Customers can rent Synchronous Digital Hierarchy/Synchronous Optical Network (SDH/SONET) links from carriers and provide their Layer 2 protocol/encapsulation of choice or directly deploy PoS (Packet over SDH/SONET). These links can be acquired either protected (spare port) or unprotected.
For larger enterprise customers, the carriers usually deploy add/drop multiplexers to deliver fractional STM1/OC3 bandwidth. Depending on the linecards of these multiplexers, channels as small as 56/64 kbps could be extracted/injected (added/dropped in multiplexer lingo). T1/E1 fractions are the most common when looking at the total number of deployed units, most of them at carrier edge facilities. Due to the increasing bandwidth needs of larger enterprises, such trunks are normally sold as multiples of full T1/E1 rates. In carrier backbones, state-of-the art photonic networks based on optical cross-connects and dense wavelength-division multiplexing (DWDM) technology transport aggregated traffic of multiple 10 to 100 Gbps.