-This is just a start really, you need to consider how many times the modem packet sent will go from analog to digital only to come back in analog again. When you send a signal through your phone line if your lucky it will be turned into a digital signal through a switching station. Then from here we hope that it will get to your ISP and make a even better digital connection. This is were you get the best 53k download connection from you ISP. Now this is a good download but when you send information the data stream wont be so fast, you will still poke around at 33.6 sending on a different frequency. If you remember people were saying that modems could never make it past 33.6. The reason is its a analog line. Why then can you download at 53k now? Your modem is made to take advantage of the digital switching of your phone company and ISP (internet service provider). If the ISP isn't up to date an your phone lines are bad a fast connection will not be present. If its the other way around you will get to take advantage of your ISP digital backbone such as a T1,T3 and even a ISDN.
-Most of your modem woes will be contributed to a bad phone line or poor switching.
-Now, how does your 56k modem know how to talk to say a 33.6 modem down the street? This is where we get into standards. This may also unveil a little mystery to the whole V.90 and so on. Check out ITU Standards below.
Modems - Little On 56k 
