[Mobile Internet Technology] Medium Access Control

 Medium Access Control

Notes from RWTH Aachen University course 
“Mobile Internet Technology” Summer semester 2020
professor: Drik Thißen

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
  • Contention-based access
  • No central coordination
  • Carrier sense:
  • listen to wire before transmission
  • avoid collision with active transmission
  • Collision detection:
  • listen while transmitting
  • In wireless:
  • signal strength decreases with distance
  • A transmission is not necessarily received by all hosts

Hidden station

• senders cannot detect each other
• CS fails (C sends to B), then CD fails (A cannot hear the collision)
• collision at receiver
• A is hidden for C
  C is hidden for A

Exposed station

• B \(\rightarrow\) A, C\(\xrightarrow[]{\text{wants}}\) D
• CS signals the medium is used (but the other medium isn’t) \(\rightarrow\) C has to wait
• C’s waiting is not necessary
• C is exposed to B

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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) guarantee 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 (power saving)
• 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
• Goal:
  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
    \(\Rightarrow\) small overlaps destroy both packets
    
    
    
• Slotted Aloha:
  • Fixed packet length
  • If collision, retransmit with probability \(p\), until successful

Collision avoidance by “Carrier Extension”

• Receiver informs potential interferer
• during an ongoing transmission  \(\Rightarrow\) Waste of bandwidth
• before a transmission \(\Rightarrow\) can use the same channel
• busy tone on signaling channel

Busy Tone Protocol

• Busy tone multiple access (BTMA)
  • Each receiving 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 (all stations: "collision detect") 
  • after decoding receiver address  
  • then RI-BTMA (receiver: "feedback", others: stop "collision detect")

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