The ISO {International Organization for
Standardization} developed the OSI architectural model [www.iso.org].
The model is
divided in seven layers, with layers 1 to 4 called the lower layers and 5
to 7 the upper layers.
Layer Responsibilities: |
Layer 6. Presentation Layer: Determines how computers
represent data [ASCII, GIF..]. Protocol Data Unit [PDU] is called Data at
this layer. This layer responds to service requests from the Application
Layer and issues service requests to the Session Layer. The Presentation
Layer relieves the Application Layer of concern regarding syntactical
differences in data representation within the end-user systems. Note: An
example of a presentation service would be the conversion of an
EBCDIC-coded text file to an ASCII-coded file.
Layer 5. Session Layer: Establishing a communication
session, Security, Authentication. NetBIOS is a layer 5 protocol.
Protocol Data Unit [PDU] is called Data at this layer. This layer
responds to service requests from the Presentation Layer and issues
service requests to the Transport Layer. The Session Layer provides the
mechanism for managing the dialogue between end-user application
processes. It provides for either duplex or half-duplex operation and
establishes check-pointing, adjournment, termination, and restart
procedures.
Layer 4. Transport Layer: Provides transfer correctness,
data recovery, and flow control. TCP is a layer 4 protocol. Protocol Data
Unit [PDU] is called a Segment at this layer. This layer responds to
service requests from the Session Layer and issues service requests to
the Network Layer. The purpose of the Transport Layer is to provide
transparent transfer of data between end users, thus relieving the upper
layers from any concern with providing reliable and cost-effective data
transfer.
Layer 3. Network Layer: Provides address assignment, and
Packet's forwarding methods. Protocol Data Unit [PDU] is called a Packet
at this layer. This layer responds to service requests from the Transport
Layer and issues service requests to the Data Link Layer. The Network
Layer provides the functional and procedural means of transferring
variable length data sequences from a source to a destination via one or
more networks while maintaining the quality of service requested by the
Transport Layer. The Network Layer performs network routing, flow
control, segmentation/desegmentation, and error control functions.
Layer 2. Data Link layer: Frame format, Transmitting
frames over the net [additional bit/byte stuffing, start / stop flags,
checksum, and CRC]. CAN bus, ATM, StarLAN, LocalTalk and HDLC are layer 2 protocols.
Different network and protocol characteristics are defined by different
data link layer specifications. The Data Link layer is subdivided into
the Media Access Control (MAC) which controls access and encodes data
into a valid signaling format [for the physical layer], and the Logical
Link Control (LLC), which provides the link to the network [for the
Network layer] Protocol Data Unit [PDU] is called a Frame at this layer.
This layer responds to service requests from the Network Layer and issues
service requests to the Physical Layer. The Data Link Layer provides the
functional and procedural means to transfer data between network entities
and to detect and possibly correct errors that may occur in the Physical
Layer. Note: Examples of data link protocols are HDLC and ADCCP for
point-to-point or packet-switched networks and LLC for local area
networks.
Layer 1. Physical Layer: Defines the physical [hardware] implementation and the electrical [signal level] implementation of the bus; network cabling, connector type, pin-out, physical data rates, maximum transmission distances, and data transmission encoding. At this layer information is placed on the physical network medium. RS-232, and RS422 are examples of a physical layer specification. Protocol Data Unit [PDU] is called a Bit at this layer. The Physical layer performs services requested by the Data Link Layer. The major functions and services performed by the physical layer are: (a) establishment and termination of a connection to a communications medium; (b) participation in the process whereby the communication resources are effectively shared among multiple users, e.g., contention resolution and flow control; and, (c) conversion between the representation of digital data in user equipment and the corresponding signals transmitted over a communications channel. Other physical layer specifications are listed on the Electronic Bus page.
Related Topics: Protocol Acronyms, Protocol Definitions
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