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Chapter 14. Asynchronous Transfer Mode (... > 28. What kind of technology is neede... - Pg. 438

438 Networking Explained, Second Edition gence technology.) To satisfy all three network types, some basic functions must be cre- ated to allow all three types of network technologies to coexist. 28. What kind of technology is needed for data, voice, and video? Data networking allows traffic characteristics that can be bursty in nature, and data can have variable-length packets or frames. Data traffic also can tolerate a certain amount of transmission delay, especially for non-real-time traffic. This implies that data arrival rates can be variable in nature, which suggests that variable-bit rate (VBR) transmissions are acceptable to data applications. Voice traffic, on the other hand, is more sensitive to the arrival time of traffic. It's a good idea for packets carrying part of a conversation to arrive quickly enough to avoid blank spots of time ("dead air") in the conversation. This suggests a constant-bit rate (CBR) transmission be used for voice. There also is a problem with full duplex speech, where two or more people speak at exactly the same time and can be heard simulta- neously. Networks don't necessarily agree with this concept very well. Voice, however, is not a big bandwidth hog, as a rule, so fast data networks with VBR can support voice com- munications as long as the network is swift and does not suffer congestion or loss delays. Video is even more sensitive to timing. Such transmissions expect the number of frames transmitted from one site to another to arrive in order and in a very specific time frame (usually measured in milliseconds). Technologies such as constant-bit rate, where the num- ber of arrival bits in a transmission are constant and consistent, are essential to making a standard traffic arrival rate possible. However, CBR is not enough. The frames must be transmitted in the proper order and must arrive at the correct speed within a specific time frame. To do this, the network must reserve bandwidth in the path from the source to the destination to ensure that all the bits arrive in order and on time. This general method of providing CBR with a guaranteed delivery sequence in a specific time frame while reserving path "space" is called isochronous communications and is common to all ATM networks. When you add to the discussion that different network applications will ultimately require more and more bandwidth in the future to satisfy consumer needs, ATM is the only network presently on the drawing board that provides the transmission technologies, trans- mission rates, and quality of service (QoS) required to address user needs . . . at least for now (e.g., it is still too early to determine 10 Gigabit Ethernet's impact). 29. What you're saying, then, is that the convergence of network functions, which is the "wave of the future," requires technologies analogous to ATM. Is this correct? Yes. 30. What about Gigabit Ethernet and 10 Gigabit Ethernet? Aren't these two technol- ogies (especially 10 GbE) fast enough for all of this? Although 1 GbE and 10 GbE are capable of transmitting data and voice at acceptable levels, they are still a VBR technology and have problems with video equipment. This becomes acutely evident in the presence of network congestion, or when a specific deliv- ery time frame is required. A good example of this is High Definition Television (HDTV), which will eventually appear on global networks as a standard transmission method.