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Table Of Contents  The TCP/IP Guide
 9  The Open System Interconnection (OSI) Reference Model
      9  Key OSI Reference Model Concepts

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Interfaces: Vertical (Adjacent Layer) Communication
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Data Encapsulation, Protocol Data Units (PDUs) and Service Data Units (SDUs)
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Protocols: Horizontal (Corresponding Layer) Communication
(Page 2 of 2)

Horizontal Communication

Let's consider how these corresponding layers communicate using protocols. First, recall that every layer in the model, except the bottom (physical) layer, is really a program or algorithm running on a computer. There is no way for, say, a Web browser and a Web server to actually connect together directly—they are just software programs, after all. Instead, the software running at various layers communicates logically. That is to say, through the use of software and procedures, a process running at layer 5 on one machine can accomplish logical communication with a similar process running at layer 5 on another machine.

Since machines are only physically connected at layer 1, this means that in order for a protocol at layer 5 to function, the data on the sending machine must “pass down” the data through the layers between layer 5 and layer 1. The data is then transmitted over the physical connection to layer 1 of the other machine, and “passed up” the protocol stack of the receiving machine to layer 5. This is how the two machines are logically linked at layer 5, even though they have no physical connection at that layer.

Thus, with the exception of the actual physical connection at layer 1, all horizontal communication also requires vertical communication—down the stack on one machine, and then back up the stack on the other. This process is illustrated in Figure 14. (The communication doesn’t always go all the way back up the stack for each connection, however, as in the case of routing.)


Figure 14: OSI Reference Model Protocols: Horizontal Communication

The term “protocol” has many meanings; in the context of the OSI Reference Model, it refers specifically to software or hardware elements that accomplish communication between corresponding layers on two or more devices. For example, the Internet Protocol is said to be a layer 3 protocol because each device uses IP software to communicate at layer 3. The actual transmission and reception of data only occurs at the lowest, physical layer; higher-layer protocols communicate logically, by passing data down interfaces until it reaches layer 1, transmitting at layer 1, and then passing the data back up to the appropriate layer at the recipient.

 


Key Concept: In the OSI Reference Model, a protocol refers specifically to a set of rules or procedures that define communication between software or hardware elements running at the same layer on network devices. Physical layer protocols are responsible for the actual transmission and reception of data at layer one. Protocols at higher layers pass data down through the layers below them to layer one for transmission, then across the network and back up to the corresponding entity at the same layer on the receiving device. The result is that software processes running at say, layer four on each of two devices can communicate logically as if they were directly connected at layer four, when they are not.



Previous Topic/Section
Interfaces: Vertical (Adjacent Layer) Communication
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Pages in Current Topic/Section
1
2
Next Page
Data Encapsulation, Protocol Data Units (PDUs) and Service Data Units (SDUs)
Next Topic/Section

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