The Standard will define a Dual Bus subnetwork incorporating a shared Medium physical layer of two unidirectioonal buses, a non-isochronous media access control sublayer based on Distributed Queueing and an isochronous media access control sublayer based on centralized time slot allocation. The standard will allow for bridges to interconnect two or more Dual Bus subnetworks to form a Metropolitan Area Network (MAN). The Dual Bus subnetwork and MAN shall be capable both of operating in the public telecommunications network environment and of incorporation in the public network structure, and shall meet the operations, administration, maintenance and performance requirements of this environment for criteria such as reliability, availablity and security. The Dual Bus subnetworks and MAN shall operate at selected rates exceeding 1 Mbps and consistent with public network rates. The Dual Bus subnetworks and MAN shall be capable of operating over areas in excess of 50 kilometers in diameter. The performance and efficiency shall be essentially independent of subnetwork size and rate.