The Open System Interconnection (OSI) model defines a networking framework to implement protocols in seven layers. You must at first understand that OSI model is not tangible rather it is conceptual.
LAYER 1- PHYSICAL LAYER
The physical layer, the lowest layer of the OSI model, is concerned with
the transmission and reception of the unstructured raw bit stream over a
physical medium. It provides the hardware means of sending and
receiving data on a carrier network.
Networking Device – Hub, Network Interface Card (NIC), repeater, gateway
Protocol Data Unit - Bit
Some Protocols - Ethernet
The physical layer of the network focuses on hardware elements, such as
cables, repeaters, and network interface cards. By far the most common
protocol used at the physical layer is Ethernet. For example, an
Ethernet network (such as 10BaseT or 100BaseTX) specifies the type of
cables that can be used, the optimal topology (star vs. bus, etc.), the
maximum length of cables, etc.
LAYER 2 – DATA LINK LAYER
When obtaining data from the Physical layer, the Data Link layer checks
for physical transmission errors and packages bits into data "frames". The
data link layer provides error-free transfer of data frames from one
node to another over the physical layer, allowing layers above it to
assume virtually error-free transmission over the link.
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.
Networking Device – Bridge, Ethernet Switches and multi layer switches, proxy server, gateway
Protocol Data Unit - Frame
Some Protocols – Ethernet, Point to Point Protocol (PPP)
LAYER 3 – NETWORK LAYER
The network layer controls the operation of deciding which physical path
the data should take based on network conditions, priority of service,
and other factors. When data arrives at the Network layer, the source
and destination addresses contained inside each frame are examined to
determine if the data has reached its final destination. If the data
has reached the final destination, then network layer formats the data
into packets delivered up to the Transport layer. Otherwise, the Network
layer updates the destination address and pushes the frame back down to
the lower layers.
Networking Device – Router, multi layer switches, gateway, proxy server
Protocol Data Unit - Packets
Some Protocols – Address Resolution Protocol (ARP), IPv4/IPv6, Internet Protocol, Routing Information Protocol (RIP), IPX.
LAYER 4 – TRANSPORT LAYER
The Transport Layer provides transparent transfer of data between end
systems, or hosts, and is responsible for end-to-end error recovery and
flow control. It relieves the higher layer protocols from any concern with the transfer of data between them and their peers.
The transport layer controls the reliability of communications through
flow control, segmentation, and error control. Two great examples of
transport protocols are TCP (as in TCP/IP) and UDP.
Networking Device – proxy server, gateway
Protocol Data Unit – Segments for TCP, Datagram for UDP
Some Protocols – SPX, TCP
TCP, paired with IP, is by far the most popular protocol at the
transport level. If the IPX protocol is used at the network layer, then
it is paired with SPX at the transport layer.
LAYER 5 – SESSION LAYER
The session layer sets up, coordinates and terminates conversations. Services
include authentication and re-connection after an interruption. It
allows session establishment between processes running on different
stations.
Networking Device – gateway, Logical Ports
Protocol Data Unit – Data/Session
Some Protocols – AppleTalk Data Stream Protocol, Remote Procedure Call Protocol (RPC)
LAYER 6 – PRESENTATION LAYER
As the sixth layer of the OSI model, the presentation layer is primarily responsible for managing two networking characteristics: protocol and architecture.
Whereas, protocol defines a standard set of guidelines under which the
network operates, the network's architecture determines what protocol
applies. Encryption is typically done at this level too.
Networking Device – gateway
Protocol Data Unit – Data/ Encoded User Data
Some Protocols – Musical instrument digital interface (MIDI), Moving picture experts group (MPEG)
LAYER 7 – APPLICATION LAYER
The application layer serves as the window for users and application
processes to access network services. 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.
Networking Device – gateway
Protocol Data Unit – Data
Some Protocols – DNS, FTP, SMTP, POP3, IMAP, Telnet, HTTP
Takeaways from Study Notes
- Layer 7: Application layer - provides access to available network resources
- Layer 6: Presentation layer - translates, encrypts, and compresses data
- Layer 5: Session layer - establishes, manages, and terminates communicative sessions
- Layer 4: Transport layer - provides reliable process-to-process message delivery and error recovery
- Layer 3: Network layer - moves packets from source to destination providing inter networking capabilities
- Layer 2: Data link layer - organizes bits into frames providing node-to-node delivery
- Layer 1: Physical layer - transmits bits over a medium establishing mechanical and electrical specifications
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