This chapter is built from the official PCTB textbook Computer Science and Entrepreneurship, Class 9 (based on the Revised National Curriculum of Pakistan 2023, approved by the Punjab Curriculum and Textbook Board, Lahore). The exercise questions below are the book's own exercise questions; the answers are model answers written for this guide.
Chapter titles and exercise questions are taken from the official book's contents and exercise pages. No page numbers are claimed for individual questions.
A computer network connects two or more computers so they can share resources and communicate. Its main objectives are resource sharing (printers, files, internet), data communication (fast exchange of information), and cost saving (one printer for many users instead of one each).
Every data communication system has five components: a sender, a receiver, a message, a transmission medium (cable or wireless), and a protocol — the agreed rules for how data is sent and understood.
کمپیوٹر نیٹ ورک دو یا زیادہ کمپیوٹرز کو جوڑتا ہے تاکہ وہ وسائل بانٹ سکیں اور رابطہ کر سکیں۔ اس کے اہم مقاصد وسائل کی شراکت، ڈیٹا کی ترسیل اور لاگت کی بچت ہیں۔ ہر ڈیٹا کمیونیکیشن سسٹم میں پانچ اجزاء ہوتے ہیں: بھیجنے والا، وصول کرنے والا، پیغام، ترسیلی ذریعہ اور پروٹوکول یعنی طے شدہ اصول۔
In simplex mode data flows one way only (for example a keyboard to a computer). In half-duplex both sides can send, but only one at a time (for example a walkie-talkie). In full-duplex both sides send and receive at the same time (for example a phone call).
Circuit switching reserves a dedicated path for the whole conversation (old telephone calls). Packet switching breaks data into packets that travel independently and are reassembled at the destination — this is how the internet works, and it uses the network far more efficiently.
سمپلیکس میں ڈیٹا صرف ایک طرف جاتا ہے، ہاف ڈوپلیکس میں دونوں طرف مگر ایک وقت میں ایک، اور فل ڈوپلیکس میں دونوں طرف بیک وقت بات ہو سکتی ہے۔ سرکٹ سوئچنگ میں پوری گفتگو کے لیے مخصوص راستہ محفوظ ہوتا ہے جبکہ پیکٹ سوئچنگ میں ڈیٹا چھوٹے پیکٹوں میں ٹوٹ کر الگ الگ سفر کرتا ہے اور منزل پر دوبارہ جڑتا ہے — انٹرنیٹ اسی طرح کام کرتا ہے۔
A router connects different networks and directs packets between them; a switch connects devices inside one network and sends data only to the right device; a hub broadcasts to all devices; a modem connects a home network to the internet service provider.
In a star topology every device connects to a central hub or switch — if one cable fails, only that device is affected. In bus all devices share one cable; in ring each device connects to two neighbours in a circle. The OSI model has seven layers; the Data Link layer handles node-to-node transfer and error detection, and the Network layer handles routing between networks.
راؤٹر مختلف نیٹ ورکس کو جوڑتا اور پیکٹوں کی سمت طے کرتا ہے، سوئچ ایک نیٹ ورک کے اندر آلات جوڑتا اور ڈیٹا صرف درست آلے کو بھیجتا ہے، ہب سب کو بھیجتا ہے اور موڈیم گھر کے نیٹ ورک کو انٹرنیٹ سے جوڑتا ہے۔ اسٹار ٹوپولوجی میں ہر آلہ مرکزی ہب سے جڑتا ہے اس لیے ایک کیبل خراب ہو تو صرف وہ آلہ متاثر ہوتا ہے۔ او ایس آئی ماڈل کی سات تہیں ہیں؛ ڈیٹا لنک تہہ نوڈ سے نوڈ ترسیل اور خرابی کی نشاندہی کرتی ہے اور نیٹ ورک تہہ روٹنگ کرتی ہے۔
TCP gives reliable, ordered delivery (it resends lost packets); UDP is faster but unreliable, used for live video and games. DNS translates domain names like punjabtest.com into IP addresses. DHCP assigns IP addresses automatically to devices joining a network.
Security layers protect the network: a firewall monitors and controls traffic in and out, and encryption scrambles data so that even if someone intercepts it, they cannot read it. Both are needed — a firewall guards the door, encryption protects the message.
ٹی سی پی قابل اعتماد اور ترتیب وار ترسیل دیتا ہے اور گم شدہ پیکٹ دوبارہ بھیجتا ہے، یو ڈی پی تیز مگر غیر یقینی ہے اور لائیو ویڈیو اور گیمز میں استعمال ہوتا ہے۔ ڈی این ایس ڈومین نام کو آئی پی ایڈریس میں بدلتا ہے اور ڈی ایچ سی پی آلات کو خودکار آئی پی ایڈریس دیتا ہے۔ فائر وال آنے جانے والی ٹریفک کو کنٹرول کرتا ہے اور انکرپشن ڈیٹا کو ایسا بگاڑ دیتی ہے کہ چوری ہونے پر بھی پڑھا نہ جا سکے۔
اس chapter کی exercise میں book کے MCQs, short questions اور long questions ہیں — تمام سوال کتاب کی exercise کے ہیں, جواب کے ساتھ.
1. The primary objective of computer networks is:
Answer: (b) — The primary objective of networks is resource sharing and data communication.
2. Which device is used to connect multiple networks and direct data packets between them?
Answer: (c) — A router connects multiple networks and directs packets between them.
3. Which layer of the OSI model is responsible for node-to-node data transfer and error detection?
Answer: (b) — The Data Link layer handles node-to-node transfer and error detection.
4. The function of the Domain Name System (DNS) is to:
Answer: (b) — DNS translates domain names into IP addresses.
5. Which method of data transmission uses a dedicated communication path?
Answer: (b) — Circuit switching uses a dedicated communication path for the whole session.
6. Which protocol is used for reliable data transfer in the TCP/IP model?
Answer: (c) — TCP is the protocol for reliable data transfer in the TCP/IP model.
7. The main purpose of a firewall in network security is to:
Answer: (b) — A firewall monitors and controls network traffic.
8. Which network topology connects all devices to a central hub?
Answer: (d) — In a star topology all devices connect to a central hub.
9. What is a key benefit of using computer networks in businesses?
Answer: (b) — A key business benefit is resource sharing and efficient communication.
Answer: Data communication is the exchange of data between a sender and a receiver through a transmission medium, following agreed rules (protocols).
Answer: Its key components are: sender, receiver, message, transmission medium, and protocol.
Answer: A router connects different networks and forwards each data packet towards its destination network, choosing the best path.
Answer: The Network layer provides logical addressing (IP addresses) and routing: it selects the path data take across networks.
Answer: DHCP (Dynamic Host Configuration Protocol) automatically assigns IP addresses and network settings to devices when they join a network, so no manual setup is needed.
Answer: TCP is connection-oriented and reliable: it checks delivery and resends lost data. UDP is connectionless and faster but unreliable: it does not resend lost packets, so it suits live video and games.
Answer: Encryption scrambles data into unreadable form so that anyone who intercepts it cannot understand it; only the intended receiver with the key can read it.
Answer: In a star topology every device connects to a central hub or switch; all communication passes through the centre.
Answer: A firewall monitors and controls incoming and outgoing network traffic, blocking unauthorised access while allowing legitimate traffic through.
Answer: In half-duplex, data flow both ways but only one direction at a time (walkie-talkie). In full-duplex, both directions work simultaneously (phone call).
Answer: The objectives of computer networks are: resource sharing (printers, files, software and internet shared by many users); data communication (fast exchange of information within and between organisations); cost saving (shared resources cost less than one per user); reliability (copies of data on several machines protect against one machine failing); and centralised management (updates and backups can be done in one place).
Answer: Time = amount of data ÷ rate. a. 10 kilobits = 10,000 bits. Time = 10,000 ÷ 500 = 20 seconds. b. 10 kilobytes = 10 × 1,000 × 8 = 80,000 bits (1 byte = 8 bits). Time = 80,000 ÷ 500 = 160 seconds. So the kilobyte message takes eight times as long as the kilobit message, because a byte is eight bits.
Answer: A protocol is a set of agreed rules that lets devices communicate: how a connection starts, how data are formatted, how errors are handled, and how it ends. Types include: communication protocols (TCP/IP, HTTP, FTP) that move data; network management protocols that monitor the network; and security protocols (HTTPS, SSL/TLS) that protect data. TCP is connection-oriented and reliable; UDP is connectionless and fast; HTTP transfers web pages; FTP transfers files.
Answer: In business, networks let branches share stock and sales data, let staff work on the same files, support online selling and video meetings, and cut costs through shared printers and storage. In education, networks give students shared learning material, online classes and research, let teachers distribute and collect work, and connect schools to libraries and resources worldwide.
Answer: A network topology is the arrangement of devices and links in a network. Star: every device connects to a central hub/switch; it is easy to add devices and one cable failure affects only one device, but the hub is a single point of failure. Bus: all devices share one main cable; it is cheap and simple, but a break in the main cable affects the whole network and fault-finding is hard. (Diagrams in the book show the centre-and-spokes of star and the single backbone of bus.)
Answer: The OSI model is a seven-layer framework for how data travel from one device to another: Physical, Data Link, Network, Transport, Session, Presentation, Application. Three layers in detail: Physical — the actual cables, signals and hardware that carry bits. Data Link — node-to-node delivery on one link, framing and error detection. Network — logical addressing and routing of packets across different networks to their destination.
Networks exist to share resources and communicate; data travel in simplex, half-duplex or full-duplex modes, in packets; routers, switches and topologies organise the connections; and TCP, DNS, firewalls and encryption keep them working and safe.
Revision guidance: پہلے MCQs خود حل کرو, پھر جواب ملاؤ; short سوالوں کے جواب اپنے الفاظ میں لکھ کر practice کرو; long سوالوں کے لیے ہر section کی headings اور مثالیں یاد رکھو.
نیٹ ورکس وسائل بانٹنے اور رابطے کے لیے ہیں؛ ڈیٹا سمپلیکس، ہاف ڈوپلیکس یا فل ڈوپلیکس میں اور پیکٹوں میں سفر کرتا ہے؛ راؤٹر، سوئچ اور ٹوپولوجیز رابطوں کو ترتیب دیتے ہیں؛ اور TCP، DNS، فائر وال اور انکرپشن انہیں محفوظ رکھتے ہیں۔
رہنمائی: پہلے ایم سی کیوز خود حل کریں پھر جوابات ملائیں؛ مختصر سوالات کے جوابات اپنے الفاظ میں لکھ کر مشق کریں؛ تفصیلی سوالات کے لیے ہر سیکشن کی سرخیاں اور مثالیں یاد رکھیں۔
Is chapter ke topics: Why Networks? Objectives and Components · Transmission Modes and Switching · Devices, Topologies and the OSI Model · Protocols, DNS, DHCP and Network Security · مشق
Topics (Urdu): نیٹ ورکس کیوں؟ مقاصد اور اجزاء · ترسیل کے طریقے اور سوئچنگ · آلات، ٹوپولوجیز اور او ایس آئی ماڈل · پروٹوکولز، ڈی این ایس، ڈی ایچ سی پی اور نیٹ ورک سیکیورٹی
Revision guidance: پہلے MCQs خود حل کرو, پھر جواب ملاؤ; short سوالوں کے جواب اپنے الفاظ میں لکھ کر practice کرو; long سوالوں کے لیے ہر section کی headings اور مثالیں یاد رکھو.
Chapter-wise tests, fresh random papers اور instant score کے لیے PunjabTest.com کا میں test section دیکھو:
PunjabTest.com — Complete Guide Book • Class 9 Computer Science, Chapter 3 (official PCTB book exercise, Session 2026–27) • English + Urdu