CPSC 826 Internetworking. Wireless and Mobile Networks. Wireless Networks Wireless Hosts

Transcription

1 1 CPSC 826 Intering Wireless and Mobile Networks Michele Weigle Department of Computer Science Clemson University November 29, 200 Wireless and Mobile Networks Background Number of wireless (mobile) phone subscribers now exceeds number wired phone subscribers! Computer s» laptops, palmtops, PDAs, Internet-enabled phone promise anytime un-tethered Internet access Two important (but different) challenges: 1. communication over wireless link 2. handling mobile user who changes point of attachment to 2 Wireless and Mobile Networks Roadmap Wireless Hosts Introduction Wireless links, characteristics» CDMA IEEE wireless LANs ( wi-fi ) Mobility Principles: addressing and routing to mobile users Mobile IP Mobility and higher-layer protocols infrastructure wireless hosts laptop, PDA, IP phone run applications may be stationary (nonmobile) or mobile» wireless does not always mean mobility 3

2 5 Base Station Wireless Link infrastructure base station typically connected to wired relay - responsible for sending packets between wired and wireless host(s) in its area» e.g., cell towers access points infrastructure 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 6 Characteristics of selected wireless link standards Infrastructure Mode 5 Mbps 5-11 Mbps 1 Mbps 38 Kbps 56 Kbps {a,g} b UMTS/WCDMA, CDMA2000 IS-95 CDMA, GSM.11 p-to-p link 3G 2G infrastructure infrastructure mode base station connects mobiles into wired handoff: mobile changes base station providing connection into wired Indoor Outdoor Mid range outdoor Long range outdoor 10 30m m 200m Km 5Km 20Km 7 8

3 9 Ad-hoc Mode Wireless Link Characteristics Ad hoc mode no base stations nodes can only transmit to other nodes within link coverage nodes organize themselves into a : route among themselves Differences from wired link.» decreased signal strength radio signal attenuates as it propagates through matter (path loss)» interference from other sources standardized wireless frequencies (e.g., 2. 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 10 Characteristics Multiple wireless senders and receivers create additional problems (beyond multiple access): A 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 B A B C A s signal strength space C s signal strength Signal fading: B, A hear each other B, C hear each other A, C can not hear each other interfering at B Code Division Multiple Access (CDMA) Used in several wireless broadcast channels (cellular, satellite, etc) standards Unique code assigned to each user» 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 ) 11 12

4 13 Wireless and Mobile Networks Roadmap IEEE Wireless LAN Introduction Wireless links, characteristics» CDMA IEEE wireless LANs ( wi-fi ) Mobility Principles: addressing and routing to mobile users Mobile IP Mobility and higher-layer protocols b» 2.-5 GHz unlicensed radio spectrum» up to 11 Mbps» direct sequence spread spectrum (DSSS) in physical layer all hosts use same chipping code» widely deployed, using base stations a» 5-6 GHz range» up to 5 Mbps g» 2.-5 GHz range» up to 5 Mbps All use CSMA/CA for multiple access All have base-station and ad-hoc versions 1 IEEE LAN Architecture IEEE Channels, Association BSS 1 AP Internet hub, switch or router AP 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 b: 2.GHz-2.85GHz spectrum divided into 11 channels at different frequencies» AP admin chooses frequency for AP» interference possible: channel can be same as that chosen by neighboring AP! Host: must associate with an AP» scans channels, listening for beacon frames containing AP s name (SSID) and MAC address» selects AP to associate with» may perform authentication» will typically run DHCP to get IP address in AP s subnet BSS

5 17 IEEE Multiple Access IEEE MAC Protocol CSMA/CA Avoid collisions: 2 + nodes transmitting at same time : CSMA - sense before transmitting» don t collide with ongoing transmission by other node : no collision detection!» difficult to receive (sense collisions) when transmitting due to weak received signals (fading)» can t sense all collisions in any case: hidden terminal, fading» goal: avoid collisions: CSMA/C(ollision)A(voidance) A C B A B C A s signal strength C s signal strength Sender If sense channel idle for DIFS then» transmit entire frame (no CD) 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 Receiver if frame received OK» return ACK after SIFS (ACK needed due to hidden terminal problem) DIFS sender data ACK receiver SIFS space 18 IEEE MAC Protocol Avoiding Collisions Collision Avoidance RTS-CTS exchange Idea: allow sender to reserve channel rather than random access of data frames: avoid collisions of long data frames Sender first transmits small request-to-send (RTS) packets to BS using CSMA» RTSs may still collide with each other (but they re short) BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes» sender transmits data frame» other stations defer transmissions A RTS(A) RTS(A) CTS(A) DATA (A) AP reservation collision CTS(A) B RTS(B) defer Avoid data frame collisions completely using small reservation packets! time ACK(A) ACK(A) 19 20

6 21 IEEE Frame Addressing IEEE Frame Addressing frame duration address address control 1 2 address 3 seq control address payload CRC H1 R1 router Internet 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 Address 3: MAC address of router interface to which AP is attached Address : used only in ad hoc mode AP R1 MAC addr AP MAC addr dest. address source address frame AP MAC addr H1 MAC addr R1 MAC addr address 1 address 2 address frame 22 IEEE Frame IEEE Mobility Within Same Subnet frame duration address address control 1 2 Protocol version Type duration of reserved transmission time (RTS/CTS) Subtype To AP address 3 From AP seq control More frag frame seq # (for reliable ARQ) address 1 1 Power Retry mgt payload More data WEP CRC Rsvd H1 remains in same IP subnet: IP address can remain same Switch: which AP is associated with H1?» self-learning: switch will see frame from H1 and remember which switch port can be used to reach H1 BBS 1 AP 1 router hub or switch AP 2 H1 BBS 2 frame type (RTS, CTS, ACK, data) 23 2

7 25 IEEE Personal Area Network Wireless and Mobile Networks Roadmap Less than 10 m diameter Replacement for cables (mouse, keyboard, headphones) Ad-hoc: no infrastructure Master/slaves:» slaves request permission to send (to master)» master grants requests : evolved from Bluetooth specification» GHz radio band» up to 721 kbps S S P M P S P M Master device S Slave device radius of coverage P Introduction Wireless links, characteristics» CDMA IEEE wireless LANs ( wi-fi ) Mobility Principles: addressing and routing to mobile users Mobile IP Mobility and higher-layer protocols P Parked device (inactive) 26 What is Mobility? Mobility Vocabulary Spectrum of mobility, from the perspective: no mobility high mobility home : permanent home of mobile (e.g., /2) home agent: entity that will perform mobility functions on behalf of mobile, when mobile is remote mobile wireless user, using same access point mobile user, connecting/ disconnecting from using DHCP. mobile user, passing through multiple access point while maintaining ongoing connections (like cell phone) Permanent address: address in home, can always be used to reach mobile e.g., wide area 27 28

8 29 Mobility More Vocabulary Mobility How do you contact a mobile friend? Permanent address: remains constant (e.g., ) Care-of-address: address in visited. (e.g., 79, ) wide area visited : in which mobile currently resides (e.g., /2) Consider friend frequently changing addresses, how do you find her? search all phone books? call her parents? expect her to let you know where he/she is? I wonder where Alice moved to? : wants to communicate with mobile home agent: entity in visited that performs mobility functions on behalf of mobile. 30 Mobility Approaches Let routing handle it:» routers advertise permanent address of mobile-nodesin-residence via usual routing table exchange.» routing tables indicate where each mobile located» no changes to end-systems Let end-systems handle it:» indirect routing: communication from to mobile goes through home agent, then forwarded to remote» direct routing: gets foreign address of mobile, sends directly to mobile Mobility Approaches Let routing handle it:» routers advertise permanent address of mobile-nodesin-residence via usual routing table exchange. not scalable» routing tables indicate where each mobile located to millions of» no changes to end-systems mobiles Let end-systems handle it:» indirect routing: communication from to mobile goes through home agent, then forwarded to remote» direct routing: gets foreign address of mobile, sends directly to mobile 31 32

9 33 Mobility Registration End result: home 2 wide area foreign agent contacts home agent home: this mobile is resident in my Foreign agent knows about mobile Home agent knows location of mobile visited 1 mobile contacts foreign agent on entering visited Mobility via Indirect Routing home addresses packets using home address of mobile home agent intercepts packets, forwards to foreign agent 1 wide area 2 foreign agent receives packets, forwards to mobile 3 visited mobile replies directly to 3 Indirect Routing Comments Mobile uses two addresses:» permanent address: used by (hence mobile location is transparent to )» care-of-address: used by home agent to forward datagrams to mobile Foreign agent functions may be done by mobile itself Triangle routing» -home--mobile» inefficient when, mobile are in same Indirect Routing Moving Between Networks Suppose mobile user moves to another» registers with new foreign agent» new foreign agent registers with home agent» home agent update care-of-address for mobile» packets continue to be forwarded to mobile (but with new care-of-address) Mobility, changing foreign s transparent: on going connections can be maintained! 35 36

10 37 Mobility via Direct Routing Mobility via Direct Routing home forwards to foreign agent foreign agent receives packets, forwards to mobile visited Overcome triangle routing problem Non-transparent to : must get care-of-address from home agent» what if mobile changes visited? 2 wide area 3 requests, receives foreign address of mobile 1 mobile replies directly to 38 Accommodating Mobility with Direct Routing Wireless and Mobile Networks Roadmap Anchor foreign agent: FA in first visited Data always routed first to anchor FA When mobile moves: new FA arranges to have data forwarded from old FA (chaining) Introduction Wireless links, characteristics» CDMA Mobility Principles: addressing and routing to mobile users Mobile IP wide area 1 anchor foreign agent foreign net visited at session start 2 IEEE wireless LANs ( wi-fi ) Mobility and higher-layer protocols agent 5 new foreign agent 3 new foreign 39 0

11 1 Mobile IP RFC 3220 Has many features we ve seen:» home agents, foreign agents, foreign-agent registration, care-ofaddresses, encapsulation (packet-within-a-packet) Three components to standard:» indirect routing of datagrams» agent discovery» registration with home agent Mobile IP Indirect Routing packet sent by home agent to foreign agent: a packet within a packet dest: dest: Permanent address: dest: packet sent by foreign-agent-to-mobile packet dest: Care-of address: Mobile IP Agent Discovery Mobile IP Registration Example agent advertisement: foreign/home agents advertise service by broadcasting ICMP messages (typefield = 9) H,F bits: home and/or foreign agent R bit: registration required type = 9 code = 0 = 9 type = 16 registration lifetime router address checksum = 9 length sequence # RBHFMGV bits 0 or more care-ofaddresses reserved standard ICMP fields mobility agent advertisement extension home agent HA: time foreign agent COA: registration req. COA: HA: MA: Lifetime: 9999 identification: 71 encapsulation format. registration reply HA: MA: Lifetime: 999 Identification: 71 encapsulation format. visited : /2 ICMP agent adv. Mobile agent COA: MA: registration req. COA: HA: MA: Lifetime: 9999 identification:71. registration reply HA: MA: Lifetime: 999 Identification: 71. 3

12 5 Wireless and Mobility Impact on Higher Layer Protocols Logically, impact should be minimal» best effort service model remains unchanged» TCP and UDP can (and do) run over wireless, mobile but performance-wise:» packet loss/delay due to bit-errors (discarded packets, delays for link-layer retransmissions), and handoff» TCP interprets loss as congestion, will decrease congestion window unnecessarily» delay impairments for real-time traffic» limited bandwidth of wireless links