er 5 messages. Data frame created from data packet + Layer 3 Frame headers removed, payloads network addresses + Layer 3 header Network layer reassembled into data packets Data frame encrypted, frame control Layer 2 Bit stream structured into frames, header added, network addresses decrypted, and checked for destination translated into MAC addresses Data Link layer MAC addresses Access gained to physical medium, bit Layer 1 Received signals are continuously stream coded and modulated onto PHY demodulated, decoded and bits stream layer signals and transmitted Physical layer are set to Data Link Layer Figure 2-1: The OSI Model in Practice - an E-mail Example Chapter Two IP address. At the Data Link layer the IP address is "resolved" to determine the physical address of the first device that the sending computer needs to transmit frames to the so-called MAC or media access control address. In this example, this device may be a network switch that the sending computer is connected to or the default gateway to the Internet from the sending computer's LAN. At the physical layer, also called the PHY layer, the data packets are encoded and modulated onto the carrier medium - a twisted wire pair in the case of a wired network, or electromagnetic radiation in the case of a wireless network - and transmitted to the device with the MAC address resolved at Layer 2. Transmission of the message across the Internet is achieved through a number of device-to-device hops involving the PHY and Data Link layers of each routing or relaying device in the chain. At each step, the Data Link layer of the receiving device determines the MAC address of the next immediate destination, and the PHY layer transmits the packet to the device with that MAC address. On arrival at the receiving computer, the PHY layer will demodulate and decode the voltages and frequencies detected from the transmission medium, and pass the received data stream up to the Data Link layer. Here the MAC and LLC elements, such as a message integrity check, will be extra