bridges (IEEE 802.1D) to further enhance efficiency in point to multipoint applications and VLAN tagging (IEEE 802.1Q). One drawback of the first version of RPR was that it did not provide spatial reuse for frame transmission to/from MAC addresses not present in the ring topology. This was addressed by IEEE 802.17b, which defines an optional spatially aware sublayer (SAS). This allows spatial reuse for frame transmission to/from MAC address not present in the ring topology. See also Ethernet Automatic Protection Switching Spatial Reuse Protocol (Cisco) Metro Ring Protocol (Foundry Networks) Open Transport Network (Nokia Siemens Networks) Dynamic Packet Transport (Cisco) Ethernet Ring Protection Switching (ITU-T) References External links IEEE 802.17 Resilient Packet Ring Working Group IEEE 802 Network architecture IEEE standards