Explain about OSI reference model.
The OSI, or Open System Interconnection, model
defines a networking framework to implement protocols in seven layers. Control
is passed from one layer to the next, starting at the application layer in one
station, and proceeding to the bottom layer, over the channel to the next
station and back up the hierarchy.
The OSI model takes the task of internetworking
and divides that up into what is referred to as a vertical stack that consists
of the following layers:
Physical (Layer 1)
This layer conveys the bit stream - electrical
impulse, light or radio signal -- through the network at the electrical and
mechanical level. It provides the hardware means of sending and receiving data
on a carrier, including defining cables, cards and physical aspects. Fast
Ethernet, RS232, and ATM are protocols with physical layer components.
Layer 1 Physical examples include Ethernet,
RJ45.
Data Link (Layer 2)
At this layer, data packets are encoded and
decoded into bits. It furnishes transmission protocol knowledge and management
and handles errors in the physical layer, flow control and frame
synchronization. The data link layer is divided into two sub layers: The Media
Access Control (MAC) layer and the Logical Link Control (LLC) layer. The MAC
sub layer controls how a computer on the network gains access to the data and
permission to transmit it. The LLC layer controls frame synchronization, flow
control and error checking.
- Layer 2 Data Link examples include PPP, ATM,
IEEE, Frame Relay,
Network (Layer 3)
This layer provides switching and routing
technologies, creating logical paths, known as virtual circuits, for
transmitting data from node to node. Routing and forwarding are functions of
this layer, as well as addressing, internetworking, error handling, congestion
control and packet sequencing.
- Layer 3 Network examples include AppleTalk
DDP, IP, IPX.
Transport (Layer 4)
This layer provides transparent transfer of
data between end systems, or hosts, and is responsible for end-to-end error
recovery and flow control. It ensures complete data transfer.
- Layer 4 Transport examples include SPX, TCP,
UDP.
Session (Layer 5)
This layer establishes, manages and terminates
connections between applications. The session layer sets up, coordinates, and
terminates conversations, exchanges, and dialogues between the applications at
each end. It deals with session and connection coordination.
- Layer 5 Session examples include NFS, NetBios
names, RPC, SQL.
Presentation (Layer 6)
This layer provides independence from
differences in data representation (e.g., encryption) by translating from
application to network format, and vice versa. The presentation layer works to
transform data into the form that the application layer can accept. This layer
formats and encrypts data to be sent across a network, providing freedom from
compatibility problems. It is sometimes called the syntax layer.
- Layer 6 Presentation examples include
encryption, ASCII, EBCDIC, TIFF, GIF, PICT, JPEG, MPEG, MIDI.
Application (Layer 7)
This layer supports application and end-user
processes. Communication partners are identified, quality of service is
identified, user authentication and privacy are considered, and any constraints
on data syntax are identified. Everything at this layer is
application-specific. This layer provides application services for file
transfers, e-mail, and other network software services. Telnet and FTP are
applications that exist entirely in the application level. Tiered application
architectures are part of this layer.
- Layer 7 Application examples include WWW
browsers, NFS, SNMP, Telnet, HTTP, FTP.
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