Computer Networks

┌─────────────────────────────────┐
│         LLC (Logical Link       │ ← Flow control, error detection
│            Control)             │ ← Interface to Network layer
├─────────────────────────────────┤
│        MAC (Media Access        │ ← Physical addressing
│            Control)             │ ← Channel access control
└─────────────────────────────────┘

┌────────┬──────────────┬─────────────┬────────┬─────────┐
│ Header │ Source MAC   │ Dest MAC    │ Data   │ Trailer │
│ (Flag) │ (6 bytes)    │ (6 bytes)   │(Payload)│ (CRC)  │
└────────┴──────────────┴─────────────┴────────┴─────────┘

Flag: 01111110

Data: 011111110 (problematic - looks like flag!)
After stuffing: 0111110110 (add 0 after five 1s)
Receiver removes extra 0s

Even Parity: Total 1s should be even
Data: 1011001 (four 1s) → Parity bit = 0 → 10110010
Data: 1011011 (five 1s) → Parity bit = 1 → 10110111

Limitation: Only detects odd number of errors!

# Add all data bytes, take complement
data = [0x1234, 0x5678, 0x9ABC]
total = sum(data)  # = 0x1E248
# Fold overflow and complement
checksum = ~total & 0xFFFF

# Receiver adds all including checksum
# Result should be 0xFFFF (all 1s)

CRC Process:
1. Append n-1 zeros to data (where n = divisor bits)
2. Divide by generator polynomial using XOR
3. Remainder = CRC (append to data)
4. Receiver divides: remainder 0 = no error

Example:
Data: 1010, Generator: 1011 (CRC-3)
Append 3 zeros: 1010000
Divide by 1011 → Remainder = 011
Send: 1010011

Format: XX:XX:XX:XX:XX:XX (48 bits / 6 bytes)
Example: 00:1A:2B:3C:4D:5E

First 3 bytes (OUI): Manufacturer ID
Last 3 bytes (NIC): Device ID

Special Addresses:
FF:FF:FF:FF:FF:FF = Broadcast (all devices)

Sender              Receiver
  |---Frame 0--->|
  |<----ACK------|
  |---Frame 1--->|
  |<----ACK------|

Simple but slow (waits for each ACK)

Window Size = 4
Sender can send frames 0,1,2,3 without waiting

Sender              Receiver
  |---Frame 0--->|
  |---Frame 1--->|
  |---Frame 2--->|
  |<----ACK 2----|
  |---Frame 3--->|
  |---Frame 4--->|

Efficient use of bandwidth!

Pure ALOHA:
- Send whenever you have data
- Collision? Wait random time, resend
- Efficiency: 18.4%

Slotted ALOHA:
- Send only at slot boundaries
- Efficiency: 36.8% (double!)

1. Listen before sending (Carrier Sense)
2. If busy, wait
3. If idle, send
4. While sending, listen for collision (CD)
5. Collision? Send jam signal, wait random time

1. Listen before sending
2. Send RTS (Request to Send)
3. Receive CTS (Clear to Send)
4. Send data
5. Receive ACK