# MIT-MAC Layer

## Medium Access Control
- Controlling *when* to receive and send data
- idle listening wastes energy
- **Multiplexing**
- Inflexible
- cannot support bursty traffic
> **Medium Access Control:**
> Suitable for wireless data networks
> energy-efficent

## CSMA/CD
- Ethernet MAC: CSMA/CD
- Carrier sense multiple access / collisions detection
- MAC in wireless
- **Carrier sense:**
- listen to wire before transmission
- avoid collision with active transmission
- **Collision detection:**
- listen while transmitting

### Hidden station
- senders cannot detect each other
- CS fails, then CD fails
- collision at receiver

### Exposed station
- CS signals the medium is used (but the other medium isn't)
- sender's waiting is not neccesary

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- Medium Access in wireless is **difficult** because
- Interference situation at receiver
- Impossible to send and receive at the same time

### Requirements
- high throughput, low overhead, low error-rate
- energy-efficiency

### Energy problems
- Collisions
- Overhearing
- receiving a packet to another node
- Idle listening
- protocol overhead

## Centralized Medium Access
- **Central station**
- **Advantages:**
- Simple, efficient
- TDMA: voice network
- Polling: small number of devices e.g. Bluetooth
- **Disadvantages:**
- Burdens the central station
- produces overhead and delays
- Not feasible for non-trivial network **sizes**

### Schedule based MAC
- A schedule exists $\Rightarrow$ time synchronization needed
- collisions, overhearing, idle listening no issues
### Demand Assigned Multiple Access (DAMA)
- **Reservation** can increase efficiency $\Rightarrow$ but higher delays
- Example for reservation algorithms:
- Explicit reservation: Reservation Aloha
- Implicit reservation: PRMA
- Reservation-TDMA
#### Explicit reservation
- Aloha mode: competition, collisions possible
reservation mode: no collisions possible
- All stations have to keep a **list** $\Rightarrow$ Synchronization

#### Implicit reservation Packet Reservation MA (PRMA)
- slots form a frame
- frame repeated
- stations compete for **empty slots**
- the station can use the following frame
- a slot of frame is empty $\Rightarrow$ competition

#### Reservation Time-division MA (TDMA)
- $N$ mini-slots
$x$ data-slots
- $x=N\cdot k$
reserve up to $k$ data-slots
- use mini-slots for reservation
- unused data-slots: Round-robin for other stations

### IEEE 802.15.4
- Goal: low-to-medium bit rates, moderate delays without too stringent(嚴格) guarentee requirements, low energy consumption
- **Beacon mode:**
- Star network: devices are associated with coordinators
- coordinator sends beacon for synchronization/network identification
- active phase (16 slots) + inactive phase
- active phase = CAP + CFP (GTS+GTS)
- CAP: contention access period
- CFP: contention free period
- GTS: guarenteed time slots

### Contention based MAC
- handle collisions
- Hope: coordination overhead can be saved
- randomization used

- *How to detect collisions?*
- *How to recover from collisions?*
- e.g.
- CSMA/CD
- ALOHA/ Slotted ALOHA

### ALOHA
- No central station
- Receiver sends ACK
- Detecting collisions for timing out ACK
- recover: retransmitting
- **Pure Aloha:**
- no coordination
- **no common packet length**
- risk of collisions
- **Slotted Aloha:**
- Fixed packet length
- If collision, retransmit with probability $p$, until successful

### Collision avoidance by "Carrier Extension"
- use seperated channel for signaling
- busy tone on signaling channel

### Busy Tone Protocol
- **Busy tone multiple access (BTMA)**
- Each station sends busy tone
- all stations around the sender will wait
- many *exposed station*
- **Receiver Initiated-BTMA (RI-BTMA)**
- only receiver sends busy tone
- few *exposed station*
- **Wireless Collision Detect (WCD)**
- BTMA, after decoding receiver address then RI-BTMA

## Multiple Access with Collision Avoidance (MACA)
- Use
- RTS (request to send)
- sender
- CTS (clear to send)
- receiver
- optional mechanism within CSMA/CA
- **Problem: Idle listening**
- sleeping will break the protocol (listening RTS/CTS)
- IEEE 802.11 Solution: sleeping and synchronized wake-up
- stations buffer data
- send at pre-arranged points in time