The Ansible is the common name for a type of Inter-dimensional communications technology built and maintained by Eul-Speak, Inc., a division of the Gudersnipe Foundation. While other examples can and presumably do exist, Ansible is the most common.
Only the Foundation is known to possess the ability to create Ansibles, and provides the equipment at a fee to other entities. Interestingly, this is one of the few technologies they build but do not themselves use. Because of the operational complexity and cost, Ansibles are seldom owned by private entities or individuals, most of the customers are governments.
The Ansible allows for inter-dimensional communication, in real-time, and is a mainstay of the Alliance communications network. While the Alliance is not able to produce new Ansible sets, they can repair and maintain them. Though most of this is done by the customer service department of Eul-Speak.
The Ansible itself can be anywhere from the size of room to the the size of a small building, and is usually housed somewhere out of the way. Users interact with it through a receiver, and usually they are simply linked to a telephone system. Due to an odd quirk of the technology, voice communication is very clear and reliable, but data transfer rates are exceptionally slow, not enough to support video or send digital files of any substantial size. The Ansible is typically used in one of three modes: voice, fax, or as a very slow text transmitter/receiver.
While the limitations do present problems, the technology is effective enough. Ansible sets have been known to function without maintenance for centuries, meaning they can even be installed on lower-tech worlds and still be used.
The underlying technology has been stated to be "both very complicated, and fairly simple". The operation of the machine involves conducting sound waves through an aperture, and does so with rudimentary implements. The aperture itself, however, exists outside of normal space-time and enables "the other end" to be in another dimension. The design makes the Ansible exceptionally robust and good for transmitting voice, but places severe limits on data throughout. Two people can have a very clear conversation, but transmission of live video is not possible. A system analogous to a fax machine exists for sending documents, and teletype-style systems can also provide written communication.
The technological limitations are due to the vagaries of time between worlds. Some worlds move at faster or slower rates. While not substantial, this is known for playing hell with any sort of real-time data transmission. It's also apparent that there is some sort of limit to the number of ansibles that can be in use at any one time (though no one knows precisely).