(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.