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

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OSI Reference Model Layer Mnemonics
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TCP/IP Protocol Suite and Architecture
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OSI Reference Model Layer Summary

To assist you in quickly comparing the layers of the OSI Reference Model, and understanding where they are different and how they relate to each other, I have created the summary chart shown in Table 18. It shows each layer's name and number, describes its key responsibilities, talks about what type of data is generally handled at each layer, and also what the scope of each layer is in approximate terms. I also show some of the more common protocols that are associated with each layer.

I will say here that “standard disclaimers apply”. I don't want to repeat all of the caveats I mentioned elsewhere in discussions of the OSI model, but know that they apply here as well. Namely: the layers aren't always hard-fast, I haven't listed every single protocol here, some may really fit into more than one layer, and so on. In particular, note that many of the technologies listed as being in the data link layer are there because that is the layer where their primary functionality resides. In reality, most of these technologies include components in other layers, especially the physical layer.


Table 18: OSI Reference Model Layer Summary

Group

#

Layer Name

Key Responsibilities

Data Type Handled

Scope

Common Protocols and Technologies

Lower Layers

1

Physical

Encoding and Signaling; Physical Data Transmission; Hardware Specifications; Topology and Design

Bits

Electrical or light signals sent between local devices

(Physical layers of most of the technologies listed for the data link layer)

2

Data Link

Logical Link Control; Media Access Control; Data Framing; Addressing; Error Detection and Handling; Defining Requirements of Physical Layer

Frames

Low-level data messages between local devices

IEEE 802.2 LLC, Ethernet Family; Token Ring; FDDI and CDDI; IEEE 802.11 (WLAN, Wi-Fi); HomePNA; HomeRF; ATM; SLIP and PPP

3

Network

Logical Addressing; Routing; Datagram Encapsulation; Fragmentation and Reassembly; Error Handling and Diagnostics

Datagrams / Packets

Messages between local or remote devices

IP; IPv6; IP NAT; IPsec; Mobile IP; ICMP; IPX; DLC; PLP; Routing protocols such as RIP and BGP

4

Transport

Process-Level Addressing; Multiplexing/Demultiplexing; Connections; Segmentation and Reassembly;
Acknowledgments and Retransmissions;
Flow Control

Datagrams / Segments

Communication between software processes

TCP and UDP; SPX; NetBEUI/NBF

Upper Layers

5

Session

Session Establishment, Management and Termination

Sessions

Sessions between local or remote devices

NetBIOS, Sockets, Named Pipes, RPC

6

Presentation

Data Translation; Compression and Encryption

Encoded User Data

Application data representations

SSL; Shells and Redirectors; MIME

7

Application

User Application Services

User Data

Application data

DNS; NFS; BOOTP; DHCP; SNMP; RMON; FTP; TFTP; SMTP; POP3; IMAP; NNTP; HTTP; Telnet


 


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OSI Reference Model Layer Mnemonics
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TCP/IP Protocol Suite and Architecture
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Version 3.0 - Version Date: September 20, 2005

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