FHSS DSSS / CDMA Frequencies Multiple Single User's frequency Hard to find Always the same Frequency reuse Allowed Not allowed Sender wait Not needed Needs to wait if busy Signal strength High Low Penetration/Strength Stronger, penetrates obstacles Weaker compared to FHSS Interference Never affected Can be affected Cost Cheaper Expensive Usage Commonly used Not frequently used

Advantages of Spread Spectrum: Cross-talk elimination, better output with data integrity, reduced multipath fading, better security, noise reduction, coexistence with other systems, longer operative distances, hard to detect, not easy to demodulate/decode, difficult to jam.

1.8 MAC Protocols

Objectives: Enforce discipline in shared channel access, maximize channel utilization, minimize average latency, ensure fairness, support various traffic types, robust against failures.

Issues in Wireless MAC:
- Difficult to detect collisions due to weak received signals.
- Hidden and exposed terminal problems make MAC inefficient.
Taxonomy of MAC Protocols:
- Fixed Assignment Schemes: Resources assigned for entire call duration (circuit-switched).
- Random Assignment Schemes: No resource reservations; nodes transmit when they have a packet (packet-switched).
- Demand-based Schemes: Node reserves channel explicitly before transmitting.
Fixed Assignment MAC Protocols:
- Frequency Division Multiple Access (FDMA): Divides bandwidth into narrower frequency bands (channels). Each user gets two channels (forward/reverse link).
- Time Division Multiple Access (TDMA): Divides timeline into fixed-sized time slots, allocated to nodes in a round-robin manner. All sources use the same channel, taking turns.
- Code Division Multiple Access (CDMA): Multiple users allotted different codes (sequences of 0s and 1s) to access the same channel. Codes must be orthogonal (inner product = 0). Uses pseudo-random number generators (PRSG) for unique keys.
Random Assignment Schemes:
- ALOHA: Node transmits if it has data. If frame fails, it's sent again. Works for low contention.
- Slotted ALOHA: Time divided into equal-sized slots. Packet transmission starts only at slot beginning. Reduces collision chances. Requires synchronization.
- Carrier Sense Multiple Access (CSMA): Node senses medium before transmitting. Defers if busy. Reduces collision probability but hidden terminals can still cause collisions.
- Non-persistent CSMA: Senses, sends if idle. If busy, pauses random time, senses again.