Wireless Medium Access Control Protocols

1
Wireless Medium Access Control Protocols

• A Survery by Ajay Chandra V. Gummalla and John O.
  Limb

2
Introduction

• Survey
• Distributed vs. Centralized Networks
• Wireless MAC Issues
• Low Power Sensor Nodes
• Random Access
• Guaranteed Access
• Hybrid Access

3
Introduction Contd.

• Distributed MAC Protocols
• Distributed Foundation Wirelesss MAC (DFWMAC)
• Eliminate Yield Non-Preemptive Priority
  Multiple Access (EY-NPMA)

4
Introduction Contd.

• Centralized MAC Protocols
• Random Access
• Idle Sense Multiple Acces (ISMA)
• Randomly Addressed Polling (RAP)
• Resource Auction Multiple Access (RAMA)
• Guaranteed Access
• Zhangs and Acamporas Proposals
• Disposable Token MAC Protocol (DTMP)

5
Introduction Contd.

• Hybrid Access
• Random Reservation Protocols (RRA)
• Packet Reservation Multiple Access (PRMA)
• Random Reservation Access Independent Stations
  Algorithm (RRA-ISA)
• Distributed Queuing Request Updated Multiple
  Access (DQRUMA)
• Moble Access Scheme based on Contention and
  Reservation for ATM (MASCARA)

6
Introduction Contd.

• Dynamic Slot Assignment (DSA)

7
Distributed Wireless Network

• ad hoc network
• No central administration
• Multi-hop wireless networks
• Wireless Sensor Nets

8
Centralized Wireless Network

• Last Hop Network
• Very common
• Corporate, Academic, and Cellular uses.
• Has a controlling Base Station, with variable
  intelligence
• Wireless Access Point
• Cellular Tower

9
Wireless MAC Issues

• Half-Duplex
• No Collision Detection
• Uplink and Downlink must be multiplexed
• Time Varying Channel
• Reflection, Diffraction, and Scattering
• Different version of signal are superimposed on
  each other
• Multipath Propagation
• Coherence Time time signal strength changes by
  3dB

10
Wireless MAC Issues Contd.

• Burst Channel Errors
• Higher BER
• Errors occur in long bursts
• Link Layer retransmission based on immediate ACKs

11
Wireless MAC Issues Contd.

• Location Dependent Carrier Sensing
• Hidden Nodes Node A doesnt know Node B is also
  talking to BS
• Exposed Nodes Node A knows node B is talking,
  but doesnt know that it will not affect Node As
  conversation with BS
• Capture Node A and B are both transmitting to
  BS, but since Node As signal strength is
  stronger Node As transmission is used no
  collision is detected.

12
Random Access

• Random Access is based on a Talk whenever you
  want way of thinking
• Collisions are resolved by a contention
  resolution algorithm
• Distributed Networks

13
Guaranteed Access

• Access to medium is scheduled
• Round Robin
• Master/Slave (Polling)
• Tokens
• TDMA, FDMA

14
Hybrid Access

• Melds best qualities of Random and Guaranteed
  Access
• Request-Grant mechanisms
• Requests are Random Access, and once reserved
  transmission is guaranteed
• Random Reservation Access
• Demand Assignment

15
Distributed Foundation Wireless MAC (DFWMAC)

• 802.11 Standard
• 4-way exchange RTS-CTS-DATA-ACK
• No ACK causes sender to retransmit
• No CTS causes exponential backoff
• RTS and CTS contain a NAV which details how much
  data is to be sent

16
Elimination Yield Non-Preemptive Priority
Multiple Access (EY-NPMA)

• HIPERLAN
• Sense channel for time to send (TTS) 1700 bits,
  if clear, then send
• If busy, N slots When done listen again
• If still busy, abort Else listen again, and if
  not busy then transmit until finished

17
Idle Sense Multiple Access (ISMA)

• Carrier Sensing and Collision detection are
  performed by the BS
• When medium is idle BS broadcasts idle signal
  (IS)
• Nodes with data send
• If collision BS cannot decode signal, does not
  send ACK and broadcasts IS again
• Otherwise BS sends ACK/ISA (ISA)
• Efficiency is improved by using small Reservation
  packets

18
Randomly Address Polling (RAP)

• Nodes with data broadcast orthogonal codes
  simultaneously
• BS receives all codes, using a CDMA receiver
• BS then polls each code
• All nodes with that code transmit
• If only one node the BS sends ACK
• More than one node with code causes BS to send
  NACK
• Reservation RAP supports nodes with streaming
  traffic

19
RAP
20
Resource Auction Multiple Access (RAMA)

• Each node has and N-bit ID and transmits it, in
  contention phase
• BS then echos back ID it heard bit-by-bit
• Once a node receives a bit it did not transmit,
  it drops out
• Since BS does an OR operation on received IDs
  then node with highest ID always wins

21
Zhangs Proposal

• BS polls each node for data, round robin
• Node responds with data request, or a keep alive
  if queue is empty
• BS then polls each node that responded with a
  data request

22
Disposable Token MAC Protocol (DTMP)

• Improves on Zhangs proposal
• When polling nodes BS indicates if it has data to
  send to nodes
• If no data, then remain silent
• Otherwise send short message
• Transmit any data to send
• Channel is assumed to be reciprocal

23
Acamporas Proposal

• Poll, request, data phases
• BS polls each node, if the node has data to sends
  it responds
• The BS the broadcasts this nodes ID so that all
  nodes know the order in which to send
• BS then polls nodes each node in turn for its data

24
Various Proposals
25
Random Reservation Protocols (RRA)

• Uplink is time slotted
• Each slot large enough to carry one voice packet
• Downlink is broadcast channel
• Nodes use random access to request reservations
  for data to send
• BS enforces a policy of reservations
• Stream Reservation
• Complete BS scheduling

26
Packet Reservation Multiple Access (PRMA)

• A node with a back-logged voice packet transmits
  with probability p
• If successful, reserves that slot for following
  packets
• Data is similar, though no reservations are made
• Different access probabilities are used for voice
  and data
• Introduction of data packets into voice only
  network decreases efficiency
• Improvements include limited data reservations,
  separating voice and data channels (FRMA),
  separating request and data channels (PRMA)
• Centralized PRMA uses scheduling to achieve QOS

27
Random Reservation Access Independent Stations
Algorithm (RRA-ISA)

• BS polls a subset of all nodes
• Subset is defined by the probability of a single
  transmission in a slot is maximized
• BS uses channel history to compute subsets

28
Distributed-Queuing Request Update Multiple
Access (DQRUMA)

• Uplink and Downlink are duplexed
• Uplink has request channel and packet channel
• Request channel is for contention requests
• Packet channel is for data (and piggyback new
  contention requests)
• Downlink has 3 messages ACK for current slot,
  transmission permission for node to use next
  uplink slot, and data to the nodes
• Better than RAMA and PRMA

29
Mobile Access Scheme based on Contention and
Reservation for ATM (MASCARA)

• Frame consists of three periods broadcast,
  reserved, and contention
• Broadcast informs nodes of structure of current
  frame and scheduled uplink transmissions
• Reserved period consists of downlink data, and
  uplink data as defined in broadcast period
• Contention is random access and used to send new
  requests to BS

30
Dynamic Slot Assignment (DSA)

• MAC on uplink is TDMA
• Both uplink and downlink are slotted
• Each downlink slot contains some data and a MAC
  message
• MAC message contains ACK for transmission on
  previous uplink slot and a reservation for next
  uplink slot
• BS collects all requests and schedules uplink
  transmissions

31
Hybrids
32
Comparison Summary
33
Comparisons

• QoS guarantees are not suited to Random Access
  protocols because delay cannot be bounded
• Demand Assignment protocols are best suited to
  multimedia applications
• Random Access lends itself to large networks
• Polling protocols are efficient only for smaller
  networks
• TDD protocols perform poorly at high data rates
  due to increase in switching