OSI stands for Open Systems Interconnection. It has been developed by ISO – ‘International Organization of Standardization‘, in the year of 1974. It is 7 Physical layers in OSI model the layer architecture with each of layer having specific functionality to perform. All these 7 layers work collaboratively to transmit the data from one person to another across the globe.
The OSI model is the layered model and a conceptual standard used for defining the standards to promote multi-vendor integration as well as to maintain the constant interfaces and isolate changes of implementation to a single layer. It is NOT only an application or protocol specific. In order to pass any Cisco exam, you must have to know the OSI model inside and out.
Provides network access for applications, flow control and error recovery. Provides communications services to applications by identifying and establishing the availability of other computers as well as to determine if sufficient resources exist for communication purposes. * Device:- Gateway * Protocol:- NCP, SMB, SMTP, FTP, SNMP, Telnet, AppleTalk.
Performs protocol conversion, encryption and data compression. Device:- Gateway and redirectors. Protocol:- NCP, AFP, TDI.
Allows 2 applications to communicate over a network by opening a session and synchronizing the involved computers. Handles connection establishment, data transfer and connection release. *Device:- Gateway. *Protocol:- NetBIOS.
Repackages messages into smaller formats, provides error free delivery and error handling functions. Device:- Gateway. Protocol:- NetBEUI, TCP, SPX, and NWLink.
Handles addressing, translates logical addresses and names to physical addresses, routing and traffic management. Device:- Router and brouter. Protocol:- IP, IPX, NWLink, NetBEUI.
Packages raw bits into frames making it transmitable across a network link and includes a cyclical redundancy check(CRC). It consists of the LLC sublayer and the MAC sublayer. The MAC sublayer is important to remember, as it is responsible for appending the MAC address of the next hop to the frame header. On the contrary, LLC sublayer uses Destination Service Access Points and Source Service Access Points to create links for the MAC sublayers..
Device:- Switch, bridge and brouter. Protocol:- None.
Physical layer works with the physical media for transmitting and receiving data bits via certain encoding schemes. The lowest layer of the OSI Reference Model is layer is the physical layer; it is commonly abbreviated “PHY”. The physical layer is special compared to the other layers of the model, because it is the only one where data is physically moved across the network interface.
All of the other layers perform useful functions to create messages to be sent, but they must all be transmitted down the protocol stack to the physical layer, where they are actually sent out over the network.
It also includes specifications for certain mechanical connection features, such as the adaptor connector. Device: - Multiplexer and repeater. Protocol:- None.
Here is an simplest way to learn the order of the layers:
All People Seem To Need Data Processing. The first letter of each word corresponds to the first letter of one of the layers. It is a little corny, but it works.
The above table mentions the word "MAC Address". A MAC address is the 48 bit address which is usually used for uniquely identifying devices on the network. Like, 00-00-12-33-FA-BC, in this way we call it presenting the address in 12 hexadecimal digits format. The first 6 digits are to specify the manufacture, while the remainders are for the host itself. ARP Protocol is used for determine the IP to MAC mapping. And of course, MAC address cannot be duplicated in the network or problems will occur.
Now if we are discussing the OSI model it is important to keep in mind those differences between "Connection-oriented" and "Connectionless" communications. A connection oriented communication is having the following characteristics:
- A session is the guaranteed.
- A guaranteed rate of the throughput occupies resources without any flexibility of dynamic allocation.
- Phrases in the connection-oriented service involves the Call Setup, Data transfer and Call termination.
- Acknowledgements are issued and received at the transport layer, meaning if the sender does not receive an acknowledgement before the timer expires, the packet is retransmitted.
- Reliable = SLOW (this is always the case in networking).
In contrast, a connectionless communication has the following characteristics:
- Often used for voice and video applications.
- Dynamic bandwidth allocation.
- NO guarantee nor acknowledgement.
- Unreliable = FAST.
So this short summary on OSI model.
I wish that this would help you.