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22.1 INTRODUCTION TO ATM NETWORKS AND INTERFACES 723Figure 22.1-2. UNI and NNI interfaces.Cell characteristics ensure superior latency and jitter characteristics (compared toIP-based packet communication), making ATM well suited not only for data but alsofor real-time multimedia applications such as voice and video. The same characteristicsmake highly efficient switching of broadband data flows possible: ATMsystems can easily support rates of up to multigigabits per second [3].ATM is connection-oriented: a virtual (logical) connection with a fixed path isestablished by signaling messages at the start of a data flow and released at theend, thus ensuring cell sequence integrity. Thanks to virtual connections and the predictabilityof fixed-length cells, ATM can emulate traditional TDM circuits whenconfigured to transfer cells at a fixed rate. Circuit emulation introduces a small performancepenalty, compared to TDM, due to header overhead, but is of interest toservice providers with legacy TDM circuits who want to phase-in packet technologyand minimize interworking problems.As in TDM, no checks are done on the payload, which is transferred transparentlyfrom source to destination.ATM supports variable data rates by allocating bandwidth based on demandand transmitting cells only when there is data to send. Cells can be sent on unframedtransmission systems in cell-based mode, where cells are placed on the data linkback-to-back; or they can be sent on framed systems where a frame structure ispresent. Examples of framed systems are the Synchronous Digital Hierarchy(SDH) and the Synchronous Optical Network (SONET), discussed in Section 1.5.3.ATM also supports elaborate service class and bandwidth management capabilities,which ensure efficient handling of data rates with both predictable and