OSI and seven layers of OSI (open system interconnection) model

Q.2 seven layers of osi model.

A.2 OSI: The OSI, or Open System Interconnection, model defines a networking framework for implementing protocols in seven layers. Control is passed from one layer to the next, starting at the application layer in one station, proceeding to the bottom layer, over the channel to the next station and and back up the hierarchy. 

 

1. Physical Layer
   • Concerned with the transmission of bits.
   • How many volts for 0, how many for 1?
   • Number of bits of second to be transmitted.
   • Two way or one-way transmission
   • Standardized protocol dealing with electrical, mechanical and signaling interfaces.
   • Many standards have been developed, e.g. RS-232 (for serial communication lines).
   • Example: X.21

(2) Data Link Layer

• Handles errors in the physical layer.

• Groups bits into frames and ensures their correct delivery.
• Adds some bits at the beginning and end of each frame plus the checksum.
• Receiver verifies the checksum.
• If the checksum is not correct, it asks for retransmission. (send a control message).
• Consists of two sub layers:
  • Logical Link Control (LLC) defines how data is transferred over the cable and provides data link service to the higher layers.
  • Medium Access Control (MAC) defines who can use the network when multiple computers are trying to access it simultaneously (i.e. Token passing, Ethernet [CSMA/CD]).

(3) Network Layer

• Concerned with the transmission of packets.
• Choose the best path to send a packet ( routing ).
• It may be complex in a large network (e.g. Internet).
• Shortest (distance) route vs. route with least delay.
• Static (long term average) vs. dynamic (current load) routing.
• Two protocols are most widely used.
• X.25
  • Connection Oriented
  • Public networks, telephone, European PTT
  • Send a call request at the outset to the destination

If destination accepts the connection, it sends an connection identifier

• IP (Internet Protocol)
  • Connectionless
  • Part of Internet protocol suite.
  • An IP packet can be sent without a connection being established.
  • Each packet is routed to its destination independently.

(4) Transport Layer

• Network layer does not deal with lost messages.
• Transport layer ensures reliable service.
• Breaks the message (from sessions layer) into smaller packets, assigns sequence number and sends them.
• Reliable transport connections are built on top of X.25 or IP.
• In case IP, lost packets arriving out of order must be reordered.
• TCP : (Transport Control Protocol) Internet transport protocol.
• TCP/IP Widely used for network/transport layer (UNIX).
• UDP (Universal Datagram Protocol) : Internet connectionless transport layer protocol.
• Application programs that do not need connection-oriented protocol generally use UDP.

(5) Sessions Layer

• Just theory! Very few applications use it.
• Enhanced version of transport layer.
• Dialog control, synchronization facilities.
• Rarely supported (Internet suite does not).

(6) Presentation Layer

• Just theory! Very few applications use it.
• Concerned with the semantics of the bits.
• Define records and fields in them.
• Sender can tell the receiver of the format.
• Makes machines with different internal representations to communicate.
• If implemented, the best layer for cryptography.

(7) Application Layer

• Collection of miscellaneous protocols for high level applications
• Electronic mail, file transfer, connecting remote terminals, etc.
• E.g. SMTP, FTP, Telnet, HTTP, etc.