A computer network is two or more computers joined so they can share data, files, printers and an internet connection. Networks are named by size (PAN, LAN, MAN, WAN), by layout (topology) and by the rules they follow (protocols such as TCP/IP). Every device on a network has an IP address. VoIP uses the same network to carry phone calls. This page covers all of it, as Punjab Police SI and Constable papers ask it.
What a network is, and PAN, LAN, MAN, WAN
Networks are named by how far they reach.
Computer communication is the process of transferring data between computers. It has five parts: a sender, a receiver, the message, the medium (cable or air) and the protocol (the agreed rules).
The end-user device, such as a computer or smartphone. It asks for services.
A powerful computer that gives services: web pages, files, email.
Joins devices of one LAN and sends data to the right one using its MAC address.
Joins different networks and chooses the path using IP addresses.
Topologies and transmission media
How devices are arranged, and what carries the signal.
Topology is the physical arrangement of network devices and cables, the "shape" of the network. Routing, switching and surfing are actions, not arrangements.
| Topology | Shape | Good point | Weak point |
|---|---|---|---|
| Bus | All devices on one main cable | Cheap, little cable | A break in the main cable stops everyone |
| Star | Every device to one central switch or hub | Easy to add devices; one bad cable affects one device | If the centre fails, all stop |
| Ring | Each device joined to two neighbours in a circle | Orderly: data goes round in turn | One break can stop the ring |
| Mesh | Devices joined to many or all others | Most reliable: many paths | Costly, lots of cable |
| Tree | Stars joined in levels, like branches | Easy to grow | Top link failing cuts a branch |
Very short range, in a straight line. Example: TV remote.
Wi-Fi for a wireless LAN; Bluetooth for a PAN.
Tower to tower, needs a clear line of sight.
The C band (about 4–6 GHz) was the first band for commercial satellite links.
Network hardware: NIC, Ethernet, MAC and modem
The parts that plug a computer into a network.
| Part | What it does |
|---|---|
| NIC (Network Interface Card) | The card that connects a device to a network. It gives the LAN port, even in a VoIP phone. |
| Ethernet card | The network adapter used to set up a wired network |
| Ethernet port | The socket built into computers for a wired network (RJ45) |
| MAC address | Media Access Control address: a 48-bit hardware number fixed in every NIC. Ethernet needs it to deliver data. |
- ComputerDigital data (0 and 1)
- senderModulationDigital → analog, to travel on the line
- receiverDemodulationAnalog → digital, at the other end
- ComputerDigital data again
MOD = Make it Old (digital → old analog). DEMOD = DEcode to Modern (analog → digital).
Rules of the network: OSI, TCP/IP, packets and IP addresses
Layers, protocols, packets, addresses and ports.
- 7. ApplicationWhat the user's app talks: HTTP, FTP, SMTP, DNS.
- 6. PresentationData format, encryption, compression.
- 5. SessionOpens, keeps and closes a conversation.
- 4. TransportTCP (reliable) or UDP (fast).
- IP3. NetworkIP addresses and routing. Router works here.
- MAC2. Data linkMAC addresses. Switch works here.
- 1. PhysicalCables, signals, bits. Hub works here.
Bottom to top: "Please Do Not Throw Sausage Pizza Away" = Physical, Data link, Network, Transport, Session, Presentation, Application.
Data is sent over the internet in small, manageable pieces called packets. Each packet carries the sender's and receiver's IP addresses and a number. Packets may take different roads and are put back together at the end. This is called packet switching.
| Service | Port number |
|---|---|
| HTTP | 80 |
| HTTPS | 443 |
| FTP | 20, 21 |
| SSH | 22 |
| SMTP (sending email) | 25 |
| DNS | 53 |
| Connection-oriented services | Connectionless services |
|---|---|
| Reliable message stream | Unreliable datagram |
| Reliable byte stream | Acknowledged datagram |
| Unreliable connection | Request-reply |
ARPANET, ISPs, VPN and mobile networks
Where the internet came from, how it is built, and the phone network.
- 1969ARPANET
Started by the US Department of Defense (ARPA). First four nodes: UCLA, SRI, UC Santa Barbara, University of Utah.
- 1973Satellite links
ARPANET reaches Hawaii and London.
- 1984OSI model
ISO publishes the 7-layer reference model.
- 1995First VoIP software
VocalTec's "Internet Phone" lets one PC call another over the internet.
- BackbonesVery fast fibre-optic trunk lines across countries and under oceans.
- NAPNetwork Access PointsMeeting points where networks join the backbones.
- ISPInternet Service ProvidersISPs connect to the backbones at NAPs with high-speed lines.
- YouYour home or phone connects to the ISP.
A VPN extends a private network across a public network like the internet. It makes an encrypted connection (a "tunnel") from your device to the network. Staff use it to reach the office network from home.
| Mobile term | Full form | Identifies |
|---|---|---|
| IMEI | International Mobile Equipment Identity | The phone (handset) itself. 15 digits; dial *#06# to see it. |
| SIM | Subscriber Identity Module | The subscriber: the small card from your mobile company |
| IMSI | International Mobile Subscriber Identity | The subscriber number stored on the SIM |
| MCC / MNC | Mobile Country Code / Mobile Network Code | The country and the mobile company |
VoIP: phone calls over the internet
The most-asked topic on this page: about 30 past questions.
VoIP lets you make voice calls over a broadband internet connection instead of a regular (analog) phone line. Its other name is IP telephony. You need a computer or mobile device (or an IP phone) and a good internet connection.
- You speakThe microphone picks up an analog sound wave.
- codecVoice → digital signalA codec turns it into digital data and compresses it.
- UDPPackets over IPSent in packets, usually with UDP for speed.
- Back to soundThe other end joins the packets and plays the voice.
- Low cost, even for long-distance calls
- Accessibility: call from any connected device
- Portability: your number goes where you go
- Scalability: add lines in software
- Multitasking: voice, video and files together; extra features such as call notification
- Needs a good internet connection; quality depends on its speed
- Latency and jitter: delay and broken sound
- Needs a continuous power supply
- Security risks on the open internet
- Weak support for emergency calls; often no directory / white-page listing
- "Requirement for an internet connection" is a disadvantage, never an advantage.
- Low cost and portability are advantages, so they are "NOT disadvantages".
- Latency and jitter are a disadvantage, so they are "NOT an advantage".
- "Call notification is not possible with VoIP" is false.
| VoIP protocol | What to remember |
|---|---|
| SIP | Session Initiation Protocol. A signalling protocol: it starts, changes and ends a call, like the old phone system's dialling and ringing. |
| Transport protocols (e.g. RTP over UDP) | Carry the voice packets made by the codec |
| UDP | Used instead of TCP for voice: it does not wait to resend lost packets, so it is faster |
| SCCP (Skinny) | Skinny Client Control Protocol. Made by Selsius Systems; now a Cisco product (Cisco bought Selsius in 1998). |
Quick revision
Full forms, traps and one-line facts.
- LAN
- Local Area Network
- MAN
- Metropolitan Area Network
- WAN
- Wide Area Network
- PAN
- Personal Area Network
- NIC
- Network Interface Card
- MAC
- Media Access Control
- OSI
- Open System(s) Interconnection
- TCP/IP
- Transmission Control Protocol / Internet Protocol
- UDP
- User Datagram Protocol
- VoIP
- Voice over Internet Protocol
- SIP
- Session Initiation Protocol
- VPN
- Virtual Private Network
- ISP
- Internet Service Provider
- SIM
- Subscriber Identity Module
- IMEI
- International Mobile Equipment Identity
- ARPANET
- Advanced Research Projects Agency Network
- IMEI vs SIM: IMEI identifies the phone; SIM identifies the subscriber.
- IP vs MAC: IP address can change with the network; the MAC address is fixed in the card.
- IPv4 = 32 bits, not 32 bytes. IPv6 = 128 bits.
- VoIP: "Intranet" and "Internal" are wrong; it is Voice over Internet Protocol.
- VoIP turns voice into digital signals, not analog or optical signals.
Also asked onceTap to open
- Twisted pair is used for local telephone lines and short distances.
- Fibre optics is the usual technology for long-distance WAN links.
- Using 4–6 GHz for satellite transmission and retransmission is C-band transmission.
- The Ethernet port is built into computers to connect them to a wired network.
- HTTPS works on port 443.
- The first VoIP service came from the company VocalTec in 1995.
- VoIP can also be called IP telephony.
- Both are true: signalling protocols handle the call functions taken from the telephone system, and transport protocols carry the codec's voice packets.
- Both are true: any TCP/IP network can run VoIP, and UDP helps VoIP run on any type of network.
- In a VoIP phone, the NIC does provide the LAN port, and a MAC address is needed to join an Ethernet network.
- The quality of a voice call over a network depends on the speed of the internet connection.
Name the network size for each: (a) your phone and smartwatch, (b) a school computer lab, (c) the internet. Then say which medium carries the internet across oceans.
Show answer
(a) PAN, (b) LAN, (c) WAN, the biggest one. Optical fibre cables carry it across oceans.