The International Standards Organization (ISO) developed the Open Systems Interconnection (OSI) Reference Model to define functional communications standards. This reference model is widely used by equipment manufacturers to assure their products will interoperate with products from other vendors. The OSI model describes the functions that are performed in data communications. The model architecture is seven layers those are listed below with each layer defining specific functions.
The Application layer provides three basic services to applications:
The Application layer is used to support the following services:
In addition, the Application layer advertises any services that are being offered and determines whether requests made by the client should be processed locally or remotely (through another network resource).
The main function of the presentation layer is to define the data formats used to provide a number of services to the Application layer.
Included in these services the ones you need to know are:
Conversions standards defined on the Presentation Layer for data conversion and formatting:
| Category | Standards |
| Data Conversion | ASCII, EBCDIC, encryption |
| Audio/video conversion | MIDI, MPEG, QuickTime, AVI |
| Graphics conversion | GIF, JPEG, PICT, TIFF |
Layer 5 of the OSI model is the Session Layer, which establishes, manages, and terminates sessions between applications.
Following are some of the communication tasks performed at this layer:
When you create a connection, you authenticate the user account at the sending and receiving computers. Connection creation also involves determining the type of communication that will take place and the protocols that will be used by the lower layers.
Data transfer and dialogue control are used to determine which computer is making requests and which computer is making responses. This also determines whether acknowledgments are required for data transmission.
A session is a series of related connection-oriented transmissions between network nodes. Another way to look at it is that a session is the interrelated communications between two or more presentation entities, which emphasizes that the Session layer provides services to the Presentation layer.
The four main functions of the Transport layer are:
In carrying out its duties, the Transport layer performs a range of support activities, including:
The Network layer of the OSI Model basically defines logical addressing and the ways that packets are moved from source to destination on a network. The functions of the Network layer can be broken down as follows:
Logical Address Construction
| Protocol | Total Address Length | Bits in Network Portion | Bits in Host Portion |
| TCP/IP | 32 | Class A – 8 Class B – 16 Class C – 24 |
Class A – 24 Class B – 16 Class C – 8 |
| IPX | 80 | 32 or less (only significant digits listed) |
48 bits (MAC address) |
| AppleTalk | 24 | 16 or less (indicates one or many in cable range) |
8 bits or less (cynamically assigned) |
Routing protocols support routed protocols. A routing protocol is used to pass messages between routers for maintaining and updating routing tables. Examples of routing protocols are RIP, IGRP, OSPF, EIGRP.
Routed protocols are used to carry end-user traffic across the internetwork. Examples of routed protocols are IP and IPX Read more »
The Data Link layer performs a number of separate activities, including:
Different data link layer specifications define different network and protocol characteristics, including physical addressing, network topology, error notification, sequencing of frames, and flow control.
Physical addressing, is not to be confused with network or IP addresing. The physical address defines how devices are labeled in the data link layer. This physical address is most commonly called the Media Access Control (MAC) address. The MAC address is a unique number assigned by the manufacturer. This numbering system is actually administered by one of the networking governing bodies.
The Physical Layer has two responsibilities; it sends bits and receives bits.
The Physical layer specifications specify the electrical, me
chanical, procedural, and functional requirements for activating, maintaining, and deactivating a physical link between end systems.
The physical layer is usually a combination of software and hardware programming and may include electromechanical devices.
All wireing, power, cabling and connections are part of the physical layer. Without the physical layer functioning properly none of the upper layers will respond correctly.
* transmits raw bit stream over physical cable
* defines cables, cards, and physical aspects
* defines NIC attachments to hardware, how cable is attached to NIC
* defines techniques to transfer bit stream to cable
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