One Comment on “which of the following OSI model layers?”

1. Michael McNeil says:

   June 18, 2014 at 1:16 am

   APPLICATION

   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: TELNET, HTTP, FTP, TFTP, SMTP, NTP, SNMP, EDI.

   PRESENTATION

   This layer ‘represents’ the data in a particular format to the Application layer. It defines encryption, compression, conversion and other coding functions.
   Specifications defined at this layer include: GIF, TIFF, JPEG, MPEG, MIME, and ASCII.

   SESSION

   Establishes, maintains and terminates end-to-end connections (sessions) between two applications on two network nodes. It controls the dialogue between the source and destination node, which node can send when and how long. Also provides error reporting for the Application, Presentation and Session layer.
   Protocols/API’s that operate on this layer include: RPC, SQL, NETBIOS.

   TRANSPORT

   This layer converts the data received from the upper layers into segments. The Transport layer is responsible for end-to-end (also called source-to-destination) delivery of entire messages. Provides end-to-end connectivity, it allows data to be transferred reliably and sequencing to guarantee that it will be delivered in the same order that it was sent. Provides services such as error checking and flow control (software).
   Protocols that operate on this layer: TCP, UDP, NETBEUI, SPX.

   These protocols are either connectionless or connection-oriented:

   Connection-oriented means that a connection (a virtual link) must be established before data can be exchanged. This can guarantee that data will arrive, and in the same order it was sent. It guarantees delivery by sending acknowledgements back to the source when messages are received. TCP is an example of an connection-oriented transport protocol.

   A common example of connection-oriented communication is a telephone call: you call, the ‘destination’ picks up the phone and acknowledges and you start talking (sending data). When a message or a piece of it doesn’t arrive, you say: “What!?” and the sender will retransmit the data.

   Connectionless is the opposite of connection-oriented; the sender does not establish a connection before it sends data, it just sends without guaranteeing delivery. UDP is an example of an connectionless transport protocol.

   NETWORK

   This layer converts the segments from the Transport layer into packets (or datagrams) and is responsible for path determination, routing, and the delivery of these individual packets across multiple networks without guaranteed delivery. The network layer treats these packets independently, without recognizing any relationship between those packets, it relies on upper layers for reliable delivery and sequencing.
   Also this layer is is responsible for logical addressing (also known as network addressing or Layer 3 addressing) for example IP addresses
   Examples of protocols defined at this layer: IP, IPX, AppleTalk, ICMP, RIP, OSPF, BGP, IGRP, EIGRP, NLSP, ARP, RARP, X.25
   Devices that operate on this layer: Routers, Layer 3 Switches.

   Network layer addresses
   Also known as Layer 3 or Logical addresses. These type of addresses are protocol-dependent, for example if the network protocol is IP, IP addressing will be used which is made up of a network part and a host part and needs a subnet mask to determine the boundaries of these parts. An example of an IP address is: 172.16.0.1 and a subnet mask: 255.255.0.0
   Another example is Novell’s IPX addressing, which uses a combination of a hexadecimal network address + the layer 2 MAC address to form a network layer address, for example” 46.0010E342A8BC

   DATA LINK

   The Data Links provides transparent network services to the Network layer so the Network layer can be ignorant about the physical network topology and and provides access to the physical networking media. Responsible for reassambling bits taken of the wire by the Physical layer to frames, makes sure they are in the correct order and requests retransmission of frames in case an error occurs. Provides error checking by adding a CRC to the frame, and flow control. Examples of devices that operate on this layer are switches, bridges, WAPs, and NICs.

   
   
   
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