OSI model

(redirected from ISO model)

OSI Model

OSI model

(Open Systems Interconnection model) The International Standards Organization's OSI model serves as a standard template for describing a network protocol stack (see OSI and ISO). For comparisons between the OSI model and other protocol stacks, see TCP/IP, NetWare, ATM, SNA and SS7.



The Protocol Stack
Using TCP/IP as a model, the sending application hands data to the transport layer, which breaks it up into the packets required by the network. It stores the sequence number and other data in its header. The network layer adds source and destination data in its header, and the data link layer adds station data in its header. On the other side, the corresponding layer reads and processes the headers and discards them.



Upper Layers


Layers 7 through 4 comprise the upper layers of the OSI protocol stack. They are more geared to the type of application than the lower layers, which are designed to move packets, no matter what they contain, from one place to another.

Application Layer 7
This top layer defines the language and syntax that programs use to communicate with other programs. The application layer represents the purpose of communicating in the first place. For example, a program in a client workstation uses commands to request data from a program in the server. Common functions at this layer are opening, closing, reading and writing files, transferring files and email messages, executing remote jobs and obtaining directory information about network resources.

Presentation Layer 6
When data are transmitted between different types of computer systems, the presentation layer negotiates and manages the way data are represented and encoded. For example, it provides a common denominator between ASCII and EBCDIC machines as well as between different floating point and binary formats. Sun's XDR and OSI's ASN.1 are two protocols used for this purpose. This layer is also used for encryption and decryption.

Session Layer 5
Provides coordination of the communications in an orderly manner. It determines one-way or two-way communications and manages the dialog between both parties; for example, making sure that the previous request has been fulfilled before the next one is sent. It also marks significant parts of the transmitted data with checkpoints to allow for fast recovery in the event of a connection failure.

In practice, this layer is often not used or services within this layer are sometimes incorporated into the transport layer.

Transport Layer 4
This layer is responsible for overall end-to-end validity and integrity of the transmission. The lower layers may drop packets, but the transport layer performs a sequence check on the data and ensures that if a 12MB file is sent, the full 12MB is received.

"OSI transport services" include layers 1 through 4, collectively responsible for delivering a complete message or file from sending to receiving station without error.

Lower Layers


Layers 3 through 1 are responsible for moving packets from the sending station to the receiving station.

Network Layer 3
The network layer establishes the route between the sender and receiver across switching points, which are typically routers. The most ubiquitous example of this layer is the IP protocol in TCP/IP (see TCP/IP). IPX, SNA and AppleTalk are other examples of routable protocols, which means that they include a network address and a station address in their addressing system. This layer is also the switching function of the dial-up telephone system. If all stations are contained within a single network segment, then the routing capability in this layer is not required. See layer 3 switch.

Data Link Layer 2
The data link is responsible for node to node validity and integrity of the transmission. The transmitted bits are divided into frames; for example, an Ethernet, Token Ring or FDDI frame in local area networks (LANs). Frame relay and ATM are also at Layer 2. Layers 1 and 2 are required for every type of communications. For more on this layer, see data link protocol.

Physical Layer 1
The physical layer is responsible for passing bits onto and receiving them from the connecting medium. This layer has no understanding of the meaning of the bits, but deals with the electrical and mechanical characteristics of the signals and signaling methods. For example, it comprises the RTS and CTS signals in an RS-232 environment, as well as TDM and FDM techniques for multiplexing data on a line. SONET also provides layer 1 capability.



The Protocol Stack
Using TCP/IP as a model, the sending application hands data to the transport layer, which breaks it up into the packets required by the network. It stores the sequence number and other data in its header. The network layer adds source and destination data in its header, and the data link layer adds station data in its header. On the other side, the corresponding layer reads and processes the headers and discards them.
References in periodicals archive ?
Tenders are invited for Supply Of Dosing Pump Plant Make Simcotech Iso Model Sta 10 Capacity 0 10Kg Lph 06Ux 100Ltr Water Tank 12 Ux Fitting Charges With All Materials Ie Nipple Non Return Valve Welding Etc Complete With 18 Percent Gst
Obtained data in our study have revealed that in this ISO model adiponectin kinetic is similar to BNP.
Other risk management standards that are of interest to the food industry that are either ISO standards or management systems certification standards loosely based on the ISO model include OHSAS 18001--Occupational health and safety; which provides a framework for managing occupational health and safety in the workplace and holds great potential for saving on costs related to health and safety failures and routinely reduces insurance premium costs for business owners.
Tim is one of the first and one of the best in implementing the ISO model and changing the lives of youth for positive improvement," said Guillory.
Under the ISO model, the transmission system owner is to finance investments decided by the ISO and approved by the regulatory authority.
The ACC has taken the concept of continuous improvement and applied it to the ISO model and its own previous verification venture, producing an integrated certification process that allows member-companies to meet membership and business requirements.
During the ongoing quality operation, there should be routine opportunities for aligning the company's systems with the ISO model.
This ISO model may provide supportive services to consumers, such as skills and self-advocacy training, assistance with criminal background checks, development of emergency backup service plans, and maintenance of an attendant registry that assists consumers in performing the employment-related administrative and supervisory tasks associated with their CD-PAS.
The ISO model refers to management systems and not products made at the site.
This blocks the way for eight member states, including France and Germany, who had proposed the alternative to full ownership unbundling and the highly restrictive ISO model.
Do you really have the expertise in the Commission to ensure that the ISO model functions properly," asked Morgan.
Similar to Well-Established Practices Concerning Wireline Networks and Layer-1 of the ISO Model --Wireless Networks Require a Solid, Conditioned RF Physical Layer for Maximum Coverage and Capacity