, were specified to transmit ATM cells. However, because LAN and IP switching technology has developed to manage higher data rates, the importance of ATM is decreasing. ATM was also defined to be the initial network technol- ogy for UMTS, introduced in Chapter 5. It will be replaced by evolving IP technology later in the evolution of the third generation mobile networks. 6.9 Problems and Review Questions Problem 6.1 Compare parallel and serial data transmission principles and applications. Data Communications Problem 6.2 Explain what is meant by asynchronous and synchronous data transmission. Problem 6.3 ASCII strings "A" [1000001] and "2" [1000110] (first bit on the left) are sent and a parity bit for even parity is added in the end of each character. What are the transmitted 8-bit strings? Problem 6.4 Synchronous frames use bit sequences or flags (0111110) to indicate the start and end of the frame. Explain why user data sequence ..01111110. is not detected as the end flag. Problem 6.5 If the data bit string 0111101111101111110 is bit stuffed, what is the out- put string to be framed with flags? Problem 6.6 Compare circuit-switched service and packet-switched networks. What are their advantages and disadvantages? Problem 6.7 What do we mean by physical circuits and virtual circuits? How does the packet-switching principle based on datagram transmission differ from the switching based on virtual circuits? Compare their advantages and disadvantages. Problem 6.8 Explain how frames are switched in a packet-switched node of ATM or frame-relay network. Problem 6.9 List three examples of both circuit- and packet-switched networks. Problem 6.10 What do we mean by polling in data communications? Introduction to Telecommunications Network Engineering Problem 6.11 What does protocol mean in data communications? Give some examples of protocols. Problem 6.12 Why do we use a layered protocol structure in data communications? Problem 6.13 Explain the basic principle and structure of the OSI reference model. Problem 6.14 Compa