OSI Model Layers

The following sections provide descriptions and typical examples of each OSI layer. Furthermore, examples of communication methods and functions follow each layer description.

Layer 1: Physical Layer

The Physical layer consists of standards that describe bit ordering, bit transmission rates, connector types, and electrical specifications. Information is transmitted as binary bits (ones and zeros). Examples of Physical layer standards include the following:

Layer 2: Data Link Layer

The Data Link layer will focus on getting data reliably across any particular kind of link. Flow control and error notifications are other functions of the Data Link layer, as well. The Data Link layer applies to all access methods whether they are LAN or WAN methods. Information being processed at this layer is commonly known as frames. Examples of data link frame types include the following:

Layer 3: Network Layer

The Network layer is used to determine the best path to a destination. Device addressing, packet fragmentation, and routing all occur at the Network layer. Information being processed at this layer is commonly known as packets. Examples of Network layer protocols include the following:

► Internetwork Packet Exchange (IPX)

At the Network layer, a packet is associated with a connection-oriented protocol, while a datagram is associated with a connectionless protocol.

Layer 4: Transport Layer

The Transport layer is responsible for segmenting upper-layer applications and establishing end-to-end connections between devices. Other functions of the Transport layer include providing data reliability and error-free delivery mechanisms. Information being processed at this layer is commonly known as segments. Examples of Transport layer protocols include the following:

► Transmission Control Protocol (TCP)

► Novell's Sequenced Packet Exchange (SPX)

► User Datagram Protocol (UDP)

Layer 5: Session Layer

The Session layer performs several major functions, including managing sessions between devices, and establishing and maintaining sessions. Examples of Session layer protocols include the following:

► NetBIOS Name Queries

Layer 6: Presentation Layer

The Presentation layer handles data formats and code formatting. The functions of this layer are normally transparent to the end user, because this layer will take care of code formats and present them to the Application layer (layer 7) where the end user can examine the data. Examples of Presentation layer protocols include the following:

Layer 7: Application Layer

The Application layer is closest to the end user, which means that the application is being accessed by the end user. The major function of this layer is to provide services to end users. Examples of Application layer services include the following:

► File Transfer Protocol (FTP)

► HTML browsers

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