Description of OSI layers
According to recommendation X.200, there are seven layers, each generically known as an N layer. An N+1 entity requests services from the N entity.
At each level, two entities (N-entity peers) interact by means of the N protocol by transmitting protocol data units (PDU).
A Service Data Unit (SDU) is a specific unit of data that has been passed down from an OSI layer to a lower layer, and which the lower layer has not yet encapsulated into a protocol data unit (PDU). An SDU is a set of data that is sent by a user of the services of a given layer, and is transmitted semantically unchanged to a peer service user.
The PDU at any given layer, layer N, is the SDU of the layer below, layer N-1. In effect the SDU is the ‘payload’ of a given PDU. That is, the process of changing a SDU to a PDU, consists of an encapsulation process, performed by the lower layer. All the data contained in the SDU becomes encapsulated within the PDU. The layer N-1 adds headers or footers, or both, to the SDU, transforming it into a PDU of layer N-1. The added headers or footers are part of the process used to make it possible to get data from a source to a destination.
Some orthogonal aspects, such as management and security, involve every layer.
Security services are not related to a specific layer: they can be related by a number of layers, as defined by ITU-T X.800 Recommendation.
These services are aimed to improve the CIA triad (i. e. confidentiality, integrity, availability) of transmitted data. Actually the availability of communication service is determined by network design and/or network management protocols. Appropriate choices for these are needed to protect against denial of service.
Layer 1: Physical Layer
The Physical Layer defines the electrical and physical specifications for devices. In particular, it defines the relationship between a device and a transmission medium, such as
a copper or optical cable. This includes the layout of pins, voltages, cable specifications, hubs, repeaters, network adapters, host bus adapters (HBA used in storage area networks) and more.
To understand the function of the Physical Layer, contrast it with the functions of the Data Link Layer. Think of the Physical Layer as concerned primarily with the interaction of a single device with a medium, whereas the Data Link Layer is concerned more with the interactions of multiple devices (i. e., at least two) with a shared medium. Standards such as RS-232 do use physical wires to control access to the medium.
The major functions and services performed by the Physical Layer are:
Establishment and termination of a connection to a communications medium.
Participation in the process whereby the communication resources are effectively shared among multiple users. For example, contention resolution and flow control.
Modulation, or conversion between the representation of digital data in user equipment and the corresponding signals transmitted over a communications channel. These are signals operating over the physical cabling (such as copper and optical fiber) or over a radio link.
Parallel SCSI buses operate in this layer, although it must be remembered that the logical SCSI protocol is a Transport Layer protocol that runs over this bus. Various Physical Layer Ethernet standards are also in this layer; Ethernet incorporates both this layer and the Data Link Layer. The same applies to other local-area networks, such as token ring, FDDI, ITU-T G. hn and IEEE 802.
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