Optical Networks - _J_1 Functions of ATM_171

Optical Networks - _J_1 Functions of ATM_171 - 802...

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802 Asynchronous Transfer Mode Another advantage of ATM is that it employs switching even in a local-area environment, unlike other LAN technologies like Ethernets, token rings, and FDDI, which use a shared medium such as a bus or a ring. This enables it to provide quality-of-service guarantees more easily than these other technologies. The fixed size of the packets used in an ATM network is particularly advantageous for the development of low-cost, high-speed switches. Various lower or physical layer standards are specified for ATM. These range from 25.6 Mb/s over twisted-pair copper cable to 622.08 Mb/s over single-mode optical fiber. Among the optical interfaces is a 155.52 Mb/s optical interface that operates over distances up to 2 km using LEDs over multimode fiber in the 1300 nm band. Using the specified minimum transmit and receive powers, the loss budget for this interface is 9 dB. The line code used in this case is the ( 8 , 10 ) line code specified by the Fibre Channel standard. These interfaces are called private user–network interfaces in ATM terminology, since they are meant for interconnecting ATM users and switches in networks that are owned and managed by private enterprises. A number of public user–network interfaces, which are meant for connecting ATM users and switches to the public or carrier network, are also defined. In these latter interfaces, ATM uses either PDH or SONET/SDH as the immediately lower layer. These interfaces are defined at many of the standard PDH and SONET/SDH rates shown in Tables 6.1 and 6.2, respectively. Among these are DS3, STS-3c, STS-12c, and STS-48c interfaces. In the terminology of the ATM standards, since the layer below ATM is called the physical layer, these
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Optical Networks - _J_1 Functions of ATM_171 - 802...

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