wireless-mobile

null Wireless and Mobile Networks Wireless and Mobile Networks金志刚 zgjin@tju.edu.cnThe three wireless “waves”The three wireless “waves”Wave #1: cellular telephony Still, biggest profit maker Wave #2 : wireless Internet access Most Internet access on US campuses is wireless Hot spots are rapidly proliferating in the US; Europe and Asia to follow soon 2.5 G, 3G and 4G trying to keep up; competitive edge? Wave #3: ad hoc wireless nets (now) Set up in an area with NO infrastructure; to respond to a specific, time limited neednullnullWhat do we use wireless for?What do we use wireless for? Device-to-device communication Data transmission from one device to the other Device to infrastructure Access to the Internet Phone callsCellular applicationsCellular applicationsThe first killer application – SMS text messaging Then ring tones… … and with data communication a dominant use Always-on connectivity…  … while on the moveFind where your friends are – Where, Whrrl, LooptLocalization through GPSWiFi applicationsWiFi applicationsWireless access to the Internet! In home Through hotspots While on the move… Free Internet access through mesh WiFi WiFi in developing regionsCurrent Wireless InternetCurrent Wireless InternetInternetWireless WAN: 2G/3G cellular infrastructure Wireless LAN: IEEE 802.112G/3G WAN InfrastructureMoving: Wireless and Mobile NetworksMoving: Wireless and Mobile NetworksBackground: # wireless (mobile) phone subscribers now exceeds # wired phone subscribers! computer nets: laptops, palmtops, PDAs, Internet-enabled phone promise anytime untethered Internet access two important (but different) challenges communication over wireless link handling mobile user who changes point of attachment to networkOutlineOutline 1 Introduction Wireless 2 Wireless links, characteristics CDMA 3 Cellular Internet Access architecture standards (e.g., GSM) 4 IEEE 802.11 wireless LANs (“wi-fi”) Mobility 5 Principles: addressing and routing to mobile users 6 Mobile IP 7 Handling mobility in cellular networks 8 Mobility and higher-layer protocols 9 SummaryElements of a wireless networkElements of a wireless networkElements of a wireless networkElements of a wireless networkElements of a wireless networkElements of a wireless network 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 distanceElements of a wireless networkElements of a wireless networkCharacteristics of selected wireless link standardsCharacteristics of selected wireless link standardsIndoor 10-30mOutdoor 50-200mMid-range outdoor 200m – 4 KmLong-range outdoor 5Km – 20 Km.056.384145-1154IS-95, CDMA, GSM2GUMTS/WCDMA, CDMA20003G802.15802.11b802.11a,gUMTS/WCDMA-HSPDA, CDMA2000-1xEVDO3G cellular enhanced802.16 (WiMAX)802.11a,g point-to-point200802.11nData rate (Mbps)dataElements of a wireless networkElements of a wireless networkAd hoc mode no base stations nodes can only transmit to other nodes within link coverage nodes organize themselves into a network: route among themselvesWireless Link CharacteristicsWireless Link CharacteristicsDifferences from wired link …. decreased signal strength: radio signal attenuates as it propagates through matter (path loss) 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 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 characteristicsWireless network characteristicsMultiple wireless senders and receivers create additional problems (beyond multiple access):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 Signal fading: B, A hear each other B, C hear each other A, C can not hear each other interferring at BCode Division Multiple Access (CDMA)Code Division Multiple Access (CDMA)used in several wireless broadcast channels (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/DecodeCDMA Encode/Decodeslot 1slot 0Zi,m= di.cmslot 0 channel outputslot 1 channel outputchannel output Zi,msendercodedata bitsslot 1slot 0slot 0 channel outputslot 1 channel outputreceivercodereceived inputCDMA: two-sender interferenceCDMA: two-sender interferencenullComponents of cellular network architectureCellular networks: the first hopCellular networks: the first hopTwo 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 accessCellular standards: brief surveyCellular standards: brief survey2G systems: voice channels IS-136 TDMA: combined FDMA/TDMA (north america) GSM (global system for mobile communications): combined FDMA/TDMA most widely deployed IS-95 CDMA: code division multiple accessIS-136GSMIS-95GPRSEDGECDMA-2000UMTSTDMA/FDMADon’t drown in a bowl of alphabet soup: use this oor reference onlyCellular standards: brief surveyCellular standards: brief survey2.5 G systems: voice and data channels for those who can’t wait for 3G service: 2G extensions general packet radio service (GPRS) evolved from GSM data sent on multiple channels (if available) enhanced data rates for global evolution (EDGE) also evolved from GSM, using enhanced modulation Date rates up to 384K CDMA-2000 (phase 1) data rates up to 144K evolved from IS-95 Cellular standards: brief surveyCellular standards: brief survey3G systems: voice/data Universal Mobile Telecommunications Service (UMTS) GSM next step, but using CDMA CDMA-2000 ….. more (and more interesting) cellular topics due to mobility (stay tuned for details)IEEE 802.11 Wireless LANIEEE 802.11 Wireless LAN802.11b 2.4-5 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 802.11a 5-6 GHz range up to 54 Mbps 802.11g 2.4-5 GHz range up to 54 Mbps All use CSMA/CA for multiple access All have base-station and ad-hoc network versions 802.11n draft 108M-300M MIMO802.11 LAN architecture802.11 LAN architecturewireless 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 onlyBSS 1BSS 2hub, switch or routerScanningScanningScanning required for many functions finding and joining a network finding a new AP while roaming initializing an ad hoc network 802.11 MAC uses a common mechanism Passive scanning by listening for Beacons Active Scanning probe + response802.11: passive scanning802.11: passive scanningAP 2AP 1H1BBS 2BBS 1Passive Scanning: beacon frames sent from APs association Request frame sent: H1 to selected AP association Response frame sent: H1 to selected AP Passive scanning is mandatory where each NIC scans individual channels to find the best access point signal.  Periodically, access points broadcast a beacon, and the radio NIC receives these beacons while scanning and takes note of the corresponding signal strengths. The beacons contain information about the access point, including service set identifier (SSID), supported data rates, etc. The radio NIC can use this information along with the signal strength to compare access points and decide upon which one to use. 802.11: active scanning802.11: active scanningAP 2AP 1H1BBS 2BBS 1Active Scanning: Probe Request frame broadcast from H1 Probes response frame sent from APs Association Request frame sent: H1 to selected AP Association Response frame sent: H1 to selected APHost send the Probe beacon each time with a different channel Active scanning enables a radio NIC to receive immediate response from access points, without waiting for a beacon transmission. The issue, however, is that active scanning imposes additional overhead on the network because of the transmission of probe and corresponding response frames. Summary of ScanningSummary of ScanningHow many to scan? 11 channels in 802.11b 10 channels to be scanned, except the one currently used by the old AP How to perform the scan? Passively listen to beacons save power Actively exchange probe messages save timeftT100 msbeacons802.11 Association 802.11 Association Once authenticated, the radio NIC must associate with the access point before sending data frames. Association is necessary to synchronize the radio NIC and access point with important information, such as supported data rates. The radio NIC initiates the association by sending an association request frame containing elements such as SSID and supported data rates. The access point responds by sending an association response frame containing an association ID along with other information regarding the access point. Once the radio NIC and access point complete the association process, they can send data frames to each other. 802.11: Channels, association802.11: Channels, association802.11b: 2.4GHz-2.485GHz 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 [Chapter 8] will typically run DHCP to get IP address in AP’s subnet IEEE 802.11: multiple accessIEEE 802.11: multiple accessavoid collisions: 2+ nodes transmitting at same time 802.11: CSMA - sense before transmitting don’t collide with ongoing transmission by other node 802.11: 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)Ethernet CSMA/CD algorithmEthernet CSMA/CD algorithm1. Adaptor receives datagram from net layer & creates frame 2. If adapter senses channel idle, it starts to transmit frame. If it senses channel busy, waits until channel idle and then transmits 3. If adapter transmits entire frame without detecting another transmission, the adapter is done with frame !4. If adapter detects another transmission while transmitting, aborts and sends jam signal 5. After aborting, adapter enters exponential backoff: after the mth collision, adapter chooses a K at random from {0,1,2,…,2m-1}. Adapter waits K·512 bit times and returns to Step 2 IEEE 802.11 MAC Protocol: CSMA/CAIEEE 802.11 MAC Protocol: CSMA/CA802.11 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 2 802.11 receiver - if frame received OK return ACK after SIFS (ACK needed due to hidden terminal problem) senderreceiverAvoiding collisions (more)Avoiding collisions (more)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 Avoid data frame collisions completely using small reservation packets!Collision Avoidance: RTS-CTS exchangeCollision Avoidance: RTS-CTS exchangeAPABtimedefer802.11 frame: addressing802.11 frame: addressingAddress 2: MAC address of wireless host or AP transmitting this frameAddress 1: MAC address of wireless host or AP to receive this frameAddress 3: MAC address of router interface to which AP is attachedAddress 3: used only in ad hoc mode802.11 frame: addressingH1R1802.11 frame: addressing802.11 frame: more802.11 frame: moreduration of reserved transmission time (RTS/CTS)frame seq # (for reliable ARQ)frame type (RTS, CTS, ACK, data)802.11: mobility within same subnetH1 remains in same IP subnet: IP address can remain same switch: which AP is associated with H1? self-learning (Ch. 5): switch will see frame from H1 and “remember” which switch port can be used to reach H1hub or switchAP 2AP 1H1BBS 2BBS 1802.11: mobility within same subnet802.15: personal area networkless 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 802.15: evolved from Bluetooth specification 2.4-2.5 GHz radio band up to 721 kbps radius of coverage802.15: personal area networkWhat is mobility?What is mobility?spectrum of mobility, from the network perspective:mobile wireless user, using same access pointmobile user, passing through multiple access point while maintaining ongoing connections (like cell phone)mobile user, connecting/ disconnecting from network using DHCP. Mobility: VocabularyMobility: Vocabularyhome 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., 128.119.40.186home agent: entity that will perform mobility functions on behalf of mobile, when mobile is remotewide area networkcorrespondentMobility: more vocabularyMobility: more vocabularyCare-of-address: address in visited network. (e.g., 79,129.13.2) wide area networkvisited network: network in which mobile currently resides (e.g., 79.129.13/24)Permanent address: remains constant (e.g., 128.119.40.186)foreign agent: entity in visited network that performs mobility functions on behalf of mobile. correspondent: wants to communicate with mobileHow do you contact a mobile friend:How do you contact a mobile friend:search all phone books? call her parents? expect her to let you know where he/she is?I wonder where Alice moved to?Consider friend frequently changing addresses, how do you find her?Mobility: approachesMobility: approachesLet routing handle it: routers advertise permanent address of mobile-nodes-in-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 correspondent to mobile goes through home agent, then forwarded to remote direct routing: correspondent gets foreign address of mobile, sends directly to mobileMobility: approachesMobility: approachesLet routing handle it: routers advertise permanent address of mobile-nodes-in-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 correspondent to mobile goes through home agent, then forwarded to remote direct routing: correspondent gets foreign address of mobile, sends directly to mobilenot scalable to millions of mobilesMobility: registrationMobility: registrationEnd result: Foreign agent knows about mobile Home agent knows location of mobilewide area networkhome networkvisited networkMobility via Indirect RoutingMobility via Indirect Routingwide area networkhome networkvisited networkIndirect Routing: commentsIndirect Routing: commentsMobile uses two addresses: permanent address: used by correspondent (hence mobile location is transparent to correspondent) care-of-address: used by home agent to forward datagrams to mobile foreign agent functions may be done by mobile itself triangle routing: correspondent-home-network-mobile inefficient when correspondent, mobile are in same networkIndirect Routing: moving between networksIndirect Routing: moving between networkssuppose mobile user moves to another network 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 networks transparent: on going connections can be maintained!Mobility via Direct RoutingMobility via Direct Routingwide area networkhome networkvisited networkcorrespondent requests, receives foreign address of mobilecorrespondent forwards to foreign agentMobility via Direct Routing: commentsMobility via Direct Routing: commentsovercome triangle routing problem non-transparent to correspondent: correspondent must get care-of-address from home agent what if mobile changes visited network?Accommodating mobility with direct routingAccommodating mobility with direct routinganchor foreign agent: FA in first visited network data always routed first to anchor FA when mobile moves: new FA arranges to have data forwarded from old FA (chaining)wide area networkforeign net visited at session startanchor foreign agentnew foreign agentcorrespondent agentcorrespondentnew foreign networkMobile IPMobile IPRFC 3220 has many features we’ve seen: home agents, foreign agents, foreign-agent registration, care-of-addresses, encapsulation (packet-within-a-packet) three components to standard: indirect routing of datagrams agent discovery registration with home agent Mobile IP: indirect routingMobile IP: indirect routingPermanent address: 128.119.40.186Care-of address: 79.129.13.2Mobile IP: agent discoveryMobile IP: agent discoveryagent advertisement: foreign/home agents advertise service by broadcasting ICMP messages (typefield = 9)R bit: registration requiredH,F bits: home and/or foreign agentMobile IP: registration exampleMobile IP: registration exampleSummarySummaryWireless wireless links: capacity, distance channel impairments CDMA IEEE 802.11 (“wi-fi”) CSMA/CA reflects wireless channel characteristics cellular access architecture standards (e.g., GSM, CDMA-2000, UMTS)Mobility principles: addressing, routing to mobile users home, visited networks direct, indirect routing care-of-addresses case studies mobile IP mobility in GSM impact on higher-layer protocols