Many technologies have been used to deliver metro and wide-area services. Layer 1 TDM technologies used to deliver private line services include E1 and E3 copper circuits, and SDH/SONET-based optical circuits. Layer 2 technologies used to deliver MAN/WAN services include Frame Relay, ATM and PPP. These legacy technologies provide inflexible bandwidth scalability because the bandwidth is dictated by the technology. When a service provider or enterprise needs to add bandwidth, they either bond multiple circuits together or upgrade their network and equipment to support a new technology.
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