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08.89
ISO layers
The layer model defined by ISO includes 7 layers:
The transport-oriented layers (1 to 4) and the user-oriented layers (5 to 7). Each of these 7
layers provides services. In order to make its services available, a layer of a network entity
must communicate with the same layer of its peer. This communication process, including the
means used for it, is referred to as a protocol.
Layer 1:
Physical Layer = Physical characteristics of the transmission path, e.g.
Yellow Cable
This layer defines the physical operation such as electrical (current-voltage level) and
mechanical parameters between 2 peers, in order to be able to transmit data.
It provides services in order to activate, maintain and deactivate physical connections for
transmission of data between two data-link entities.
"A medium must be available, for example air, telephone line etc.".
Layer 2:
Data Link Layer = 2a Access method,
such as CSMA/CD 2b Reliable transfer, e.g CRC check
On this level, the access mechanism is defined that ensures that only one node sends data
over the bus at a given time. The layer provides functional and procedural services in order to
establish, maintain and release data link connections. It detects and possibly corrects errors
which may occur in the subordinate physical layer. It enables the network layer to control the
interconnection of data circuits within the physical layer, e.g. to interconnect partial sections
and to switch to alternate paths using different combinations.
Layer 1 and 2 ensure that information can be transmitted and that access to the transmission
medium is controlled.
Layer 3:
Network Layer = Network connections/addressing of other networks
The network layer provides the means to establish, maintain and terminate connections
between systems as well as networks.
"Rules for routing must be specified when using several transmission sections, e.g. route
switching of several sections in a communication network".
Layer 4:
Transport Layer = Transport control: Flow control, blocking,
acknowledgement, establishment and release of connection, transmission
of the conversation text.
The function of the transport layer is to provide a universal transport service in cooperation
with the services of the lower-level layers.
It separates the application-oriented from the transmission-oriented layers.
© Siemens AG 1989 All Rights Reserved
SINUMERIK 840/880 (PJ)
6FC5 197-0AB20-0BP0
2 Local Area Networks
2.1.1 The ISO seven layer model
2–5
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