tailieunhanh - Internetworking with TCP/IP- P9

Internetworking with TCP/IP- P9: TCP/IP has accommodated change well. The basic technology has survived nearly two decades of exponential growth and the associated increases in traffic. The protocols have worked over new high-speed network technologies, and the design has handled applications that could not be imagined in the original design. Of course, the entire protocol suite has not remained static. New protocols have been deployed, and new techniques have been developed to adapt existing protocols to new network technologies | 48 Review Of Underlying Network Technologies Chap. 2 Point-To-Point Networks We said that Wide Area Networks are usually composed of dedicated packet switches interconnected by data circuits leased from a telephone company. Phone companies originally designed such circuits to carry digitized voice calls only later did their use in data networks become important. Consequently the data rates of available circuits are not powers of ten. Instead they have been chosen to carry multiples of 64 Kbps because a digitized voice call uses an encoding known as Pulse Code Modulation PCM which produces 8000 samples per second where each sample is 8 bits. The table in Figure lists a few common data rates used in North America and Europe. Name Bit Rate Voice Circuits Location Mbps 1 T1 Mbps 24 North America T2 Mbps 96 North America T3 Mbps 672 North America E1 Mbps 30 Europe E2 Mbps 120 Europe E3 Mbps 480 Europe Figure Example data rates available on digital circuits leased from a telephone company. The rates were chosen to encode multiple voice calls. Higher rate digital circuits are also available. In addition to standards that specify the transmission of high data rates over copper the phone companies have developed standards for transmission of the same rates over optical fiber. The table in Figure contains examples. Of course circuits that operate at such high data rates are considerably more expensive than circuits that operate at lower rates. Standard Name Optical Name Bit Rate Voice Circuits STS-1 OC-1 Mbps 810 STS-3 OC-3 Mbps 2430 STS-12 OC-12 Mbps 9720 STS-24 OC-24 1 Mbps 19440 STS-48 OC-48 2 Mbps 38880 Figure Example data rates of high-capacity circuits that can be leased from phone companies. Optical fiber is used to achieve such high rates over long distances. Sec. Other Technologies Over Which TCP IP Has Been Used 49 From TCP IP s point of view any .