Network vendors brew Ethernet/SONET hybrid

System vendors are welcoming new technologies for carrying Ethernet over synchronous optical networks (SONET), a development that will enable additional services on their previously SONET-only platforms. DWDM system vendors see the technologies as a means of increasing their client signal density without having to increase the number of wavelengths supported per fiber, and carriers see them as way of adding services on top of the existing infrastructure. And, end customers will finally get a wide area network (WAN) interface that looks just like one of their familiar LAN ports.

Vendors and carriers want small incremental additions to existing systems. For customers, the immediate value is high: a reliable, high-performance WAN connection that is as simple to manage as any other port on their LAN.

The benefit of Ethernet over SONET is that it is evolutionary, using existing protocols and networking equipment at customer premises. No changes are required to the data packets being transported between a customer's sites, or between a customer site and a service provider's point of presence. The majority of network data today exists at some point as an Ethernet packet most often originating from a PC. Network servers (file servers, application servers or Web hosting servers) typically connect to networks with 100 or 1000 megabit per second Ethernet.

From a PC to a file server, or from a PC to a Web site, each packet will start and end its life as an Ethernet packet. It is logical and desirable to keep this Ethernet format end-to-end to prevent the need for protocol conversions. The problem, however, is that Ethernet was designed for LANs and doesn't readily scale to WANs extending beyond a building or campus. Even though there are a number of initiatives to improve the scalability and reliability of Ethernet networks, it will be some time before native Ethernet networks can stretch around the globe and offer the predictable performance and reliability that carriers and their customers demand.

Today's networks already span metro areas, countries and continents — these networks are based on SONET / SDH technology, standards adopted in North America and the rest of the world respectively. It has taken significant investment to build out these networks and carriers will be reluctant to rebuild the networks again to support new services. Instead, the requirement is to add new services without increasing, and even reducing, capital expenses.

SONET / SDH owes its success to its high performance and scalable nature, combined with manageability and high reliability. SONET channelization and multiplexing enables a hierarchical network structure where individual client signals are kept independent and managed end-to-end.

Meanwhile, Ethernet owes its success in the LAN due to its consistent nature. The IT manager of an organization can be confident that new equipment will integrate into an existing Ethernet network and that the same management techniques will still apply. No staff retraining is required to support increased network speeds. However, interconnecting the LAN to the WAN has traditionally been awkward, needing a router to convert the LAN Ethernet packets into Packet over SONET (POS) for transport across the WAN. The router must have knowledge of all protocols that are being carried on the Ethernet LAN and must be configured to handle IP as well as any enterprise-specific legacy protocols.

Management of this system and the SONET WAN interface requires specialized IT skills beyond those needed to manage just the Ethernet LAN. Fortunately, standards now exist that enable Ethernet packets to be carried directly over the SONET/SDH WAN with no protocol conversion.

Two new standards have been developed that increase the efficiency of Ethernet over SONET/SDH. The first, a technique called Virtual Concatenation (ITU-T G.707 2000), enables SONET/SDH bandwidth to be better partitioned to efficiently carry Ethernet traffic. Rather than using contiguously concatenated SONET payloads, Virtual Concatenation uses the base SONET/SDH payloads and groups these payloads to create a larger "right-sized" aggregate payload. In addition to right-sizing the payload to match the data rate of the client service, Virtual Concatenation enables the payload capacity to be varied. This sizing allows a greater number of Ethernet channels to be mapped into the SONET/SDH signal. These subrate channels allow carriers to offer a tiered transport service where customers only pay for the required bandwidth, yet they still use the same connection into the WAN.

The virtually concatenated SONET payload is viewed as a set of byte lanes, where each byte is carried in a separate channels. The data stream to be transported is mapped, byte by byte, into the lanes of the virtually concatenated payload. The individual SONET/SDH channels that make up the virtually concatenated group can then be independently transported over the SONET/SDH network.

Aligning the SONET paths requires buffering each path. The buffering required should be sufficient to store all paths up to the point that the slowest path is received. The slowest path is the path that experiences the greatest network latency between the transmitter and the receiver. The difference in latency between the fastest and the slowest path is a result of the different paths taking diverse routes through the network. Once the slowest path has been received, then the alignment can begin and the data can be extracted.

The second standard recently developed is the Generic Framing Procedure (GFP, ITU-T G.7041) to provide a means of delineating packet data in a SONET/SDH payload without variable bandwidth expansion. Each data packet has an 8 byte header added which indicates the start, type and length of the data frame. GFP supports an extended header, for future uses that may need addressing and multiplexing functions.

SONET Virtual Concatenation and Generic Framing Procedure may seem like a lot of added complexity. Will this result in expensive data solutions? Fortunately, the answer is no. Virtual Concatenation and GFP together with necessary Ethernet and SONET path processing functions can be handled in a single device, which will sit on a line card in an Add/Drop Multiplexer (ADM) or a SONET/SDH cross connect. This adds Ethernet transport services to the existing SONET network. The expensive alternative is replacing the SONET network with an all-packet infrastructure, requiring that every box in the network be replaced, and each and every packet be inspected and switched at every node. Finally, supporting traditional TDM services becomes very difficult in a packet-only network. It will be some time before packet technologies are able to approach the characteristics of today's SONET-based carrier networks — high performance, reliability and manageability.