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Protocols: Horizontal (Corresponding Layer) Communication
(Page 2 of 2)
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 directlythey 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 communicationdown the stack on one machine, and then
back up the stack on the other. This process is illustrated in Figure 14.
(The communication doesnt 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.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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