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2.3 | |
Networking Models | |
| 2.3.5 | |
Peer-to-peer communications |
In order for data to travel from the
source to the destination, each layer of the OSI model at the source
must communicate with its peer layer at the destination. This form of
communication is referred to as peer-to-peer. During this process, the
protocols of each layer exchange information, called protocol data
units (PDUs). Each layer of communication on the
source computer communicates with a layer-specific PDU, and with its
peer layer on the destination computer as illustrated in Figure
 .
Data packets on a network originate at a
source and then travel to a destination. Each layer depends on the
service function of the OSI layer below it. To provide this service, the
lower layer uses encapsulation to put the PDU from the upper layer into
its data field; then it adds whatever headers and trailers the layer
needs to perform its function. Next, as the data moves down through the
layers of the OSI model, additional headers and trailers are added.
After Layers 7, 6, and 5 have added their information, Layer 4 adds more
information. This grouping of data, the Layer 4 PDU, is called a
segment. The network layer provides a service to the transport layer, and the transport layer presents data to the internetwork subsystem. The network layer has the task of moving the data through the internetwork. It accomplishes this task by encapsulating the data and attaching a header creating a packet (the Layer 3 PDU). The header contains information required to complete the transfer, such as source and destination logical addresses. The data link layer provides a service to the network layer. It encapsulates the network layer information in a frame (the Layer 2 PDU). The frame header contains information (for example, physical addresses) required to complete the data link functions. The data link layer provides a service to the network layer by encapsulating the network layer information in a frame. The physical layer also provides a service to the data link layer. The physical layer encodes the data link frame into a pattern of 1s and 0s (bits) for transmission on the medium (usually a wire) at Layer 1.
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