T3 to E3 conversions
Sean M. Doran
smd at clock.org
Tue Jul 14 09:47:34 UTC 1998
| How do carriers handle E3 to T3 conversions?
That is, they don't convert.
It is possible to carry an E3 inside a PDH T3, but it's very ugly.
These days an E3 or a T3 is carried inside an SDH VC3, which is
then carried in a VC4, which is then carried inside an STM-1 or an
OC3, and broken out of the final SDH or SONET ADM by using an
appropriate trib card. (At the "final" SDH or SONET ADM, a T3
payload may be digitally cross-connected to a T3 that will be
carried in a disjoint SONET network. This avoids dedicating
an OC3 to carry the VC4/VC3 structure).
If you are fortunate and have an end-to-end SDH/SONET path, then
you can use a VC3 in any way you want. Typically, however, for
transatlantic traffic, you have to choose one or the other, or an
ugly hybrid that uses an E3-in-T3 scheme, because someone will be
using PDH gear in the end-to-end path. Usually, the reason you
can't do an entire VC3 is because the US carrier doesn't want to
waste bandwidth (as above). The commoner case is that you
can't do a T3 end-to-end because a non-US carrier has PDH gear in
the path that can only do E3s, as SDH has a formally specified and
widely implemented means of carrying around T3s.
| What equipment is used?
The ICM E3 path formerly used by NORDUNET was an E3 from
Stockholm to London, connecting to a Cisco router ("speed
converter"), and a DS3 from London to Pennsauken, connected
to another port on the "speed converter". The "speed converter"
ended up being a "TDM" of sorts, since an 8Mbps circuit was
eventually stuck to yet another port. This was not an SDH/SONET
issue, however; SDH-using or SONET-using cables are quite new.
| have heard that trans-Atlantic carriers sell E3s to Europeans when in fact
| it is a T3 with T3 costs passed on the customer and merely convert the line
| from T3 to E3 - thereby causing 11Mb/sec of b/w to be wasted. Any truth to
The T3 and E3 tarriffs are probably the same, but nobody in their
right mind would waste even more of a VC3 deliberately by forcing
you to take an E3 instead, not even (most) PNOs, since they'd have
to coordinate PDH encapsulation. But not all cables and land paths
are SDH or SONET yet...
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