Wireless and Mobile Networks EECS 489 Computer Networks

Wireless and Mobile Networks EECS 489 Computer Networks http: //www. eecs. umich. edu/courses/eecs 489/w 07 Z. Morley Mao Wednesday March 14, 2007 Acknowledgement: Some slides taken from Kurose&Ross

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

Elements of a wireless network infrastructure mode base station connects mobiles into wired network handoff: mobile changes base station providing connection into wired network

Elements of a wireless network Ad hoc mode no base stations nodes can only transmit to other nodes within link coverage nodes organize themselves into a network: route among themselves

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”

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

Review on FDMA, TDMA, CDMA

Code Division Multiple Access (CDMA) r used in several wireless broadcast channels r r r (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”)

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

CDMA in the presence of multiple users

CDMA: two-sender interference

IEEE 802. 11 Wireless LAN r 802. 11 b m 2. 4 -5 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 -6 GHz range m up to 54 Mbps r 802. 11 g m 2. 4 -5 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

802. 11 LAN architecture Internet AP hub, switch or router BSS 1 AP BSS 2 wireless host communicates with 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

802. 11: Channels, association r 802. 11 b: 2. 4 GHz-2. 485 GHz 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 m will typically run DHCP to get IP address in AP’s subnet

IEEE 802. 11: multiple access r avoid collisions: 2+ nodes transmitting at same time 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) A C A B B C C’s signal strength A’s signal strength space

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 sender receiver DIFS 802. 11 receiver - if frame received OK return ACK after SIFS (ACK needed due to hidden terminal problem) data SIFS ACK

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!

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)

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 Address 2: MAC address of wireless host or AP transmitting this frame 2 6 seq address 4 control 0 - 2312 4 payload CRC Address 3: used only in ad hoc mode Address 3: MAC address of router interface to which AP is attached

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

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 frame type (RTS, CTS, ACK, data) 1 Retry 1 0 - 2312 4 payload CRC 1 Power More mgt data 1 1 WEP Rsvd

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 router hub or switch BBS 1 AP 2 H 1 BBS 2

802. 15: personal area network r less than 10 m diameter r replacement for cables (mouse, keyboard, headphones) r ad hoc: no infrastructure r master/slaves: m m slaves request permission to send (to master) master grants requests r 802. 15: evolved from Bluetooth specification m m 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)

Components of cellular network architecture MSC cell q connects cells to wide area net q manages call setup (more later!) q handles mobility (more later!) q covers geographical region q base station (BS) analogous to 802. 11 AP q mobile users attach to network through BS q air-interface: physical and link layer protocol between mobile and BS Mobile Switching Center Public telephone network, and Internet Mobile Switching Center wired network

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

Cellular standards: brief survey 2 G systems: voice channels r IS-136 TDMA: combined FDMA/TDMA (north america) r GSM (global system for mobile communications): combined FDMA/TDMA m most widely deployed r IS-95 CDMA: code division multiple access TDMA/FDMA 0 CDMA-200 GPRS EDGE UMTS IS-136 GSM IS-95 Don’t drown in a bowl of alphabet soup: use this oor reference only

Cellular standards: brief survey 2. 5 G systems: voice and data channels r for those who can’t wait for 3 G service: 2 G extensions r general packet radio service (GPRS) m evolved from GSM m data sent on multiple channels (if available) r enhanced data rates for global evolution (EDGE) m also evolved from GSM, using enhanced modulation m Date rates up to 384 K r CDMA-2000 (phase 1) m data rates up to 144 K m evolved from IS-95

Cellular standards: brief survey 3 G systems: voice/data r Universal Mobile Telecommunications Service (UMTS) m GSM next step, but using CDMA r CDMA-2000 …. . more (and more interesting) cellular topics due to mobility (stay tuned for details)

What is mobility? r spectrum of mobility, from the network perspective: no mobility mobile wireless user, mobile user, using same access connecting/ point disconnecting from network using DHCP. high mobility mobile user, passing through multiple access point while maintaining ongoing connections (like cell phone)

Mobility: Vocabulary home network: permanent “home” of mobile (e. g. , 128. 119. 40/24) Permanent address: address in home network, can always be used to reach mobile e. g. , 128. 119. 40. 186 home agent: entity that will perform mobility functions on behalf of mobile, when mobile is remote wide area network correspondent

Mobility: more vocabulary Permanent address: remains constant (e. g. , 128. 119. 40. 186) visited network: network in which mobile currently resides (e. g. , 79. 129. 13/24) Care-of-address: address in visited network. (e. g. , 79, 129. 13. 2) wide area network correspondent: wants to communicate with mobile home agent: entity in visited network that performs mobility functions on behalf of mobile.

How do you contact a mobile friend: Consider friend frequently changing addresses, how do you find her? r search all phone books? r call her parents? r expect her to let you know where he/she is? I wonder where Alice moved to?

Mobility: approaches r Let routing handle it: routers advertise permanent address of mobile-nodes-in-residence via usual routing table exchange. m routing tables indicate where each mobile located m no changes to end-systems r Let end-systems handle it: m indirect routing: communication from correspondent to mobile goes through home agent, then forwarded to remote m direct routing: correspondent gets foreign address of mobile, sends directly to mobile

Mobility: approaches r Let routing handle it: routers advertise permanent not address of mobile-nodes-in-residence via usual scalable routing table exchange. to millions of m routing tables indicate mobiles where each mobile located m no changes to end-systems r let end-systems handle it: m indirect routing: communication from correspondent to mobile goes through home agent, then forwarded to remote m direct routing: correspondent gets foreign address of mobile, sends directly to mobile

Mobility: registration visited network home network 2 1 wide area network foreign agent contacts home agent home: “this mobile is resident in my network” End result: r Foreign agent knows about mobile r Home agent knows location of mobile contacts foreign agent on entering visited network

Mobility via Indirect Routing foreign agent receives packets, forwards to mobile home agent intercepts packets, forwards to foreign agent home network visited network 3 wide area network correspondent addresses packets using home address of mobile 1 2 4 mobile replies directly to correspondent

Indirect Routing: comments r Mobile uses two addresses: m permanent address: used by correspondent (hence mobile location is transparent to correspondent) m care-of-address: used by home agent to forward datagrams to mobile r foreign agent functions may be done by mobile itself r triangle routing: correspondent-home-networkmobile m inefficient when correspondent, mobile are in same network

Indirect Routing: moving between networks r suppose mobile user moves to another network m registers with new foreign agent m new foreign agent registers with home agent m home agent update care-of-address for mobile m packets continue to be forwarded to mobile (but with new care-of-address) r mobility, changing foreign networks transparent: on going connections can be maintained!

Mobility via Direct Routing correspondent forwards to foreign agent receives packets, forwards to mobile home network 4 wide area network 2 correspondent requests, receives foreign address of mobile visited network 1 3 4 mobile replies directly to correspondent

Mobility via Direct Routing: comments r overcome triangle routing problem r non-transparent to correspondent: correspondent must get care-of-address from home agent m what if mobile changes visited network?

Accommodating mobility with direct routing r anchor foreign agent: FA in first visited network r data always routed first to anchor FA r when mobile moves: new FA arranges to have data forwarded from old FA (chaining) foreign net visited at session start wide area network anchor foreign agent 1 2 4 5 correspondent agent 3 new foreign agent new foreign network