Open Systems Interconnect (OSI) Model


Jarret W. Buse

Open Systems Interconnect (OSI) Model

The Open Systems Interconnect (OSI) Model is the basis for most network protocols. Two examples are TCP/IP and IPX/SPX.

The OSI Model was started by the International Organization for Standards (ISO) in the early 1980’s. It was Honeywell Information Services which came up with the seven layer approach to the OSI Model. The layers are as follows:

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

NOTE: The layers are usually represented in an upside-down order with Layer 7 being at the top.

From a user standpoint, everything starts at Layer 7. Layer 7 is where the applications reside and make requests to network services. Network services consist of file sharing, printer sharing, e-mail services, instant messaging, etc.

Before we get into the individual layers and what they accomplish, let’s look at what type of information each one manages.

The Application, Presentation and Session Layer all deal with data. Data can be files loaded, saved or printed, etc. The data is necessary information needed by the application or sent to the Network Services.

The Transport Layer handles Segments of information. Large files or data cannot be sent over the network medium, so the data is segmented into smaller sections. Each network protocol has its own specification as to the segment size.

The Network Layer deals with Packets and Datagrams. Packets allow for error correction, for guaranteed delivery and destination addressing. Packets are used for transfer of data between two systems. Datagrams, on the other hand, are the opposite in that there is no error correction or addressing. Datagrams are useful in streaming data that can allow for non-guaranteed delivery of some data to multiple systems.

The Data Link Layer consists of Bit and/or Frames. The frames are basically a Packet or Datagram which has had a header and trailer containing the MAC Addresses of the source and destination system. It is also possible to have the Frame size set by the network bus type: bus, Token Ring, Ethernet, etc.

The Physical Layer deals with bits. As the name suggests, this is the layer where the data is sent over the physical medium. The medium can be wired or wireless, but the information is sent serially as bits (ones and zeroes).

Now that we understand what is being transferred in each of the seven Layers, let’s look at each Layer to see what is going on.

Starting at Layer 7, the Application Layer, you may already understand that this is where the user applications reside. Any application which can connect to a network service, usually any of them, is on this layer. The simplest is any application which can open a file. If the file resides on another system, such as on a network or the Internet, the application is requesting and receiving the file by a network protocol. As stated before, there are numerous network services and not all applications can access every service. Layer 7 deals with data such as files.

Layer 6, the Presentation Layer, may encrypt and convert data as needed. If the requesting system requires a file in specific format, this layer will change the data as needed. For example, data can be converted from ASCII (PC format) to EBCDIC (mainframe format).

The Session Layer, or Layer 5, establishes connections between systems. Once the transmission is complete, the Session Layer also terminates the connection. The Layer is also responsible for managing the connection by determining if the transmission is full or half-duplex. Full-duplex is when two systems can send and receive simultaneously. Half-duplex allows two systems to only “listen” or “talk”, but not at the same time. Full-duplex can be thought of as being similar to a telephone. Each caller can listen and talk at once. Half-duplex is like a walkie-talkie. Only one person can talk at a time while the other listens.

For Layer 4, or the Transport Layer, this is where the addressing occurs. Each protocol has its own addressing scheme. TCP/IP, for example, uses TCP/IP addresses to handle the addressing. The addressing may also contain port numbers. Each Segment sent/received is tracked/acknowledged. Any Segment not received and acknowledged will be resent by the sending system’s Transport Layer.

Now that we can see where the data needs to go, Layer 3 or the Network Layer can determine if the Segments from Layer 4 need to remain on the Local Area Network (LAN). Segments may need to go to another network such as on a Wide Area Network (WAN). If the Segments remain on the LAN, the required addressing is duplicated from Layer 4. If the Segments need to go to a WAN, then they should be sent to the Gateway address of the proper network. In the case of a WAN, the Gateway address is used here; otherwise, the destination system address is used since it is local.

NOTE: a Gateway can be a routing device of some type which connects the LAN to another LAN or even the Internet. The device allows bits to leave the LAN and go to another network. The device also receives bits and transmits them to the LAN.

Layer 2, or the Data Link Layer, is responsible for transmission of data on the network medium. The Layer can detect if the network is available for sending data across the medium. For example, Layer 2 can determine if another system is sending data and will wait to try delivery again after a certain time has passed. The method is not true for Token Ring networks where a system cannot transmit data until it holds the token. Once data is sent, the receiving system can send a small acknowledgment packet, if required, to show that the Frame was received intact. This layer is also responsible to listen to network medium for frames addressed to it.

Layer 1, the Physical Layer, is responsible for the physical transmission of the bits which make up the encapsulated data and necessary headers and trailers. If the medium is a wired network, the electrical pulses are generated with proper voltage across the medium. If the medium may be wireless, the proper frequencies are generated to travel through the air to a wireless hub.

As data travels from the upper layer (Layer 7) to the lower layers (Layer 1), each Layer attaches its own header and trailer information to the original data. At the receiving system, as each Layer receives the data unit, it checks the header and trailer information attached by the sending system. The header and trailer information is removed before it is sent on to the next Layer. Here, the process occurs again until all the data is received, if it is larger than a frame, and rejoined together as one file. Here, the data is checked by the specified application, and it is handled as needed. This process occurs at both systems until all requests are made and all data requested is sent, in the case of a file request.


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