he buffer is sufficiently full. This allows time SO that lost packets can be retransmitted and late packets can be used. The media player examines the status of the buffer and can even adjust the playout speed to avoid an empty buffer and the associated break in provision. Video can be played at a frame rate and quality that might not otherwise be achieved given current network conditions. A typical example is the video-share site YouTube. Video file transfer is where a server has data that is requested by the client, and that data is delivered in packets to the receiver. The transfer has to complete before the file is of use to the receiver. This is the type of thing that happens when you request an MPEG file or an AVI file from a Web site. Data Transfer Data transferred across any medium is subject to the possibility of loss. In modern networks the primary cause of data loss is the finite resource, which has to be shared between multiple users. There is considerable variation in the types of traffic on the Internet, and the interac- tions between them can often be complex and difficult to predict. Nevertheless, a commu- nication can be simply defined by its probability of loss, its delay, and its jitter. The main task of a communication network is to enable end-to-end information transfer. The Internet is a ubiquitous communication medium, and a simple diagram of communica- tion is shown in Figure 3.19. The Internet uses the TCP/IP reference model, which has four readily identifiable layers-namely, application, transport, Internet, and network interface (Pearson, 1975). The application layer consists of protocols such as HTTP, FTP, and Telnet, while the transport layer provides two services, Transmission Control Protocol (TCP) and User Datagram Pro- tocol (UDP). TCP offers a reliable, connection-oriented transfer for exchanging data and uses retransmissions to handle packet loss in the network. UDP, on the other hand, provides best-effort connectionless transfer of individual messages and offers no error recove