The OSI Model - Part II

OSI model is the designation for a model for the functional stratification of communication protocols in computer networks. OSI is the abbreviation of Open Systems Interconnection reference model, often also called ISO/OSI. This designation makes reference to the fact that the model has beens standardized by the International Organization for Standardization.

7 Layers

The OSI model provides for a structure of seven layers. Here, every layer except the top layer provides functions for the layer above it. With this, each layer only has to know the interface definition of the next layer down, but not the concrete implementation of this lower layer or the layers below this.

1st Layer: Bit transmission
The lowest layer of the OSI model is called the bit transmission layer. The designation as a physical layer clearly indicates what actually happens on this layer: Here, it is a matter of actual transmission of individual bits and how this transmission actually occurs on the physical medium. While the implementation of this layer is quite simple for classic electrical conductors as the medium, for other media like glass fiber or for transmission via electromagnetic waves (W-LAN), they must be capable of representing bits or packets of bits for example as specific wavelengths. Together with the second layer, this layer is the only layer where the actual nature of the medium must be taken into consideration.

2nd Layer: Security layer
The security layer, also called the link layer, assures the consistency of the transmission. Amongst others, this layer provides the transmitted bit packets (frames) with checksums, which enable the receiver to check whether a frame has been transmitted correctly or whether an error has occurred during the transmission. Depending on the type of the error, it is either possible to correct the frame itself or to request the sender to repeat the transmission. The sending of receipt confirmations also takes place on this layer. If the sender does not receive such feedback from the receiver, it assumes that a transmission error has occurred and repeats the transmission.

3rd Layer: Network layer
The network layer of the OSI model is in charge of routing in the network. While with packet-based services, for example TCP/IP, a path is searched for each bit of data (packet), which may differ strongly from packet to packet, for line-based services a unique path is selected between the two communication partners, and it remains the same during the same transmission, unless a line failure or a similar error occurs. A widely distributed protocol on this layer is the Internet Protocol (IP), which should be known even to less versed users because of the IP address assigned to each device in an IP-based network, like for example the Internet.

4th Layer: Transport layer
The transport layer of the OSI model combines the individual packets again into the so-called segments. A cross-packet error correction may also take place in this layer, i.e. further errors are detected and corrected if possible or a repeat sending is requested from the sender.

5th Layer: Session Layer
In the OSI model, the session layer is in charge of establishing a session between the two communication partners and to manage it. This also includes the re-establishment of any interrupted communication and to continue at the point where it was interrupted.

6th Layer: Presentation layer
The tasks of the presentation layer include the offering of cross-protocol functions. Accordingly, with text-based protocols, individual bytes again become a readable text in this layer, or in the case of transmission with special protocols for video transmission, images and similar are processed. A cross-system processing of umlaut characters can also be done in this layer.

7th Layer: Application layer
The top layer is the application layer, which interacts directly with the end-user program and is finally operated by the user. Considering the World Wide Web as a very frequently used service, then it is the Hypertext Transport Protocol (HTTP), used amongst others in the application layer, that interacts directly with the browser. This again is operated by the user.

TCP/IP Protocol Stack as Application of the OSI Model

The Internet is based on the TCP/IP protocol stack. Here, the first two layers of the OSI model are combined to form the network access layer. Protocols like Ethernet or IEEE 802.11 (for WLAN) are used in these layers. The naming protocols IP and TCP are used on the network or the transport layer. The top three layers of the OSI model are finally combined to the application layer of the TCP/IP protocol stack.

Advantages of the Model

The advantages of the OSI model are obvious on closer inspection. Thus, the division of the communication into individual layers stacking onto each other has the advantage that it is possible, for example to change the physical medium without having to change the protocol layers above the data link layer. This modularization also makes it easier to determine that implementation is correct, as the layers can first be checked by themselves, so that any errors can already be corrected.