[ILUG] Subscription ISDN ISPs
Niall O Broin
niall at linux.ie
Fri Jan 25 14:20:38 GMT 2002
On Thu, Jan 24, 2002 at 01:42:47AM -0000, Enda wrote:
> Questin buggin me in the back of my mind for some time, maybe someone can
> enlighten me... does ISDN compress or bloat the data being transferred along
> the channels or is the data transferred in the nip?
Naked data, all the way :-) And to add to it, ISDN is a synchronous protocol
so 64Kbits/s = 8KBytes/s. Modem protocols are asynchronous so even if you get a
claimed 50Kbits/s connection your data transmission rate = 5KBytes/s
(because every 8 bits of data are preceded by a start and stop bit hence 25%
of the bits transmitted are pure waste from the point of view of
throughput). There are types of cmpression used on the link - TCP header
compression, for instance - but they can be used by modem or ISDN. There's
also the protocol overhead but again, that applies to both modem and ISDN.
> Reason for the question is to decide on whether or not the pain of
> configuring an internal TA is worth it.... external ta's in my mind are
> going to lose speed when they communicate at 115K with the com port (ok...
> USB is different) unless the data is bloated between the two ends of the PPP
I don't like serial attached TAs both because they limit your speed (you
can't use the full capacity of both channels if needed/wanted) and because
all that conversion down the serial line offends my engineer's soul AND if
you're a gamer, it leads to higher latency :-)
For any modern distro. configuring an internal TA with ISDN4Linux is really
easy and setting up dial on demand is much easier than with diald and a
modem BUT make sure you're monitoring the damn thing - a 3cm thick ISDN bill
is a terrible thing to behold.
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