control transmission Ethernet, Wi-Fi, of the data stream over a particular Bluetooth, WiMAX medium, at the level of coding and modulation methods, voltages, signal durations and frequencies. Wireless Network Logical Architecture how these layers combine to achieve a task such as sending and receiving an e-mail between two computers on separate local area networks (LANs) that are connected via the Internet. The process starts with the sender typing a message into a PC e-mail application (Figure 2-1). When the user selects "Send", the operating system combines the message with a set of Application layer (Layer 7) instructions that will eventually be read and actioned by the corresponding operating system and application on the receiving computer. The message plus Layer 7 instructions is then passed to the part of sender's operating system that deals with Layer 6 presentation tasks. These include the translation of data between application layer formats as well as some types of security such as Secure Socket Layer (SSL) encryption. This process continues down through the successive software layers, with the message gathering additional instructions or control elements at each level. By Layer 3 - the Network layer - the message will be broken down into a sequence of data packets, each carrying a source and destination Sender writes e-mail message Recipient reads e-mail message Message is prepared and Layer 7 Message is received by the e-mail "sent" from an e-mail application Application layer application and read by the addressee Layer 6 Message is broken into presentation and Session and Presentation layer control session elements. Presentation and Presentation layer headers are successively removed. session layer control headers are Layer 5 Messages reassembled into a specific successively added format for the receiving e-mail application Session layer Message is broken into packets and Layer 4 Packet reception and sequencing controlled, transport layer control header added Transport layer data reassembled into Lay