[ Pobierz całość w formacie PDF ]
.Thisapproach leads to a flexible client-server architecture thatcan be implemented in a variety of ways but which also pro-vides interoperability and portability at the network inter-faces.The WAP protocol stack is depicted in Figure W-1.WAP solves the problem of using Internet standards suchas HTML, HyperText Transfer Protocol (HTTP), TLS, andTransmission Control Protocol (TCP) over mobile networks.These protocols are inefficient, requiring large amounts ofmainly text-based data to be sent.Web content written withHTML generally cannot be displayed in an effective way onthe small-sized screens of pocket-sized mobile phones andpagers, and navigation around and between screens is noteasy with one hand.Furthermore, HTTP and TCP are not optimized for theintermittent coverage, long latencies, and limited band-width associated with wireless networks.HTTP sends itsheaders and commands in an inefficient text format insteadof compressed binary format.Wireless services using theseprotocols are often slow, costly, and difficult to use.The TLSsecurity standard, too, is problematic, since many messagesneed to be exchanged between client and server.With wire-less transmission latencies, this back-and-forth traffic flowresults in a very slow response for the user.WAP has been optimized to solve all these problems.Itmakes use of binary transmission for greater compression ofdata and is optimized for long latency and low to mediumbandwidth.WAP sessions cope with intermittent coverageand can operate over a wide variety of wireless transportsusing the Internet Protocol (IP) where possible and other Application Wireless ApplicationLayer Environment (WAE)Wireless SessionSessionProtocol (WSP)LayerWireless TransactionTransactionsProtocol (WTP)LayerWireless TransportSecurityLayer Security (WTLS)LayerTransportDatagrams (UDP/IP)LayerWireless Bearers:NetworkLayerPHS GSM CSD TDMA CDMA iDENFigure W-1 The Wireless Application Protocol (WAP) stack.396 WIRELESS APPLICATION PROTOCOL 397optimized protocols where IP is impossible.The WML usedfor WAP content makes optimal use of small screens, allowseasy navigation with one hand without a full keyboard, andhas built-in scalability from two-line text displays through tothe full graphic screens on smart phones and communicators.WAP Applications EnvironmentWAP applications are built within the Wireless ApplicationEnvironment (WAE), which closely follows the Web contentdelivery model, but with the addition of gateway functions.Figure W-2 contrasts the conventional Web model with theWAE model.All content is specified in formats that are simi-lar to the standard Internet formats and is transported usingstandard protocols on the Web while using an optimizedServerClientCGI Scripts,RequestJava, etc.UserAgentGetResponse (Content) ContentGateway ServerClientCGI Scripts,Request RequestJava, etc.UserEncodersAgentand Decoders GetEncoded Response Response (Content) ContentFigure W-2 The standard Web content delivery model (top) and theWireless Application Environment mode (bottom). 398 WIRELESS APPLICATION PROTOCOLHTTP-like protocol in the wireless domain (i.e., WAP).Thearchitecture is designed for the memory and CPU processingconstraints that are found in mobile terminals.Support forlow-bandwidth and high-latency networks is also included inthe architecture as well.Where existing standards were notappropriate due to the unique requirements of small wirelessdevices, WAE has modified the standards without losing thebenefits of Internet technology.The major elements of the WAE model includeWAE user agents These client-side software components pro-vide specific functionality to the end user.An example of auser agent is a browser that displays content downloadedfrom the Web.In this case, the user agent interprets networkcontent referenced by a Uniform Resource Locator (URL).WAE includes user agents for the two primary standard con-tent types: encoded WML and compiled WML Script.Content generators Applications or services on serversmay take the form of Common Gateway Interface (CGI)scripts that produce standard content formats in responseto requests from user agents in the mobile terminal
[ Pobierz całość w formacie PDF ]
