OSI model

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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 ?
Secret communication can take place at each of the OSI layers.
For mission-critical links, a WAN monitoring tool should perform statistical analysis of link performance across all seven OSI layers and report performance over a period of time.
Expert System categorizes the problems detected by OSI layers and summarizes the address or name of the stations involved and the position of frames in the capture file that trigger the system to identify the problem.
Combining advanced 0,18um semiconductor technology to benefit from optimized flow through architecture for network processing covering OSI layers till level 7 with dedicated Virtual Machine implementation enables to run up to 100Mbps full duplex VPN, open level firewall and other applicative level networking features into the xNSP processor.
This dimension of the framework consists of the basic seven OSI layers plus a Subject layer and an Operations layer.
1) high interoperability between layers--by adhering to its logical link control (LLC) interface of the token-ring network typology, IBM can interconnect with OSI layers three through seven, and SNA protocols can reside on both token-bus or Ethernet networks [2, 14];
1] DPI refers to packet inspection at OSI layers that reveal information about the types of data being transported.
The proposed work will focus on key thematic areas within the PACs domain, for example, security concerns across different OSI layers, emerging mobile and telecoms security, security monitoring and incident response, emerging notions of privacy and identity, embedded security, and emerging managed security services models.
Cross-layer design is a new approach in optimization of parameters of communication networks which should provide access to parameters of several OSI layers.
Some schemas present fewer levels--and not all OSI layers are distinct, but these classic definitions are still foundational to data network architectures.
The new architecture eliminates data dependencies among the OSI layers to facilitate parallel processing.
In between installation and troubleshooting, volunteers and eNet team members are encouraged to sit in classes headed by the various network experts and sponsors, eNet leaders explain everything, from the basic OSI layers to understanding routing protocols and the mechanics of hubs and switches.