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Layers in a nutshell
✦ Top layers build an application data payload that is divided by the lower transport layer into several small data chunks called segments. Each segment is numbered so that the receiving host can reassemble the application data. ✦ The transport segments are then forwarded down to the network layer, which tags each segment with logical source and destination addresses and some control information, and hands over the resulting shippable data packet to the lower data link layer. ✦ The data link layer adds the physical source address of the sender and the physical destination address of the receiver if the receiver is located in the same local network as the sender. If the receiver is not located in the same local network as the sender, the data link layer adds the physical destination address of the gateway in the local network. The gateway of a local network is usually a router that connects the local network to other networks.
encapsulation
✦ Each layer encapsulates the data and controls the data of upper layers
within its own control data.
✦ The data chunk encapsulated within the control data of each layer
travels from the sending host to the receiving host.
✦ The receiving host unwraps the successive control information layers
that encapsulate the data.
✦ Top layers on the sending host hand off the data to the transport layer
and trust the transport layer (and the layers beneath transport) to ship
it to the receiving host.
✦ The data ends up being sliced into smaller chunks. The data is also
augmented with control information at each layer. The control information
added by each layer is wrapped around by the control data of the lower
layers on the sending host.
✦ The data is unwrapped on the receiving host.
Layer 7: Application
manages the user interface, and processes the data before handing it off to the user.
Layer 6: Presentation
converts, encrypts and translates the data.
Layer 5: Session
opens and maintains communication channels, and authenticates data communications.
Layer 4: Transport
segments and delivers the data, and corrected any transmission errors.
Layer 3: Network
routes data packets between networks and manages global logical addressing.
Layer 2: Data link
routes data frames locally and manages local physical addressing.
Layer 1: Physical
manages electrical, optical and over the air transmission of data bits.
Benefits of the OSI Reference Model
- Independently operating layers with clearly defined interlayer interfaces
allow layers to evolve internally without impact on other layers
- The network communication problem is divided into smaller problems.
By dividing the network communication process into several precise
tasks and by assigning a specific layer to each task, it’s easier to manage
the whole process. It also allows each layer to specialize to specific
network communication contexts.
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