CCNP Wireless CUWSS 642-732 Summary Notes 10 : Designing a mesh network

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Design Considerations (especially outdoors)

Consider the data rate required at the edge of the cell

  • Larger cells offer binary phase shift keying (BPSK) or quadrature phase shift keying (QPSK) data rates at the edge but require fewer hops to wired network.
  • Smaller cells may offer quadrature amplitude modulation (QAM) (16QAM or even 64 QAM) at the edge hence increased performance BUT
    • More APs are needed to cover the area
    • More APs means more backhaul traffic leading to need for multiple backhauls to increase throughput and reduce hop count to wired closet.
  • Approximate AP radius per speed offered and area of the cell
    • AP with QPSK at the edge can have 0.619 m radius (in open space)
    • 16-QAM rate AP offers  0.324-m radius per cell at the edge
    • 64-QAM rate AP offers  0.226-m radius per cell at the edge
  • Common distances
    • MAP to MAP: 100 to 200 m
    • Client to MAP: 80 to 150 m
    • RAP to first MAP: 200 to 800 m ( If RAP is high, APs at the foot of the building are in the shadow area, and need to go through another MAP to reach the RAP)
    • Maximum recommended number of hops to RAP is 4
  • AP cell area can be approximated as

    Area = Pi (3.14159) x Square of the radius

    • For Mesh, neighbouring APs overlap forming a hexagon with aan AP on each side

Area = 2.598 x Square of the radius

Number of APs per Km= 1 / (2.598 x Square of the radius)

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Planning for Outdoor Surveys

  • MAPs are commonly located on light poles and RAPs on towers or rooftops
  • Issues with mounting on the pole
    • The mounting hardware might not fit the pole.
    • The antenna might end up too close to the pole.
    • Power might not be available all the time.
    • The pole might not handle the AP weight and wind load.
    • Aesthetics
  • All outdoor APs must be grounded
  • Make sure that nothing is blocking the first Fresnel zone, even the mounting point itself.
    • Poles placed near the antenna can also affect the antenna radiation pattern and create dead spots in the direction of the pole.
    • The minimum distance between the antenna and the pole depends on:
      • Pole thickness
      • Antenna height
      • the street angle, the lower pole antenna radiation may be too low to be in range with the next AP up the street.
      • Frequency to be used (5 GHz APs need more clearance)
    • the antenna should be placed far enough so that the pole does not block more than 5 degrees in the antenna field of view.
  • Outdoor mesh APs should not be used to provide indoor coverage. Indoor APs should be used to relay signal from the mesh AP
  • Surrounding environment will also affect the design
    • Buildings in the path change the cell shape
    • Light pole positions determine where MAPs can be placed
  • APs and antennas in the same physical space interfere with each other even if they are not using the same channel soAPs should be separated to limit interference.
    • Vertical separation is more efficient than horizontal separation.

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References:

  1. CCNP Wireless  (642-732 CUWSS) Quick Reference Guide by Jerome Henry

  2. IPexpert’s CCNP CUWSS Wireless Voice on Demand (642-731)

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