The fixed bit-rate signals traveling through the transmission channels are
neatly carved into smaller TDM signals at the endpoints, efficiently supporting voice
and other TDM payloads. Conversely, data networks, with inherently ???bursty??? characteristics,
have developed much differently. The differences between voice-optimized
and data-optimized networks have resulted in difficult interoperability between the
two network types. A conversion, or mapping, is required to support data on the TDM
links.
But mapping protocols onto other formats is neither simple nor efficient. If a signal
with a different bit rate and protocol is mapped onto the next higher bit rate, the bandwidth
difference between the lower and higher rate is lost. For example, when you map
Gigabit Ethernet (1 Gbps) into OC-48 SONET (2.5 Gbps), you lose the difference between
the two??”1.5 Gbps, or over half the OC-48??™s capacity. This inefficiency represents
a new cost for carriers.
This approach to mapping also assumes the data signal has a lower bit rate than the
signal it is being mapped into. In a TDM-multiplexing hierarchy, this is always true.
With data-networking interfaces such as Ethernet, however, this is much less likely to
be the case??”and this causes problems.
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