Layers, Massages, Segments, Datagram's and Frames
Figure 2.15 show the physical path data takes down a sending end systems protocol stack up and down the protocol stacks of an intervening link layer switch and router and then up the protocol stack at the receiving end systems.
Host Routers and link layer switches each contains a different set of layer reflecting their differences in functionality
As show in figure2.15 link layer switches implement layers 1 and 2 routers implement layer 1 through 3. This means for example that internet routers are capable of implementing the IP protocol ( a layer 3 protocol) while link layer switch are not .
Host routers and link layer switches each contains a different set of layer reflecting their different in functionality.
Figure 2.15 also illustrates the important concept of encapsulation. At the sending host an application layer messages(M) is passed to the transport layer. In the simple case the transport layer takes the massage and appends additional information( so - called transport layer header information. H1) that will be used by the receiver side transport layer. The application layer message and the transport layer header information together constitute the transport layer segment. The transport layer segment thus encapsulates the application layer message. The added information might include information allowing the receiver side transport layer to deliver the message up to the appropriate application and error detection bits that allow the receiver to determine whether bits in the message have been changed in route. The transport layer then passes the segment to the network layer which adds networks layer header information(H n) such as source and destination end system address creating a network layer datagram. The datagram is then passed to the link layer. Which will add its own link layer header information and create link layer frame.