DCN — the 20% that covers 80%

Every item below is confirmed on your professor's actual written syllabus. Nothing extra, nothing missing. Read top to bottom once, out loud if you can — that's the whole plan.

1Data Comm Basics

5 components

Message, Sender, Receiver, Transmission Medium, Protocol.

Protocol = 3 things

Syntax (format), Semantics (meaning), Timing (when/how fast).

4 effectiveness criteria

Delivery, Accuracy, Timeliness, Jitter (variation in arrival delay).

Topologies

Mesh: links = n(n-1)/2, ports = n-1 → max fault tolerance, most cabling Star: links = n, ports = 1 (hub = n) → single point of failure at hub Bus: 1 backbone, n taps, 2 terminators → one break kills everything Ring: n links, 2 ports/device → token circulates one direction

Real-life: Star = your home Wi-Fi router. Bus = old 10BASE2 office Ethernet (one cable, one cut kills it). Ring = FDDI campus backbones (dual ring, self-healing). Mesh = Tier-1 internet backbone routers.

2Layering

OSI 7 layers (top → bottom)

7 Application — HTTP, DNS (what the program does) 6 Presentation — encryption, compression 5 Session — dialog control, sync 4 Transport — process-to-process, TCP/UDP, ports 3 Network — IP addressing, routing 2 Data Link — framing, MAC address, media access 1 Physical — raw bits, voltages

Mnemonic: "All People Seem To Need Data Processing"

TCP/IP 5 layers + encapsulation

Application, Transport, Network, Data Link, Physical (merges OSI 5–7 into one).

Data → Segment (Transport, +ports) → Packet (Network, +IP) → Frame (Data Link, +MAC) → Bits

Real-life: Alice mails a letter to Bob — she writes the message (Application), addresses the envelope (Network), the postal service routes it hop-by-hop (Data Link, changes at every relay), the truck just carries paper (Physical). MAC address changes every hop; IP address stays the same end-to-end.

3Switching & Delay

Circuit vs Packet switching

Circuit: dedicated path reserved first (setup phase), guaranteed bandwidth, wastes capacity if idle. Packet: no reservation, each packet routed independently, efficient for bursty data.

Real-life: Old landline phone call = circuit switching (64kbps reserved whether you talk or not). WhatsApp/web browsing = packet switching (IP datagrams routed independently, sharing the link).

Delay components

Nodal delay = d_proc + d_queue + d_trans + d_prop d_trans (transmission) = L / R ← pushing bits onto the wire (size ÷ bandwidth) d_prop (propagation) = d / s ← travel time across medium (distance ÷ ~2×10^8 m/s) Traffic Intensity I = (a·L)/R — must stay < 1 or queue explodes

Real-life: Satellite internet has huge propagation delay (~119ms one-way — distance) even with tiny transmission delay (fast link) — that's the video-call lag. Campus Wi-Fi during registration week = traffic intensity → 1, portal "hangs" as arrival rate nears capacity, not because bandwidth is literally gone.

4Physical Layer

Capacity formulas

Nyquist (noiseless): C = 2·B·log2(L) B=bandwidth(Hz), L=signal levels Shannon (noisy): C = B·log2(1+SNR) SNR(dB) = 10·log10(SNR) → SNR = 10^(SNR_dB/10)

Use Shannon when SNR/dB is given. Use Nyquist when levels L is given.

Line coding — recognize these 5

NRZ-L: 0=high, 1=low NRZ-I: 1=transition at start, 0=no transition Manchester: transition at MID-bit; 0=high→low, 1=low→high Diff. Manchester: ALWAYS mid-bit transition; 0=extra transition at start, 1=none AMI: 0=zero volts, 1=alternating +V/−V

3 causes of impairment

Attenuation (energy loss, fixed by amplifiers) · Distortion (frequencies travel at different speeds) · Noise (thermal/crosstalk/impulse).

Multiplexing

FDM: separate frequency bands, all sources transmit simultaneously — cable TV, each channel its own frequency slice. TDM: separate time slots, cycling round-robin — T1 phone trunks, 24 calls in fixed repeating slots.

5Medium Access Control

Only ALOHA is on your syllabus. CSMA/CD/CA are NOT (verified against both the written syllabus and the actual lecture slides).

ALOHA formulas

Pure ALOHA: vulnerable time = 2×Tfr S = G·e^(−2G) max = 18.4% at G=0.5 Slotted ALOHA: vulnerable time = Tfr S = G·e^(−G) max = 36.8% at G=1.0

Real-life: Pure ALOHA = the original 1970s Hawaii radio network (AlohaNet). Slotted ALOHA = LoRaWAN IoT sensors synced to a shared clock — halves the vulnerable window, doubles throughput.

Controlled Access

Reservation: minislots reserve a turn before sending. Polling: primary asks "data?" (Poll) / "ready?" (Select). Token Passing: only the token-holder may transmit.

Real-life: Reservation = VSAT satellite links (bank branches). Polling = old IBM mainframe terminals / industrial SCADA. Token Passing = IBM Token Ring, FDDI.

6Framing

Only Framing is on your syllabus. Error Control (CRC/Hamming/Checksum) and HDLC/PPP are NOT.

Bit stuffing (flag 01111110): after FIVE consecutive 1s, insert a 0 Byte stuffing (FLAG/ESC bytes): if FLAG or ESC appears in data, prepend ESC

Both exist so the flag pattern marking frame boundaries never collides with real data.

Real-life: Byte stuffing = PPP/PPPoE DSL modems (escape byte 0x7D). Bit stuffing = HDLC leased lines / cellular backhaul.

The byheart floor — memorize these exactly, verbatim