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In this section we'll begin to dissect the OSI (Open Systems Interconnect) reference model. As its name implies, it's an open standard maintained by the ISO (International Standards Organization) used to ensure interoperability of network devices. Such interoperability means that a wireless router from Linksys can be swapped out for similar products from the likes of Netgear or Edimax without missing a beat. As previously discussed, the various layers of the OSI model define specific functions handled exclusively at each layer. This specialization is referred to as "encapsulation." So much has to happen in order to transfer data, that no one device or protocol is expected to handle it all. We'll see how various network devices map to the individual OSI layers. The data payload size remains the same as it's passed down and up through the 7 layers of the reference model. Headers are added to the data payload as it gets passed up the layers. This packaging up of data is the encapsulation process. At the upper layers (5-7) the data payload plus headers is referred to as a PDU (protocol data unit). As we move down towards the physical layer, headers are stripped off during the sending process. We have a segment at the transport layer, packet at the network layer, frame at the datalink layer, and finally, bits at the physical layer. The unpackaging process is referred to as "decapsulation." It should be noted, that in terms of the CISSP exam, it's important to understand the security threats and specific attacks that exist at each layer. There are several nifty mnemonic devices for remembering each of the OSI layers. One of the more popular is "All People Seem To Need Data Processing" which translates to: Application, Presentation, Session, Transport, Network, Datalink, and Physical layers. Feel free to come up with one that works for you!