Peerto Peer Communication

Each layer of the OSI model has its own function and interaction with the layers above and below it. Furthermore, there is also peer-to-peer communication between end devices through each corresponding layer of the OSI model. Peer-to-peer communication means that each layer of the OSI model uses its own protocol to communicate with its equivalent peer layer in another system. For example,

Table 2.1 The OSI reference model.

Layer

Service

Layer 7

Application

Layer 6

Presentation

Layer 5

Session

Layer 4

Transport

Layer 3

Network

Layer 2

Data Link

Layer 1

Physical

the Transport layer of Device A in Figure 2.1 will communicate with the Transport layer in Device B, assuming there are no intermediate devices. The layers between the two end stations communicate via protocol data units (PDUs).

In other words, each layer communicates to the corresponding layer above and below it and also exchanges protocol data units (PDU is an OSI term for a packet) between end systems. Figure 2.1 shows how each layer of the OSI model provides services to the layers above and below. The PDU exchanges are represented by the horizontal lines in Figure 2.1.

Note: Layers 1 and 2 of the OSI model are implemented with hardware. Layers 3 through 7 are implemented in software.

Layer Communication

*

l Application

1 t

[ Presentation

> >

' Session

i

[ Transport

[ Network

> >

[ Data Link

Device A

Peer Communication

Application

Presentation

Session

Transport

Network

Data Link

Physical

Device B

Figure 2.1 OSI layer peer-to-peer communication.

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