In the physical layer, there are several different protocols used to transmit data across the media:

• Digital Subscriber Line (DSL) provides broadband to residences and businesses using a phone line.
• Integrated Services Digital Network (ISDN) transmits voice, video, and data using the Public Switched Telephone Network (PSTN). ISDN is primarily used in the broadcasting industry.
• IEEE 802.3—the Ethernet physical layer—defines the transmission properties according to the media type, such as fast Ethernet and GB Ethernet.

The physical layer is where binary transmission takes place across the network media. At this layer, the PDU is the most basic form, which is bits.

The following chart provides a summary of the physical layer:

Prior to traveling across the media, the data must be in the correct format. The next section explores the encapsulation process, which adds headers and addresses and readies the data for transport across the media.