|  | 1.1 | The OSI Model
| 1.1.4 | Five steps of data encapsulation
As networks perform services for users, the flow and packaging of the user's original information go through several changes. In this example of internetworking, there are five conversion steps. Step 1 A computer converts an e-mail message into alphanumeric characters that can be used by the internetworking system. This is the data. Step 2 The message data is then segmented for transport on the internetwork system by the transport layer. The transport layer ensures that the message hosts at both ends of the e-mail system can reliably communicate. Step 3 The data is then converted to a packet, or datagram, by the network layer. The packet also contains a network header that includes a source and destination logical address. The address helps network devices send the packet across the network along a chosen path. Step 4 Each data-link layer device puts the packet into a frame. The frame enables the device to connect to the next directly-connected network device on the link. Step 5 The frame is changed to a pattern of 1s and 0s for transmission on the medium (usually a wire). A clocking function enables the devices to distinguish bits as they travel across the medium. The medium on the physical internetwork can vary along the path. For example, an e-mail message may originate on a LAN, cross a campus backbone, and continue through a WAN link until it reaches its destination on another remote LAN.