Computer Networks — CS3001-1 Unit-I (Animated)

01Data Communications & Network Types

A network moves data between devices. For communication to happen, five components must exist: message, sender, receiver, medium, protocol. Delivery must be on time, accurate, and error-free.

Three data-flow (duplex) modes

Simplex = one-way only · Half-duplex = both ways, one at a time · Full-duplex = both ways simultaneously.

Duplex modes — watch the direction of traffic↻ replay

Simplex: TV tower → screen. Half-duplex: walkie-talkie (press to talk, release to hear). Full-duplex: a phone call.

✦ Exam point The five components of data communication are message, sender, receiver, transmission medium, and protocol. "Protocol" is the set of rules governing exchange — remember it as the language both ends agree to speak.

02Network Types — LAN, MAN, WAN

Networks are classified by geographical span: LAN (a room/building), MAN (a city), WAN (a country / the world).

Zoom out from your desk to the planet↻ replay

A LAN connects nearby PCs through a switch; a MAN joins city LANs with routers on fibre; a WAN connects cities via the Internet backbone.

LAN

Small area: office, lab, building
High speed (Gbps), low error
Owned by one organisation

WAN

Connects cities/countries
Point-to-point leased lines + routers
Slower, higher error, expensive

03Connection-Oriented vs Connectionless

Is the path set up before the data flows, or is every packet on its own?

Three-way handshake (left) vs free datagrams (right)↻ replay

TCP: SYN → SYN-ACK → ACK reserves the channel, then data flows in order. UDP: each datagram is independent — fast, no guarantee of order.

Property	Connection-oriented (TCP)	Connectionless (UDP)
Setup	Needs handshake first	None
Order	Guaranteed	Not guaranteed
Reliability	ACKs + retransmission	Best effort
Overhead	High	Low
Used by	File transfer, web	Streaming, DNS, VoIP

04Network Devices

Modem, Hub, Switch, Router — the "physical chain" from your home to the internet. Each one works at a different layer.

Modem

MODulator–DEModulator. Converts digital ↔ analog so signals can travel over phone lines.

Hub

Physical-layer repeater. Broadcasts every frame to all ports — wasteful, collisions.

Switch

Data-link device. Learns MAC addresses, forwards only to the destination port.

Router

Network-layer device. Routes between networks using IP addresses.

Hub (broadcasts) vs Switch (learns & targets)↻ replay

Left: A sends to C — the hub sends a copy to everyone. Right: the switch already knows C lives on port 3, so only C receives it.

Router joins two LANs — a frame becomes a packet becomes a frame↻ replay

Host 192.168.1.5 sends to 10.0.0.8: the frame goes to the router, which strips the old frame, repackages the IP packet in a new frame, and sends it onward.

05Network Models — OSI & TCP/IP

Both are layered architectures. Each layer talks only to its neighbours and adds its own header — that's encapsulation.

OSI — 7 layers (reference model)

Layer	What it does
7 Application	Network services to apps (HTTP, FTP)
6 Presentation	Encoding, encryption, compression
5 Session	Dialogue control, checkpoints
4 Transport	Segments, end-to-end delivery, ports
3 Network	Packets, logical (IP) addressing, routing
2 Data link	Frames, MAC addressing, error control
1 Physical	Bits, signals, media

TCP/IP — 5 layers (practical model)

Layer	OSI equivalent
5 Application	7 + 6 + 5 combined
4 Transport	4
3 Network	3
2 Data link	2
1 Physical	1

✓ Mnemonic OSI from bottom: Please Do Not Throw Sausage Pizza Away.

Encapsulation — headers stack up going down, peel off going up↻ replay

At the sender: data → +TCP header = segment → +IP header = packet → +MAC header/trailer = frame → bits. The receiver reverses the process, stripping headers layer by layer.

⚠ Common mistake OSI is a reference model; the Internet actually runs on TCP/IP. Don't say "OSI is the protocol" — it's a blueprint.

06Physical Layer — Digital Signals

Computers speak in digital signals — discrete voltage pulses (0/1). A periodic digital signal has amplitude, period/frequency, and phase.

One period, three knobs — drag and watch the wave change

Amplitude A Frequency f Phase φ

Amplitude = strength (volts), frequency = cycles/sec (Hz), phase = starting angle. The screen shows one period; T = 1/f.

Transmission impairment

Signals weaken and deform as they travel: attenuation (loss of energy), distortion (components arrive out of sync), noise (unwanted signals added on top).

Attenuation, distortion, noise — three ways a signal gets hurt↻ replay

Left: amplitude fades with distance. Middle: a square wave becomes a rounded, lagging mess. Right: random noise rides on the signal.

✓ Worked — dB cascade Signal is amplified +30 dB, then +20 dB, then a cable loses 10 dB. Net gain = 30 + 20 − 10 = 40 dB. As a ratio that is 10^40/10 = 10,000×. dB = 10·log₁₀(P2/P1) — always add dB, multiply ratios.

07Data-Rate Limits & Performance

Two hard ceilings govern any channel: Nyquist (noiseless) and Shannon (noisy). Performance is judged by throughput, latency, and bandwidth.

Nyquist & Shannon calculators

Bandwidth B (Hz) Signal levels L SNR (dB)

Nyquist:
Shannon:

Latency = propagation + transmission

Distance (km) Frame size (bits) Rate (Mbps)

Propagation:
Transmission:
Total latency:

⚠ Common mistake Bit rate ≠ baud rate. Bit rate = baud × log₂(L). With 4 levels (L=4), each signal element carries 2 bits, so bit rate is double the baud rate. Nyquist is 2B·log₂L — many students forget the factor of 2.

08Line Coding — Digital-to-Digital

Converting a bit stream into voltage signals. Unipolar = one polarity; polar = positive/negative. Watch all six schemes draw themselves on the bits 0 1 0 0 1 1 1 0.

Comparing the schemes

Scheme	Self-clocking?	DC component?	Used for
Unipolar NRZ	No	Yes	Rare (baseline wander)
NRZ-L	No	Yes (long runs)	Simple digital links
NRZ-I	No	Reduced	USB, 100Base-FX
Manchester	Yes	No	Ethernet (10Base-T)
Differential Manchester	Yes	No	Token Ring

✦ Exam point Manchester is self-clocking because there is always a transition at the middle of every bit — the receiver extracts its clock from the signal. Unipolar NRZ has a DC component because it never goes below zero.

09Transmission Media (Guided)

Guided media = the signal is contained in a conductor: twisted-pair, coaxial cable, or optical fibre.

Three guided media, three ways to carry a signal↻ replay

Twisted pair: two insulated copper wires, twisted to cancel interference. Coax: centre conductor + braided shield. Fibre: light pulses bounce down the core by total internal reflection.

Medium	Speed class	Typical use
Twisted pair (STP/UTP)	10 Mbps – 10 Gbps	Telephone, Ethernet
Coaxial cable	10–100 Mbps	CATV, older Ethernet
Optical fibre	Gbps–Tbps	Backbones, WAN, submarine cables

10Switching — Packet Switching

In packet switching, data is chopped into packets that travel independently. Two flavours: datagram (each packet routed on its own) and virtual circuit (path reserved first).

Datagram vs Virtual Circuit across three routers↻ replay

Datagram: packets 1, 2, 3 take different routes and may arrive out of order. VC: a setup packet reserves the path, then all data follows it in order — then it's torn down.

	Datagram	Virtual circuit
Setup	None	Needed (setup phase)
Routing decision	Per packet	Once, at setup
Ordering	May be lost/reordered	In order
Best for	Email, web	Voice, video

11Data-Link Layer — Addressing & ARP

Four levels of addressing exist on the internet. At the link layer we use MAC (physical) addresses, and ARP translates an IP address into a MAC address.

Level	Address	Length	Example
Application	Port number	16 bits	443 (HTTPS)
Network	IP address	32 bits (v4)	141.23.56.23
Data link	MAC address	48 bits	AA:BB:CC:11:22:33
Physical	—	—	—

ARP in action — broadcast the question, unicast the answer↻ replay

A (IP .2, MAC AA) needs to send to IP .23 (MAC BB). A broadcasts: "who has .23? tell .2". Only B answers (unicast): "BB is at .23". A stores it in its ARP cache — next time, no broadcast needed.

⚠ Common mistake ARP works within one LAN only — it never crosses a router. MAC addresses are not routable; IP addresses are. The MAC of a host is set at manufacture (or by the OS) — not leased like IP.

12Block Coding & Error Detection — CRC

Block coding adds redundancy by mapping m data bits to n codeword bits (e.g. 4B/5B). CRC (Cyclic Redundancy Check) appends a checksum so the receiver can detect corrupted frames.

CRC in one sentence

Append (divisor_len − 1) zeros to the data, divide the whole thing by the divisor using XOR, and attach the remainder. The receiver re-divides; a non-zero remainder means the frame is corrupt.

✓ Worked — CRC-3 Data 1011001, divisor 1011 (x³+x+1). Append 3 zeros → 1011001000. Divide by 1011 → remainder 011. Transmit 1011001 011.

Shift-register division — each step shifts and XORs↻ replay

The 3-bit register shifts in the next data bit; if a 1 falls out the left, the register is XORed with the divisor (without its leading 1). The final register content is the CRC remainder.

✦ Exam point CRC detects errors, it does not correct them. A single error in the data is caught because the remainder comes out non-zero. The minimum Hamming distance d_min decides what a code can detect/correct.

13CSMA / CSMA-CD

Carrier Sense Multiple Access: listen before you talk. In Ethernet (CSMA/CD), if two stations transmit at once → collision → both stop, wait a random backoff, and retry.

CSMA/CD — sense, collide, back off, retry↻ replay

A and B both sense idle and transmit. Their frames collide mid-cable, both send a jam signal, then each waits a random time before retransmitting — this time they succeed one after the other.

Persistence method	Behaviour
1-persistent	If busy, keep listening; send immediately when free (aggressive — collisions)
Non-persistent	If busy, wait a random time then listen again (calmer)
p-persistent	If free, send with probability p; else wait for the next slot

14Formula Sheet & 5 Near-Certain Questions

Formulas you must own

Quantity	Formula
Data rate (Nyquist)	C = 2B·log₂L bps
Data rate (Shannon)	C = B·log₂(1 + SNR) bps
dB	dB = 10·log₁₀(P₂/P₁)
Bit rate ↔ baud	bit rate = baud × log₂L
Transmission time	T = frame size / rate
Propagation time	T = distance / speed
Total latency	T = propagation + transmission + queueing
CRC remainder	data·2^(n−1) mod divisor (XOR division)
Throughput	Th = actual bits delivered / time

Q1 · A channel has B = 1 MHz, L = 4 levels. Max data rate? answer

Nyquist: C = 2(1×10⁶)·log₂4 = 2×10⁶ × 2 = 4 Mbps.

Q2 · B = 3000 Hz, SNR = 35 dB. Max rate?

SNR = 10^3.5 = 3162. Shannon: C = 3000·log₂(3163) ≈ 3000 × 11.63 ≈ 34.9 kbps.

Q3 · Draw NRZ-I for 01001110. Level starts low.

Low, High, High, High, Low, High, Low, Low — transition only on 1 (start Low). Check the line-coding widget above for the visual.

Q4 · When does a hub differ from a switch?

Hub = physical layer, broadcasts to all ports, can't filter → collisions & waste. Switch = data-link layer, learns MAC→port and forwards only to the destination.

Q5 · Explain the ARP exchange when A (.2) talks to B (.23) on the same LAN.

A broadcasts "Who has .23? Tell .2". B (and only B) replies unicast "BB is at .23". A caches it. Later sends are direct — no more broadcast.