Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme

Transcription

1 Chapter 2 Technical Basics: Layer 1 Methods for Medium Access: Layer 2 Chapter 3 Wireless Networks: Bluetooth, WLAN, WirelessMAN, WirelessWAN Mobile Telecommunication Networks: GSM, GPRS, UMTS Chapter 4 Mobility on the network layer Mobility on the transport layer Mobility support on the application layer Application Layer Mobility Support Conclusion 1

2 Application Example: WWW and Mobility Protocol (HTTP, Hypertext Transfer Protocol) and language (HTML, Hypertext Markup Language) of the WWW have not been designed for mobile applications and mobile devices, thus creating many problems Typical transfer sizes HTTP request: byte Responses on the average <10 kbyte, header 160 byte, GIF 4.1 kbyte, JPEG 12.8 kbyte, HTML 5.6 kbyte, but also many large files that cannot be ignored The Web is no file system Web pages are not simple files to download Static and dynamic content, interaction with servers via forms, content transformation, push technologies etc. Many hyperlinks, automatic loading and reloading, redirecting A single click might have big consequences Mobile devices usually have small displays HTML content usually consists of frames each one too large for the display Interaction is more difficult like with fixed computers 2

3 HTTP 1.0 and Mobility HTTP was designed for fixed networks Designed for high capacity (compared to wireless access) and low delay Big and redundant protocol headers (readable for humans, stateless, therefore big headers in ASCII) Uncompressed content transfer Huge overhead per request compared with the content (e.g., a GET request) since using TCP (3-way-handshake) DNS lookup by client causes additional traffic Caching could reduce network traffic, but is quite often disabled by information providers to be able to create user profiles, usage statistics etc. more and more dynamic objects which cannot be cached Security problems: how to use SSL/TLS together with proxies? POST messages (i.e., sending to a server) can typically not be buffered, very problematic if currently disconnected 3

4 How to support Mobility in WWW? You cannot change the WWW but why not adopting content for mobile clients? Application gateways (like proxies), enhanced servers Simple clients, pre-calculations in the fixed network: compression, filtering, content extraction Automatic adaptation to network characteristics, e.g. picture scaling, color reduction, transformation of document format (e.g., PS to TXT), automatic abstract generation Maybe also use variants of HTTP and HTML: HDML (handheld device markup language): simple language similar to HTML requiring a special browser HDTP (handheld device transport protocol): transport protocol for HDML Problems Proprietary approaches, require special enhancements for browsers Heterogeneous devices make approaches more complicated 4

5 WAP - Wireless Application Protocol Goals of WAP: Deliver Internet content and enhanced services to mobile devices and users (mobile phones, PDAs) Independence from wireless network standards Open for everyone to participate, protocol specifications will be proposed to standardization bodies Applications should scale well beyond current transport media and device types and should also be applicable to future developments Platforms: e.g., GSM (900, 1800, 1900), CDMA IS-95, TDMA IS-136, 3G systems (IMT-2000, UMTS, W-CDMA, cdma2000, ) Scope of standardization: Browser: micro browser, similar to existing, well-known browsers in the Internet Script language: similar to Java script, adapted to the mobile environment WTA/WTAI: Wireless Telephony Application (Interface): access to all telephone functions Content formats: e.g., business cards (vcard), calendar events (vcalender) Protocol layers: transport layer, security layer, session layer etc. WAP 2.0: use of wireless TCP 5

6 WAP - Network Elements Combined with proxies, content can be adopted: fixed network wireless network Internet HTML filter WML WAP proxy Binary WML HTML web server HTML WML HTML filter/ WAP proxy Binary WML PSTN WTA server Binary WML Binary WML (Wireless Markup Language): binary file format for clients 6

7 WAP Architecture (not to be recognized) Service discovery External services EFI Security services Crypto libraries Multimedia Messaging ( ) WAE/WTA User Agent (WML, XHTMLMP) Content formats Push Application framework Provisioning Navigation Discovery Authentication Identification Push OTA Capability Negotiation Synchronisation Cookies Session Service Lookup PKI Secure transport Hypermedia transfer (WTP+WSP, HTTP) Datagrams (WDP, UDP) Streaming Connections (TCP with wireless profile) MMS Transfer Transport Protocol framework Secure bearer IPv4 CSD USSD GPRS... IPv6 SMS FLEX MPAK... Bearer 7

8 Conclusion Lehrstuhl für Informatik 4 Mobile Communications considers may aspects It is about wireless networks (It is about design of mobile devices, not treated here) It also deals with problems on higher layers caused by wireless network device mobility Many wireless networks exist Data networks: WLAN, WMAN (WiMAX), Bluetooth, UWB, Telephony networks: DECT, GSM, GPRS, UMTS, (Broadcast networks like DVB and DAB, as well as satellite communications) 4 th generation: somehow unifying all wireless networks with an IP-based core Not sufficient solved mobility problems on higher layers Mobile IP and variants of TCP are not really good in practice Also other protocols (e.g. routing) are still under development The application layer needs to be redesigned not only because of other transmission situation, but also because of another category of devices 8

9 Following Lectures This lecture was about communication basics; what s next? Distributed Systems: general application layer protocols, cooperation of application processes as addition to communication, support of developing distributed applications using middleware (SS 08) Modeling and Evaluation of Communication Systems: methods for analytic evaluation of new systems or protocols before implementation (SS 08) Simulation: simulative evaluation as addition to analytic evaluation (SS 08) Security in Communication Networks: security-related aspects of communication, e.g. encryption, authentication, anonymity, (?) Multimedia Systems: media formats and coding, quality of service mechanisms, and transfer/storage of multimedia data (?) Mobile Multimedia (Ericsson): media formats, mobile multimedia streaming, mobile broadcasting and mobile TV, IP Multimedia Subsystem IMS, (?) 9