Wi-Fi MAC basics: IEEE 802.11(1)

Centralized multiple access

Why we need multiple access scheme

• bandwidth is a scarce resource at the air interface (頻寬是珍貴的資源)
• there are a numbers of users that want to transmit （有很多users想要transmit）

Centralized vs. Random access

• Centralized: master contorls assignment
  • Wireless Wide Area Netowrk (WWAN)
  • Wireless Metropolitan Area Networks(WMAN)
• Random: Station compete for access
  • Wireless Local Area Network (WLAN)
  • Wireless Personal Area Network (WPAN)
  • Wireless Body Area Network (WBAN)
• Therera are total four centralised multiple Access schemes
  • Frequency Division Multiple Access (FDMA)
  • Time Division Multiple Access (TDMA)
  • Code Division Multiple Access (CDMA)

Frequency Division Multiple Access( FDMA)

• Share avaliable bandwidth in the frequency domain

• avaliable bandwidth is divided into a number of channels;

• there should be a guard band between adjacent channels;

  • guard band —> inefficient use of the spectrum

• each transmitter/receiver pair is assigned the same channel for operation

Time Division Multiple Access (TDMA)

• Share avaliable bandwidth in the time domain
• frequency band is divided into a number of time slots
• a set of periodically repeated time slots is known as TDMA frame
• each node is assigned a slot in each frame and transmits only in this slots

Spread Spectrum Techniques

• every user uses the entire spectrum
• individual transmission are encoded with pseudo-random sequences
• assigned codes are orthogonal so that the simultaneous transmission are possible
• Two types
  • Frequency Hopping Spread Spectrum (FHSS)
    • Spectrum is divided into many subchannels
    • two communicating systems hop on same frequencies
  • Direct Sequence Spread Spectrum (DSSS)
    • station are assigned orthogonal codes
    • use these codes for transmission
    • other stations transmission appears an noise

Random Access Schemes

• provideds access to a channel for multiple concurrent stations
• is not needed when there is a centralized control
• required when the access is distributed

• can be used for decentralised in TDMA and FDMA channels

• ALOHA and slotted ALOHA
• Carrier Sense Multiple Access (CSMA)
• Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)
• Carrier Sense Multiple Access with Collision Detection (CSMA/CD)

ALOHA

Pure Aloha

• a terminal transmits whenver the user data is ready
• if the sender finds that the packet get collided
  • it waits for a random period of time and sends the packet again

Slotted Aloha

• time is slotted, length on the slot is the time to transmit a packet
• node starts transmission in the beginning of slots only
• if collisions occurs:
  • sneder waits for a random number of slots
  • transmits packet again

Throughput: Slotted Aloha is better than Pure Aloha but still low

Carrier Sense Multiple Access

• Throughput of ALOHA is low
• should listen for packet transmissions
• 3 types of CSMA
  • 1-persistent CSMA
  • non-persistent CSMA
  • p-persistent CSMA

1-persistent CSMA

• when the packet is ready for transmission, the sender listens to the channel

• if the channel is free, packet is immediately transmitted

• if not ,the senders continues to listen till the channel becomes free

• If two are more stations becomes ready at the same times, collision happens

  • Probability of starting transmission when the channel is free: 1.

• Two bad effect

  • Wrong “Channel free” effect

    • an arbitray node starts transmitting
    • a node near the destination sense the channel and finds it free since packet has not yet arrived

  • Synchronization effect

    • the propogation delay

Non-persistent CSMA

• to solve the synchronization problem
  • when the packet is ready for transmission , the sender listens the channel
  • if the channel is busy , the sender goes in the waiting state for a randomly chosen time
  • after this time , the sender sense the channel again

p-persistent CSMA

• the cahnnel is slotted
• transmission is free channel is preformed with Probability $p$
• When the packet is ready for transmission , the sender listens the channel
• if the channel is busy , the sender keeps listens the cahnnel until it finds the channel idel;
  • if idle
    • the sender transmits the packet in this slot with probability $p$
    • defers transmission to the next slot with probability $q=1-p$

Carrier sense multiple access with collision detection(CDMA/CD)

• if the collision is detected , the nodes immediately aborts its current transmission
• then, the node sends a brief jamming signal
• any other transmitting node on hearing the jamming signal abort their tranmissions
• after transmitting the jamming signal the node waits for a random time and repeats the CSMA.