CD-ROMS are great things. After all they have allowed a whole new genre of software to be developed which, because it incorporates sound, video, still images and searchable data, requires hundreds of megabytes of previously unavailable storage space. But, as these optical discs proliferate, users are coming up against a barrier: they can only access one disc at a time and must switch discs to change titles. This can lead to problems where users reasonably expect to need more than one disc, such as in multiple-disc titles, or when accessing large file collections. Also, in a networked environment where many users need to access multiple discs, single CD-ROM drives can be limiting. Some companies have addressed this with CD jukeboxes. These have one or two reading mechanisms but can hold many discs. The problem, some in the industry say, with this solution is that only one or two discs can be accessed at once, not ideal for an on-line network environment. By repeatedly changing discs they become over-used, being grabbed by mechanical 'hands' and inserted into or removed from drives. One solution is a CD-tower, which allows many CD devices to be housed in one enclosure with one connection to a PC. In a multimedia or reference environment this can allow many users to access multiple discs. It is useful for libraries, businesses with large databases and environments with regular, heavy multimedia requirements. Hong Kong's BIOS Solutions distributes for California-based Procom Technology, which produces a range of network storage solutions and several multimedia-orientated packages including multimedia upgrade kits and CD-ROM towers. Procom sells towers that can hold from seven CD-ROM drives to 21 CD-ROM drives. Both the CD-Tower7 and CD-Tower21Plus can use double-, triple-, or quad-speed CD-ROM drives. The basic models connect to a file server or workstation through the SCSI port. If the server is connected to the network, then using peer-to-peer networking software, the drives can be made available to all network users. The seven-drive model can use separate SCSI IDs for each drive or, where more devices will be needed in the SCSI chain, one model can be used with a SCSI interface card that supports logical unit numbering (LUN). LUN allows up to seven devices to be mapped to one SCSI-ID. Likewise, the 21-drive model uses this smart SCSI mapping to map the maximum of 21 drives to three SCSI IDs. There is also a seven-drive model available to be connected directly to an Ethernet network, but because of the SCSI-to-Ethernet address mapping scheme used, it is impossible to directly connect a 21-disc model to an Ethernet network. Still, there are 21-drive towers available with built-in motherboards and monitors, keyboards, hard drives and floppy drives so that they can act as stand-alone servers not requiring a dedicated PC to act as the server on a Token Ring or Ethernet network. Of course the multimedia user is limited in a networked environment - and this is to be expected - by network throughput. If a user needs to make heavy use of video from a CD-ROM, a high-speed LAN backbone or switching Ethernet would be desirable. Otherwise a dedicated, SCSI-connected tower can provide the desired throughput for one workstation. Even so, with quad-speed drives installed, the 21-drive tower provides surprisingly fast access times, even if the video clips are a bit choppy. For mission-critical data access, the Procom towers provide several fault-tolerant features, including hot-pluggable power supplies for each bank of seven drives, a redundant power supply that will kick in if a power supply fails, as well as hot-pluggable fans. Perhaps the biggest drawback of the towers, at least for multimedia users, is that they do not support audio CD playback, because it is not possible to connect speakers directly to the drive audio ports. Even so, as the number of available CD titles grows into the thousands, the need for towers will increase, especially as organisations make significant investments in large numbers of packages.