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ATM is designed to switch these small cells through an ATM network very quickly. It does this by setting up a virtual connection between the source and destination nodes; the cells may go through multiple switching points before ultimately arriving at their final destination. The cells may also arrive out of order, so the receiving system may have to reassemble and correctly order the arriving cells. ATM is a connection-oriented service in contrast to most network architectures, which are broadcast-based.


Data rates are scalable and start as low as 1.5Mbps, with speeds of 25Mbps, 51Mbps, 100Mbps, 155Mbps, and higher. The common speeds of ATM networks today are 51.84Mbps and 155.52Mbps. Both of these speeds can be used over either copper or fiber-optic cabling. An ATM with a speed of 622.08Mbps is also becoming common but is currently used exclusively over fiber-optic cable. ATM supports very high speeds because it is designed to be implemented by hardware rather than software; faster processing speeds are therefore possible. Soon, fiber-based ATM networks will be operating at data rates of 10Gbps.


In the U.S., the standard for synchronous data transmission on optical media is SONET (Synchronous Optical Network); the international equivalent of SONET is SDH (Synchronous Digital Hierarchy). SONET defines a base data rate of 51.84Mbps; multiples of this rate are known as optical carrier (OC) levels, such as OC-3, OC-12, etc. Table 7.3 gives common OC levels and their associated data rates.


7.3 Common Optical Carrier levels (OC-x)


Level


Data Rate


OC-1


51.84Mbps


OC-3


155.52Mbps


OC-12

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