The first game featured the tower of Ar tonelico and the second features a new system: Infel Phira.
Both of these systems are Song Magic Servers and have the same basic fundamental features:

• An H-wave receptor that can interpret instructions and produce D-waves
• The ability to support connections from at least one Reyvateil (daughter)
• Hymmnos-language-processing capabilities

Ar tonelico was constructed during the First Era. Its basic functionality is much like a core that produces magical effects upon receiving a Reyvateil's song. The magical effects are caused by the emission of dynamic D-waves upon receiving thoughts; the net effect of this process is that its users, the Reyvateils, can command powers far beyond their actual capabilities.

The differences between Ar tonelico and Infel Phira are not unlike the differences between an Intel-based system and a Sparc.

For each system, the basic logic is the same: CPU receives instructions in a format it can understand -> CPU acts accordingly.

However, Intel processors, and the format they work with, store and transport data differently from the machinations of a Sparc processor, so you could not simply send messages intended for one to the other without some sort of translator.

The same problem arises when attempting to foster communication between different systems like Ar tonelico and Infel Phira: both perform similar tasks, but their formats and means of encoding messages (the Hymmnos they recognize) are very different. Additionally, like the systems themselves, there are no fundamental differences between the roles of the Reyvateils of one system and the Reyvateils of the other: the only difference is which Hymmnos dialect they natively speak. The relationship is much like different partition formats on hard drives: they are accessed differently and have varying, often incompatible designs, but they can all handle the same basic data.

As designed, neither system is inherently able to communicate with the other. To get around this, they need to agree on a compatible protocol for exchanging data, somewhat like how your Web browser used TCP/IP and HTTP to access this page. Once a protocol has been agreed on, simple tasks like sharing files to complex ones like entering Cosmospheres and establishing links in the Binary Field may be accomplished.