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.