Already finished all layers in the stack, why a separate chapter for wireless and mobile networks?

Transcription

1 Wireless and Mobile Networks Yanmin Zhu Department of Computer Science and Engineering Mobile lobal ISP Home Regional ISP Institutional Already finished all layers in the stack, why a separate chapter for wireless and mobile s? Key differences: Wireless and mobile Considering wireless, what shall be different on link layer, layer, transport layer and application layer? Considering mobility, what shall be different on link layer, layer, transport layer and application layer? 7. Introduction 7. Wireless links and 7. WiFi: 80. wireless LANs 7. Cellular Access 7. Introduction 7. Wireless links and 7. WiFi: 80. wireless LANs 7. Cellular Access 5 6

2 Elements of a wireless Elements of a wireless infrastructure wireless hosts laptop, PDA, IP phone run applications may be stationary (non-mobile) or mobile wireless does not always mean mobility 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, 80. access points 7 8 Elements of a wireless Characteristics of wireless standards 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 ac a,g 80.b Data rate (Mbps)50 80.n 80.a,g point-to-point : LTWE WIMAX : UMTS/WCDMA-HSPDA, CDMA000-xEVDO.5: UMTS/WCDMA, CDMA000 : IS-95, CDMA, SM 9 Indoor 0-0m Outdoor 50-00m Mid-range outdoor 00m Km Long-range outdoor 5Km 0 Km 0 Modes of wireless infrastructure infrastructure mode base station connects mobiles into wired handoff: mobile changes base station providing connection into wired Modes of wireless ad hoc mode no base stations nodes can only transmit to other nodes within link coverage nodes organize themselves into a : route among themselves

3 Wireless taxonomy Mesh Networks infrastructure (e.g., s) no infrastructure single hop host connects to base station (WiFi, WiMAX, cellular) which connects to larger no base station, no connection to larger (Bluetooth, ad hoc nets) multiple hops host may have to relay through several wireless nodes to connect to larger : mesh net no base station, no connection to larger. May have to relay to reach other a given wireless node MANET, VANET Vehicular Ad Hoc Networks Why VANET? VV IV VI Introduction 7. Wireless links and 7. WiFi: 80. wireless LANs 7. Cellular Access Wireless Link Characteristics () 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.,. Hz) 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 Thus, these differences make communication across (even a point to point) wireless link much more difficult 7 8

4 Wireless Link Characteristics () FDMA, TDMA, CDMA SNR: signal-to-noise ratio larger SNR easier to extract signal from noise (a good thing ) SNR versus BER tradeoffs given physical layer: increase power -> increase SNR->decrease BER given SNR: choose physical layer that meets BER requirement, giving highest thruput SNR may change with mobility: dynamically adapt physical layer (modulation technique, rate) BER SNR(dB) QAM56 (8 Mbps) QAM6 ( Mbps) BPSK ( Mbps) 9 0 Basic Idea of CDMA CDMA Encode/Decode: One Sender Code Division Multiple Access (CDMA) unique code assigned to each user; i.e., code set partitioning all users share same frequency, but each user uses own code to encode data A receiver uses the sender s code to recover the sent bits encoded signal = (original data) X (chipping sequence) decoding: inner-product of encoded signal and chipping sequence sender receiver data bits d = - Z i,m = d i. c m d 0 = code slot slot 0 received input code slot slot 0 channel output Z i,m slot channel output slot 0 channel output d = - slot channel output M D i = Z i,m. c m m= d 0 = slot 0 channel output M CDMA Encode/Decode: Tow Senders A 0 CDMA A B C What C transmits? E B D SCENARIO 0 Reception from A SCENARIO SCENARIO Code of A

5 Codes 7. Introduction 7. Wireless links and 7. WiFi: 80. wireless LANs 7. Cellular Access 5 6 IEEE 80. Wireless LAN WLAN architecture 80.b 80.a.-5 Hz unlicensed spectrum 5-6 Hz range up to Mbps up to 5 Mbps direct sequence spread 80.g spectrum (DSSS) in physical.-5 Hz range layer up to 5 Mbps all hosts use same chipping 80.n: multiple antennae code.-5 Hz range up to 00 Mbps all use CSMA/CA for multiple access BSS switch or router wireless host communicates with base station base station = access point () Basic Service Set (BSS) (aka cell ) in infrastructure mode contains: wireless hosts access point (): base station ad hoc mode: hosts only all have base-station and ad-hoc versions 7 BSS 8 80.b Channels Association before Transmission 80.b:.Hz-.85Hz spectrum divided into channels at different frequencies admin chooses frequency for interference possible: channel can be same as that chosen by neighboring! host: must associate with an scans channels, listening for beacon frames containing s name (SSID) and MAC selects to associate with may perform authentication will typically run DHCP to get IP in s subnet Question: what is the maximum throughput if you deploy multiple WiFi s in a room? 9 0 5

6 Two types: passive/active scanning Multiple Access for Wireless Channel BBS BBS BBS BBS H H Passive Scanning: () beacon frames sent from s ()association Request frame sent: H to selected ()association Response frame sent: H to selected Active Scanning: () Probe Request frame broadcast from H ()Probes response frame sent from s ()Association Request frame sent: H to selected ()Association Response frame sent: H to selected H H H CSMA may fail! Why? Due to Hidden Terminal Problem! Multiple access control protocol BSS A B C A B C A s signal strength space C s signal strength H H H A->B C->B Collision happen at B A transmits C transmits time time avoid collisions: + nodes transmitting at same time CSMA - sense before transmitting don t collide with ongoing transmission by other node CSMA for 80. WiFi CSMA vs Ethernet Protocol operations () Sense the channel a) If idle for DIFS, transmit entire frame (no CD), wait for ACK b) If busy - start random backoff time - timer counts down while channel idle - transmit when timer expires () Wait for ACK a) If ACK received, done b) If no ACK, increase random backoff interval, repeat DIFS sender data ACK receiver SIFS 5 What are the key differences? () WiFi has ACK, since radio links are more error prone () WiFi starts random backoff as soon as it senses a busy channel, why? No collision detection. More conservative! But no collision detection, Why? a. Difficult to receive while transmitting b. Anyway, cannot detect all collisions, due to hidden terminal problem 6 6

7 Avoiding collisions (more) Collision Avoidance: RTS-CTS exchange idea: allow sender to reserve channel rather than random access of data frames: avoid collisions of long data frames A B 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 CTS heard by all nodes sender transmits data frame other stations defer transmissions time DATA (A) reservation collision defer avoid data frame collisions completely using small reservation packets! 7 8 Network allocation vector (NAV) Two Animations DEMO DEMO 9 0 Three Modes of Service Sets Three Modes of Service Sets Mode : Independent Basic Service Set (IBSS) Independent Basic Service Set (IBSS) allows two or more devices to communicate directly with each other without a need for a central device Mode : Basic Service Set (BSS) All mobile devices do not communicate directly, instead through the access point (). BSS H H H H H H 7

8 Three Modes of Service Sets 80.: mobility within same subnet Mode : Extended Service Set (ESS) ESS is created by connecting multiple Basic Service Set (BSS) via a distribution system SSID: sameid H H H switch H5 SSID: sameid H H remains in same IP subnet: IP can remain same switch: which is associated with H? self-learning (Ch. 5): switch will see frame from H and remember which switch port can be used to reach H ood, but what about switching? BBS router switch H BBS BSS BSS 80. frame: ing Sending Frames between H & R frame control duration seq control payload CRC Address : MAC of wireless host or to receive this frame Address : MAC of router interface to which is attached Address : used only in ad hoc mode Note: Two link-layer technologies Address : MAC of wireless host or transmitting this frame frame: ing 80. frame: ing H router R H router R H MAC addr R MAC addr R MAC addr H MAC addr dest. source dest. source 80. frame 80. frame H MAC addr MAC addr R MAC addr Frame conversion MAC addr H MAC addr R MAC addr Frame conversion 80. frame 80. frame 7 8 8

9 80. frame: more 80.: advanced capabilities Byte Bit frame control Protocol version duration Type duration of reserved transmission time (RTS/CTS) Subtype To From seq control More frag Retry frame seq # (for RDT) Power mgt payload frame type (RTS, CTS, ACK, data) More data WEP CRC Rsvd 9 Rate Adaptation base station, mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves, SNR varies QAM56 (8 Mbps) QAM6 ( Mbps) BPSK ( Mbps) operating point BER SNR(dB). SNR decreases, BER increase as node moves away from base station. When BER becomes too high, switch to lower transmission rate but with lower BER : advanced capabilities 80.: advanced capabilities Power Management node-to-: I am going to sleep until next beacon frame knows not to transmit frames to this node node wakes up before next beacon frame beacon frame: contains list of mobiles with -tomobile frames waiting to be sent node will stay awake if -to-mobile frames to be sent; otherwise sleep again until next beacon frame A A beacon beacon beacon beacon The beacon frame contains the list of hosts with pending frames to receive B A, B 5 5 From to 7. Introduction 7. Wireless links and 7. WiFi: 80. wireless LANs 7. Cellular Access 5 5 9

10 Components of cellular cell covers geographical region base station (BS) analogous to 80. mobile users attach to through BS air-interface: physical and link layer protocol between mobile and BS connects cells to wired tel. net. manages call setup (more later!) handles mobility (more later!) Mobile Switching Center Mobile Switching Center telephone wired 55 Cellular s: 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 56 (voice) architecture Base station system (BSS) BTS BSC Legend ateway Base transceiver station (BTS) Base station controller (BSC) Mobile Switching Center () Mobile subscribers telephone 57 (voice+data) architecture radio controller Key insight: new cellular data operates in parallel (except at edge) with existing cellular voice voice unchanged in core data operates in parallel SSN telephone ateway SN Serving PRS Support Node (SSN) ateway PRS Support Node (SN) 58 (voice+data) architecture radio controller radio interface (WCDMA, HSPA) radio access Universal Terrestrial Radio Access Network (UTRAN) SSN core eneral Packet Radio Service (PRS) Core Network ateway SN telephone public 59 versus LTE architecture -LTE radio access Universal Terrestrial Radio Access Network (UTRAN) radio controller SSN HSS MME Evolved Packet Core (EPC) ateway SN S-W P-W telephone

11 : differences from all IP core: IP packets tunneled (through core IP ) from base station to gateway no separation between voice and data all traffic carried over IP core to gateway Mobility Home Subscriber Management Server(HSS) Serving Entity (MME) (like HLR+VLR) UE enodeb ateway (user element) (base station) HSS (S-W) MME radio access Universal Terrestrial Radio Access Network (UTRAN) data Evolved Packet Core (EPC) S-W P-W Packet data ateway (P-W) 7-6 Wireless and Mobile Networks 7. Introduction 7. Wireless links and 7. WiFi: 80. wireless LANs 7. Cellular Access 6 What is mobility? spectrum of mobility, from the perspective: no mobility mobile wireless user, using same access point mobile user, connecting/ disconnecting from using DHCP. high mobility mobile user, passing through multiple access points while maintaining ongoing connections (like cell phone) How do we support mobile access? 6 6 Mobility: Vocabulary Mobility: more vocabulary home : permanent home of mobile (e.g., 8.9.0/) home agent: entity that will perform mobility functions on behalf of mobile, when mobile is remote Permanent : remains constant (e.g., ) Care-of-: in visited. (e.g., ) visited : in which mobile currently resides (e.g., 79.9./) Permanent : in home, can always be used to reach mobile e.g., wide area 65 : wants to communicate with mobile wide area foreign agent: entity in visited that performs mobility functions on behalf of mobile. 66

12 How do you contact a mobile friend? Mobility: approaches () Consider friend frequently changing es, 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? Let routing handle it (the core approach): routers advertise permanent of mobile-nodes-in-residence via usual routing table exchange. routing tables indicate where each mobile located no changes to end-systems not scalable to millions of mobiles Mobility: approaches () let end-systems handle it: indirect routing: communication from to mobile goes through home agent, then forwarded to remote direct routing: gets foreign of mobile, sends directly to mobile Mobility: registration home wide area Eventually: Foreign agent knows about mobile Home agent knows location of mobile foreign agent contacts home agent home: this mobile is resident in my visited mobile contacts foreign agent on entering visited Mobility via Indirect Routing Indirect Routing home es packets using home of mobile home agent intercepts packets, forwards to foreign agent wide area foreign agent receives packets, forwards to mobile visited mobile replies directly to corresponde nt 7 Mobile uses two es: permanent : used by (hence mobile location is transparent to ) care-of-: used by home agent to forward datagrams to mobile foreign agent functions may be done by mobile itself 7

13 Can a mobile move to another? Indirect routing, any problem? suppose mobile user moves to another ) registers with new foreign agent ) new foreign agent registers with home agent ) home agent update care-of- for mobile ) packets continue to be forwarded to mobile (but with new care-of-) mobility, changing foreign s transparent: ongoing connections can be maintained! triangle routing problem: -home-mobile inefficient when, mobile are in same 7 7 Mobility via Direct Routing home requests, receives foreign of mobile forwards to foreign agent wide area foreign agent receives packets, forwards to mobile visited mobile replies directly to corresponde nt 75 Mobility via Direct Routing: comments overcome triangle routing problem non-transparent to : must get care-of- from home agent what if mobile changes visited? 76 Any new problem arise? Accommodating mobility with direct routing 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 new foreign agent new foreign wide area anchor foreign agent agent 5 new foreign agent foreign net visited at session start new foreign 77 78

14 7. Introduction 7. Wireless links and 7. WiFi: 80. wireless LANs 7. Cellular Access Mobile IP RFC has many features we ve seen: home agents, foreign agents, foreign-agent registration, care-of-es, encapsulation (packetwithin-a-packet) three components to standard: indirect routing of datagrams agent discovery registration with home agent Mobile IP: indirect routing Mobile IP: agent discovery packet sent by home agent to foreign agent: a packet within a packet foreign-agent-to-mobile packet dest: agent advertisement: foreign/home agents advertise service by broadcasting ICMP messages (typefield = 9) dest: dest: type = 9 code = 0 checksum Permanent : dest: packet sent by Care-of : H,F bits: home and/or foreign agent R bit: registration required router type = 6 length sequence # RBHFMV registration lifetime reserved bits 0 or more care-ofes standard ICMP fields mobility agent advertisement extension 8 8 Mobile IP: registration example home agent HA: time foreign agent COA: registration req. COA: HA: MA: Lifetime: 9999 identification: 7 encapsulation format. registration reply HA: MA: Lifetime: 999 Identification: 7 encapsulation format. visited : 79.9./ ICMP agent adv. Mobile agent COA: MA: registration req. COA: HA: MA: Lifetime: 9999 identification:7. registration reply HA: MA: Lifetime: 999 Identification: Introduction 7. Wireless links and 7. WiFi: 80. wireless LANs 7. Cellular Access 8 8

15 Components of cellular Handling mobility in cellular s recall: wired public telephone home : of cellular provider you subscribe to (e.g., Sprint PCS, Verizon) home location register (HLR): database in home containing permanent cell phone #, profile information (services, preferences, billing), information about current location (could be in another ) different cellular s, operated by different providers visited : in which mobile currently resides visitor location register (VLR): database with entry for each user currently in could be home SM: indirect routing to mobile SM: handoff with common home consults HLR, gets roaming number of mobile in visited mobile user HLR home home Mobile Switching Center VLR Mobile Switching Center visited call routed to home switched telephone home sets up nd leg of call to in visited in visited completes call through base station to mobile old BSS VLR Mobile Switching Center old routing new routing new BSS Handoff goal: route call via new base station (without interruption) reasons for handoff: stronger signal to/from new BSS (continuing connectivity, less battery drain) load balance: free up channel in current BSS SM doesn t mandate why to perform handoff (policy), only how (mechanism) handoff initiated by old BSS SM: handoff with common SM: handoff between s old BSS VLR Mobile Switching Center new BSS. old BSS informs of impending handoff, provides list of + new BSSs. sets up path (allocates resources) to new BSS. new BSS allocates radio channel for use by mobile. new BSS signals, old BSS: ready 5. old BSS tells mobile: perform handoff to new BSS 7. mobile, new BSS signal to activate new channel 7. mobile signals via new BSS to : handoff complete. reroutes call 8 -old-bss resources released home Home anchor PSTN (a) before handoff anchor : first visited during call call remains routed through anchor new s add on to end of chain as mobile moves to new IS- allows optional path minimization step to shorten multi- chain

16 SM: handoff between s Mobility: SM versus Mobile IP SM element Comment on SM element Mobile IP element home Home anchor PSTN (b) after handoff anchor : first visited during call call remains routed through anchor new s add on to end of chain as mobile moves to new IS- allows optional path minimization step to shorten multi- chain 9 Home system ateway Mobile Switching Center, or home. Home Location Register (HLR) Visited System Visited Mobile services Switching Center. Visitor Location Record (VLR) Mobile Station Roaming Number (MSRN), or roaming number Network to which mobile user s permanent phone number belongs Home : point of contact to obtain routable of mobile user. HLR: database in home system containing permanent phone number, profile information, current location of mobile user, subscription information Network other than home system where mobile user is currently residing Visited : responsible for setting up calls to/from mobile nodes in cells associated with. VLR: temporary database entry in visited system, containing subscription information for each visiting mobile user Routable for telephone call segment between home and visited, visible to neither the mobile nor the. Home Home agent Visited Foreign agent Care-of 9 7. Introduction 7. Wireless links and 7. WiFi: 80. wireless LANs 7. Cellular Access What is the impact of wireless links and mobility on high layer protocols? 9 9 Wireless, mobility: impact on higher layer protocols Chapter 7 summary 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 un-necessarily delay impairments for real-time traffic limited bandwidth of wireless links Wireless wireless links: capacity, distance channel impairments CDMA IEEE 80. ( Wi- Fi ) CSMA/CA reflects wireless channel cellular access architecture standards (e.g., SM,, LTE) Mobility principles: ing, routing to mobile users home, visited s direct, indirect routing care-of-es case studies mobile IP mobility in SM impact on higher-layer protocols

17 Homework #6 Lab #6 Chapter 6: P5, P0, P, P Chapter 7 : P, P7, P, P 80. Lab No submission is required!