Date: Mon, 10 Jan 2005 07:41:14 +0200 From: Ian FREISLICH <if@hetzner.co.za> To: martes.wigglesworth@earthlink.net Cc: ipfw-mailings <freebsd-ipfw@freebsd.org> Subject: Re: Viable FreeBSD Network Access Server projects...? Message-ID: <E1CnsIk-0006Un-00@hetzner.co.za> In-Reply-To: Message from Martes Wigglesworth <martes.wigglesworth@earthlink.net> <1105221596.683.387.camel@Mobile1.276NET>
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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 conversion. 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. Ian -- Ian Freislich
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