Introduction to the OSI Model

by Jason Zandri - June 2, 2010

The Open System Interconnection Reference Model (OSI) is a seven layer model that was developed as part of the effort to standardize networking that was started in the late 1970's as part of the Open Systems Interconnection (OSI) initiative.

This article will be a brief overview of the model itself and the tie in to the Cisco Certified Network Associate Routing & Switching exam (640-802 CCNA), as well as the Network+ exam.

OSI Model's place in the CCNA and Network+ exams

Both the 640-802 CCNA exam and the Network+ exam test for some of the common knowledge of the OSI Model.

For the CCNA 640-802 exam this information is tested as part of the "Describe how a network works" domain as part of the following subtopics:

• Use the OSI and TCP/IP models and their associated protocols to explain how data flows in a network
• Describe the purpose and basic operation of the protocols in the OSI and TCP models
• Select the components required to meet a network specification

Beyond these subtopics you'll need to have at least a general understanding of the model across some of the other exam topics as well but this is the primary area of focus.

For the Network+ exam most of this relates to subtopic  4.1 Explain the function of each layer of the OSI model.

For both certification exams you'll need to know where in the model certain protocols function as well as knowing at what layers hardware devices such as routers, switches, bridges, et cetera work. Additionally, you'll need to have a good understanding of how security of data is handled through the devices and what security features are offered to the data in transit and at which levels of the OSI model offer what types of security.

The Seven Layers of the OSI Model

In summary the four layers of the OSI model are broken as follows:

The Physical Layer defines the electrical and physical properties and the operating specifications for the devices and media in use. The main job of the Physical Layer is the physical "connection" or attachment of given media and how it is configured (e.g. Token Ring cable, size of cable used, termination in place etc.). In some instances, there may be secondary responsibilities of this layer depending on the device for things such as flow control, modulation/demodulation and so forth. The protocol data unit in use at this level of the OSI model is referred to as a "bit."

The Data Link Layer provides the practical means to transfer data between network nodes as its main job is to transfer data between network nodes in a wide area network or between nodes on the same local area network segment/subnet. It has the secondary responsibility to detect and correct errors (as permissible) that may take place at the Physical Layer. The protocol data unit in use at this level of the OSI model is referred to as a "frame."

The Network Layer handles the forwarding and routing of data along logical paths between network connected nodes. In addition to routing and forwarding functions of this layer of the model is also performs addressing, error handling, quality of service control, congestion control and packet sequencing. The protocol data unit in use at this level of the OSI model is referred to as a "packet."

The Transport Layer is responsible for the reliable, end to end transfer, recovery and flow control of the segments between the nodes. The protocol data unit in use at this level of the OSI model is referred to as a "segment."

The Session Layer addresses the build up and tear down of the connection sessions between nodes on a network. The protocol data unit in use at this level (and all of the subsequent levels) of the OSI model is referred to simply as "data."

The Presentation Layer is responsible for taking the data from applications at the application layer and breaking it down for use on the session layer as well as the reverse. It also has the task of formatting the data so that it can be sent to other nodes.

The Application Layer handles the initial connection of a given application to the network. It is where applications and application type activities such as browsing the web, sending and receiving email and performing file transfers take place. There are applications that wholly reside at the level such as Telnet and FTP.

 

Protocol Use at each of the TCP/IP Model Layers

At each layer of the OSI Model there are associated protocols that are in use.

These are not fully comprehensive lists but are examples of the more common protocols that are functioning at these different levels of the OSI Model.

At the Application layer you can find many but some of the more common ones include:

• DHCP - Dynamic Host Configuration Protocol
• FTP - File Transfer Protocol
• HTTP - HyperText Transfer Protocol
• IMAP - IMAP4, Internet Message Access Protocol (version 4)
• LDAP - Lightweight Directory Access Protocol
• LPD - Line Printer Daemon Protocol
• MIME (S-MIME) - Multipurpose Internet Mail Extensions and Secure MIME
• NFS - Network File System
• NNTP - Network News Transfer Protocol
• NTP - Network Time Protocol
• POP - POP3, Post Office Protocol (version 3)
• RDP - Remote Desktop Protocol
• RPC - Remote Procedure Call
• SMTP - Simple Mail Transfer Protocol
• SNMP - Simple Network Management Protocol
• SNTP - Simple Network Time Protocol
• SSH - Secure Shell
• TELNET - Terminal Emulation Protocol of TCP/IP
• TFTP - Trivial File Transfer Protocol

At the Presentation layer you can find these common protocols:

• MIME - Multipurpose Internet Mail Extensions
• SSL - Secure Sockets Layer
• TLS - Transport Layer Security
• XDR - eXternal Data Representation

At the Session layer you can find socket driven connections and session establishment in Transmission Control Protocol (TCP), Session Initiation Protocol (SIP), and Real-time Transport Protocol (RTP).

You can also find Named Pipe sessions, a protocol in the Server Message Block (SMB) suite as well as the NetBIOS (Network Basic Input/Output System) application Programming Interface (since NetBIOS is not formally a true networking protocol).

Session Announcement Protocol (SAP) is a protocol for broadcasting multicast session information and it is also found at the Session layer.

 

At the Transport layer you can find these common protocols:

• SPX - Sequenced Packet Exchange
• TCP - Transmission Control Protocol
• UDP - User Datagram Protocol
• SCTP - Stream Control Transmission Protocol

At the Network Layer you can find these common protocols:

• ATP - AppleTalk Transaction Protocol
• IPv4 - Internet Protocol v4
• IPv6 - Internet Protocol v6
• IPX - Internetwork Packet Exchange
• ICMP - Internet Control Message Protocol
• IGMP - Internet Group Management Protocol
• OSPF - Open Shortest Path First

At the Data Link Layer you can find these common protocols:

• PPP - Point-to-Point Protocol
• PPTP - Point-to-Point Tunneling Protocol
• SLIP - Serial Line Internet Protocol
• L2TP - Layer 2 Tunneling Protocol

Since the Physical Layer is really for defining the physical "connection" or attachment of given media and how it is configured as well as the electrical and physical properties and the operating specifications for the devices and media in use there are no actual TCP/IP common protocols that are in use.

You can find certain combinations of media and standards at this layer such as RS-232 (Recommended Standard 232) which is the standard for data and control signals connecting between a DTE (Data Terminal Equipment) and a DCE (Data Circuit-terminating Equipment) and Digital Subscriber Line (DSL) which provides digital data transmission over local telephone lines.

In this article we reviewed the tie in of the OSI Model to the CCNA and Network+ exams as well as took a look at the breakdown of the seven layers of the OSI Model

We wrapped up with a quick look at some of the protocols that are in use at each of the OSI Model Layers

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