CCNA
Cisco Certified Network Associate
OSI Layer
1. Application Layer
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 transfers-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.
Eg: WWW Browser, FTP, HTTP, Telnet,
Nfs, Snmp
2. Presentation layer
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.
Eg: encryption, ASCII,
EBCDIC, TIFF, GIF, PICT, JPEG, MPEG, MIDI.
3. Session Layer
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.
Eg: NFS, NetBios names, RPC, SQL.
4. Transport Layer
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.
Eg: SPX, TCP, UDP.
5. Network Layer
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.
Eg: AppleTalk DDP, IP, IPX.
6. Data link layer
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.
Eg: PPP, FDDI, ATM, IEEE
802.5/ 802.2, IEEE 802.3/802.2, HDLC, Frame Relay.
7. Physical layer
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.
Eg: Ethernet, FDDI, B8ZS, V.35,
V.24, RJ45
