The Open Systems Interconnection (OSI) was developed by the International Organization for Standardization in 1984 to enable different types of networks to be linked together.
The model contains seven layers, as shown in the Figure below.
These layers describe networking functions from the physical network interface to the software applications interfaces. The intent of the OSI model is to provide a framework for networking that ensures compatibility in the network hardware and software and to accelerate the development of new networking technologies.
Physical layer
Provides the electrical and mechanical connection to the network. Examples of technologies working in this layer are Electronic Industries Alliance/Telecommunications Industry Association (EIA/TIA)–related technologies, UTP, fiber, and network interface cards (NICs).
It uses topologies: BUS, STAR, RING, MESH
Communication modes: Simplex, Half Duplex, Full Duplex
Hardware including networking devices, antennas, cables, modem, and intermediate devices such as repeaters and hubs.
Data link layer
Handles error recovery, flow control (synchronization), and sequencing (which terminals are sending and which are receiving data with physical devices, ex:switch). It is considered the “media access control layer” and is where media access control (MAC) addressing is defined. The Ethernet 802.3 standard is defined in this area, which is why the MAC address is sometimes called the Ethernet address.
Network layer
Accepts outgoing messages and combines messages or segments into packets, adding a header that includes routing information. It acts as the network controller. Examples of protocols working in this layer are Internet Protocol (IP) and Internetwork Packet Exchange (IPX).
It functions to encapsulate and fragment, congestion controls and error handling.
Hardware: routes, bridge routers, 3-layer switches, and protocols such as Internet (IPv4) Protocol version 4 and Internet Protocol version 6 (IPv6).
Transport layer
Is concerned with message integrity between source and destination. It also segments/reassembles (the packets) and handles flow control. Examples of protocols working in this layer are Transmission Control Protocol (TCP) and User Datagram Protocol (UDP).
Mechanism: error control, flow control, and congestion control to keep track of the data packets, check for errors and duplication, and resend the information that fails delivery. Packet segmentation & Reassembly.
Session layer
Provides the control functions necessary to establish, manage, and terminate the connections as required to satisfy the user request. Examples of technologies working in this layer are Network File System (NFS) and Structured Query Language (SQL).
Functions: open, closing and re-establishing activities. synchronizing data streams.
Presentation layer
Accepts and structures the messages for the application. It translates the message from one code to another, if necessary. This layer is responsible for data compression and encryption. Examples of technologies working in this layer are American Standard Code for Information Interchange (ASCII) and Joint Photographic Experts Group (JPEG).
Functions: Data conversion, character code translation
Application layer
Interacts with application programs that incorporate a communication component such as your Internet browser and email. This layer is responsible for logging the message in, interpreting the request, and determining what information is needed to support the request. Examples are Hypertext Transfer Protocol (HTTP) for web browsing, File Transfer Protocol (FTP) for transferring files, and Simple Mail Transfer Protocol (SMTP) for email transmission.
APPLYING THE OSI MODEL
A network administrator needs to have a good understanding of all seven layers of the OSI model. Knowledge of the layers can help to isolate network problems. There are three basic steps in the process of isolating a network problem:
Step 1 Is the connection to the machine down? (layer 1)
Step 2 Is the network down? (layer 3)
Step 3 Is a service on a specific machine down? (layer 7)
A network administrator uses the OSI model to troubleshoot network problems by verifying the functionality of each layer. In many cases, troubleshooting network problems requires the network administrator to isolate at which layer the network problem occurs.
For example, assume that a network is having problems accessing an email server that uses SMTP - a layer 7 application. The first troubleshooting step for the network administrator is to ping the IP address of the email server (layer 3 test). A “ping” to an IP address can be used to quickly check whether there is a network connection.
A “reply from” response for the ping indicates that the connection to the server is up. A “request timed out” response indicates that the network connection is down. This could be due to a cabling problem (layer 1) or a problem with a switch (layer 2) or a router (layer 3), or the email server could be completely down (layer 7). In the case of “request timed out,” the network administrator has to go directly to the telecommunications closet or the machine to troubleshoot the problem. In this case, the administrator should first check for layer 1 (physical layer) problems. Many times this just requires verifying that a network cable is connected. Cables do get knocked loose or break.
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