Sunday, March 22, 2009

The OSI Reference Model

Connecting open systems that are open for communication with other system.
Seven layers.
  • Physical layer:
Transmit raw bits from one system to another.
Amount of volts needed to represent 0 or 1.
Type of transmission medium whether Guided or Unguided
Guided(copper wire, fiber optics) and Unguided(laser or radio waves).
  • Data link layer
Accepts the raw bits which are received from the physical layer.Divides the sequence of bit stream received in parts called as frames and recompute the check sum for checking errors.for detecting errors it uses the CRC or Polynomial is used and for error correcting hamming distance is used.
it also performs flow control.
Node to Node transmission takes place.
ARP(Address Resolution Protocol),RARP(Reverse Address Resolution Protocol),
Frame relay(x.25),ISDN,PPP,LLC etc.
  • Network layer
This layer deals with the end to end transmission.
This layer mainly adds source and destination address in the header.
Route the data to different LANs and WANs depending on network Address.
Example : IP,ICMP.

  • Transport layer
This provides efficient reliable and cost effective service to the users.
This ensures the transmission of data to the correct process.
TCP and UDP packets comes under this.
  • Session Layer
This layers allows users on different machines to establish session between them.
This checks two stations do not attempt the same operation at same time.
Critical operation functionality also present here(synchronization).
Starts and stops session and maintains order.
  • Presentation layer
This layer deals with the syntax and semantics of the information to be transmitted.
It encode data in a format so that receiver can easily understand.
This layer also deals with the encryption and compression of the data.
eg: ASCII to EBCDID , Binary to BCD.
  • Application layer
Deals with the protocols that are need for the transfer file,mail etc.
Examples are : SMPT/IMAP4(E-MAIL),Telenet,HTTP(WWW),FTP(File Transfer),
RIP(Route Information Protocol),OSPF(Open Shortest Path First),TFTP (Trivial File Transfer Protocol),DHCP(Dynamic Host Configuration Protocol),BOOTP(Bootstrap protocol).

Names of various PDU in each layer.
Application Protocol: APDU
Presentation Protocol:PPDU
Session protocol:SPDU
Transport Protocol:TPDU
Network : Packet
Datalink: frame
Physical: bit

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