I’ve been having a lot of fun lately with my Sierra Wireless HSDPA data card for my notebook which reaches speeds I still have difficulties to believe that they are possible over a wireless network. While many people probably prefer data cards for wireless Internet access I’d rather like to use my mobile phone to connect the notebook to the Internet. This is because I use connected applications both on the mobile phone and the notebook and don’t want to have two subscriptions for mobile data access. While the data card acts like a network adapter and thus has no speed limitation on the interface between the data card and the notebook, mobile phones simulate a modem over a serial port, also called a COM port. The question I thus had was if those virtual COM ports have the same speed limitations as their real counterparts. If so, this could be a problem for mobile phones that support HSDPA, WiMAX and other future wireless broadband technologies which can deliver higher data rates than real COM ports can transport.
The maximum data rate that can be set for a real or a virtual COM port is 921.600 bits/s. This is far too slow for modern HSDPA mobiles that support downlink speeds of up to 3.6 MBit/s. For real serial ports this limit is real as the speed setting in the modem dialog translate into commands to a UART chip to send bits over a serial wire at exactly this speed. Most if not all high end phones today, however, have no serial connector anymore. Instead they use USB for communicating with a PC. In order not to modify the dial up networking software of operating systems, serial port emulations over USB are used. To the dial up network softwae it looks like the ‘modem’ in the mobile phone is connected via a serial port. The question now is whether the same speed limitations apply to these virtual COM ports over USB as well. At least in the dial up networking menus, the maximum speed that can be set for them is also 921.600 bits/s. USB is much faster than HSDPA or any other wireless wide area network. Even USB 1.1 can already handle a speed of up to 12 MBit/s, so physically no limitation exists.
As I don’t have an HSDPA capable mobile phone yet I did the next best thing and used a Nokia 6680 UMTS mobile. Instead of setting the COM port speed to 921.600 bit/s, which is twice as high as what UMTS can deliver, I set it to 19.200 bit/s which equals a maximum transfer rate of 2.4 kBytes/s. This is much less than what a UMTS connection can deliver, about 40 kBytes/s. Then, I established a UMTS connection, started a download of a big file and monitored the throughput. As can be seen in the picture on the left, the COM port speed is set to 19.2 kbit/s = 2.4 kBytes/s. The download rate, however is 40 kByte/s.
This proves that the speed setting for a virtual COM port has no influence on the actual speed of the data transfer over the virtual connection. Thus, if both the driver on the PC and the mobile phone support higher speeds over the virtual COM port, HSDPA and WiMAX speeds will be supported without a need for modifying the dial up networking software of the operating system. Very well, so I am looking forward to testing things with a real HSDPA mobile phone!