IBM will announce upgrades to its mainframe computer on Friday, refreshing a high-end server line many had given up as extinct. The upgrades improve some of the features of the z900 mainframe, introduced a year ago, though it leaves the heart of the refrigerator-size machine unchanged. The changes make the mainframe better able to run the Linux operating system, help it screen out some types of computer attacks, improve the system's ability to conduct encrypted communications, and make it possible to add memory without shutting the machine down. Mainframes are powerful but expensive computers that can handle fast communications with other computers, making them good for heavy-duty tasks such as processing credit card or bank account transactions. Mainframes also can be partitioned into several independent computers, a useful feature for consolidating work handled by numerous servers into an easier-to-manage system. Mainframes once were sold by many companies, including Hitachi Data Systems, Amdahl and Unisys, but IBM is for the most part the last company to pursue the strategy. Sun Microsystems and others, though, have been putting some pressure on the mainframe market with high-end systems that are approaching mainframe capabilities. These systems, while not necessarily replacing mainframes, can unburden them by taking over some tasks that might otherwise require new mainframes. And high-end Unix servers from Sun, Hewlett-Packard and, most recently, IBM itself now feature partitioning capabilities. Mainframe revenue at IBM still is doing well, though, with 30 percent growth in IBM's most recent quarter. Among the improvements -- some of them expected -- IBM will announce

The addition of IBM's Intelligent Resource Director software, which lets the mainframe automatically add or remove more resources such as memory or communication bandwidth to adjust for changing computing demands. IRD accommodates Linux and the z/VM software that allows the creation of many partitions.

"HiperSockets," networking technology that allows high-speed connections between partitions on the mainframe. Previously, external network connections were the only way to handle such communication.

An improved expansion card that sets up encrypted communications with the Secure Sockets Layer standard used to set up secure Web pages for tasks such as transferring credit card information. The new card can handle 3,850 secure transactions per second, nearly twice that of its predecessor, and now works with Linux partitions.

The Capacity Backup feature, which lets a company gracefully move work from a backup mainframe back to the primary machine.

Faster connections using IBM's fibre-optic Ficon standard. The Ficon Express card enables communications at 120MB per second instead of the previous 70MB per second.

See ZDNet UK's Enterprise Channel for full coverage. Have your say instantly, and see what others have said. Click on the TalkBack button and go to the ZDNet news forum. Let the editors know what you think in the Mailroom. And read other letters.