Roland Turner wrote this...
> At 12:00 PM 7/31/96 +1000, Peter Meric wrote:
>> On Wed, 31 Jul 1996, I drive a go-go mobile wrote:
>>> Matthew Keenan wrote the following:
>>>> 9514 isnt Telstra it Optus.
> This isn't quite how it works.
well here are some holes in yours :)
> PSTN (analogue) calls cross the intelligent network as single
> B-channels (64Kbps). It makes no difference at all whose network it
> crosses, 64Kbps is 64Kbps, pretty much error free. (Incidentally,
> this is why V.34 is the end of the road for analogue modems. The
> Shannon liomit means that the underlying carrier must have at least
> twice the bandwidth of the signal being carried, thus for PSTN that
> limit is half of 64Kbps which is 32Kbps. Allowing for a few losses
> here and there, 28.8kbps is about the limit.)
this for the most part is correct. good to see some people actually
understand signal theory :) WOOHOO!
> The line that you are dialling in with is analogue only to the first
> optimux/exchange. It is generally safe to assume that it will have
> the same (analogue) characteristics from call to call, so if you are
> seeing different characteristics for two different paths into the
> same modem, the problem is almost certainly arising at the UTS
> end. Because 9514 is digitial all the way into UTS's premises (I
> believe) the problem (if indeed it exists) is actually with the
> equipment on UTS's premises. At a guess, this equipment belongs to
> UTS and is therefore the domain of ITD.
UTS's voice feeds are digital, and go into our (digital) PBX (a NEC
2400 IMS to be specific). the lines however are translated from
digital internally inside the PBX (or PABX if you prefer) to analogue
(so that the modem can understand it) before being pumped to the
modem. now.. i think you should be lucky to get 28.8 because we have a
2400 SDS here at work (an older model of the 2400 range), and it cant
punch anything beyond 20,100 baud. we have had to get external lines
right off the MDF to get faster speeds than that.
i think you might find though that it is Optus' equipment that makes
the diff if Telstra's 330 switch (which redirects off to the Optus
9514 switch, which it has been doing for some 6 months or so now) runs
faster. and i guess you would probably need to look and find where
Optus' main trunk line to Telstra's sydney local network is, and this
may be different from the trunk it is using to get from 330 to
9514. Maybe Telstra are deliberately (conspiracy theory here :)) doing
funny business on all traffic heading down to Optus on this main
trunk, keep people using Telstra local services maybe? *shrug*
a couple of ways to test this would be to talk to a design engineer
and find out what trunks go where, and also to try dialing into the
9514 switch from an Optus local service (since it would stay inside
their network).
another point is that it is not unheard of for PCM (the same
modulation modems use for sending data) compression to be used on
voice line over long distance or heavy use trunks. this _will_ affect
the maximum speed you can get out of a modem on a voice line across
such a trunk. data lines and hence ISDN lines are typically guaranteed
not to have such compression.
> Perhaps they bought bargain basement equipment.
at the time the PBX was installed it was only the second installation
of such equipment in *.au (after the ABC), and such equipment is
extensively used throughout *.au (99% of line men/engineers got their
initial PBX training on them). the NEC 2400 boxen are quite amazing
for what they achieve, even more so for when they were released. and
if you had ever seen the regulations you need to hop through in order
to get telephony equipment approved you probably appreciate it more.
Matt
-- Matthew Keenan Network Administrator First Pacific Stockbrokers Sydney, Australia