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The Fiber Optic Association - Tech Topics

What is the OSI (Open Systems Interconnection) Network Model?

These are networking standards that separate networking protocols into seven layers. Cabling, including fiber optics, is covered in the Layer 1, the PHY or physical layer. For a complete description, all seven layers consist of:

Layer 1 - ­Physical Layer (the PHY)
The electrical and mechanical hardware level where the light, electrical or radio signals are transported over the network. It consists of the cabling (cable, connectors and network interface cards). The PHY refers to anything relating to the hardware that sends and receives data. Network protocols have physical layer components like Ethernet operating over twisted pair or fiber optic cabling or wireless links.

Note: A physical data structure refers to the organization of the data on a storage device. The "logical" or "virtual" data structure refers to the software, or how the information appears to the user.

Layer 2 - ­Data Link Layer
Here data packets have 3 purposes:
1) handle errors in the physical layer;
2) provide flow control, and
3) provide frame synchronization.
There are two sublayers:
1) Media Access Control (MAC) ­ that controls how we access data and gives permission for its transmission. Every device connected to the network must have a unique MAC address.
2) Logical Link Control (LLC) layer that controls frame synchronization, flow control and error checking.

Layer 3 - Network Layer
For switching and routing to send data from node to node. It can also perform forwarding, addressing, internetworking, error handling, congestion control and packet sequencing.

Layer 4 - Transport Layer
For the transfer of data between systems/hosts. End-to-end error recovery and flow control ensure complete data transfer.

Layer 5 ­ Session Layer
The management of connections between applications-setting up and handling of conversations, exchanges, and dialogues between applications at each end.

Layer 6 ­ Presentation Layer
Here any compatibility problems are handled and "different looking" data is formatted or encrypted so that it's accepted over the network.

Layer 7 ­ Application Layer
Here is where users and quality of service (QOS) are identified, and authentication and privacy occurs. This layer is application specific and it's where file transfers, e-mail, and other network software services are provided.

What do those 7 layers do in a typical user's world?
1. Layer 7: document creation.
2. Layer 6: encoding into ASCII before being "zipped."
3. Layer 5: attaching that document/file to an e-mail sent via SMTP (Simple Mail Transfer Protocol).
4. Layer 5: sending the e-mail. (Sometimes layers 4 and 5 are considered to be one.)
5. Layer 4: the start and end of a session.
6. Layer 3: routing the packets across the Internet from data contained in the packet headers, specified in the IP (Internet Protocol).
7. Layer 2: sending the data from your computer to enter the network. The size of the data packets is also determined by the IP.
8. Layer 1: (the Physical Layer or PHY) puts the packets, frames, or cells onto a Ethernet LAN cabling, a DSL (Digital Subscriber Line) circuit, or a SONET (Synchronous Optical Network) loop.
9. Layer 1: Also comes into action again at the receiving end where the process is reversed and the data begins at their Layer 1 and terminates in Layer 7 where the receiver sees your e-mail on their computer monitor.

OSI Model
Layers Data Unit Layer Function
Host Layers Data 7. Application Network process to application
6. Presentation Data representation and encryption

5. Session Interhost communication (SMTP)

Segment/Datagram 4. Transport End to end connections and reliability
Media Layers Packet 3. Network Path determination and logical addressing
Frame 2. Data Link Physical addressing
(MAC and LLC)

Bit 1. Physical Media (PHY), signal and transmission

Where does the Ethernet topology come in?
Ethernet is the physical layer LAN technology primarily used today-it's speedy and its installation is uncomplicated. Ethernet is the IEEE 802.3 standard which defines the rules for configuring an Ethernet network as well as specifying how elements in that network interact with each other. Other LANs could be Token Ring, Fast Ethernet, or Asynchronous Transfer Mode (ATM).

The seven-layer OSI model provides an overall architecture for the LAN that can operate over a topology that meets Ethernet specs. When all the components of the network are designed, manufactured, and installed correctly, the senders and receivers can communicate. This is why "interoperability" is so important in the communications industry.

(C) 2002-12 The FOA and Business Communications Services

This information was gathered through research into standards applying to local area networks and what they do. For other information on industry performance standards applying to the PHY by TIA in the US and ISO/IEC internationally.

More detailed information can be found on the FOA Online Reference Guide.

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