Unit 1 Exercise When using an application that sends data, such as your Web browser, email client, instant messenger or a game, you are at the highest level of data transfer known as the "application layer." This layer is the application's human interface, where commands for sending and receiving data can be initiated by the user who clicks a link or makes a move in a computer game. This layer handles many services including file transfer protocols, email protocols, domain naming and dynamic host configuration. After information has been made more computer friendly, or vice versa when receiving data, a connection must be made with the destination host, whether a Web server, another computer on the network or a device such as a networked printer. Various services are used at this layer; but no matter the technology used, the basic principle is the same. The computer sends a small amount of data requesting a connection to the destination. This data is routed to the destination by lower-layer processes, and then a reply is sent to the computer. This reply may be "OK, you can send me data now," "the destination could not be found" or "OK, but first I need a password." The presentation layer translates this information and the application layer will ask the user for a password, communicate an error or acknowledge that a connection has been made. When the connection is established, the transport layer determines what type of data transfer will take place and begins the transfer. If the transfer requires all data to be received and assembled in order, or not at all, Transmission Control Protocol, or TCP, is used. TCP requires the computer to send a request to the destination before sending any data and to receive an acknowledgement of each segment. If a segment is missed, the destination will not acknowledge receipt and the computer will send it again after a designated amount of time. For transfers that require a fast connection and will not be harmed by losing a small portion of data here and there, such as a streaming video or a VOIP session, User Datagram Protocol, or UDP, is used. UDP does not require acknowledgement and will not attempt to resend lost data. Streaming data from videos or VOIP would experience a delay due to TCP processing and acknowledgments, and a portion of video or voice from a second before would not make sense if added into the data later. The network layer is made up of routers and certain types of switches; it assigns IP addresses and other logical addresses to computers in the local network. This layer also retrieves IP addresses from systems the computer is sending data to or retrieving data from. The network layer handles path determination, the process of finding the destination address through many routers outside the local network. A header is attached to each segment designating the type of transport used, the length and other information. Once a header is attached, the segment is known as a packet and is sent to the data link layer. Data link layer devices work with information on the local network, converting the IP address from the network layer into the physical addresses of each device known as MAC address, DLCI or in some cases a device phone number. The packet from the network layer is broken down and converted into a format compatible with the physical medium for data transfer on the physical layer, whether Ethernet or otherwise. All information is finally broken down into bits -- a series of ones and zeroes.