e 1980s. The original idea was that it should be used embedded on full-size ISO card (the standard form factor for credit cards) but as phone form factors became smaller through the 1990s it became normal to use the device on its own. The device is 0.8 mm high, which apart from the Smart Media Card (see below) makes it the thinnest of any of the plug-in memories and one of the smallest (26 mm X 15 mm). Early SIMs consisted of an 8-bit microcontroller, 8 k of ROM and 250 or SO bytes of RAM and were used primarily to store the user's phone number (IMSI/TMSI) and to provide the basis for the A3/A5/A8 challenge and response authentication and encryption algorithms. Early SIMS also had a limited amount of data bus bandwidth, typically 100 kbps. A SIM in 2005 had the same mechanical form factor but used a 16-bit or 32-bit micro- controller. Most of the vendor product roadmaps suggested then that one gigabyte of data storage would be technically and economically feasible within 2 to 3 years coupled with a faster bus (up to 5 Mbits/s) and faster clock speed (up to 5 MHz). These devices were termed HC SIMS (high-capacity SIMS), or MegaSIMS or SuperSIMS depending on the vendor and Making Telecoms Work were targeted at the image storage market. A SIM used with a UMTS phone was known as a USIM (universal subscriber identity module) with additional functionality in terms of device and service identification. An ISIM was a SIM optimised to work with the IP multimedia subsystem intended to provide the basis for IPQOS management and control. Although the SIM remains a mandated component within UMTS and a mandated part of the 3GPP1 and 3GPP2 standardisation process, it's longer-term role in managing authentication, access and service control is being challenged by microprocessor and memory manufacturers. ARM11 microprocessors for example were promoted in terms of their ability to manage user keys and as a mechanism for providing network virus and M commerce protection. ARM/TI dual-core microprocessor /DSPs were promoted in terms 