Microwave...
Glen Turner
glen.turner at aarnet.edu.au
Thu Mar 29 02:11:18 UTC 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
glen.turner at aarnet.edu.au http://www.aarnet.edu.au/
--
The revolution will not be televised, it will be digitised
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