I recently overheard some students in the halls of the Syracuse University School of Information Studies ruminating about Layers 5 and 6 of the OSI model. The students are part of an independent study group preparing to take the CCNA (Cisco Certified Network Associate) exam, and I admire their ambition to obtain this industry certification on top of their undergraduate education. I'm agitated, however, by the fact that these students must spend time studying such an obfuscated model of networking when they could be pursuing many more vital and relevant networking topics. Unfortunately, the CCNA exam requires knowledge of all the OSI layers -- even though some are no longer applicable in the real world.
Let's face it, networking is a tough discipline to master. It can take a good, long while before any of it even starts to click. Back in the 1980s when I first began teaching myself about networking, the only book I could find on the topic was Andrew Tanenbaum's first edition of Computer Networks. Although this book helped a lot, its detailed explanations of the layers also confused me; I remember driving myself nuts trying to conceptualize the purpose of each layer. I eventually concluded that the most mysterious layers -- Layer 5 (the Session Layer) and Layer 6 (the Presentation Layer) -- didn't serve much purpose for a network practitioner, and I ignored them. These days, aside from the layered model, everyone pretty much ignores the ISO's OSI. It certainly was a noble idea to define standards-based, detailed profiles of networked protocols layer by layer. In the late 1980s, the ISO's OSI (which has a symmetry unrivaled by any other networking acronyms) even had the backing of the federal government, which mandated that the procurement of all networking equipment adhere to the standard. Remember GOSIP (Government Open Systems Interconnection Profile)? But OSI never caught on and TCP/IP did.
Many networking textbooks are guilty of propagating this conundrum of networking. This is unfortunate. It's also almost as bad as Microsoft continuing to include detailed coverage of Ethernet barrel connectors, terminators and the like in its Networking Essentials test-training material. There simply is no excuse for it -- nor for a networking giant like Cisco to include material in its networking industry certification exam that serves no purpose but to confuse and bamboozle.
So why do network-certification exams and college textbooks continue to exhibit such mindless loyalty to the OSI model? One argument is that it helps us understand the important concepts of layering, modularity and encapsulation of network protocols. These concepts are indeed fundamental to comprehending how networks function -- which is my point exactly. Why make it more confusing than necessary?
My decision to ignore Layers 5 and 6 has served me well. I've been successfully engineering, troubleshooting and managing networks for about 15 years and have spent countless hours with developers to diagnose difficult problems with multilayered, networked-based client/server applications. Layers 5 and 6 have never been an issue. Sure, things happen on a network that fit somewhere between the application and the transport layers, but that doesn't mean we should attempt to force them into the contrived explanations of Layers 5 and 6 in college textbooks and CCNA prep materials.
My advice to Cisco: Remove requirements for knowledge of Layers 5 and 6 from the CCNA test immediately. You have no excuse for distracting future network engineers with this drivel. And for those of you writing a textbook that covers the OSI layers, replace the usual contrivances that attempt to make sense out of Layers 5 and 6 with the following: Historical artifacts; no longer relevant.
Peter Morrissey is a full-time faculty member of Syracuse University's School of Information Studies, and a contributing editor and columnist for Network Computing. Send your comments on this column to him at ppmorris@syr.edu.
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Let's face it, networking is a tough discipline to master. It can take a good, long while before any of it even starts to click. Back in the 1980s when I first began teaching myself about networking, the only book I could find on the topic was Andrew Tanenbaum's first edition of Computer Networks. Although this book helped a lot, its detailed explanations of the layers also confused me; I remember driving myself nuts trying to conceptualize the purpose of each layer. I eventually concluded that the most mysterious layers -- Layer 5 (the Session Layer) and Layer 6 (the Presentation Layer) -- didn't serve much purpose for a network practitioner, and I ignored them. These days, aside from the layered model, everyone pretty much ignores the ISO's OSI. It certainly was a noble idea to define standards-based, detailed profiles of networked protocols layer by layer. In the late 1980s, the ISO's OSI (which has a symmetry unrivaled by any other networking acronyms) even had the backing of the federal government, which mandated that the procurement of all networking equipment adhere to the standard. Remember GOSIP (Government Open Systems Interconnection Profile)? But OSI never caught on and TCP/IP did.
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