A web transfer relates to the transfer of a resource (data file, audio sample, video clip, multimedia presentation, webcast, etc.) over the internet. It is always initiated by a user and serviced by a server(s), unless the case where a user is being hacked.

The performance of web transfers is affected by a multitude of factors. These factors are related to what is being transferred, how it is being transferred and using which tools, and the status of the network between the user and the server.

Here is a list of those factors:

• The type of resource (streamed or unstreamed);
• The format of the resource (RealVideo or Flash presentations);
• The design of the resource ( SMIL file or not);
• The type and loading of the server delivering the resource (Media Server or Web Server);
• The type or version of the application requesting and receiving the resource;
• The transport protocol being used and its settings;
• The applications layer protocol being used and its version;
• The round trip time between the user and the server;
• The use of a proxy (intermediary server between user and server)
• The security settings of the network between the server and the client.
• The bandwidth of the weakest link between the user and the server.
• The status of the Network Congestion between the user and the server.

TCP/HTTP Acceleration

Many common applications and services used across IP networks require reliable service to ensure data is delivered correctly. For this purpose the Transmission Control Protocol (TCP) is most often used, for example for email, Web browsing and file transfers among other things. TCP does however have well known and researched issues in any GEO satellite environment. Specifically, the long propagation delays slow all of TCP’s closed loop control mechanisms. Also the combination of long delay and broadband capacity requires the use of large buffers if high performance and reliability are not to be mutually exclusive. Bit errors and terrestrial congestion (even when very minimal) often degrade TCP performance by orders of magnitude in a GEO environment. Finally, using bandwidth on demand mechanisms for bandwidth efficiency purposes presents an environment that TCP is not well suited to. Fortunately, the solution to these problems is well known. A Performance Enhancing Proxy (PEP) is a device used to intercept all TCP connections at either side of the satellite link and allow the use of a dedicated satellite protocol over the satellite hop. This offers optimum performance, reduced hardware costs and automatic configuration at the IDU.

Even with a TCP-level fast connection start, Internet browsing via satellite can result in poor performance despite the broad bandwidth offered by satellite links. The core reason for this is the interactivity required by the Hypertext Transfer Protocol (HTTP) to download pages with many embedded objects. These are most frequently images on media rich web sites, a very common occurrence among the top web sites, where there can be twenty or more objects.

The process of downloading a web page using a web browser consists of downloading a primary HTML page from an HTTP server. The web browser parses the HTML page and requests, one by one, the embedded web objects in that page.

Many HTTP acceleration features can be implemented to improve the user experience when browsing the web.

• First, the PEP can pre-fetch the embedded web objects in an HTML page from multiple web servers. The pre-fetching algorithm sends multiple web object requests simultaneously to the web servers instead of doing it one by one.
• HTTP pipelining and HTTP keepalive found in the HTTP 1.1 specification can also be implemented. The HTTP pipelining feature allows a client to send multiple sequenced HTTP requests instead of waiting for a complete reply before making another object request. The HTTP keepalive feature allows a client to send multiple HTTP requests over the same TCP connection.
• The compression of the HTML and text of a web page is another feature of the PEP solution offered by Advantech Satellite Networks. Compressing the content of a web page reduces the bandwidth required to download a web page over the satellite link.