Wireless Mobile Communications Chapter 8 Support for

Wireless & Mobile Communications Chapter 8: Support for Mobility File systems Ø Data bases Ø WWW and Mobility Ø WAP - Wireless Application Protocol Ø Ch 8 - Mobility Support Winter 2001

File systems - Motivation Ø Goal q Ø Problems q q q Ø efficient and transparent access to shared files within a mobile environment while maintaining data consistency limited resources of mobile computers (memory, CPU, . . . ) low bandwidth, variable bandwidth, temporary disconnection high heterogeneity of hardware and software components (no standard PC architecture) wireless network resources and mobile computer are not very reliable standard file systems (e. g. , NFS, network file system) are very inefficient, almost unusable Solutions replication of data (copying, cloning, caching) q data collection in advance (hoarding, pre-fetching) q Ch 8 - Mobility Support Winter 2001 2

File systems - consistency problems Ø THE main problem of distributed, loosely coupled systems are all views of the data the same? q how and when should changes be propagated and to which users? q Ø Weak consistency many algorithms offering strong consistency (e. g. , via atomic updates (locking the data file, updating caches after a lock is released)) cannot be used in mobile environments q Updating of invalid data located in caches by a central server is very problematic if the mobile computer is currently not connected to the network q weak consistency consists of having to live with invalid data for short durations of time q means that occasional inconsistencies have to be tolerated, but conflict resolution strategies must be applied afterwards to reach consistency again q Ø Conflict detection content independent: version numbering, time-stamps q content dependent: dependency graphs q Ch 8 - Mobility Support Winter 2001 3

Database systems in mobile environments Ø Request processing power conserving, location dependent, cost efficient q example: find the fastest way to a hospital q Ø Replication management q Ø similar to file systems Location management tracking of mobile users to provide replicated or location dependent data in time at the right place (minimize access delays) q example: with the help of the HLR (Home Location Register) in GSM a mobile user can find a local towing service q Ø Transaction processing “mobile” transactions can not necessarily rely on the same models as transactions over fixed networks (ACID: atomicity, consistency, isolation, durability) q therefore models for “weak” transaction q Ch 8 - Mobility Support Winter 2001 4

World Wide Web and mobility Protocol (HTTP, Hypertext Transfer Protocol) and language (HTML, Hypertext Markup Language) of the Web have not been designed for mobile applications and mobile devices, thus creating many problems! Ø Typical transfer sizes Ø HTTP request: 100 -350 byte q responses avg. <10 kbyte, header 160 byte, GIF 4. 1 k. Byte, JPEG 12. 8 kbyte, HTML 5. 6 kbyte q but also many large files that cannot be ignored q Ø The Web is no file system Web pages are not simple files to download q static and dynamic content, interaction with servers via forms, content transformation, push technologies etc. q many hyperlinks, automatic loading and reloading, redirecting q a single click might have big consequences! q Ch 8 - Mobility Support Winter 2001 5

HTTP 1. 0 and mobility I Ø Characteristics stateless, client/server, request/response q needs a connection oriented protocol (TCP), one connection per request (some enhancements in HTTP 1. 1) q primitive caching and security q Ø Problems designed for large bandwidth (compared to wireless access) and low delay q large and redundant protocol headers (readable for humans, stateless, therefore large headers in ASCII) q uncompressed content transfer q using TCP q huge overhead per request (3 -way-handshake) compared with the content, e. g. , of a GET request slow-start problematic as is without having to deal with the wireless problem q DNS lookup by client causes additional traffic and delays Ch 8 - Mobility Support Winter 2001 6

HTTP 1. 0 and mobility II Ø Caching quite often disabled by information providers to be able to create user profiles, usage statistics etc. q dynamic objects cannot be cached q numerous counters, time, date, personalization, . . . mobility quite often inhibits caches q security problems q caches cannot work with authentication mechanisms that are contracts between client and server and not the cache q Ø POSTing (i. e. , sending to a server) q Ø today: many user customized pages, dynamically generated on request via CGI, ASP, . . . can typically not be buffered, very problematic if currently disconnected Many unsolved problems! Ch 8 - Mobility Support Winter 2001 7

HTML and mobile devices Ø HTML designed for computers with “high” performance, color highresolution display, mouse, hard disk q typically, web pages optimized for design, not for communication q Ø Mobile devices q Ø often only small, low-resolution displays, very limited input interfaces (small touch-pads, soft-keyboards) Additional “features” animated GIF, Java AWT, Frames, Active. X Controls, Shockwave, movie clips, audio, . . . q many web pages assume true color, multimedia support, highresolution and many plug-ins q Ø Web pages ignore the heterogeneity of end-systems! q e. g. , without additional mechanisms, large high-resolution pictures would be transferred to a mobile phone with a lowresolution display causing high costs Ch 8 - Mobility Support Winter 2001 8

Approaches toward a WWW for mobile devices Ø Application gateways, enhanced servers simple clients, pre-calculations in the fixed network q Compression, transcoding, filtering, content extraction q automatic adaptation to network characteristics q Ø Examples q q q Ø picture scaling, color reduction, transformation of the document format (e. g. , PS to TXT) Present only parts of the image: detail studies, clipping, zooming headline extraction, automatic abstract generation HDML (handheld device markup language): simple language similar to HTML requiring a special browser, developed by Unwired Planet HDTP (handheld device transport protocol): transport protocol for HDML, developed by Unwired Planet Problems proprietary approaches, require special enhancements for browsers q heterogeneous devices make approaches more complicated q Ch 8 - Mobility Support Winter 2001 9

Some new issues that might help mobility? Ø Push technology q Ø HTTP/1. 1 q q q Ø real pushing, not a client pull needed, channels etc. client/server use the same connection for several request/response transactions multiple requests at beginning of session, several responses in same order enhanced caching of responses (useful if equivalent responses!) semantic transparency not always achievable: disconnected, performance, availability -> most up-to-date version. . . several more tags and options for controlling caching (public/private, max-age, no-cache, etc. ) encoding/compression mechanism, integrity check, security of proxies, authentication, authorization. . . Cookies: well. . . , stateful sessions, not really integrated. . . Ch 8 - Mobility Support Winter 2001 10

System support for WWW in a mobile world I Ø mobile client Enhanced browsers Pre-fetching, caching, off-line use q e. g. Internet Explorer q integrated enhancement browser web server Ch 8 - Mobility Support Winter 2001 11

System support for WWW in a mobile world II Ø mobile client Client Proxy Pre-fetching, caching, off-line use q e. g. , Caubweb, Tele. Web, Weblicator, Web. Whacker, Web. Ex, Web. Mirror, . . . q browser client proxy web server Ch 8 - Mobility Support Winter 2001 12

System support for WWW in a mobile world III Ø Client and network proxy mobile client Ø client proxy network proxy Special network subsystem adaptive content transformation for bad connections, pre-fetching, caching q e. g. , Mowgli q Ø browser web server combination of benefits plus simplified protocols q e. g. , Mobi. Scape, Web. Express q Additional many proprietary server extensions possible q mobile client browser client proxy web server network proxy “channels”, content negotiation, . . . Ch 8 - Mobility Support Winter 2001 13

WAP - Wireless Application Protocol Ø Goals deliver Internet content and enhanced services to mobile devices and users (mobile phones, PDAs) q independence from wireless network standards q open for everyone to participate, protocol specifications will be proposed to standardization bodies q applications should scale well beyond current transport media and device types and should also be applicable to future developments q Ø Platforms q Ø e. g. , GSM (900, 1800, 1900), CDMA IS-95, TDMA IS-136, 3 rd generation systems (IMT-2000, UMTS, W-CDMA) Forum WAP Forum, co-founded by Ericsson, Motorola, Nokia, Unwired Planet q further information http: //www. wapforum. org q Ch 8 - Mobility Support Winter 2001 14

WAP - scope of standardization Ø Browser q Ø Script language q Ø e. g. , business cards (v. Card), calendar events (v. Calender) Protocol layers q Ø Wireless Telephony Application (Interface): access to all telephone functions Content formats q Ø similar to Java script, adapted to the mobile environment WTA/WTAI q Ø “micro browser”, similar to existing, well-known browsers in the Internet transport layer, security layer, session layer etc. Working Groups q WAP Architecture Working Group, WAP Wireless Protocol Working Group, WAP Wireless Security Working Group, WAP Wireless Application Working Group Ch 8 - Mobility Support Winter 2001 15

WAP - reference model and protocols Internet HTML, Java A-SAP WAP Application Layer (WAE) S-SAP additional services and applications Session Layer (WSP) HTTP TR-SAP Transaction Layer (WTP) SEC-SAP SSL/TLS Security Layer (WTLS) T-SAP TCP/IP, UDP/IP, media Transport Layer (WDP) WCMP Bearers (GSM, CDPD, . . . ) WAE comprises WML (Wireless Markup Language), WML Script, WTAI etc. Ch 8 - Mobility Support Winter 2001 16

WAP - network elements fixed network Internet HTML wireless network WML filter WAP proxy Binary WML HTML web server HTML filter/ WAP proxy WTA server Binary WML PSTN Binary WML: binary file format for clients Ch 8 - Mobility Support Winter 2001 17

WDP - Wireless Datagram Protocol Ø Protocol of the transport layer within the WAP architecture uses directly transport mechanisms of different network technologies q offers a common interface for higher layer protocols q allows for transparent communication using different transport technologies q if IP is used, then WDP more or less translates to UDP q Ø Goals of WDP create a worldwide interoperable transport system with the help of WDP adapted to the different underlying technologies q transmission services such as SMS in GSM might change, new services can replace the old ones q Ch 8 - Mobility Support Winter 2001 18

WTLS - Wireless Transport Layer Security Ø Goals q data integrity prevention of changes in data q privacy prevention of tapping q authentication creation of authenticated relations between a mobile device and a server q protection against denial-of-service attacks protection against repetition of data and unverified data Ø WTLS is based on the TLS (Transport Layer Security) protocol (former SSL, Secure Sockets Layer) q optimized for low-bandwidth communication channels q Ch 8 - Mobility Support Winter 2001 19

WTP - Wireless Transaction Protocol Ø Goals q different transaction services, offloads application can select reliability, efficiency q support of different communication scenarios class 0: unreliable message transfer class 1: reliable message transfer without result message class 2: reliable message transfer with exactly one reliable result message supports peer-to-peer, client/server and multicast applications q low memory requirements, suited to simple devices (< 10 kbyte ) q efficient for wireless transmission q segmentation/reassembly selective retransmission header compression optimized connection setup (setup with data transfer) Ch 8 - Mobility Support Winter 2001 20

WSP - Wireless Session Protocol Ø Goals q HTTP 1. 1 functionality Request/reply, content type negotiation, . . . support of client/server, transactions, push technology q key management, authentication, Internet security services q session management (interruption, resume, . . . ) q Ø Services session management (establish, release, suspend, resume) q capability negotiation q content encoding q Ø WSP/B (Browsing) HTTP/1. 1 functionality - but binary encoded q exchange of session headers q push and pull data transfer q asynchronous requests q Ch 8 - Mobility Support Winter 2001 21

WAE - Wireless Application Environment Ø Goals network independent application environment for low-bandwidth, wireless devices q integrated Internet/WWW programming model with high interoperability q Ø Requirements device and network independent, international support q manufacturers can determine look-and-feel, user interface q considerations of slow links, limited memory, low computing power, small display, simple user interface (compared to desktop computers) q Ø Components q q q architecture: application model, browser, gateway, server WML: XML-Syntax, based on card stacks, variables, . . . WMLScript: procedural, loops, conditions, . . . (similar to Java. Script) WTA: telephone services, such as call control, text messages, phone book, . . . (accessible from WML/WMLScript) content formats: v. Card, v. Calendar, Wireless Bitmap, WML, . . . Ch 8 - Mobility Support Winter 2001 22

WAE logical model Origin Servers web server other content server Gateway response with content encoders & decoders push content encoded response with content encoded push content request Ch 8 - Mobility Support Client encoded request Winter 2001 WTA user agent WML user agent other WAE user agents 23

Wireless Markup Language (WML) Ø WML follows deck and card metaphor WML document consists of many cards, cards are grouped to decks q a deck is similar to an HTML page, unit of content transmission q WML describes only intent of interaction in an abstract manner q presentation depends on device capabilities q Ø Features text and images q user interaction q navigation q context management q Ch 8 - Mobility Support Winter 2001 24

Examples for WAP protocol stacks WAP standardization WAE user agent outside WAP WAE WSP transaction based application WTP WTLS datagram based application WTLS UDP WDP IP non IP (GPRS, . . . ) (SMS, . . . ) 1. (GPRS, . . . ) (SMS, . . . ) 2. typical WAP application with complete protocol stack Ch 8 - Mobility Support (GPRS, . . . ) (SMS, . . . ) 3. pure data application with/without additional security Winter 2001 25