Wireless, Mobile Networks 6-1 Ch. 6: Wireless and Mobile Networks Background:  # wireless (mobile) phone subscribers now exceeds # wired phone subscribers (5-to-1)!  # wireless Internet-connected devices equals # wireline Internet-connected devices  laptops, Internet-enabled phones promise anytime untethered Internet access  two important (but different) challenges  wireless: communication over wireless link  mobility: handling the mobile user who changes point of attachment to network

Wireless, Mobile Networks 6-2 Elements of a wireless network network infrastructure

Wireless, Mobile Networks 6-3 wireless hosts  laptop, smartphone  run applications  may be stationary (non- mobile) or mobile  wireless does not always mean mobility Elements of a wireless network network infrastructure

Wireless, Mobile Networks 6-4 base station  typically connected to wired network  relay - responsible for sending packets between wired network and wireless host(s) in its “area”  e.g., cell towers, access points Elements of a wireless network network infrastructure

Wireless, Mobile Networks 6-5 wireless link  typically used to connect mobile(s) to base station  also used as backbone link  multiple access protocol coordinates link access  various data rates, transmission distance Elements of a wireless network network infrastructure

6: Wireless and Mobile Networks 6-6 Characteristics of selected wireless link standards Indoor 10-30m Outdoor m Mid-range outdoor 200m – 4 Km Long-range outdoor 5Km – 20 Km G: IS-95, CDMA, GSM 2.5G: UMTS/WCDMA, CDMA b a,g 3G: UMTS/WCDMA-HSPDA, CDMA2000-1xEVDO 4G: LTWE WIMAX a,g point-to-point n Data rate (Mbps) Mesh

6: Wireless and Mobile Networks 6-7 WiMax Vs. Wireless Mesh r WiMax 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 Wireless LAN m Use unlicensed public spectrum m ’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 over multi-hop wireless links, bad QoS

6: Wireless and Mobile Networks 6-8 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 Networks 6-9 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 6-10 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

Wireless, Mobile Networks 6-11 Wireless network taxonomy single hop multiple hops infrastructure (e.g., APs) no infrastructure host connects to base station (WiFi, WiMAX, cellular) which connects to larger Internet no base station, no connection to larger Internet (Bluetooth, ad hoc nets) host may have to relay through several wireless nodes to connect to larger Internet: mesh net no base station, no connection to larger Internet. May have to relay to reach other a given wireless node MANET, VANET

6: Wireless and Mobile Networks 6-12 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 Networks 6-13 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

Wireless, Mobile Networks 6-14 IEEE Wireless LAN b r GHz unlicensed spectrum r up to 11 Mbps r direct sequence spread spectrum (DSSS) in physical layer m all hosts use same chipping code a  5-6 GHz range  up to 54 Mbps g  GHz range  up to 54 Mbps n: multiple antennae  GHz range  up to 200 Mbps  all use CSMA/CA for multiple access  all have base-station and ad-hoc network versions

Wireless, Mobile Networks LAN architecture  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 BSS 1 BSS 2 Internet hub, switch or router

Wireless, 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

Wireless, Mobile Networks : passive/active scanning AP 2 AP 1 H1 BBS 2 BBS passive scanning: (1)beacon frames sent from APs (2)association Request frame sent: H1 to selected AP (3)association Response frame sent from selected AP to H1 AP 2 AP 1 H1 BBS 2 BBS active scanning: (1)Probe Request frame broadcast from H1 (2)Probe Response frames sent from APs (3)Association Request frame sent: H1 to selected AP (4)Association Response frame sent from selected AP to H1

6: Wireless and Mobile Networks 6-18 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 2 else /* received ack */ return back to 2 (why?) to transmit next frame receiver - if frame received OK return ACK after SIFS (ACK needed due to hidden terminal problem) (no ack in ethernet!!) sender receiver DIFS data SIFS ACK DIFS: distributed inter-frame spacing, SIFS: short inter-frame spacing

6: Wireless and Mobile Networks 6-19 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 long data frame collisions using small reservation packets!

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

6: Wireless and Mobile Networks 6-21 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 Maximum frame length

6: Wireless and Mobile Networks 6-22 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 Networks 6-23 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 Networks 6-24 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 Networks 6-25 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 Networks 6-27 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 Networks 6-28 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 Networks 6-29 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 6-30 CDMA example

6: Wireless and Mobile Networks 6-31 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 Networks 6-32 Why extract correctly By each user? A: user codes are orthogonal

Orthogonal Explanation r We can use the orthogonal vectors in 3-D or 2-D space to explain why signal from another user will not affect your received data? 6: Wireless and Mobile Networks 6-33 On the 2-D space, if we add noise To the X-axis, it does not affect The node’s Y-axis position value

Wireless, Mobile Networks 6-34 Mobile Switching Center Public telephone network Mobile Switching Center Components of cellular network architecture  connects cells to wired tel. net.  manages call setup (more later!)  handles mobility (more later!) MSC  covers geographical region  base station (BS) analogous to AP  mobile users attach to network through BS  air-interface: physical and link layer protocol between mobile and BS cell wired network

Wireless, Mobile Networks 6-35 Cellular networks: the first hop Two techniques for sharing mobile-to-BS radio spectrum  combined FDMA/TDMA: divide spectrum in frequency channels, divide each channel into time slots  CDMA: code division multiple access frequency bands time slots

Wireless, Mobile Networks 6-36 BSC BTS Base transceiver station (BTS) Base station controller (BSC) Mobile Switching Center (MSC) Mobile subscribers Base station system (BSS) Legend 2G (voice) network architecture MSC Public telephone network Gateway MSC G

Wireless, Mobile Networks G (voice+data) network architecture radio network controller MSC SGSN Public telephone network Gateway MSC G Serving GPRS Support Node (SGSN) Gateway GPRS Support Node (GGSN) Public Internet GGSN G Key insight: new cellular data network operates in parallel (except at edge) with existing cellular voice network  voice network unchanged in core  data network operates in parallel GPRS: Generalized Packet Radio Service

Wireless, Mobile Networks 6-38 radio network controller MSC SGSN Public telephone network Gateway MSC G Public Internet GGSN G radio access network Universal Terrestrial Radio Access Network (UTRAN) core network General Packet Radio Service (GPRS) Core Network public Internet radio interface (WCDMA, HSPA ) 3G (voice+data) network architecture