Design and Evaluation of a New MAC Protocol for LongDistance 802'11 Mesh Networks

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Design and Evaluation of a New MAC Protocol for
Long-Distance 802.11 Mesh Networks

• Bhaskaran Raman (IIT, Kanpur, IN), Kameswari
  Chebrolu (IIT, Kanpur, IN)
• MobiCom 2005

Presented by Chunyu Hu
Friday, 23 July 2005
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The assumed mesh network

• 3 relevant characteristics
• Multiple radios per node
• Use of high-gain directional antenna
• Long-distance point-to-point links (several kms)
• Use one single channel

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Outline

• Contributions
• Background on SynRx/SynTx
• The proposed MAC protocol 2P
• Enabling technique topology construction
• Evaluation

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Contributions

• Propose a MAC protocol 2P
• Can achieve 3-4 times saturation throughput than
  CSMA/CA
• Can achieve up to 20 times TCP throughput of that
  by CSMA/CA
• Key idea switch between a transmission phase and
  a reception phase at each node
• Perform an experimental study
• Exploiting the Prism2-based chipsets
• A successful case of exploring multiple
  transceivers operating at one channel

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Background on SynRx/SynTx

• Syn-Rx
• R1 and R2 receive simultaneously
• Feasible physically
• Syn-Tx
• T1 and T2 transmit simultaneously
• Feasible physically
• Mix-Rx-Tx
• R1 receives and T2 transmits
• Not feasible because the signals that T2 and T1
  transmit interfere with each other at receiver R1

3 possible operating ways of node N. Node N has
two transceivers for links with A and B,
respectively.
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Protocol feasibility of SynRx/SynTx?

• Assume the IEEE 802.11 MAC DCF protocol
• Problem with SynRx
• If R2 finishes receiving first, N is supposed to
  reply an ACK immediately. However, the ACK will
  incur a collision at the other receiver R1
• Problem with SynTx
• If T1 starts transmission earlier than T2, then
  the other transmitter T2 will carrier-sense T1s
  signal and thus invoke the backoff procedure
• The key reason
• The presence of side-lobes due to the
  imperfection of directional antenna

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Tackling the problems encountered

• Conquer the problem by physical means?
• Costly in using better antennas with no or
  negligible side-lobes (600 vs. 20-50 parabolic
  grid antenna)
• Besides, antenna has the near-field effect (that
  is, the directionality comes into play only at
  longer distances)
• Specialize the MAC protocol to detour the problem
  the 2P MAC protocol
• Each node switches its transceivers between two
  phases SynRx and SynTx in another word, each
  node synchronizes operations of its links
• Have to disable immediate ACKs and the
  carrier-sense mechanism

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Challenges

• How to achieve 2P on off-the-shelf hardware?
• Disable ACK
• Disable carrier-sense
• How to achieve 2P without tight synchronization
  across the network?
• Synchronization of links means synchronization
  among nodes
• What kind of topology that a network has to have
  to be able to operate in 2P?
• Bipartite requirement

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Outline

• Contributions
• Background on SynRx/SynTx
• The proposed MAC protocol 2P
• Enabling technique topology construction
• Evaluation

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Enable SynOp in existed hardware

• To disable immediate ACKs
• Operate all interfaces in the IBSS (ad-hoc) mode
  with individual SSIDs
• Convert IP unicast packets to MAC broadcast
  packets at the driver level
• Send ACKs in the LLC implemented in the driver
• Piggy-back ACKs in DATA packets
• To avoid carrier-sense backoff during SynTx
• Explore the feature provided by the Intersil
  Prism chipsets two antenna connectors for
  diversity
• See next page

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How to avoid carrier-sensing

• Many 802.11 radio cards have two antenna
  connectors for diversity, say LEFT and RIGHT
• An antsel_rx command is provided to select the
  receiving antenna
• Connect the actual antenna to one of them, e.g.,
  LEFT
• During tx, set the receiving antenna to RIGHT,
  thereby enforces the carrier-sense is performed
  on the unconnected RIGHT. Then only noise will be
  sensed
• Introduce a switch overhead between SynTx and
  SynRx

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Achieving synchronization

• Synchronization is needed among links and thus
• Among all nodes
• 2P operation on a single link
• Among all the wireless interfaces owned by each
  node
• Communication across all interfaces at a node

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2P operation on a single link

• A node in steady state
• An interface sends B bytes in SynTx, then
• Sends a marker packet
• Enter the SynRx phase
• Switch to SynTx upon receiving a marker packet or
  upon timeout

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Avoid deadlock in synchronized switching

• Problem two ends of a link get out of synchrony
  and timeout at the same time
• Solution add some random perturbation to the
  timeout value each time

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Communication across interfaces at a node

• Coordination of interfaces to switch from SynRx
  to SynTx
• Once an intf. Decides to switch to Tx, it sends a
  notification (NOTIF) to other intfs.
• Means shared memory or ethernet messages
• Aware of UP/DOWN status of other intfs.
  (observation of 3 consecutive time-outs implies
  DOWN)
• Coordination of interfaces to switch from SynTx
  to SynRx
• Not necessary since all intfs. begin Tx
  simultaneously and with the same duration (B
  bytes)

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Remarks

• Overhead in 2P operation
• Each round (SynTxSynRx) has fixed overhead time,
  including the change of antsel_rx and the marker
  packet
• For B10KB, the per-round overhead is 6
• Time-out is set to 25-50ms
• In case that there is no data packet to transmit
• Send dummy filler bytes to maintain synchrony
• Implementation
• Implement 2P by modifying the HostAP open-source
  Linux driver (v0.2.4) for Intersil Prism based
  802.11 chipsets
• Experiment with various PCMCIA as well as PCI
  prism-based devices

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Outline

• Contributions
• Background on SynRx/SynTx
• The proposed MAC protocol 2P
• Enabling technique topology construction
• Evaluation

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Constraints in topology

• The bipartite constraint
• Because at any time, if a node is in SynTx, all
  its neighbors must be in SynRx and vice versa.
• Power constraint
• The signal level has to be above the RXThreshold
• SINR has to be above SIR_Threshold
• For a specific topology, they are expressed by

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Expression of power constraint for a specific
topology
consider transmission power
consider interference from all other nodes
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Parameters involved

• P_min -85 dB for 11Mbps reception
• SIR_reqd 10 dB for the 10-6 BER level, set to
  14-16 dB in topology construction
• The antenna radiation pattern that decides the
  gain in different angles

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Topology formation

• Given a network, it is an open question whether
  its topology is feasible to be operated in 2P
• Solution construct a tree topology that
  satisfies the two constraints
• Suppose all (or most) traffic passes through the
  land-line node and dont do multi-path routing
• A tree rooted at the land-line node satisfies the
  bipartite constraint
• Consider the power constraint in constructing the
  tree a heuristic algorithm (see next slide)

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A heuristic algorithm of creating a tree topo

• Highlight
• First, seek to connect nodes that are closest to
  the land-line (1-hop) node
• Then, extend it to the next set of closest nodes
  (2-hop) and so on
• Three heuristics used
• H1 reducing the length of links chosen
• Long links tend to interfere with more links
• H2 avoid small angles between links
• Avoid effect of side-lobes
• H3 reducing the hop-count
• Reduce latency

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Evaluation of the tree formation algorithm

• The evaluate purpose
• The effectiveness of the algorithm
• The effect of the parameter SIR_reqd
• Evaluation subjects
• 4 collections of villages from a local district
  map
• Q1, Q2, Q3 and Q4
• Q1 has 31 nodes
• Q2-Q4 have 32 nodes, respectively
• Topologies randomly generated
• 50 nodes in an area of 44Km X 44Km

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Topology formation results set 1
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Topology formation results set 2
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Outline

• Contributions
• Background on SynRx/SynTx
• The proposed MAC protocol 2P
• Enabling technique topology construction
• Evaluation

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Simulation study of 2P

• Goals to investigate
• The impact step by step that the link
  establishment has on a already loosely
  synchronized network
• The saturation performance compared to the
  traditional CSMA/CA protocol
• The TCP performance over the 2P operated network
• Simulation tool
• ns-2 extended in several aspects
• Multiple interface support
• Directional antenna support
• MAC modifications
• LLC modifications

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Simulation results set 1

• Link establishment
• Method add links one after another to a network
  that is already synchronized
• Result
• It took 12.9ms for the first link to join in
• It took 4.9ms for all other links to join in
• No noticeable disturbance in the throughput
• Explanation for the first link, the first
  transmission of both ends coincide (comment
  still vague)

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Simulation results set 2

• Saturation throughput
• UDP traffic
• One packet every 2ms
• Packet size 1400 bytes
• Results
• Nodes operated in 2P achieve around 3-4 times
  more bandwidth than operated in the CSMA/CA
  protocol

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Simulation results set 3

• TCP performance
• ftp traffic
• Up to 20 times better performance than CSMA/CA

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Experimental study of 2P

• Testbed
• Hardware
• Prism2 cards
• Power setting
• 0dBm at A, 5dBm at B and 0dBm at both interfaces
  at N
• HostAP v0.2.4 driver on Linux v2.4.20-8

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Experimental results

• Confirmation of the feasibility of SynTx and
  SynRx and infeasibility of Mix-Rx-Tx
• 2P performance on a single link
• Saturation throughput 5.6Mbps on link A-N
• 2P performance on a pair of links

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Comments

• 2P is attractive in performance
• Concerns
• Require one dedicated transceiver for each link
• Expensive
• Very limited in flexibility
• Have to reconfigure if a node joins, or is
  removed or is re-located
• The topology is constructed in a centralized way
• Transmit even having no packets
• Create interferences for other networks/devices