UGC NET Computer Science Data Communication and Computer Networks Syllabus

Differentiate LAN, MAN, and WAN by their geographic scope.

LAN (Local Area Network) covers a building/campus; MAN (Metropolitan Area Network) covers a city; WAN (Wide Area Network) spans large areas like countries or continents (e.g., the Internet).

List the seven layers of the OSI reference model from bottom to top.

Physical, Data Link, Network, Transport, Session, Presentation, Application.

Which OSI layers handle routing, reliable end-to-end delivery, and framing respectively?

Network layer handles routing/logical addressing; Transport layer handles reliable end-to-end (process-to-process) delivery; Data Link layer handles framing and node-to-node delivery.

What is the function of the OSI Presentation layer?

It handles translation (data format conversion), encryption/decryption, and compression so that data is presented in a form usable by the Application layer regardless of system differences.

Name the four layers of the TCP/IP protocol suite and a key protocol of each.

Application (HTTP, DNS, SMTP), Transport (TCP, UDP), Internet/Network (IP, ICMP, ARP), and Network Access/Link (Ethernet). (Some models split Link into Physical + Data Link.)

Identify physical, logical, port, and specific addresses with the layer each belongs to.

Physical (MAC) address — Data Link layer; Logical (IP) address — Network layer; Port address — Transport layer; Specific (application/user) address such as an email address — Application layer.

Compare circuit switching, packet switching, and message switching.

Circuit switching reserves a dedicated path for the whole session (e.g., telephone). Packet switching breaks data into packets routed independently (datagram) or over a virtual circuit. Message switching forwards a whole message store-and-forward without a dedicated path.

Differentiate datagram and virtual-circuit approaches in packet switching.

Datagram: each packet is routed independently and may follow different paths and arrive out of order (connectionless). Virtual circuit: a path is set up first and all packets follow it in order (connection-oriented).

What problem does framing solve, and name two byte/bit framing techniques?

Framing separates a continuous bit stream into distinct data units (frames). Techniques: byte stuffing (flag bytes with escape characters) and bit stuffing (insert a 0 after five consecutive 1s within data).

Compare Stop-and-Wait and Sliding Window flow control protocols.

Stop-and-Wait sends one frame then waits for its ACK (low efficiency, especially over long links). Sliding Window allows multiple unacknowledged frames (up to a window size) in transit, greatly improving channel utilization.

For a Go-Back-N protocol with window size W, what is the sender window size limit in terms of sequence-number bits m?

The maximum sender window size is 2^m − 1 (Go-Back-N), whereas Selective Repeat requires window size ≤ 2^(m−1), where m is the number of bits used for the sequence number.

What is HDLC and what are its three frame types?

HDLC (High-Level Data Link Control) is a bit-oriented data link protocol. Its three frame types are I-frames (Information, carry user data), S-frames (Supervisory, for flow/error control), and U-frames (Unnumbered, for link management).

Differentiate CSMA/CD and CSMA/CA.

CSMA/CD (Collision Detection) is used in wired Ethernet—stations detect collisions during transmission and back off. CSMA/CA (Collision Avoidance) is used in wireless (Wi-Fi)—stations try to avoid collisions using inter-frame spacing, RTS/CTS, and random backoff since collisions can't be reliably detected.

Name the three controlled-access protocols for medium access.

Reservation (stations reserve slots in advance), Polling (a primary station invites secondaries to send), and Token Passing (a token circulates and only its holder may transmit).

Compare FDMA, TDMA, and CDMA channelization techniques.

FDMA assigns each user a separate frequency band; TDMA assigns each user a separate time slot on a shared frequency; CDMA lets all users share the same frequency and time, separated by unique orthogonal codes.

State the operating layer of a repeater, hub, bridge, switch, router, and gateway.

Repeater and hub — Physical layer (Layer 1); bridge and switch — Data Link layer (Layer 2); router — Network layer (Layer 3); gateway — operates across all layers up to the Application layer.

What is the difference between a hub and a switch?

A hub is a Layer-1 device that broadcasts incoming data to all ports (shared collision domain, half-duplex). A switch is a Layer-2 device that learns MAC addresses and forwards frames only to the destination port (separate collision domains, full-duplex).

What is a VLAN and its main benefit?

A VLAN (Virtual LAN) is a logical grouping of devices into separate broadcast domains independent of physical location/switch. Benefits: improved security, reduced broadcast traffic, and flexible administration without re-cabling.

What is a backbone network?

A backbone is a high-capacity network that interconnects multiple LANs or network segments, acting as the central transmission path; common types are bus backbones and star backbones.

List the IPv4 classful address ranges (Classes A, B, C) by first octet.

Class A: 0–127 (default mask /8). Class B: 128–191 (/16). Class C: 192–223 (/24). Class D: 224–239 (multicast). Class E: 240–255 (reserved/experimental).

What is classless (CIDR) addressing and how is a block notated?

CIDR (Classless Inter-Domain Routing) removes fixed class boundaries and uses a prefix length to define the network portion, written as address/n (e.g., 192.168.1.0/26). The block size = 2^(32−n) addresses.

In IPv4, what is fragmentation and which header fields control it?

Fragmentation splits a datagram larger than a link's MTU into smaller fragments. Controlled by the Identification field (same for all fragments), the Flags field (DF/MF bits), and the Fragment Offset field (position in units of 8 bytes).

How is the IPv4 header checksum computed and what does it cover?

The header is divided into 16-bit words, summed using one's-complement arithmetic, and the one's complement of the sum is stored as the checksum. It covers only the header (not the data) and is recomputed at each router because TTL changes.

State key facts about the IPv6 packet format: address size, header size, and a notable change from IPv4.

IPv6 uses 128-bit addresses and a fixed 40-byte base header. Unlike IPv4 it has no header checksum and no fragmentation by routers (only source fragments); optional information is carried in extension headers.

What is the purpose of ARP (Address Resolution Protocol)?

ARP maps a known logical (IP) address to its physical (MAC) address on a local network by broadcasting an ARP request; the target host replies with its MAC address (unicast).

Differentiate direct and indirect delivery at the network layer.

Direct delivery occurs when the destination is on the same physical network as the sender (final hop). Indirect delivery occurs when the destination is on a different network, so the packet is forwarded router-to-router until the last (direct) hop.

Compare distance-vector and link-state routing algorithms.

Distance-vector (e.g., RIP, Bellman-Ford) shares its entire routing table with neighbors periodically and knows only distance/direction. Link-state (e.g., OSPF, Dijkstra) floods link information to all routers so each builds a complete topology map and computes shortest paths.

Describe TCP's congestion-control phases of slow start and congestion avoidance.

Slow start: the congestion window grows exponentially (doubles each RTT) until it reaches the slow-start threshold (ssthresh). Congestion avoidance: the window then grows linearly (additive increase). On loss, TCP multiplicatively decreases the window (AIMD).

Name two features of SCTP that distinguish it from TCP.

SCTP supports multi-streaming (multiple independent streams in one association, avoiding head-of-line blocking) and multi-homing (a single endpoint can use multiple IP addresses for reliability); it is also message-oriented rather than byte-stream-oriented.

Break down the components of a URL: http://www.example.com:80/path/page.html

Scheme/protocol (http), host/domain (www.example.com), port (80), and path to the resource (/path/page.html). The port is optional when the default for the scheme is used.

What is DNS and how does forward vs reverse resolution differ?

DNS (Domain Name System) is a hierarchical distributed database that maps names to IP addresses. Forward resolution maps a domain name to an IP address (A/AAAA records); reverse resolution maps an IP address back to a name using the in-addr.arpa (PTR) domain.

State the roles and default ports of SMTP, POP3, and IMAP in email.

SMTP (port 25) pushes/sends mail between servers and from client to server. POP3 (port 110) downloads mail to the client, typically removing it from the server. IMAP (port 143) lets clients manage and synchronize mail kept on the server.

Why does FTP use two TCP connections and what are their ports?

FTP separates control and data: a control connection on port 21 stays open for commands/responses, and a separate data connection on port 20 (active mode) transfers the actual files. This separation allows commands during transfer.

Differentiate symmetric (secret-key) and asymmetric (public-key) cryptography.

Symmetric uses a single shared secret key for both encryption and decryption (fast; e.g., DES, AES) but has key-distribution problems. Asymmetric uses a public key to encrypt and a private key to decrypt (e.g., RSA), solving key distribution but slower.

How does a digital signature provide authentication and integrity?

The sender hashes the message and encrypts the hash with their private key (the signature). The receiver decrypts it with the sender's public key and compares with a freshly computed hash—matching proves authentication, integrity, and non-repudiation.

What is steganography and how does it differ from cryptography?

Steganography hides the very existence of a message by embedding it inside another medium (image, audio, etc.), whereas cryptography makes a message unreadable but visible. Cryptography hides meaning; steganography hides presence.

What is a VPN and what core technology makes it secure over a public network?

A VPN (Virtual Private Network) creates a secure private connection over a public network (Internet) using tunneling and encryption (e.g., IPSec), giving remote users private-network-like access with confidentiality and integrity.

What is a firewall and what are its two basic filtering types?

A firewall controls incoming/outgoing traffic based on security rules. Types: packet-filter firewall (inspects header fields like IP/port at network/transport layers) and proxy/application-gateway firewall (inspects message content at the application layer).

Describe the main architectural components of a GSM network.

GSM has three subsystems: the Mobile Station (MS) with SIM; the Base Station Subsystem (BSS = BTS + BSC); and the Network Switching Subsystem (NSS = MSC with HLR, VLR, AuC, EIR databases).

What problem does Mobile IP solve and what are its two key addresses?

Mobile IP lets a device keep connectivity while moving between networks. It uses a permanent Home Address (registered with the Home Agent) and a temporary Care-of Address in the visited network (managed via the Foreign Agent); packets are tunneled from home to the current location.

What is a Mobile Ad hoc Network (MANET)?

A MANET is an infrastructure-less wireless network where mobile nodes self-organize and communicate directly, each node also acting as a router to forward packets; the topology changes dynamically as nodes move.

Define the three core cloud service models: IaaS, PaaS, and SaaS.

IaaS provides virtualized infrastructure (compute, storage, network) e.g., AWS EC2. PaaS provides a development/runtime platform e.g., Google App Engine. SaaS provides ready-to-use applications over the internet e.g., Gmail.

Differentiate public, private, and hybrid clouds.

Public cloud: shared infrastructure owned by a provider, available to the general public over the internet. Private cloud: dedicated to a single organization (on/off premises). Hybrid cloud: a combination that lets data/apps move between private and public clouds.

What is virtualization and how does a virtual server relate to it?

Virtualization uses a hypervisor to abstract physical hardware so multiple isolated virtual machines run on one physical machine. A virtual server is one such VM that behaves like a dedicated physical server while sharing the underlying hardware.

What is a Service Level Agreement (SLA) in cloud computing?

An SLA is a formal contract between provider and customer defining the expected service quality—metrics like uptime/availability (e.g., 99.9%), performance, response times, and the penalties or remedies if these guarantees are not met.

What is the Internet of Things (IoT) and its basic architectural layers?

IoT is a network of physical objects embedded with sensors, software, and connectivity that collect and exchange data over the internet. A common layered model: Perception/Sensing layer, Network/Transport layer, and Application layer (sometimes with a Processing/Middleware layer).