ltiples and others do not support HO multirate ISDN bonding. MCUs are evaluated by size and capabilities, and described by the number of sites supported. Most low-end MCUs come equipped with four ports and can grow to Group Systems eight ports. Larger systems can group up to 72 ports and be daisy-chained to link hundreds. Some smaller systems can be expanded by adding modules while others top-out at six or eight ports. Because all systems are limited by some maximum aggregate bandwidth, they may limit transmission speeds when called upon to connect too many sites. There is usually a perceptible end-to-end transmission delay in conferences in which three or more bridges are daisy-chained. There is usually a limit to the number of MCUs one can daisy-chain because doing SO consumes a port on the MCU (used to make the daisy-chain connection to other devices). MCUs generally support all common types of interfaces for public and private networks including Ethernet, T-1, F-T1, ISDN BRI, and ISDN PRI. Most allow dial-in (meet-me), dial-out or dedicated-access connections in any combination. Most also allow connection arrangements to be mixed in a hybrid conference. Today's MCUs uses tones to signal a conferee's entrance and exit. They also permit conferees to participate in the conference on an audio-only basis. MCU management is an area in which large differences still exist between products. Most are managed in a Windows environment, but some are easier to use, offer richer diagnostics, and offer many more administrative features. Administrative features include scheduling utilities. Some systems allow conference scheduling in advance. Even more valuable is the ability for an MCU to configure itself automatically at a pre-determined time and, based on that configuration, to dial out to the sites involved in the conference. Also look for event logs and the ability to account for and bill back usage. In situations in which a single organization will have many MCUs, it is important to buy products that can be remotely man