OSI Layers

OSI Layers

Layer Name
Function
Example
1.Application
(Layer 7)
(Associated word: Shared)
  1. Provides communication services to various applications e.g. word processor
  2. Deals with user interface
  3. Provides n/w services to the user e.g. email, file & print services, terminal emulation, login validation, file format translation.
File Transfer-TFTP, FTP, NFS. E-mail- SMTP
Remote Login-Telnet, r log in.
Network Management-SNMP.
2.Presentation
(Layer 6)
(Associated word:
Format)
  1. Translation of data
  2. Define data format, presentation
  3. Define encryption e.g. FTP enables us to choose binary or ASCII transfer
JPEG, ASCII*, EBCDIC, TIFF, GIFF, PICT, MPEG,
MIDI
3.Session
(Layer 5)
(Associated word:
Dialog)
  1. Keeping different application data separate.
  2. Define-how to start, how to control and how to end conversation (session) b/w presentation layer entities.
  3. Includes-control and management of multiple bi-directional messages.
  4. Coordinates-communication b/w systems and serves to organize their communication by offering three different modes: -simplex, half-duplex, and full duplex.
  5. Allows-presentation layer to have seamless view of incoming stream of data.

Basic Function: -
ü  Connection Establishment
ü  Connection Maintenance
ü  Connection teardown
RPC, SQL, NFS, Net BIOS names, SCP
e.g.ATM machines do not teardown sessions unless all the activities are performed.
4. Transport
(Layer4)
(Associated word: Reliability)
  1. Segmentation of data stream
  2. Provide-end to end data transport/delivery services
  3. Establish-logical connection b/w sending host and receiving host.
  4. Provide –choice of protocol that do or do not provide error recovery, multiplexing of data, session establishment, teardown of VCs
  5. Reordering- of incoming data stream, when out of packets are arrived.

Basic Functions: -
ü  Flow control
ü  Connection oriented communication
ü  Windowing
ü  Acknowledgement
ü  Sequencing
TCP, UDP, SPX
5.Network
(Layer 3)
(Associated word:
Datagram or Segment)
  1. Segmentation of contention networks
  2. Define: -end to end delivery of packets, logical addressing, how routing works, how to fragment a packet in to smaller packets to accommodate media with smaller MTU.
IP, IPX, Appletalk
6.Datalink
(Layer 2)
(Associated word:
Frame)
  1. Define – delivery across a one particular link
  2. Organize the streams of bits sent by physical layer into logical groups called frames.
  3. Adds up control information (SA, DA, frame length, information about upper layer protocol etc.) at the upper end called frame header.
  4. Gives following details to the traffic: -where to go, what to do on reaching destination
Basic Functions:
ü  Combines bits in to bytes and bytes into frame.
ü  Access to the media using MAC address.
ü  Error detection and error recovery
IEEE-802.2, 802.3,802.5,
HDLC, FR, PPP, FDDI

7.Physical
(Layer 1)
(Associated word:
Bits)
  1. Defines –physical characteristic (media type, connector type, signaling type) of the transmission media.
  2. Specifies- electrical, mechanical, procedural and functional requirements for activating, maintaining and deactivating the physical link between end systems.
  3. Other physical characteristics: voltage level, data rates, MTU and physical connectors.
EIA/TIA-232, V.35, EIA/TIA-449, V.24, RJ45, IEEE-802.3, 802.5,
FDDI

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