Figure 12 End System Data Flow

Encapsulation

De-Encapsutrtlion

Applicalan Pnesenifltion Session Transport

Melwork Data Link Physical

De-Encapsutrtlion

Applicalan Pnesenifltion Session Transport

Melwork Data Link Physical

Group Wise

Exchange hostb

Notice how the source GroupWise hosta encapsulates the message as it works its way from Layer 7 across the wire. Assuming that the destination Exchange hostb is on another network, lots of encap encapsulation occurs between Layers 1 through 3 until the packet gets to the destination host (rou operations). The destination host pulls the frames off the wire and processes (de-encapsulates) the stack from Layer 1 to Layer 7 so that the e-mail apelicatio n can read the b-mail. The processing ini necessary re-ordering and re-assembly of packets that result from packet routing and fragmentati<

Understanding a layered approach and packet flows is critical to being a good troubleshooter. That put all that theory stuff in toeir course s. If you don't know how things work correctly, how in the w> know what is wrong ? En d system-to-end syste m Exchange and GroupWise messaging is the main share wrth stgdents in many of my classes. Take a look again at Figure 1-2 and then at Figure 1-3 encapsulation) to review the packet flow and layer operations. For more detail, refer to materials o CCNP from C i sco Peess and otheo pub liseers. o part i cu larly like Jeff Doyln's Flouting TCP/IP, Volume think they belong on everyone's shelf.

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