Undoubtedly, Ethernet has become the technology of choice for wide-area network (WAN) connectivity for both the enterprise and the carrier. The reasons behind this are a convergence of business requirements for higher data-connectivity rates and flexible services, plus the availability of new data-transport technology.
But enterprises and carriers must account for a number of factors before implementing Ethernet over Transport. A variety of Ethernet services potentially is in the mix for delivery over public WANs, so carriers should consider the attributes and characteristics that must be defined to plan their product roadmaps or provide these services.
Ethernet-based user network interfaces (UNIs) and network-to-network interfaces (NNIs) are required for transport network equipment carrying Ethernet services as well as operations, administration, maintenance, and provisioning (OAM&P) capabilities. Certain protection and restoration technologies that can be used to guarantee carrier-grade service reliability for Ethernet WAN services also can be implemented to ensure service-level agreements are fulfilled.
Why Ethernet Over The Public WAN?
Ethernet dominates the enterprise side of the public WAN. Many reasons surround the growing interest in Ethernet WAN connection services:
- Network administrators are already familiar and comfortable with Ethernet.
- Many applications, including voice and video, are already being encapsulated into Ethernet and run over enterprise network local-area networks (LANs).
- More companies these days have multiple remote offices that need to exchange data with the headquarters office or with each other. Placing data on common servers that are accessible by all sites can bring about significant advantages.
- The increased importance of the Internet for business applications has led to a greater desire for incrementally higher WAN interface rates.
Most enterprise WAN data access today uses frame-relay connections at rates of fractional DS1, full-rate DS1, fractional DS3, and full-rate DS3. But relay-based services suffer from a lack of scalability. In some cases, the data services use asynchronous transfer mode (ATM) instead for the customer connection or as a method of transporting the data through the core network. However, ATM is both provisioning-intensive and bandwidth-inefficient.
Some larger customers use high-speed SONET OC-N/SDH STM-N connections, which typically terminate the Ethernet frames and re-encapsulate the data into point-to-point (PPP) and use packet over SONET/SDH (PoS) for transmission through a SONET/SDH pipe.
Two clear factors drive the carrier (network operator) side. First, carriers must deploy any new services on their existing SONET/SDH networks. Second, they want to generate new, revenue-bearing services.
Throughout the 1990s, carriers invested heavily in building SONET/SDH-based fiber-optic networks, including developing OAM&P systems to run them. Building an overlay network for data transport would be impractical due to the enormous capital investment required, whether it's native Ethernet or based on dense-wavelength division multiplexing (DWDM).
Fortunately, new capabilities added to SONET/SDH greatly increase its efficiency as a layer 1 network for data service transport. Some of these developments include virtual concatenation (VCAT), link capacity adjustment scheme (LCAS), and generic framing procedure (GFP). With data traffic growing faster than voice, offering WAN connectivity with higher bandwidth and enhanced service capabilities is a natural direction for new services.
In many ways, Ethernet is an obvious technology choice for WAN connectivity. Treating all of their sites as part of the same Ethernet network simplifies the job of enterprise network administration. The development of VCAT and GFP allows for efficient transport of Ethernet frames through SONET/SDH networks. Carriers will want to use the OAM capabilities inherent in the SONET/SDH backbone to implement full monitoring and protection of the transmission facilities and transport path through the SONET/SDH network.
Ethernet Connection Defined
An Ethernet virtual connection (EVC), otherwise known as an Ethernet connection (EC), supplies a connection between two or more customer UNIs. Therefore, the Ethernet frames associated with the EVC can only transfer between its associated UNIs and not to any others.
Network engineers should familiarize themselves with the three different Ethernet service areas in a multicarrier Ethernet connection:
- access (UNI-C to UNI-N)
- end-to-end/customer-to-customer (UNI-C to UNI-C)
- edge-to-edge/intracarrier (UNI-N to UNI-N)
UNI-C and UNI-N refer to the customer and network operator side of the UNI, respectively. (ITU-T Recommendation G.8012 defines UNI and NNI for Ethernet transport.) The type of customer connectivity characterizes the desirable characteristics provided by Ethernet services. The types of connectivity are point-to-point, point-to-multipoint, and multipoint-to-multipoint (Fig. 1).
Designers must take care to avoid confusing the customer connectivity (logical topology) with the physical topology of the underlying network providing that connectivity. The difference between a hub-and-spoke and a star network is that a star network supplies arbitrary multipoint-to-multipoint connectivity between all customer nodes, while a hub-and-spoke network connects the hub customer node to each of the spoke customer nodes (point-to-multipoint). A router at the customer's hub node would have to supply any connectivity between spoke nodes.
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