Carrier Ethernet


Carrier Ethernet

Carrier Ethernet: the extensions to Ethernet necessary to enable telecommunications network providers ("common carriers" in US industry jargon) to provide Ethernet services to customers and to utilise Ethernet technology in their networks.

Background

Ethernet has a long history. It has become dominant in enterprise networks. This dominance has led to high volume components, which in turn have allowed extremely low cost per bit. It has also a long history of re-inventing itself. From the original copper coaxial cable format ("thicknet") it has extended its scope to nearly all copper, optical fibre and wireless physical media. Bandwidth has continued to increase, traditionally growing by ten times each time a new rate is defined. Gigabit Ethernet interfaces are now widely deployed in PCs and servers, and 10Gb/s is the standard rate in Local Area Network(LAN) backbones. 100Gb/s rates are currently being developed.Ethernet's dominance is partly just because of the advantages to the industry of adopting a single standard to drive up volumes and drive down prices. It is also due in part to ease of deployment, using its ability to self-configure based on the key concepts of “learning bridge”, (flooding, and associating learned destination addresses with bridge ports) and “spanning tree”, (the protocol used to avoid loops).Given this ubiquity, customer LAN networks are increasingly connected to wide-area telecommunications networks over Ethernet interfaces. Moreover, customers are familiar with the capabilities of Ethernet networks, and would like to extend these capabilities to multi-site networks. Thus wide area network (WAN) providers find themselves with two needs:
#To provide their customers with Ethernet services
#To make use of the volume and cost advantages of Ethernet technologies in their networks

Carrier Ethernet services

To create a market in Ethernet services, it is necessary to clarify and standardise the services to be provided. Recognising this, the industry created the Metro Ethernet Forum [http://www.metroethernetforum.org] , which has played a key role in defining such services. The services defined are:
* E-line: a service connecting two customer Ethernet ports over a WAN.
* E-LAN: a multipoint service connecting a set of customer endpoints, giving the appearance to the customer of a bridged Ethernet network connecting the sites.
* E-tree: a multipoint service connecting one or more roots and a set of leaves, but preventing inter-leaf communication.All these services provide standard definitions of such characteristics as bandwidth, resilience and service multiplexing, allowing customers to compare service offerings and facilitating service level agreements (SLAs).

Transport of Ethernet services

The Metro Ethernet Forum does not specify how Ethernet services are to be provided in a carrier network. Despite the advantages described above, Ethernet has traditionally had a number of limitations in the WAN application. The "bridge" and "spanning tree" concepts described above do not scale to large international networks. Moreover, Ethernet has lacked some of the dependability features necessary in this application (in particular, mechanisms to isolate one customer's traffic from another, to measure performance of a customer service instance, and to rapidly detect and repair failures in large networks).Because of these limitations, and because of the need to make use of pre-existing equipment, Ethernet services have been carried across wide area networks using other technologies. Two types of technology have been widely used, while a third (Carrier-Ethernet transport) is rapidly emerging as a viable and logical option for Carrier-Ethernet services.

Ethernet over SDH/SONET

Point-to-point Ethernet links are carried over SDH/SONET networks, making use of virtual concatenation (ITU-T G.707) and LCAS (Link Capacity Adjustment Scheme - ITU-T G.7042) to create an appropriate size carrier bundle, and of Generic Framing Procedure (ITU-T G.7041) to encapsulate the Ethernet frames. This allows the re-use of the installed base of SDH equipment, and takes advantage of the management and recovery features of SDH to provide high availability and resilience to failures.

Ethernet over MPLS

Ethernet services are carried over IP/MPLS networks making use of a wide range of IP-related protocols (see IETF pseudowire standards, e.g. RFC 3985, RFC 4448). Ethernet links are transported as “pseudowires” using MPLS label switched paths (LSPs) inside an outer MPLS “tunnel”. This strategy can support both point to point (Virtual Private Wire Service - VPWS) and multipoint (Virtual Private LAN service- VPLS) services, and has recently achieved significant deployment in routed networks. It makes use of a number of basic transport protocols, including SDH and (increasingly) Ethernet.

Ethernet over Carrier-Ethernet Transport (CET)

Proponents of Carrier-Class Ethernet argue that, since all data traffic originates as Ethernet, what other technology could be better than Ethernet itself to carry this traffic in Metro Area Networks? Ethernet’s ubiquitous presence in the LANs worldwide drives down the cost of Ethernet as a technology. Thus, the use of Ethernet in a metro network allows Service Providers to take advantage of volumes that a much-larger enterprise segment commands. Carrier-Ethernet Transport (CET) usually involves an evolution of conventional Ethernet and comprises multiple Technology components. Provider Backbone Bridges (PBB) provides the scalability and a secure demarcation, while PBB-TE (commonly called PBT) provides for Traffic-Engineering and an effective transport for protected Ethernet services. Connectivity-Fault Management (CFM-OAM) provides the much-required OAM that makes Ethernet carrier-grade.

Carrier Ethernet technologies

The industry has made a concerted effort to resolve the limitations of Ethernet in the WAN described above, so as to allow the use of "native" Ethernet technologies by network providers.The key role has been played by the IEEE 802.1 standards committee, which has responsibility for the Ethernet standard. It has addressed the scalability and management issues in the standards for Provider Bridges (802.1ad) and Provider Backbone Bridges (802.1ah). These standards allow for Ethernet networks of planetary scale. Associated standards (802.1ag, and related ITU-T standard Y.1731) provide Operations and Maintenance (OAM) capabilities allowing connectivity verification, rapid recovery, and performance measurement. Current work on PBB-TE (802.1Qay: Provider Backbone Bridging-Traffic Engineering) is allowing such an Ethernet to be controlled by an external control or management application (for example, a network management application or a transport control plane such as GMPLS (IETF RFC 3945)), so as to allow the full range of traffic engineering policies and strategies to a network provider.The efficacy of this standards-making depends ultimately on its adoption into high volume components. This is not yet completely clear, but the indications are that many vendors are preparing to support the necessary addressing and OAM features in chips and systems. It seems likely that once again Ethernet will grow to encompass this new application with high volume low cost technology.


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