y a 44.84 Mbps DS-3 signal, with room left for overhead. SONET line rates are referred to as Optical Carrier (OC) speeds. The electrical signal interfaces that correspond with these speeds (known as STS) have also been identified. The 51.84 Mbps base speed for SONET is known as OC-1/STS-1. Higher Video Communications: The Whole Picture speeds are multiples of this rate. OC-3/STS-3 has a speed of 155.52 Mbps. It is expected to be an important speed level for establishing multimedia transmissions and, of the specified optical standards, it is the slowest one capable of 100 Mbps LAN speeds. SONET also includes a 622.08 Mbps, (or OC-12/STS-12), specification, and a 1.244 Gbps (OC/24/STS-24) specification. SONET's highest speed is the 2.48 Gbps OC-48/STS-48 specifications. SONET specifies not only line speeds, but also transmission encoding and multiplexing methods and establishes an optical signaling standard for interconnecting equipment from diverse suppliers. A synchronous multiplexing format carries DS-level traffic. The ITU-T developed a European-oriented version of SONET known as synchronous digital hierarchy (SDH). In short, SONET provides the physical infrastructure for a next-generation PSTN switching architecture. That architecture is known as B-ISDN and its most widely-known implementation is B-ISDN was developed by the CCITT to be a 155 or 622 Mbps public switched network service. In specifying B-ISDN, the goal of the ITU-T was to create a public network that is ideally suited to carrying the signals that are intrinsic to multimedia, which have a wide range of traffic characteristics. The CCITT's "Task Group on Broadband Aspects of ISDN" looked at a variety of interactive broadband services that would be required in the public network. Of course, they considered video telephony and video and data conferencing, HDTV, and high-speed file transfer-all topics that we cover in this book. Indeed, the B-ISDN protocol stack, and its ATM layer, would not have been necessary were it not for Real- Time interactive d