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Transport Options for Metro Ethernet

Three main options for delivering an Ethernet service to customers over the last mile are in widespread use. This articles describes the options, and where they might be used...

Metro Ethernet services are offered over three main transport technologies. Regardless of which transport is used, the customer is presented with an Ethernet interface, but the underlying layers can be established in a number of different ways.

Ethernet over SONET/SDH

During the late 1990’s and since many operators deployed SONET or SDH networks to provide a managed physical layer network across which services could be provisioned. SONET and SDH are the natural successors to the Plesiochronous Digital Hierarchy (PDH) bearer network used previously to provide access circuits to customers. SDH/SONET provides excellent management features, resilience, etc which makes it a strong candidate for offering a range of services without the need to “re-invent” various provisioning and protection mechanisms.

Resilient Packet Ring (RPR)

Resilient Packet Ring networks are constructed using a resilient ring of RPR nodes which looks at the physical layer similar to a traditional SONET/SDH ring. However whereas SONET/SDH is a physical layer technology, with multiplexing carried out at the physical layer, in RPR a modified Medium Access Control (MAC) layer replaces the traditional Ethernet MAC layer. All frames share the overall physical bandwidth between nodes, rather than this bandwidth being apportioned per signal in the SONET/SDH case. The MAC address of Ethernet frames is examined at each node, and the nodes can carry out add, drop and forward operations depending upon whether the destination address belongs to the node in question.

Therefore RPR has similarities with traditional SONET/SDH and with conventional Ethernet. The technology was initially developed by Cisco Systems and named Data Packet Transport (DPT), but was subsequently standardised by the IEEE 802.17 workgroup and re-titled RPR.

Native Ethernet Transport

Native Ethernet transport can be used to carry Ethernet services, combined with Gigabit or 10 Gigabit L2 switching and a conventional ring topology to create a Metro Ethernet network. A ring is typically used because existing fibre has been deployed in this physical structure. As with any L2 network with redundant paths, Spanning Tree Protocol (STP) or one of its variants must be used to prevent broadcast storms, but this leads to inefficient use of bandwidth and potentially slow recovery from link failures. Use of faster STP variants such as Rapid STP can help with recovery times; however native Ethernet transport is generally limited to multi-tenanted buildings, campuses and similar environments.

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