Synchronous Optical Network (SONET) is the fiber optic standard that is focused on the physical layers of the OSI
model. SONET, and its international equivalent Synchronous Digital Hierarchy (SDH), originally define how voice traffic will be carried across the carrier-built fiber optic networks deployed throughout the world. As a Layer-1, physical level protocol, SONET makes link connections along and between these fiber networks. SONET was evolved over time to include data services -- such as frame relay, T1, and OC-3 -- to connect over the fiber links. Because SONET was originally designed for voice and not variable-sized data packets, however, moving data across it was inefficient and required padding data packets with irrelevant data to make up any differences. Asynchronous transfer mode (ATM) was introduced as a solution for this inefficiency. Through the use of hardware network interface adapters, ATM networks break data into smaller cells for transport. ATM over SONET also makes home and business ISDN (Integrated Services Digital Network) data services possible.
In contrast, ATM and Multiprotocol Label Switching (MPLS) are data transport protocols, meaning that both reside above the physical data layers in the OSI model and aid in moving data from one point to another. The primary difference between ATM and MPLS is that while ATM was designed to exist in a circuit-switched environment, MPLS has its place within modern packet-switched networks such as Ethernet or IP. This difference is most apparent in how the two types of network topologies are deployed. ATM is primarily designed as a point-to-point connection, requiring an ATM adapter on each end of a physical or virtual circuit. MPLS, on the other hand, operates similar to an Ethernet switch in an any-to-any topology, allowing each of the network endpoints to be connected to the MPLS network and mesh with a particular customer’s virtual mesh. For ATM to replicate this level of meshing, multiple ATM connections would have to be installed at each of an organization’s locations. The multi-protocol nature of MPLS also enables the technology to label and pass other protocols, including ATM, across an MPLS network. Two ATM endpoints, for example, could be connected across an MPLS network, with the network itself quickly guiding traffic to each other transparently.