Specifications

TCP/IP protocol CP143 H1 / TCP/IP manual
2-4 Rev. 00/07
2.2.1.1 General
The ISO-OSI level model is based upon a suggestion that was developed by the International
Standards Organization (ISO). It represents the first step in the direction of an international standard
for the different protocols currently in use. The model is referred to as ISO-OSI level model. OSI is
the abbreviation for Open Systems Interconnection, that is communication between open systems.
The ISO-OSI level model is not a network architecture as the services and the protocols used on the
different levels have not been defined. The model simply provides information about the task that
each level must perform.
2.2.1.2 Description of the different levels
Level 1 Physical level (bit-transfer level)
The physical level transfers data bits via a communication channel. This level is generally
responsible for the mechanical, electrical and procedural interfaces and for the physical
communication medium that is used by the physical level:
• the voltages representing a logical 0 and a logical 1.
• the duration for which the voltage must be present to represent a valid bit.
• establishes a connection and terminates it when necessary.
• pin-allocation for the respective interface.
Level 2 Data link layer
The data link layer converts raw data into a data series ready for transfer. This is where the frame
delimiters are inserted during transmission and where they are detected during reception. This is
achieved by inserting a special bit-pattern at the start and at the end of a frame. In many instances a
flow-control and error detection mechanism is integrated into the data link layer.
Level 3 Network layer
The network layer is concerned with the selection of the packet path between the source and the
destination (other networks as well). Paths may be hard-wired in the network. They may also be
dynamic, that is they may be determined on the fly for every packet in order that the load on the
network optimized. The network layer must also ensure that different packets do not interfere with
each other. This layer is responsible for administrative functions for the transferred data, that is
counting of packets, characters or bits, etc.
The network layer must have the capacity to perform these tasks so that users of heterogeneous
networks may communicate with each other.
Level 4 Transport layer
The purpose of the transport layer is to accept data from the session layer, then to divide it up into
smaller portions, transfer this data to network layer and to ensure that every item arrives properly at
the destination. The transport layer generates a unique network connection for every transport link
required by the session layer. The transport layer determines the type of service that is provided to
the session layer. It can also increase the throughput, establish multiple network connections and
assign the data to these connections.
In addition to managing the use of multiple access channels by many message streams the transport
layer must also administer connections that the network establishes and relinquishes.