 GHz range m up to 54 Mbps r g m 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 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 11 channels are partial overlapping (1, 6, 11 non-overlapping) 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 Networks6-11 IEEE : multiple access 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 Networks6-12 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 2 else /* received ack */ return back to 2 (why?) to transmit next frame receiver - if frame received OK return ACK after SIFS (no ack in ethernet!!) sender receiver DIFS data SIFS ACK DIFS: distributed inter-frame spacing, SIFS: short inter-frame spacing

6: Wireless and Mobile Networks6-13 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 Networks6-14 Collision Avoidance: RTS-CTS exchange AP A B time RTS(A) RTS(B) RTS(A) CTS(A) DATA (A) ACK(A) reservation collision defer Textbook Page 522 figure DIFS CIFS

6: Wireless and Mobile Networks6-15 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 Networks6-16 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 4: used only in ad hoc mode

6: Wireless and Mobile Networks6-17 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 Networks6-18 frame control duration address 1 address 2 address 4 address 3 payloadCRC seq control Type From AP Subtype To AP More frag WEP More data Power mgt RetryRsvd Protocol version frame: more duration of reserved transmission time (data, RTS/CTS) frame type (RTS, CTS, ACK, data)

6: Wireless and Mobile Networks6-19 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 m AP2 broadcast H1’s MAC to switch

6: Wireless and Mobile Networks MAC and Bluetooth r MAC m 11 Mbps – 54 Mbps m Up to 100 meters range r 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 Networks6-21 Bluetooth r Physical layer properties: m 2.4GHz unlicensed spectrum m Frequency-hopping spread spectrum 79 channels with different frequencies TDM transmit: jump among channels with preset sequences (coding) m Up to 721bps ( is 11 Mbps to 54 Mbps)

6: Wireless and Mobile Networks6-22 Bluetooth r Ad hoc network structure r One master, <=7 slaves m Odd time slot: master m 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 Networks6-23 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 Networks6-24 CDMA example

6: Wireless and Mobile Networks6-25 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) = 2 r New signals in the air (N senders): Same decoding formula!

6: Wireless and Mobile Networks6-26 Why extract correctly By each user? A: user codes are orthogonal