Course Objectives

• Understand how two computers communicate over the Internet.
• Learn basic functionalities of computer networks.
• Gain knowledge of network programming.
• Explore the evolution and future design needs of computer networks.

Network Architecture

• Definition: The conceptual design that defines the structure and operation of a network, visualizing how two remote computers communicate.
• Requirement: Conversion of digital data to analog signals and vice versa for communication.
• End-to-End Traffic Control: Ensures reliable data transfer across the entire network, not just individual links.

Network Protocol Stack

• Layered Approach: Communication tasks are divided into layers, each responsible for specific functions.
• Key Layers (with examples):
  • Application: HTTP, FTP, SMTP
  • Transport: TCP, UDP, RTP
  • Network: IPv4, IPv6, MPLS
  • Data Link: Ethernet, WiFi, Bluetooth, UMTS, LTE
  • Physical: Transmission of raw bits over a medium

Protocols

• Definition: A controlled sequence of messages exchanged between systems to accomplish a task.
• Components: Specifies message format, sequence, and rules for communication.
• Examples: HTTP (web), FTP (file transfer), SMTP (email), TCP (reliable transport), UDP (unreliable transport).

OSI Model (Open Systems Interconnection)

• Comparison with TCP/IP:
  • TCP/IP combines Application, Presentation, and Session into one layer.
  • Both have Transport, Network, Data Link, and Physical layers.

TCP/IP Protocol Stack

• Application Layer: User processes (e.g., Telnet, FTP, SMTP, HTTP). Uses port numbers and sockets for communication.
• Transport Layer: End-to-end data transfer. Main protocols:
  • TCP: Connection-oriented, reliable, congestion and flow control.
  • UDP: Connectionless, unreliable, faster, used where some data loss is tolerable.
• Network (Internetwork) Layer: Provides a virtual network, main protocol is IP (connectionless, no error recovery). Handles routing.
• Network Interface (Link) Layer: Interface to physical network hardware (e.g., Ethernet, ATM). May or may not provide reliable delivery.

Encapsulation

• Concept: Each layer adds its own header (and sometimes trailer) to the data from the layer above.
• Order: Application Data → Transport Header → Network Header → Data Link Header → Physical Transmission

Network Devices and Their Functions

• Broadcast Domain: All devices that receive all network broadcasts on a segment.
• Collision Domain: Network area where data packets can collide (hubs = 1 domain, switches = 1 per port).

LAN and WAN

• LAN (Local Area Network): Covers small geographic area (e.g., office). Devices: NICs, repeaters, hubs, bridges, switches, routers.
• WAN (Wide Area Network): Covers large areas, connects multiple sites, often uses leased lines.

Hierarchical Design Model

• Access Layer: End-user connectivity.
• Distribution Layer: Policy-based routing, access control.
• Core Layer: Fast packet switching across the internetwork.

Switching Technologies

• Circuit Switching:
  • Dedicated path between two stations.
  • Phases: Establish, Transfer, Disconnect.
  • Channel reserved for duration, inefficient for data, best for voice.
  • Approaches: Space-Division, Time-Division, TDM Bus.
• Packet Switching:
  • Data split into packets, each routed independently.
  • Two types:
    • Datagram: Each packet independent, may take different routes, may arrive out of order.
    • Virtual Circuit: Preplanned route, all packets follow same path, connection-oriented.
  • Advantages: Efficient line use, data rate conversion, can handle busy networks, priorities possible.

Comparison Table: Circuit vs. Packet Switching

Protocols – Key Elements

• Syntax: Data formats, signal levels.
• Semantics: Control information, error handling.
• Timing: Speed matching, sequencing.

Protocol Interfaces

• Service Interface: Operations provided to the layer above.
• Peer Interface: Messages exchanged with the same layer on another system.

Internet Architecture

• Defined by IETF (Internet Engineering Task Force).
• Hourglass Model: IP at the narrow waist, many protocols above and below.
• Flexibility: Applications can bypass some layers if needed.

Network Application Programming Interface (API)

• Socket API: Standard interface for applications to communicate over the network.
• Server-Client Model: Applications exchange messages using sockets.

Application Layer Protocols (Examples and Functions)

• DNS (Domain Name System): Translates domain names to IP addresses. Examples of TLDs: .com, .in, .edu, .gov, .org, .net.
• FTP (File Transfer Protocol): Reliable, uses TCP, transfers files.
• TFTP (Trivial FTP): Simpler, uses UDP, often for device configuration.
• HTTP (Hypertext Transfer Protocol): Used for web communication.
• SMTP (Simple Mail Transfer Protocol): Used for email transfer between servers.
• SNMP (Simple Network Management Protocol): Manages network devices.
• Telnet: Remote login to another machine.

Responsibilities of Application Layer

• Identifying and establishing communication partners.
• Synchronizing applications.
• Agreeing on error recovery procedures.
• Controlling data integrity.

History of Computer Networks & Internet (Key Milestones)

Key Definitions & Concepts

• MAC Address: Unique identifier for network interfaces at Data Link Layer.
• Broadcast Domain: Network segment where all devices receive broadcast frames.
• Collision Domain: Network segment where data packets can collide (important in Ethernet).
• Encapsulation: Wrapping data with protocol-specific headers/trailers as it moves down the stack.
• Datagram: Independent packet, may take any route.
• Virtual Circuit: Predefined path for all packets in a session.

Summary Table: Devices and OSI Layers

Tips for Exam Preparation

• Understand the function and structure of each protocol layer.
• Be able to compare and contrast circuit switching and packet switching.
• Know the roles and differences between network devices (NIC, hub, switch, router).
• Memorize key milestones in the history of networks and the Internet.
• Be clear on protocol examples for each layer and their main functions.
• Practice explaining encapsulation and the flow of data through the stack.
• Review the responsibilities and examples of application layer protocols.

With these notes, you can confidently answer any question from Week 1 of the NPTEL CNIP course without needing to refer back to the PDF.