C is based on the Perceptual Audio Coder (PAC). There are many sounds that the ear cannot discern because they are masked by louder sounds. PAC discerns and discards these sounds that the ear cannot hear and that are not necessary to retain the observed quality of the transmission. This results in transmission with 15 times fewer bits. PAC was first developed at Bell Labs in the 1930s. A Joint Photographic Experts Group compression standard used mainly for photographs. The International Standards Organization (ISO) and the ITU developed JPEG. A Moving Picture Experts Group video compression standard approved in 1993 for coding and decoding video and television images. MPEG-2 uses past images to predict future images and color. It then transmits only the changed image. For example, www.TechnicalPDF.com the first in a series of frames is sent in a compressed form. The ensuing frames send only the changes. A frame is a group of bits representing a portion of a picture, text, or audio section. MPEG-3 is Layer 3 of MPEG-1. MPEG-3, also referred to as MP3, is a standard for streaming audio and music. MPEG-3 is the compression algorithm used to download audio files from the Internet. Some Internet e-commerce sites use MPEG SO that potential customers who have applications that use compression software can download samples of music to decide if they want to purchase a particular song. MPEG-4 is a standard for compression, which defined coding on audio and video. Table 1-5 OSI Layers OSI Layer Name and Number Layer Function Layer 1 is the most basic layer. Layer 1 defines the type of media (for instance, copper, wireless, or fiber optic) and how devices access media. Repeaters used to extend signals over fiber, wireless, and copper networks are Layer 1 devices. Repeaters in cellular networks extend and boost signals inside buildings and in subways SO that users can still take advantage of their cellular devices in these otherwise network-inaccessible locations. Ethernet, also known as 802.3, is a Layer 2 protocol. It prov