OSI LAYERS

          

(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 sublayers:
    • 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.