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Why an analog to analog connection between two V.90 modems cannot negotiate at speeds above 33.6K baud.
During the training sequence, V.90 modems probe the line to determine whether any downstream analog-to-digital conversions have taken place. If the V.90 modems detect any analog-to-digital conversions, they will simply connect as V.34
In an Analog to Analog dialup senerio, the upstream analog signal is converted (ADC - analog-to-digital converter) to a digital signal by the Telco and then rides along the PSTN. The signal then gets converted back (DAC - digital-to-analog converter) to analog to the recieving analog modem. The reverse occurs during the downstream communication.
The first ADC will limit the upstream connection to V.34 and the second ADC will limit the downstream connection to V.34.
Two examples of this limitation would be an Analog modem on a PC dialing into a Perle PCI-RAS modem card or an Analog modem on a PC into a Perle 833 RAS (that is using external analog modems).
The upstream/downstream connection would have to avoid any Analog-to-Digital conversions.
This would require that at least one side of the connection have a Digital interface to the Telco, such as connecting an ISDN modem to the Perle.
The Perle 833IS uses ISDN lines and the 833AS uses T1/PRI lines for connection to the Telco.
In a Analog to Digital dialup senerio (ie. dialing into an ISDN modem or 833IS/AS):
The V.90 connections employ one bidirectional channel, upstream and downstream. The V.90 analog modem's downstream (recieve) channel is capable of higher speeds because no information is lost in the digital-to-analog conversion. The V.90 analog modem's upstream (send) channel goes through an analog-to-digital conversion, which limits it to V.34 speeds.
Master / Slave modem configuration for V.90
In our 833AS/IS we have Rockwell chips that have Master (Central office) capabilities. External analog modems used for PC use are programmed as Slave devices.
To get a connection greater than V.34 you need a V.9x Slave modem dialing into a Master/CO V.9x modem setup. This is another requirement for the V.9x protocol. RAS server's used by ISP's (such as the 833AS/IS) are always Master devices, so they can offer V.90 to their customers.
So if a PC was to use the Perle Dialout client from us and then back into us they would have a Master to Master setup and they could not connect at V.90 speeds. This would also be the case if you used a PC to dialout to a different ISP (who would most likely be a Master) also.
The PC dialing out would have to dial into a Slave device to fufill the Master/Slave combination to obtain V.9x protocol negociation (note that the Analog to Digital conversions would make limitations to the upstream speed).
At present the 833AS/IS does not have dual Master/Slave functionality.
As the 833RAS uses external modems they will not encounter a Master/Master setup as these types of modems are commonly Slave devices.