multiple access links
Summary
Protocol properties
| Protocol | Collision free | Efficient | Fairness | Fully decentralized |
|---|---|---|---|---|
| Pure ALOHA | ❌ | ✅ | ✅ | ✅ |
| Slotted ALOHA | ❌ | ✅ | ✅ | ✅ |
| CSMA | ❌ | ✅ | ✅ | ✅ |
| CSMA/CD | ❌ | ✅ | ✅ | ✅ |
| Polling | ✅ | high, but polling overhead | ✅ | ❌ master node |
| Token Passing | ✅ | high, but wait for token | ✅ | ✅ |
| TDMA | ✅ | low, unused slots are wasted | ✅ | ✅ |
| FDMA | ✅ | low, unused time is wasted | ✅ | ✅ |
Concept
- point-to-point links
- no need access control
- PPP, SLIP
- broadcast links
- need access control
Shared link
- broadcast link, 1 link handle
Nnodes - collision - when 2 or more nodes transmit simultaneously
- link access control - coordination between nodes to avoid collision
- error detection - detect errors caused by noise/attenuation
Types of access control protocols
- random access protocols - allow collisions to happen, recover from collisions
- taking turns protocols - each node takes turns transmitting
- channel partitioning protocols - divide channel into a smaller part, allocate to a node for exclusive use
Properties
- for a broadcast channel rate of
- collision free
- efficient - if only one node wants to transmit, it can transmit at
- fairness - when
- fully decentralized - no dedicated coordination node
- no out-of-band signalling(mandatory)
Error detection
- detection
- single-bit parity - single/odd number of bit errors
- CRC - odd number of bit errors, small bursts
- correction