Computer Networks: DSSSB TGT Computer Science High-Yield Notes
Comprehensive revision guide aligned strictly with DSSSB TGT CS official syllabi, previous years exam frequency trends, and conceptual weightage.
🔥 High Weightage (4-7 Qs)
⚡ Moderate (2-3 Qs)
🌱 Low / Foundational (1-2 Qs)
1. Basics of Networks & Topologies
⚡ Moderate FrequencyA computer network connects autonomous devices via transmission media. Key dimensions tested by DSSSB include topology fault tolerance and cable count formulas.
- Mesh Topology: For $n$ devices, requires $\frac{n(n-1)}{2}$ duplex physical links and $(n-1)$ I/O ports per device. Highly reliable and secure, but expensive.
- Star Topology: Nodes connect to a central hub/switch. Easy to install and reconfigure; central hub represents a single point of failure.
- Ring Topology: Unidirectional/bidirectional transmission using a token. Cable break brings down the entire network.
- Bus Topology: Uses a multipoint backbone cable terminated at ends with terminators (usually 50-ohm resistors) to eliminate reflections.
DSSSB TGT CS (2018 & 2021)
Q: How many physical duplex cables are required to connect 10 computers in a fully connected mesh network?
Answer: 45 links [Formula: n(n - 1) / 2 = 10(9) / 2 = 45].
2. OSI vs TCP/IP Reference Models
🔥 High Frequency (Every Year)DSSSB recurrently tests layer-wise PDUs (Protocol Data Units), hardware mapping, and designated layer responsibilities.
| OSI Layer | PDU | Primary Function | Hardware / Protocols |
|---|---|---|---|
| Application (L7) | Data | Network virtual terminal, file/mail services | HTTP, DNS, SMTP, FTP |
| Presentation (L6) | Data | Encryption, Compression, Translation (ASCII/EBCDIC) | SSL/TLS, JPEG, MPEG |
| Session (L5) | Data | Dialog control, Checkpointing, Token management | NetBIOS, RPC |
| Transport (L4) | Segment | End-to-End delivery, Flow & Error control, Port addressing | TCP, UDP |
| Network (L3) | Packet | Logical addressing (IP), Routing, Path determination | Router, ICMP, ARP, OSPF |
| Data Link (L2) | Frame | Physical addressing (MAC), Framing, Error detection (CRC) | Switch, Bridge, Ethernet |
| Physical (L1) | Bit | Transmission of raw uninterpreted bit stream | Hub, Repeater, Cables |
DSSSB TGT CS (2014, 2021 Shift 1)
Q: Which OSI layer is responsible for data encryption, compression, and translation?
Answer: Presentation Layer (Layer 6).
DSSSB TGT CS (2021 Shift 2)
Q: At which layer does a Bridge and a Switch operate respectively?
Answer: Data Link Layer (Layer 2). (Note: Multi-layer switches operate at L2/L3).
3. IPv4 / IPv6 Addressing & Subnetting
🔥 High FrequencyQuestions commonly target class ranges, default subnet masks, and network/broadcast calculation.
- Class A: 1.0.0.0 to 126.255.255.255 (Mask: 255.0.0.0 or /8)
- Class B: 128.0.0.0 to 191.255.255.255 (Mask: 255.255.0.0 or /16)
- Class C: 192.0.0.0 to 223.255.255.255 (Mask: 255.255.255.0 or /24)
- Class D: 224.0.0.0 to 239.255.255.255 (Multicasting)
- Class E: 240.0.0.0 to 255.255.255.255 (Experimental / Research)
- Loopback IP: 127.0.0.1 (Self-test/diagnostic)
- Formula for Valid Hosts per Subnet: $2^H - 2$ (where $H$ is the number of host bits).
DSSSB TGT CS (2018)
Q: How many valid host addresses are available in a subnet with a subnet mask of 255.255.255.224?
Answer: 30 hosts. (224 in binary has 3 network bits borrowed, leaving 5 host bits: $2^5 - 2 = 32 - 2 = 30$).
4. Key Protocols & Well-Known Port Numbers
🔥 High Frequency| Protocol | Port | Type | Highlights |
|---|---|---|---|
| DNS | 53 | UDP / TCP | Uses UDP for standard queries; TCP for zone transfers. |
| HTTP / HTTPS | 80 / 443 | TCP | Stateless web protocols. HTTPS uses SSL/TLS. |
| FTP | 20 (Data), 21 (Control) | TCP | Employs out-of-band control connections. |
| SMTP / POP3 / IMAP | 25 / 110 / 143 | TCP | SMTP for sending mail; POP3/IMAP for mail retrieval. |
| DHCP | 67 (Server), 68 (Client) | UDP | DORA process (Discover, Offer, Request, Acknowledge). |
DSSSB TGT CS (2021 Shift 1)
Q: Which protocol converts an IP address into a Physical MAC Address?
Answer: ARP (Address Resolution Protocol). RARP does the reverse.
5. Routing Algorithms & Flow/Error Control
⚡ Moderate Frequency- Distance Vector Routing (DVR): Uses Bellman-Ford algorithm. Suffers from the Count-to-Infinity problem (solved using Split Horizon and Poison Reverse). Ex: RIP.
- Link State Routing (LSR): Uses Dijkstra's Shortest Path First algorithm. Each router possesses global topology knowledge. Ex: OSPF.
- Flow Control Sliding Window:
- Stop-and-Wait: Window size = 1. Sender efficiency = $\frac{1}{1 + 2a}$.
- Go-Back-N (GBN): Sender window = $N$, Receiver window = 1. Requires retransmitting all frames after an error.
- Selective Repeat (SR): Sender window = $2^{k-1}$, Receiver window = $2^{k-1}$. Only faulty frames are retransmitted.
DSSSB TGT CS (2018)
Q: What is the maximum receiver window size in Go-Back-N ARQ protocol?
Answer: 1. Only Selective Repeat allows a receiver window greater than 1.
📝 Topic-Wise Self Assessment Quiz (DSSSB Pattern)
Select an option and click "View Explanation" to check the official answer key and solution reasoning.
Q1. Which of the following layers of the OSI model coordinates dialogue control and token management? PYQ 2021
▶ View Explanation
Correct Answer: B) Session Layer
The Session layer (Layer 5) is responsible for establishing, managing, and terminating sessions between applications, including token management and synchronization checkpoints.
The Session layer (Layer 5) is responsible for establishing, managing, and terminating sessions between applications, including token management and synchronization checkpoints.
Q2. What is the broadcast address for the IP address 192.168.1.35 with a CIDR prefix of /27? PYQ 2018
▶ View Explanation
Correct Answer: A) 192.168.1.63
A /27 subnet mask has block size = $256 - 224 = 32$.
Subnets: 0-31, 32-63. IP 192.168.1.35 falls into subnet 192.168.1.32.
Network ID = 192.168.1.32; Direct Broadcast Address = 192.168.1.63.
A /27 subnet mask has block size = $256 - 224 = 32$.
Subnets: 0-31, 32-63. IP 192.168.1.35 falls into subnet 192.168.1.32.
Network ID = 192.168.1.32; Direct Broadcast Address = 192.168.1.63.
Q3. Which error detection method uses polynomial arithmetic modulo-2? PYQ 2014 & 2021
▶ View Explanation
Correct Answer: C) Cyclic Redundancy Check (CRC)
CRC operates on binary division using modulo-2 arithmetic to generate FCS (Frame Check Sequence) appended to the frame.
CRC operates on binary division using modulo-2 arithmetic to generate FCS (Frame Check Sequence) appended to the frame.
Q4. What is the size of the MAC address and IPv6 address respectively? PYQ 2021
▶ View Explanation
Correct Answer: B) 48 bits and 128 bits
MAC addresses (Physical/Hardware) are 48 bits (6 octets) represented in hex. IPv6 addresses are 128 bits (16 octets).
MAC addresses (Physical/Hardware) are 48 bits (6 octets) represented in hex. IPv6 addresses are 128 bits (16 octets).
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