North American Network Operators Group

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Re: Microwave...

  • From: Glen Turner
  • Date: Wed Mar 28 20:20:35 2001

AARNet run a comprehensive microwave network through the
Australian metropolitan areas to service universities and
research institutions.

We rarely find RF issues on the mcirowave links themselves.
The units are supplied under contract and tested bit and
burst errors between the two indoor units for a day before
we take acceptance.  The contractors provide annual maintenance,
at this time the Automatic Gain Control level is checked
against recorded values and this gives indications of
tree or buildings that may have appeared on the microwave
path.

Australia has a spectrum allocation agency that registers,
licenses and taxes point-to-point microwave links.  Thus
there is a low incidence of interference.  Australian
city centers, with the exception of Sydney and Melbourne,
are not as populated as in North America.

Sites where the RF engineering is at the nominal performance
boundaries do experience considerably more rain fade then
sites that are well within those boundaries.  Since there
is more rain in the US than in Australia, this may be
an issue for you.

Because each 'customer' site does its own networking
(ie: we don't engineer the network at both ends) we
have had considerable difficulty with, in order:

  1) getting sites to calculate G.703 electrical
     power levels.

     G.703's output power at the transmit port is
     unspecified in the standard, G.703's input
     power at the receive port is.

     The intent was that this allows manufacturers
     to design for the expected coaxial cable length.
     Most manufacturers have jumpers to select a
     "short" cable or a "long" cable.  Some manufacturers
     require you to insert attenuation (such as a RF
     test resistor) instead.

     Some equipment, notably the Cisco LS1010 ATM switch,
     has differing output levels between cards with
     differing port densities.  This complicates dealing
     with a port failure.  If you are starting from
     scratch use the same card everywhere (high density,
     with access to the clock redistribution bus).

  2) Getting sites to understand clocking.  As always
     there should be one, and only one, clock.  Preferrably
     generated by the upstream site.  If you want to
     emulate a T1/E1 circuit, then the clock should also
     be derived from a telco clock.

     Unfortunately, power level and clocking misconfigurations
     result in the same error counters incrementing.

  3) Not propoagating AIS.

     Most equipment will not propogate Alarm Indicatin Signal
     or insert RDI in their default configuration.  This allows
     the device to be tested.  Once testing is complete AIS
     propogation should be enabled and RDI insertion activated.

     Thus when a physical-layer error occurs it is instantly
     propogated to the router/switch interfaces at *both* ends.

     This is much better than relying on the routing protocol
     to discover the unreachability some minutes later, even
     if no alternative route is available (because the users
     instantly receive a Network Unreachable rather than
     timing out and the "show interface" shows the true
     status of the link).

  4) RF engineering practices on the G.703 link between
     the router/switch and the microwave indoor unit.

     In particular: long runs parallel with non-RF cables,
     small turn radii, ground currents because shields are
     terminated at both ends on long-distance hauls.

     If you are running the G.703 link some distance
     I would seriously consider using a G.703 opitcal
     modem and running multimode fiber rather than
     coaxial cable.

  5) Poor RF engineering by switch/router vendors.

     Thus the huge baluns on the Cisco-supplied coax
     cable and the warning not to manufacture your
     own (which, realistically, you *should* do as
     loops of excess G.703 coax is asking for trouble).

-- 
 Glen Turner                                 Network Engineer
 (08) 8303 3936      Australian Academic and Research Network
 [email protected]          http://www.aarnet.edu.au/
--
 The revolution will not be televised, it will be digitised