Understanding the OSI Model

The Open Systems Interconnection (OSI) model defines the ways protocols operate by breaking the different aspects of protocols into layers. The OSI model uses seven layers with different purposes to define how protocols function. Each layer may use the functionality of the first layer below it and export functionality to the next layer above it. See Table 14-10 for a detailed listing of the OSI model's layers.

Table 14-10. OSI layer reference

Layer level

Layer name

Layer 1


Layer 2

Data Link

Layer 3


Layer 4


Layer 5


Layer 6


Layer 7


OSI layer 1 covers all of the physical connectivity specifications of devices. This includes any electrical voltage, pin-outs, connectors, cables, and hubs. Layer 1 defines all network adapters, network devices that do not work in layer 2, and host bus adapters used in storage area networks (SANs). The main purpose of layer 1 includes establishing a connection or disconnection from a network medium. Layer 1 also covers modulation and flow control over the network medium.

OSI layer 2 controls the means of controlling data transfer among network entities. Layer 2 also handles the control mechanism of data transferred among network entities. Bridges and switches both work within layer 2. Although there are layer 3 switches, they work on layer 2 without the use of a router.

OSI layer 3 controls the functional means of transferring data among network entities. Layer 3 handles the variable length sequences to and from destinations among networks. It also handles QoS for the transport layer. Routing also occurs at layer 3 (in fact, routing is the most common use of layer 3).

OSI layer 4 controls the transfer of data among users, and provides reliable data transfer to the layers above itself. Layer 4 controls flow as well as errors. This layer controls the retransmit of packets lost in transport among users. TCP uses this layer as the control portion of the protocol. Layer 4 also converts data into the User Datagram Protocol (UDP) and Stream Control Transmission Protocol (SCTP) formats.

OSI layer 5 controls the networked communications between computers. This includes managing and terminating connections among machines. Layer 5 controls duplex modes on network traffic, which includes full- and half-duplex operations. TCP uses layer 5 to control the flow of data and to terminate connections.

OSI layer 6 provides a standard interface to transform data into the correct format for the application layer. Standard uses of layer 6 include data encryption, compression, and specific types of encoding, including MIME encoding. Layer 6 also allows for the transformation into and out of the XML format.

OSI layer 7 controls the means a user needs to access network resources through an application. Programs that use layer 7 include SMTP, HTTP, FTP, Telnet, IPSec, IM, and other applications.

Each layer of the OSI model handles different portions of the networking process and helps to define the process of finding errors, or just understanding how the complex process of networking actually works. Armed with the information from the OSI model, we can begin to truly understand, create, and even fix networks as well as the protocols used to transmit data across networks.

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