play of a Web page to actual data communications over LAN and other bearer networks. The aim was also to clarify how layered data communications intro- duced in Section 6.3 operate in practice. Many important aspects were not covered but this introduction should give readers a basic understanding about communications methods from the physical layer to the application layer and software applications. This helps readers extend their knowledge and use another information source for more detail. Some of these sources are listed the end of this chapter. Introduction to Telecommunications Network Engineering Frame Relay Frame-relay technology is widely used by network operators that provide long-distance data communications service to companies. It is designed for ordinary data applications and transmits data frames with variable length. The old packet-switched networks, such as X.25, were originally designed for a low-quality physical network and included data integrity checking at many protocol layers. With the present high-quality physical network, this is usu- ally unnecessary. Frame relay leaves data checking and acknowledgment pro- cedures to the network users and the protocols in use are much simpler and can support a much higher data rate. Frame relay supports data rates up to 50 Mbps. Frame-relay technology is usually used to provide semipermanent con- nections for LAN interconnections. The network operator sets up a virtual connection between endpoints and frames with circuit identifiers are routed through the network as explained in Section 6.2.4. The network capacity is shared between users and the cost for long-distance connections is much lower than cost of leased-line connections. Frame relay is a technology for data transmission and it does not sup- port isochronous transmission, such as voice or video, which requires low and constant delay. A network technology that was designed to support iso- chronous services as well is known as asynchronous transfer mode, as dis- cussed next. Most packet-switched t