OSI MODEL

Application (L7):-

• The Application layer provides services to the software through which the user requests network services. Your computer application software is not on the Application layer. This layer isn't about applications and doesn't contain any applications. In other words, programs such as Microsoft Word or Corel are not at this layer.

• The Application layer provides network services directly to the user's application such as a web browser, email software and Windows Explorer. This layer is said to be "closest to the user".

• Protocols that operate on this layer include: DNS, TELNET, HTTP, FTP, TFTP, SMTP, NTP, SNMP, EDI,

• Firewalls etc.

• SMTP and Pop3 are email protocols.

• In short, GUI (Graphical User Interface) is used at this Layer...

Presentation Layer (L6):-

• This layer is concerned with data representation and code formatting.

• Compatibility with the OS

• Proper encapsulation of data for network transmission

• Data formatting (ASCII, Binary)
• Data encryption, compression and translation.

Encapsulation:- When referring to networking, encapsulation is the process of taking data from one protocol and translating it into another protocol, so the data can continue across a network.

Encryption:-  Encryption is the coding or scrambling of information so that it can only be decoded and read by someone who has the correct decoding key. Encryption is used in secure Web sites as well as other mediums of data transfer.

Decryption:- The process of decoding data that has been encrypted into a secret format. Decryption requires a secret key or password.

Translation:-  Data translation is the process of converting data into another form of comprehensible, logical language and code according to a set of predetermined rules and patterns. Data translation is done to give meanings to the data set without interpreting the data.

Session Layer (L5):-

• The Session layer establishes, maintains, teardowns and manages the communication session between computers.

• The overall communication itself is referred to as a session.

Transport Layer (L4):-

• The transport layer ensures that messages are delivered error-free, in sequence, and with no losses or duplications. It relieves the higher layer protocols from any concern with the transfer of data between them and their peers.

• The size and complexity of a transport protocol depends on the type of service it can get from the network layer.

• It includes:-

• Message Segmentation

• Message acknowledgement

• Message traffic control

• Session multiplexing

Message Segmentation: accepts a message from the (session) layer above it, splits the message into smaller units (if not already small enough), and passes the smaller units down to the network layer. The transport layer at the destination station reassembles the message.

Message acknowledgement: provides reliable end-to-end message delivery with acknowledgments.

Message traffic control: tells the transmitting station to "back-off" when no message buffers are available.

Session multiplexing: multiplexes several message streams, or sessions onto one logical link and keeps track of which messages belong to which sessions.

Network Layer (L3):-

• The network layer controls the operation of the subnet, deciding which physical path the data should take based on network conditions, priority of service, and other factors.

• It includes:-

• Routing
• Subnet traffic control

• Frame fragmentation

• Logical-physical address mapping

• Subnet usage accounting

Routing: routes frames among networks.

Subnet traffic control: routers (network layer intermediate systems) can instruct a sending station to "throttle back" its frame transmission when the router's buffer fills up

Frame fragmentation: if it determines that a downstream router's maximum transmission unit (MTU) size is less than the frame size, a router can fragment a frame for transmission and re-assembly at the destination station.

Logical-physical address mapping: translates logical addresses, or names, into physical addresses.

Subnet usage accounting: has accounting functions to keep track of frames forwarded by subnet intermediate systems, to produce billing information.

DATA LINK LAYER (L2):-

• 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. To do this, the data link layer provides:

• Link establishment and termination

• Frame traffic control

• Frame sequencing

• Frame acknowledgment

• Frame delimiting

• Frame error checking

• Media access management

• MAC addresses deal here

Link establishment and termination: establishes and terminates the logical link between two nodes.

Frame traffic control: tells the transmitting node to "back-off" when no frame buffers are available.

Frame sequencing: transmits/receives frames sequentially.

Frame acknowledgment: provides/expects frame acknowledgments. Detects and recovers from errors that occur in the physical layer by retransmitting non-acknowledged frames and handling duplicate frame receipt.

Frame delimiting: creates and recognizes frame boundaries.

Frame error checking: checks received frames for integrity.

Media access management: determines when the node "has the right" to use the physical medium.

PHYSICAL LAYER (L1):-

• 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 describes the electrical/optical, mechanical, and functional interfaces to the physical medium, and carries the signals for all of the higher layers.
• It provides:-

• Data encoding
• Physical medium attachment, accommodating various possibilities in the medium

• Transmission technique

• Physical medium transmission

Data encoding: modifies the simple digital signal pattern (1s and 0s) used by the PC to better accommodate the characteristics of the physical medium, and to aid in bit and frame synchronization. It determines:-

• What signal state represents a binary 1

• How the receiving station knows when a "bit-time" starts

• How the receiving station delimits a frame

Physical medium attachment, accommodating various possibilities in the medium:

• Will an external transceiver (MAU) be used to connect to the medium?
• How many pins do the connectors have and what is each pin used for?

Transmission technique: determines whether the encoded bits will be transmitted by baseband (digital) or broadband (analog) signaling.

Physical medium transmission: transmits bits as electrical or optical signals appropriate for the physical medium, and determines:

• What physical medium options can be used

• How many volts/db should be used to represent a given signal state, using a given physical medium
•  
•