Open Systems Interconnection Model

Communication systems mostly consist of more than just two communication partners. These communication partners can be under the control of the same developer or com- pany, but this is not always the case. Data is transmitted over multiple nodes to reach its destination and the underlying infrastructure can differ from node to node because of different responsibilities. The transmitted data can be divided into a header, enclosing source and destination addresses, payload size, quality of service information, and the actual payload. The position of the header data and the payload has to be defined to help every developer and manufacturer to produce compatible hardware. Later in this work, protocols will be described, using the terminology of the OSI .

The International Telecommunication Union (ITU) and the International Organiza-

tion for Standardization (ISO)[29] developed the OSI model to simplify the definition

of communication protocols. Seven functional distinct layers divide the communication process. Figure 4.2 gives a graphical representation of these layers and the expected protocol flow. The flow starts at either side of the network stack. If some data shall be transmitted to another communication partner, the communication usually starts at the application layer. Every layer processes the data and passes it down to the next layer until reaching the physical layer. Each layer adds header information or transforms the data according the network requirements. Sometimes control messages are created, passed down the layers and send to their corresponding layer at the next communication partner, to create a virtual connection between them.

The physical layer transmits the data through some kind of medium (wire, air, fibre optic, . . . ) to the next node. After the transmission, the data passes the layers up. If the node is just an intermediate one the data moves up to the network layer, where it gets formatted for the transmission to the next node. If the data has arrived at its destination, it gets passed up to the application layer.

In the following sections each of the seven layers is briefly described. More information about the OSI model can be found in [29] or [30].

4.1 Open Systems Interconnection Model

physical layer data link layer network layer transport layer session layer presentation layer application layer physical layer data link layer

network layer transport layer session layer presentation layer application layer Protocol

Network Stack Network Stack

physical transmission of bits

Data Data

Figure 4.2: graphical representation of the ISO/OSI Model

4.1.1 Application Layer

The application layer is the interface between a program or application running on a PE and the communication infrastructure. It defines the interaction between two or more communication partners, such as how to request some data or how to send the partner data. For this interaction the application does not require any information about the underlying network, the destination address is enough. Very common application layer protocols used in the Internet are Hypertext Transfer Protocol (HTTP) and Post Office

Protocol Version 3 (POP3).

4.1.2 Presentation Layer

Data can be presented in multiple forms. For example some processor cores use big endian or little endian byte encoding for working with structures bigger than one byte. A higher level form ist the language encoding with ISO codes or UTF-8.

To allow the application layer to just use the passed data, the presentation layer converts and transforms the data to the required representation.

The presentation layer can be used to implement point to point encryption too.

4.1.3 Session Layer

A communication session consists of the connection establishment, the transmission and reception of multiple data and the detachment of the connection.

4 Interconnection Networks

Not every communication requires the establishment of a session. For example in a network, where every information is broadcasted to every network member, it is not pos- sible to establish a session. Sessions are always necessary, if multiple requests, belonging to the same context, have to be transmitted.

The Session layer is responsible for connection establishment before the data of session is transmitted and the tear down of the connection, when the session is finished.

4.1.4 Transport Layer

The transport layer defines at least one protocol or method, on how to transmit data to another node in the network. This protocol can be connection less or connection oriented. In a connection oriented protocol the connection establishment, data transmission and the connection tear down has to be described. In this case the data transmission ensures the reception of the data at the communication endpoint. For a connection less protocol only the data transmission is required, without acknowledgement of receipt.

Well known transport layer protocols are the User Datagram Protocol (UDP) and the

Transmission Control Protocol (TCP).

4.1.5 Network Layer

Networks can be build with different topologies. How data is transmitted from a start node to a destination node depends on this topology because it specifies if nodes are directly connected, or how many intermidate nodes exist between them. The network layer is responsible for defining routing and path finding algorithms for transmitting data beween the network nodes. If necessary, it creats an abstraction layer over all network nodes with its own distinct address range. In this logical view the nodes seem to be directly connected. Common network layer protocols are IPv4 and IPv6.

4.1.6 Data Link Layer

The data-link layer is responsible, that the entities forming the network, can communi- cate securely with each other. If the underlying physical connection is not very robust, the data link layer ensures error-detection through some kind of checksum and, if pos- sible, error-correction. This is achieved by requesting a retransmission of the data from the data-link layer on the other communication side or by recalculating lost data. If the physical transmission has a maximum number of bits, it can transmit at one time the data-link layer arranges the framing of the data.

4.1.7 Physical Layer

The physical layer of the OSI transmits data from one network entity to another one. The structure of the data is not important at this layer because just bits are transferred. The physical layer describes the electrical and physical specification for transmitting one bit. It determines the modulation of the data and which transfer medium is used. It offers the data-link layer an interface to transmit x bits of data.