Unit-I Material:
I. Types of Computer Networks
II. Reference Models- The OSI Reference Model, The TCP/IP Reference Model
III. A Critique of the OSI Model and Protocols
IV. A Critique of the TCP/IP Reference Model
V. Internet History
I. Types of Computer Networks:
Classified by size, geographical coverage, and ownership:
Local Area Network (LAN): Privately owned, single office/building/campus
LANs range from simple (two PCs, printer) to complex (audio/video devices)
Each LAN host has a unique address for packet communication
Wide Area Network (WAN): Spans towns, states, countries, or globally
WANs connect devices like switches, routers, modems; often leased
Point-to-Point WAN: Connects two devices via cable or air
Switched WAN: Multiple point-to-point WANs linked by switches
Internetwork: Connects LANs and WANs to form a private internet {Switching: Circuit-switched (dedicated) or packet-switched (store-and-forward
WAN peer-to-peer Internetwork
II. Reference Models
(a) The OSI Reference Model:
* Developed by ISO for standardizing protocols (Open Systems Interconnection)
* Seven layers, each with a specific function
* Physical Layer: Transmits raw bits, handles signals, timing, connectors
* Data Link Layer: Ensures error-free transmission via frames, acknowledgments
* Data Link includes Medium Access Control (MAC) for shared channels
* Network Layer: Manages packet routing, congestion, heterogeneous network interconnects
* Transport Layer: End-to-end delivery, splits data, ensures correct arrival
* Session Layer: Manages sessions, dialog control, token management, synchronization
* Presentation Layer: Handles data syntax/semantics, standard encoding
* Application Layer: Supports user applications (e.g., HTTP, FTP, SMTP)
Example with GMAIL usage:
OSI Layer 7 – Application Layer: i/f b/w s/w and n/w; Login/Logout/ HTTP/HTTPs connection; Compose; Send;
OSI Layer 6 – Presentation Layer: encrypt contents (PDF/Word/ Images);
OSI Layer 5 – Session Layer: Active / Inactive sessions;
OSI Layer 4 – Transport Layer: Packet lost is retransmitted using TCP
OSI Layer 3 – Network Layer: Packets sent; routing b/w sc and dt IP addresses
OSI Layer 2 – Data Link Layer: device communication to routers, using MAC address; error detection and correction;
OSI Layer 1 – Physical Layer: Binary data transmitted as either electrical signals (using Ethernet) or as light (Optical Fiber) or RadioWaves (Wi-Fi)
(b)The TCP/IP Reference Model:
* Developed for ARPANET, focuses on robust internetworking
* Four layers: Link, Internet, Transport, Application
* Link Layer: Interfaces hosts with transmission links (e.g., Ethernet)
* Internet Layer: Uses IP protocol for packet delivery across networks; Internet Layer supports connectionless communication, routing, congestion
* Transport Layer: TCP (reliable, connection-oriented), UDP (unreliable, fast); TCP ensures error-free delivery; UDP prioritizes speed (e.g., video streaming)
* Application Layer: Includes protocols like HTTP, FTP, SMTP, DNS, RTP
* Designed to survive hardware failures for continuous communication
* No session or presentation layers, unlike OSI
GMAIL Example:
* Application Layer: runs over HTTPS R/W/send emails via a browser or app.
* Transport Layer: TCP to ensure emails and attachments are delivered reliably and in order.
* Internet Layer: IP assigns addresses to route Gmail data packets across the internet from sender GSRV (Google Servers)
* Link Layer: The data travels physically via Wi-Fi, Ethernet, or mobile networks to reach GSRV
III. Critique of the OSI Model and Protocols
The OSI model appeared after the TCP/IP protocol suite.
Most experts were at first excited and thought that the TCP/IP protocol would be fully replaced by the OSI
model.
(a) OSI was completed when TCP/IP was fully in place and a lot of time and money had been spent on the suite; changing it
would cost a lot.
(b) some layers in the OSI model were never fully defined. For example, although the services provided by the presentation and the session layers were listed in the document, actual protocols for these two layers were not fully defined, nor were they fully described, and the corresponding software was not fully developed.
(c) OSI was implemented by an organization in a different application --> did not show high performance --> they switched back to TCP/IP
Bad Timing: Standardized too late, after TCP/IP adoption in academia
Bad Technology: Complex model/protocols, seven layers (some underutilized)
Session and presentation layers have minimal functionality
Data Link and Network layers overloaded with addressing, error control
Bad Implementations: Initial OSI systems were slow, unwieldy
Complexity caused poor performance, harming OSI’s reputation
Bad Politics: Seen as government-driven, not researcher-friendly
OSI protocols not widely adopted, though model remains useful
Convergence sublayers added to fix mismatches with real networks
Limited focus on internetworking, assumed single national networks
IV. Critique of the TCP/IP Reference Model
Does not distinguish services, interfaces, and protocols clearly
Model is TCP/IP-specific, not generalizable (e.g., unsuitable for Bluetooth)
Link Layer: More an interface than a true layer, lacks detail
Internet Layer: Effective for IP delivery, weak in congestion control
Transport Layer: Offers TCP (reliable) or UDP (fast), unlike OSI’s single mode
Model built after protocols, ensuring fit but limiting flexibility
Lacks session/presentation layers, leaving functions to applications
Strong in practice due to robust protocols (e.g., TCP, IP, HTTP)
Benefited from high-quality Berkeley UNIX implementation
Large user community drove improvements and adoption
V. Internet History
Pre-1960s Networks: Telegraph and telephone networks supported constant-rate communication, inadequate for bursty computer data.
1961 Packet Switching Theory: Leonard Kleinrock (MIT), Paul Baran (Rand Institute), and Donald Davies (NPL) introduced packet-switched network concepts for bursty traffic.
1967 ARPANET Proposal: ARPA proposed ARPANET at an ACM meeting, connecting diverse computers via Interface Message Processors (IMPs).
1969 ARPANET Launch: Four nodes (UCLA, UCSB, SRI, University of Utah) connected using IMPs and Network Control Protocol (NCP).
1972 Internetting Project: Vint Cerf and Bob Kahn worked on linking dissimilar networks, introducing gateways to manage diverse packet sizes and rates.
1973 TCP/IP Introduction: Cerf and Kahn’s paper outlined Transmission Control Protocol (TCP), including encapsulation, datagrams, and gateway functions.
1977 Internet Demonstration: ARPANET, packet radio, and packet satellite networks interconnected, demonstrating communication across diverse networks.
1981 TCP/IP in UNIX: UC Berkeley embedded TCP/IP in UNIX under a DoD contract, enabling widespread adoption across manufacturers.
1983 ARPANET Split: ARPANET divided into MILNET (military) and ARPANET (nonmilitary), with TCP/IP as the official protocol.
1981 CSNET Creation: National Science Foundation funded CSNET for universities without DoD ties, offering a cost-effective, slower network.
1986 NSFNET Backbone: NSFNET connected five U.S. supercomputer centers with a 1.544-Mbps T-1 line, replacing ARPANET by 1990.
1991 ANSNET Formation: IBM, Merit, and Verizon formed ANS to create ANSNET, a high-speed backbone to handle growing Internet traffic.
1990s World Wide Web: Tim Berners-Lee at CERN invented the WWW, adding commercial applications to the Internet.
Multimedia Growth: Applications like VoIP, video over IP (Skype), and video sharing (YouTube) increased Internet usage and time spent online.
Peer-to-Peer Applications: Emerging peer-to-peer networking introduced new communication models with significant potential
.png)
.png)
.png)
.png)
.png)
.png)
