Mobile and Wireless Access for Pervasive Computing Krithi

Mobile and Wireless Access for Pervasive Computing Krithi Ramamritham IIT Bombay 3/15/2018 04: 30 1

Mobile and Wireless Computing o o o Goal: Access Information Anywhere, Anytime, and in Any Way. Aliases: Mobile, Nomadic, Wireless, Pervasive, Invisible, Ubiquitous Computing. Distinction: • Fixed wired network: Traditional distributed computing. • Fixed wireless network: Wireless computing. • Wireless network: Mobile Computing. Ø Key Issues: Wireless communication, Mobility, Portability. 3/15/2018 04: 30 3

Mobile Network Architecture 3/15/2018 04: 30 4

Mobile Applications o o Expected to create an entire new class of Applications § new massive markets in conjunction with the Web § Mobile Information Appliances - combining personal computing and consumer electronics Applications: § Vertical: vehicle dispatching, tracking, point of sale § Horizontal: mail enabled applications, filtered information provision, collaborative computing… 3/15/2018 04: 30 5

Broadcast Data Dissemination o o o business data, e. g. , Vitria, Tibco election coverage data stock related data traffic information sportscasts, e. g. , Praja § Datatacycle [Herman] § Broadcast disks 3/15/2018 04: 30 6 Data Server

Wireless Communication o Cellular - GSM (Europe+), TDMA & CDMA (US) – FM: 1. 2 -9. 6 Kbps; Digital: 9. 6 -14. 4 Kbps (ISDN-like services) o Public Packet Radio - Proprietary – 19. 2 Kbps (raw), 9. 6 Kbps (effective) o o Private and Share Mobile Radio Wireless LAN - wireless LAN bridge (IEEE 802. 11) – Radio or Infrared frequencies: 1. 2 Kbps-15 Mbps o Paging Networks – typically one-way communication – low receiving power consumption o Satellites – wide-area coverage (GEOS, MEOS, LEOS) – LEOS: 2. 4 Kbps (uplink), 4. 8 Kbps (downlink) 3/15/2018 04: 30 7

Wireless characteristics o o Ø Variant Connectivity § Low bandwidth and reliability Frequent disconnections • predictable or sudden Asymmetric Communication § Broadcast medium Monetarily expensive § Charges per connection or per message/packet Connectivity is weak, intermittent and expensive 3/15/2018 04: 30 9

Portable Information Devices o PDAs, Personal Communicators § Light, small and durable to be easily carried around § dumb terminals [Info. Pad, Parc. Tab projects], palmtops, wristwatch PC/Phone, walkstations o will run on AA+ /Ni-Cd/Li-Ion batteries may be diskless I/O devices: Mouse is out, Pen is in wireless connection to information networks § either infrared or cellular phone specialized HW (for compression/encryption) o o 3/15/2018 04: 30 10

Portability Characteristics o Battery power restrictions § transmit/receive, disk spinning, display, CPUs, memory consume power Battery lifetime will see very small increase § need energy efficient hardware (CPUs, memory) and system software § planned disconnections - doze mode Ø Power consumption vs. resource utilization o 3/15/2018 04: 30 11

Portability Characteristics o Resource constraints § Mobile computers are resource poor § Reduce program size – interpret script languages (Mobile Java? ) § Computation and communication load cannot be distributed equally Small screen sizes Ø Asymmetry between static and mobile computers o 3/15/2018 04: 30 12

Mobility Characteristics o Location changes • location management - cost to locate is added to communication o Heterogeneity in services § bandwidth restrictions and variability o Dynamic replication of data • data and services follow users o o Ø Querying data - location-based responses Security and authentication System configuration is no longer static 3/15/2018 04: 30 13

Recurrent Themes o o o Handling disconnections (planned failures? ) § caching strategies § managing inconsistencies Delayed write-back and prefetch: use network idle times § increases memory requirements Buffering/batching: allows bulk transfers Partitioning and replication § triggered by relocation Compression: increase effective BW § increases battery power requirements Receiving needs less power than sending 3/15/2018 04: 30 14

Mobility in Db Applications • Need to adapt to constantly changing environment: • network connectivity • available resources and services • By varying and (re)negotiating: • the partition of duties between the mobile and static elements • the quality of data available at the mobile host Example: Fidelity (degree to which a copy of data matches the reference copy at the server) 3/15/2018 04: 30 16

C-SA-C: Server-side Agent Wireless Link Client o o Fixed Network Agent Server C-SA-C: The Client/Server-side Agent/Server Model Splits the interaction between the mobile client and server: client-agent and agent-server • different protocols for each part of the interaction • each part may be executed independently of the other 3/15/2018 04: 30 18

Responsibilities of the Agent o o Messaging and queying Manipulate data prior to their transmission to the client: § perform data specific compression § batch together requests § change the transmission order 3/15/2018 04: 30 19

Role of the Agent o Surrogate or proxy of the client § Any communication to/from the client goes through the agent § Offload functionality from the client to the agent o Application (service) specific § provides a mobile-aware layer to specifc services or applications (e. g. , web-browsing or database access) § handles all requests from mobile clients o Filters § provide agents that operate on protocols § E. g. , an MPEG-agent or a TCP-agent 3/15/2018 04: 30 20

C-CA-S: Client-side Agent Wireless Link Fixed Network Client Agent Server Mobile Host o C-SA-S: The Client/Client-side Agent/Server Model § caching § background prefetching and hoarding § various communication optimizations 3/15/2018 04: 30 21

C-I-S: Client & Server Agents Wireless Link Fixed Network Client Agent Mobile Host o C-I-S: Client/Intercept/Server Model § Caching, prefetching etc § various communication optimizations at both ends – E. g. , asynchronous queued RPC § relocate computation between the agents § Client interoperability 3/15/2018 04: 30 22 Server

Mobile Agents o o Mobile agents are migrating processes associated with an itinerary § dynamic code and state deployment Implement the agents of the previous architectures as mobile agents, E. g. , § server-side agents can relocate during handoff § client-side agent dynamically move on and off the client – Relocatable dynamic objects (RDO) [Rover] o Implement the communication using mobile agents: § clients submit/receive mobile agents to/from the server § E. g. , Compacts [Pro-Motion] 3/15/2018 04: 30 23

Outline o o o o Motivating Example Issues: Mobility, Wireless Communication, Portability Adaptability and Mobile Client-Server Models Location Management Broadcast data dissemination Disconnected database operations Mobile Access to the Web 3/15/2018 04: 30 24

Locating Moving Objects o Example of moving objects § mobile devices (cars, cellular phones, palmtops, etc) § mobile users (locate users independently of the device they are currently using) § mobile software (e. g. , mobile agents) o How to find their location - Two extremes § Search everywhere § Store their current location everywhere Ø Searching vs. Informing 3/15/2018 04: 30 25

Architectures of Location DBs o o Two-tier Schemes (similar to cellular phones) § Home Location Register (HLR): store the location of each moving object at a pre-specified location for the object § Visitor Location Register (VLR): also store the location of each moving object mo at a register at the current region Hierarchical Schemes § Maintain multiple registries 3/15/2018 04: 30 26

Two-tier Location DBs o Search § Check the VLR at your current location § If object not in, contact the object’s HLR o Update § Update the old and new VLR § Update the HLR 3/15/2018 04: 30 27

Hierarchical Location DBs Maintain a hierarchy of location registers (databases) A location database at a higher level contains location information for all objects below it 3/15/2018 04: 30 28

Hierarchical Location DBs Call caller 3/15/2018 04: 30 29

Hierarchical Location DBs Move new location old location 3/15/2018 04: 30 30

Hierarchical vs. Two-tier (+) No pre-assigned HLR (+) Support Locality (-) Increased number of operations (database operations and communication messages) (-) Increased load and storage requirements at the higher-levels 3/15/2018 04: 30 31

Locating Moving Objects o Caching § cache the callee’s location at the caller (large Call to Mobility Ratio) o Replication § replicate the location of a moving object at its frequent callers (large CMR) o Forwarding Pointers § do not update the VLR and the HLR, leave a forwarding pointer from the old to the new VLR (small CMR) § When and how forwarding pointers are purged? Ø Concurrency, coherency and recovery/checkpointing of location DBs 3/15/2018 04: 30 33

Querying Moving Objects • Besides locating moving objects, answer more advanced queries, e. g. , • find the nearest service • send a message to all mobile objects in a specific geographical reafion • Location queries: spatial, temporal or continuous • Issues: representation, evaluation and imprecision Most current research assumes a centralized location database 3/15/2018 04: 30 34

Querying Moving Objects How to represent and index moving objects? Ø Spatial indexes do not work well with dynamically changing values Ø Value-time representation • An object is mapped to a trajectory [Kollios 99] 3/15/2018 04: 30 36

Outline o o o o Motivating Example Issues: Mobility, Wireless Communication, Portability Adaptability and Mobile Client-Server Models Location Management Broadcast data dissemination Disconnected database operations Mobile Access to the Web 3/15/2018 04: 30 37

Broadcast as an air-cache for storing frequently requested data o Continoulsy adjust the broadcast content to match the database hot-spot o How? By observing the broadcast misses - requests for data not on the broadcast o 3/15/2018 04: 30 38

Information Dissemination Goal : Maximize query capacity of servers, minimize energy per query at the client. Focus: Read-only transactions (queries). – Clients send update data to server – Server resolves update conflicts, commits updates 1. Pull: PDAs demand, servers respond. § § 3/15/2018 04: 30 backchannel (uplink) is used to request data and provide feedback. poor match for asymmetric communication. 39

Information Dissemination… 2. Push: Network servers broadcast data, PDA's listen. § PDA energy saved by needing receive mode only. § scales to any number of clients. § data are selected based on profiles and registration in each cell. F G A C B . . Server 3/15/2018 04: 30 E 40 D Clients

Information Dissemination… F G A B E C . . Server D Clients 14. 4 Kbps 3. Combinations Push and Pull (Sharing the channel). o o Selective Broadcast: Servers broadcast "hot" information only. § "publication group" and "on-demand" group. On-demand Broadcast: Servers choose the next item based on requests. § FCFS or page with maximum # of pending requests. 3/15/2018 04: 30 41

Selective Tuning o o o Basic broadcast access is sequential Want to minimize client's access time and tuning time. § active mode power is 250 m. W, in doze mode 50μW What about using database access methods? Hashing: broadcast hashing parameters h(K) Indexing: index needs to be broadcast too § "self-addressable cache on the air" (+) "listening/tuning time" decreases (-) "access time" increases 3/15/2018 04: 30 44

Caching for Disconnection o o What to cache? § Entire files, directories, tables, objects § Portions of files, directories, tables, objects When to cache? Is simple LRU sufficient? § LRU captures an aspect of temporal locality § Predictive/semantic caching: based on the semantics distance between data/request E. g. , clustering of queries [Ren 99] 3/15/2018 04: 30 59

Mobile Access to the Web o o o Three-tier Architectures: Client - Web Server - Data Server Web Server can act like a server-side agent § Prefetching at its cache can hide some latency § Scripts at the Web server can perform user-specified filtering and processing. Most solutions use a Web proxy to avoid any changes to the browsers and servers. § Pythia [Fox 96] § Mobile Browser (MOWSER) [Joshi 96] – Distillation: highly lossy, real-time, datatype specific compression that preserves semantic content § Web. Express [Housel 97] 3/15/2018 04: 30 75

What Needs to be Reexamined? o o o o o Operating systems File systems Data-based systems Communication architecture and protocols Hardware and architecture Real-Time, multimedia, Qo. S Security Application requirements and design PDA design: Interfaces, Languages 3/15/2018 04: 30 114