OECG – Chapter 24


Okay – last “official” chapter of the OECG, although I still have to do the MPLS appendix. This is only 11 pages long. At a glance, we’ve all seen this material before. The 7 layers of the OSI model, how TCP/IP maps to it, plus odds and ends in router-land. Having been up since 3:45am, I’ve wore into the afternoon (about 1pm now), and I’m totally wiped out. I have few errands, too – make a run to the dump (always a festive occasion) and help move a screen house (really). But I think I can get through 11 pages of trivia before my brain collapses completely. I think I can…I think I can…I think…I…I…<zzzzzzzz>

OSI Layers

  • 7 – Application – the interface between the communications software and applications that need to communicate outside the computer.
    • Telnet
    • HTTP
    • FTP
    • NFS
    • SMTP gateways
    • SNMP
    • Content switch
  • 6 – Presentation – defines data formats like ASCII, EBCDIC, binary, BCD, JPEG, etc. Encryption is a presentation layer service.
    • JPEG
    • ASCII
    • EBCDIC
    • TIFF
    • GIF
    • PICT
    • encryption
    • MPEG
    • MIDI
    • Content switch
  • 5 – Session – defines how to start, control and end conversations, including the control and management of multiple, bidirectional messages so that the Application layer can be notified is some messages are missed or all messages completed.
    • RPC
    • SQL
    • NFS
    • NetBIOS
    • AppleTalk ASP
    • DECnet SCP
    • RADIUS
    • TACACS+
    • RTP
    • Content switch
  • 4 – Transport – focuses on delivering data to another computer through error recovery, segmentation of large blocks of application data, and reassembly of segments at the remote computer.
    • TCP
    • UDP
    • SPX
    • Content switch
  • 3 – Network – defines end-to-end delivery of packets via logical address so that endpoints can be identified. Defines how routing works and how routes are learned.
    • IP
    • IPX
    • AppleTalk DDP
    • Router
  • 2 – Data Link – delivers data across a specific type of link or medium. Each specification concerns a particular kind of medium. 802.3 and 802.2 define Ethernet, for example.
    • IEEE 802.3/802.2
    • HDLC
    • Frame Relay
    • PPP
    • FDDI
    • ATM
    • IEEE 802.5/802.2
    • Bridge or switch
  • 1 – Physical – deals with the physical characteristics of a communications medium. Connectors, pinouts, electrical currents, encoding and light modulation all fall in this layer.
    • EIA/TIA-232
    • V.35
    • EIA/TIA-449
    • RJ-45
    • Ethernet
    • 802.3
    • 802.5
    • B8ZS
    • T1
    • E1
    • Hub or repeater

OSI Layering Concepts and Benefits

  • Easier to learn – humans can grasp the concepts easier.
  • Easier to develop – reduces overall scope and complexity of a programming task, allowing for easier program changes and faster development.
  • Multivendor interoperability – everyone plays by the same rules, so they get to play together.
  • Modular engineering – one vendor writes to one layer (or layers), while someone else writes to a different layer.

OSI Terminology (mnemonics to memorize the OSI model if you haven’t already, and other notes)

  • All People Seem To Need Data Processing (7 -> 1)
  • Please Do Not Take Sausage Pizza Away (1 -> 7)
  • Pew! Dead Ninja Turtles Smell Particularly Awful (1 -> 7)
  • Please Do Not Tell Sales People Anything (1 -> 7)
  • A protocol data unit (PDU) – an OSI term generically referring to the bits, including the headers and trailers, at a certain layer. An IP packet would be a Layer 3 PDU.

TCP/IP to OSI Mapping

  • OSI – Application/Presentation/Session = TCP/IP – Application – HTTP, SMTP, POP3
  • OSI – Transport = TCP/IP – Transport – TCP, UDP
  • OSI – Network = TCP/IP – Network, IP
  • OSI – Data Link/Physical = TCP/IP – Network Interface – Ethernet, Frame Relay, PPP

OSI Layer Interactions

  • Same-layer interaction on different computers – A computer uses a protocol to communicate to the same layer on another computer. The protocol defined at each layer uses a header, transmitted between the computers, informing each other of what they want to do.
  • Adjacent-layer information on the same computer – Within a computer, one layer provides a service to the next higher layer. Software performing the higher layer function will request that the lower layer perform the requisite function.

About the author

Ethan Banks

Most people know me because I write & podcast about IT on the Packet Pushers network. I also co-authored "Computer Networks Problems & Solutions" with Russ White.

Find out more on my about page.


Most people know me because I write & podcast about IT on the Packet Pushers network. I also co-authored "Computer Networks Problems & Solutions" with Russ White.

Find out more on my about page.

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