|  | 3.3 | Basics of Data Flow Through LANs
| 3.3.1 | Encapsulation and packets review
In order for reliable communications to take place over a network, data to be sent must be put in manageable traceable packages. This is done through the process of encapsulation as covered in chapter 2. A brief review of the process states that the top three layers, Application, Presentation, Session, prepare the data for transmission by creating a common format for transmission. The Transport layer breaks up the data into manageable size units called segments. It also assigns sequence numbers to the segments to make sure the receiving host puts the data back together in the proper order. The Network layer then encapsulates the segment creating a packet. It adds a destination and source network address, usually IP to the packet. The Data-Link Layer further encapsulates the packet and creates a frame. It adds the source and destination local (MAC) address to the frame. The Data-link layer then transmits the binary bits of the frame over the Physical layer media. When the data is transmitted on just a local area network, we talk about the data units as frames, because the MAC address is all that is necessary to get from source to destination host. But if we need to send the data to another host over an Intranet or the Internet, packets become the data unit that is referred to. This is because the Network address in the packet contains the final destination address of the host the data (packet) is being sent to. The bottom three layers (Network, Data-Link, Physical) of the OSI model are the primary movers of data across an Intranet or Internet. The main exception to this is a device called a gateway. It is a device designed to convert the data from one format, created by the Application, Presentation, and Session layers, to another. So the gateway uses all seven of the OSI layers to do this. This will be explained in more detail later in the chapter.
| Web Links
| Encapsulation
| Packets
| Data Encapsulation
| Encapsulation Slide
| Encapsulation of PDUs