Viable FreeBSD Network Access Server projects...?
if at hetzner.co.za
Sun Jan 9 21:41:31 PST 2005
Martes Wigglesworth wrote:
> Also, to supply, 56K service, would I be able to use the multi-modem
> approach, or do I need to have the DSL with digital "modems" and all
> that jazz? I am reading about the digital "RAS" setups, and all the
You need digital modems to provide a 56K service. You might (or
might not) have noticed that the 56K connection is asymetric: 33.6K
max from the dial-in to the network access server (NAS) and 56K max
from the NAS to the dial-in. The reason for this is there is a low
pass filter cut-off at 4000Hz on the input to the ADC on the end
of the local loop to prevent aliasing in the analog to digital
A voice connection over the PSTN is usually provisioned over a
64kbit/s PCM channel internally in the exchange and between exchanges.
This means that the highest frequency that can be accurately
transmitted is 4KHz, half the sampling frequeny of 8KHz. This
roughly translates to 33.6Kbit/s of modulated data hence the upper
bound of an analog connection.
The 56K server modem has a digital connection to the exchange
normally over channelised E1/T1 providing ISDN or R2-MFC signalling.
The server modem can then transmit a 64Kbit/s to the exchange which
when outut to the local loop via the DAC which gives a frequency
up to 8KHz because there is no low-pass filter on the analog output
path. You could I guess theoretically get a 64K connection, but
due to losses on the local loop, the upper bound is 56K when
conditions are good.
When ran the dialup service for a large ISP (something I'm pleased
to be rid of) we used a combination of equipment from 3Com (Total
Control chasis), Ascend and Cisco AS5300. These systems provided
the total dial-in service. You might find an ISDN modem capable
of terminating an analog connection, but as far as I can recal,
there was some liscencing fee and special software required for the
56K server modem although that might have chaged - it's been years
since I've done dialup. Depending how many users you have you might
want to consider an external RADIUS or TACACS server. I personally
would put that threshold somewhere between 5 and 20.
More information about the freebsd-ipfw