OSI model
| OSI model by layer |
|---|
The O.S.I. model (O.S.I. - Open System Interconnection) is a way of sub-dividing a System into smaller parts (called layers) from the point of view of communications. A layer is a collection of conceptually similar functions that provide services to the layer above it and receives services from the layer below it. On each layer an instance provides services to the instances at the layer above and requests service from the layer below.
For example, a layer that provides error-free communications across a network provides the path needed by applications above it, while it calls the next lower layer to send and receive packets that make up the contents of the path. Conceptually two instances at one layer are connected by a horizontal protocol connection on that layer.
Description of OSI layers
| OSI Model | ||||
|---|---|---|---|---|
| Data unit | Layer | Function | ||
| Host layers |
Data | 7. Application | Network process to application | |
| 6. Presentation | Data representation,encryption and decryption | |||
| 5. Session | Interhost communication | |||
| Segments | 4. Transport | End-to-end connections and reliability,Flow control | ||
| Media layers |
Packet | 3. Network | Path determination and logical addressing | |
| Frame | 2. Data Link | Physical addressing | ||
| Bit | 1. Physical | Media, signal and binary transmission | ||
Lately the OSI model has been taught using a Mnemonic, to help in understanding the complex model, such are from layer 1 to 7, and going from layer 7 to 1:
All People Seem To Need Data Processing
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 physical medium. This includes the layout of pins, voltages, cable specifications, hubs, repeaters, network adapters, host bus adapters (HBAs 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 802.11, as well as personal area networks such as Bluetooth and IEEE 802.15.4.
Layer 2: Data Link Layer
The Data Link Layer of the OSI model is responsible for communications between adjacent network nodes. Switches operate at the Data Link Layer. It is further more responsible for monitoring,correcting the flow of data as well as errors that creep up in transmission of data . It employs the use of block and convoluted coding to check the flow and error mechanism in transmission of data. Data link layer consists of two sub-layers: 1.Logical Link Control(LLC)sublayer 2.Medium Access Control(MAC)sublayer. LLC sublayer provides interface between the media access methods and network layer protocols such as internet protocol which is a part of TCP/IP protocol suite. LLC sublayer determines whether the communication is going to be connectionless or connection-oriented at the data link layer. MAC sublayer is responsible for connection to physical media. At the MAC sublayer of data link layer, the actual physical address of the device,called the MAC address, is added to the frame (which contains the packets inside). The frame contains all the information necessary to travel from source device to destination device. Each time a frame is created while it travels the path, it gets stamped with the MAC address of the last sending device in the "source" address, whereas the "destination" address gets the MAC of the adjacent receiving device. In simple words, a frame is needed to carry packets between two adjacent devices where they get discarded and recreated each time they are received/sent. MAC address is the 12 digit hexadecimal number unique to every computer in this world. A device's MAC address is located on its Network Interface Card (NIC). In these 12 digit of MAC address, the first six digits indicate the NIC manufacturer and the last six digits are unique. For example, 32-14-a6-42-17-Oc is a 12 digit hexadecimal MAC address. Thus MAC address represents the physical address of a device in the network.
Layer 3: Network 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 functions, and might also perform fragmentation and reassembly, and report delivery errors. Routers operate at this layer—sending data throughout the extended network and making the Internet possible. This is a logical addressing scheme – values are chosen by the network engineer. The addressing scheme is hierarchical.
Careful analysis of the Network Layer indicated that the Network Layer could have at least 3 sublayers: 1.Subnetwork Access - that considers protocols that deal with the interface to networks, such as X.25; 2.Subnetwork Dependent Convergence - when it is necessary to bring the level of a transit network up to the level of networks on either side; 3.Subnetwork Independent Convergence - which handles transfer across multiple networks. The best example of this latter case is CLNP, or IPv7 ISO 8473. It manages the connectionless transfer of data one hop at a time, from end system to ingress router, router to router, and from egress router to destination end system. It is not responsible for reliable delivery to a next hop, but only for the detection of errored packets so they may be discarded. In this scheme, IPv4 and IPv6 would have to be classed with X.25 as Subnet Access protocols because they carry interface addresses rather than node addresses.
A number of layer management protocols, a function defined in the Management Annex, ISO 7498/4, belong to the Network Layer. These include routing protocols, multicast group management, Network Layer information and error, and Network Layer address assignment. It is the function of the payload that makes these belong to the Network Layer, not the protocol that carries them.
Layer 6: Presentation Layer
Interfaces
Neither the OSI Reference Model nor OSI protocols specify any programming interfaces, other than as deliberately abstract service specifications. Protocol specifications precisely define the interfaces between different computers, but the software interfaces inside computers are implementation-specific.
For example Microsoft Windows' Winsock, and Unix's Berkeley sockets and System V Transport Layer Interface, are interfaces between applications (Layer 5 and above) and the transport (Layer 4). NDIS and ODI are interfaces between the media (Layer 2) and the network protocol (Layer 3).
Interface standards, except for the Physical Layer to media, are approximate implementations of OSI Service Specifications.
Examples
Comparison with TCP/IP
In the TCP/IP model of the Internet, protocols are deliberately not as rigidly designed into strict layers as the OSI model.[3] RFC 3439 contains a section entitled "Layering considered harmful." However, TCP/IP does recognize four broad layers of functionality which are derived from the operating scope of their contained protocols, namely the scope of the software application, the end-to-end transport connection, the internetworking range, and lastly the scope of the direct links to other nodes on the local network.
Even though the concept is different from the OSI model, these layers are nevertheless often compared with the OSI layering scheme in the following way: The Internet Application Layer includes the OSI Application Layer, Presentation Layer, and most of the Session Layer. Its end-to-end Transport Layer includes the graceful close function of the OSI Session Layer as well as the OSI Transport Layer. The internetworking layer (Internet Layer) is a subset of the OSI Network Layer (see above), while the Link Layer includes the OSI Data Link and Physical Layers, as well as parts of OSI's Network Layer. These comparisons are based on the original seven-layer protocol model as defined in ISO 7498, rather than refinements in such things as the internal organization of the Network Layer document.
The presumably strict peer layering of the OSI model as it is usually described does not present contradictions in TCP/IP, as it is permissible that protocol usage does not follow the hierarchy implied in a layered model. Such examples exist in some routing protocols (e.g., OSPF), or in the description of tunneling protocols, which provide a Link Layer for an application, although the tunnel host protocol may well be a Transport or even an Application Layer protocol in its own right.
See also
- Cognitive networks
- Hierarchical internetworking model
- Internet protocol suite
- Layer 8
- OSI protocol suite
- Protocol stack
- Service layer
- TCP/IP model
- X.25 protocol suite
- WAP protocol suite
References
- ^ "X.225 : Information technology – Open Systems Interconnection – Connection-oriented Session protocol: Protocol specification". Archived from the original on 1 February 2021. Retrieved 10 March 2023.
- ^ ITU-T Recommendation Q.1400 (03/1993), Architecture framework for the development of signalling and OA&M protocols using OSI concepts, pp 4, 7.
- ^ RFC 3439
External links
- ISO/IEC standard 7498-1:1994 (PDF document inside ZIP archive) (requires HTTP cookies in order to accept licence agreement)
- ITU-T X.200 (the same contents as from ISO)
- The ISO OSI Reference Model , Beluga graph of data units and groups of layers
- Template:PDFlink, Hubert Zimmermann, IEEE Transactions on Communications, vol. 28, no. 4, April 1980, pp. 425 - 432.
- Internetworking Basics
