320 GLOSSARY IP address: In the Internet Protocol, the unique numerical address that identifies a computer in a network. Keep-alive signaling: See heartbeat signaling. Kiosk: An advanced customer terminal that allows two-way communica- tion with a customer. Kiosk systems often include smart card readers to enable ordering and purchasing products and services. Layer: A digital sign consists of virtual graphical layers superimposed on one another to form the complete screen content. Each layer is described by one or more content files. Layer is also used to describe communica- tion layers such as those in the OSI model (see Appendix B). LED: Light-emitting diode. Letter boxing: Adding black bars to the top and bottom of a screen with a regular 4:3 (1.33:1) aspect ratio to be able display widescreen 16:9 (1.78:1) programs. Media player: Device that decodes and plays media files to be displayed by various kinds of display devices. Media players also may have perma- nent storage capability, such as a hard drive or a flash memory. Some media players can be used to decode IPTV streams. Media players can be computers equipped with media player software or more dedicated