The OSI Model - Part Two

Wednesday Apr 2nd 2003 by ServerWatch Staff

A continuation of a previous article outlining an introduction to the OSI Model, focusing on each of the seven layers of the OSI model individually.

by Ryan Smith

In Part One of this article, I covered the basics of the OSI model and gave a few tips for memorizing the seven layers of the stack. In this part, I'm going to cover each of the seven layers of the OSI model individually and present more detail to how the OSI model is utilized by networking.

The OSI Model:

To refresh our memory, the OSI model takes the task of internetworking - that is, host to host networking - and divides that up into what is referred to as a vertical stack. This vertical stack consists of seven different layers:

  1. Physical
  2. Data Link
  3. Network
  4. Transport
  5. Session
  6. Presentation
  7. Application

Now let's take a closer look a the individual layers of the OSI model. We're going to look at the layers in what is referred to as bottom-up, meaning we're looking at them from a physical standpoint up to an application standpoint as opposed to top-down where you look at the application first down to the physical level.

Layer 1 - Physical

The Physical layer handles the transfer of bits (0's and 1's, remember?) from one computer to another. This is where the bits are actually converted into the electrical signals that travel across the physical circuit.  Connectors, pins, electrical currents, encoding and light modulation are all part of different physical layer specifications.

Examples include - V.35, V.24, RJ45, Ethernet, FDDI, B8ZS

Layer 2 - Data Link

The Data Link layer deals with getting data from one particular medium to another.  It provides delivery across an individual link. The Data Link layer ensures that messages are delivered to the proper device and translates messages from the Network layer into bits for the Physical layer to transmit. It formats the message into data frames and adds a customized header containing the hardware destination and source address.

Examples include - IEEE 802.3/802.2, HDLC, Frame Relay, PPP, FDDI, ATM, IEEE 802.5/ 802.2

Layer 3 - Network

The Network layer is responsible for routing through an internetwork and for network addressing. This means that the Network layer is responsible for transporting traffic between devices that are not locally attached. Routers, or other layer-3 devices, are specified at the Network layer and provide the routing services in an internetwork. When a packet is received on a router interface, the destination IP address is checked. If the packet is not destined for the router, then the router will look up the destination network address in the routing table. Once an exit interface is chosen, the packet will be sent to the interface to be framed and sent out on the local network. If the entry for the destination network is not found in the routing table, the router drops the packet.

Examples include - IP, IPX, AppleTalk DDP

Layer 4 - Transport

Services located in the Transport layer both segment and reassemble data from upper-layer applications and unite it onto the same data stream. They provide end-to-end data transport services and can establish a logical connection between the sending host and destination host on an internetwork.

Examples include - TCP, UDP, SPX

Layer 5 - Session

The Session layer is responsible for setting up, managing, and then tearing down sessions between Presentation layer entities. The Session layer also provides dialog control between devices, or nodes. It coordinates communication between systems and serves to organize their communication by offering three different modes: simplex, half-duplex, and full-duplex. The Session layer basically keeps different applications' data separate from other applications data.

Examples include - RPC, SQL, NFS, NetBios names

Layer 6 - Presentation

The Presentation layer gets its name from its purpose: It presents data to the Application layer. It's essentially a translator and provides coding and con-version functions. A successful data transfer technique is to adapt the data into a standard format before transmission. Computers are configured to receive this generically formatted data and then convert the data back into its native format for actual reading (for example, EBCDIC to ASCII). By providing translation services, the Presentation layer ensures that data transferred from the Application layer of one system can be read by the Application layer of another host.

Examples include - JPEG, ASCII, EBCDIC, TIFF, GIF, PICT, encryption, MPEG, MIDI

Layer 7 - Application

The Application layer of the OSI model is where users communicate to the computer. The Application layer is responsible for identifying and establishing the availability of the intended communication partner and determining if sufficient resources for the intended communication exist.

Examples include - Telnet, HTTP, FTP, WWW browsers, NFS, SNMP

Ryan Smith
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