A new family of standards is in development to extend the range of Ethernet to metro and access networks. Gigabit Ethernet is at the center of the effort. The original intent of the Gigabit Ethernet standard, adopted in 1998, was to interconnect local area networks running the original 10-Mbit/s Ethernet and the enhanced 100-Mbit/s Fast Ethernet. Since then, developers have proposed expanding Gigabit Ethernet—sometimes called GigE —to a broader range of "wide area networks," including backbone fiber links in metropolitan networks and access lines running to businesses, neighborhood nodes, and individual home subscribers. Gigabit Ethernet over either point-to-point fibers or passive optical networks has become a leading architecture for fiber-to-the-home systems, although formal standards are still in progress.
The success of the Ethernet standards stems largely from their use of inexpensive mass-produced hardware and their compatibility with existing cables. Ethernet has become the standard for computer networking, leading to huge production of low-cost transceivers.
Gigabit Ethernet continues that tradition, with terminal costs a small fraction of those for 2.5-Gbit/s OC-48 telephone equipment. Seeing the potential for cutting costs, developers have hopped on the Ethernet bandwagon for metro and access systems. Interest began during the telecom bubble and continues today. Realizing the potential of Ethernet in these applications required fine-tuning and new standards. The Metro Ethernet Forum is developing implementation agreements—but not formal standards—for metro applications. The Ethernet in the First Mile task force of IEEE's 803.2 standardization group is developing a set of physical layer standards for transmission over fiber and copper. The closely related Ethernet in the First Mile Alliance is developing industry support, and will host interoperability demonstrations and market the technology.
Ethernet basics
Understanding the importance of Ethernet requires a brief explanation of how it works. The central difference from standard telephone transmission is in the protocol for switching signals. The telephone network is based on circuit switching, which allocates a fixed capacity equivalent to one or more telephone circuits. Ethernet is based on packet switching, which was developed for computer data transfer in which signals come in brief bursts but delays can be tolerated. Data bits are grouped into packets, which may be of fixed or variable length. Headers indicate the address to which the bits are directed, like labels on a package. They also may indicate the length of the packet and—in some protocols—the priority it has in using network resources.
FIGURE 1: Packet switching in a router (top) holds incoming data packets in a queue, then transmits them in the data stream in sequence, filling capacity efficiently. Circuit switching (bottom) assigns a time slot to each incoming data stream, but those streams may not need all those packet slots. If there is no input on one channel (the blue data stream, for example) those slots go empty.
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When data signals arrive at a packet switch, they are queued for transmission. In a simple example, they are dropped into slots in the order they arrive, each with their own header (see Fig. 1, top). This approach can delay individual packets, but uses limited transmission resources more efficiently than circuit switching. By reserving a fixed capacity for each circuit all the time, circuit switching leaves empty space in the transmission line during quiet intervals in a conversation (see Fig. 1, bottom).
Traditional packet switching protocols lack key features that circuit switching uses to guarantee the quality of service. One is a way of assigning priorities, so services that are impaired by delays—such as voice and broadcast video—are delivered faster than delay-tolerant services. Also missing are tools that allow circuit-switched networks to recover quickly after services are interrupted by component failures or fiber damage. A major thrust of current work is to develop new standards and systems that overcome these limitations.
Ethernet standards and layers
Modern telecommunication standards are developed under the open system-interconnection structure developed by the International Standards Organization. The structure is a series of "layers," each performing a distinct function. Each layer requires specified interface formats, but the details of their implementation generally are left to the individual developer. The upper layers hide the lower ones from users. A computer user sees only the application layer, which takes packets of output data, applies headers to them, and sends them on their way to the network—actually to the next layer down. Then that layer applies its own header to the combination of user data and application header, and sends it further down the stack (see Fig. 2). The same structure applies for voice transmission.
FIGURE 2: In the layered structure of telecommunication standards each layer adds a header to packets from above and sends it to the lower layer. The whole sequence of bits is transmitted on the fiber in layer 1. Ethernet standards cover layers 3, 2, and 1.
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Ethernet standards affect the lower three layers, layers 3 (the network layer), 2 (the data link layer), and 1 (the physical or PHY layer). Layer 3 is the layer in which the Internet operates. Devices called routers collect input packets, apply the proper headers, queue the packets, and stack them together to transmit in sequence. Routers direct their output to other routers on layer 3, and they have information on the status of all other routers in the world. They use this information to decide which router to send each packet to, like a traffic cop with radio links to traffic cops at other intersections.
The fiber transmission format is specified at the physical layer. Layer 1 was established before the advent of wavelength-division multiplexing, so the output can be one optical channel transmitted on a WDM fiber, rather than an entire array of optical channels. In practice, Ethernet standards cover WDM formats as well as optical channel formats.
Metro and access standards
Three groups are collectively developing Ethernet standards for metro and access networks. The Metro Ethernet Forum (www.metroethernetforum.org) is concentrating on metro services on layers 2 and 3. A pair of allied groups—the Ethernet in the First Mile Alliance (www.efmalliance.org) and the Ethernet in the First Mile task force (grouper. ieee.org/groups/802/3/efm)—is developing physical layers standards. The Metro Ethernet Forum and the First Mile Alliance are industry groups; the First Mile Task Force is a group under the IEEE 802.3 standards board.
The Metro Ethernet Forum is working to add functions that will adapt Ethernet standards to the needs of telecommunications carriers providing metro and access services. Current Ethernet standards have no automatic recovery scheme, because they assume users will call an on-site network technician to fix the problem. The metro group is developing protection schemes to ensure the 50-ms recovery time needed for telecommunications, as well as other quality of service provisions. They are developing other operation, administration, and maintenance (OAM) tools demanded by carriers. Their standard also will define Ethernet-based service offerings, including a point-to-point Ethernet virtual private line, a point-to-multiple point Ethernet private local-area-network service, and an Ethernet service that emulates the voice circuits needed for telephone traffic. Nan Chen, MEF president, hopes the first standards will emerge early this year.
The First Mile Task Force is concentrating on physical standards for transmission over both fiber and copper. Making Ethernet work on very long lengths of existing telephone wiring is a crucial issue, says task force chairman Howard Frazier, because carriers don't want to replace all their existing cabling. To meet those goals, the task force is winnowing existing standards for Digital Subscriber Line (DSL), and converting them from the original ATM (asynchronous transfer mode) protocol to an Ethernet format. A key goal is to adapt the high-speed VDSL standard for Ethernet, but the task force has to wait for VDSL itself to be finalized. That may delay the initial IEEE standards until 2004, Frazier says.
Another task is to modify Gigabit Ethernet physical transmission standards. The original standard assumed the equipment would be housed in climate-controlled office buildings, but the new standard requires transceivers that can operate at temperatures of -40°C to +85°C found in industrial and outdoor environments. The new standard will allow for bidirectional coarse WDM transmission through a single fiber, recognizing that fiber may be scarce in parts of the access network. It also will formulate a new standard for 100-Mbit/s Fast Ethernet transmission on single-mode fiber, rather than the multimode fiber in existing standards. In addition, the standard will provide the operations and management tools that carriers need on the physical layer, complementing tools offered at layers 2 and 3.
FIGURE 3: An Ethernet PON provides downstream and upstream transmission. A passive optical splitter divides downstream signals among up to 32 fibers. All subscriber terminals receive all packets, but they discard packets addressed to other terminals, as in local area networks. Each terminal has an allocated time to transmit upstream signals, so packets from different terminals do not overlap. In single-fiber systems, upstream transmission is at 1300 nm, and downstream at 1490 nm.
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The new first-mile standard will include passive optical networks (PONs) as well as dedicated fibers, reflecting the growing interest in PONs. Downstream transmission will be an aggregate of 1 Gbit/s, split among up to 32 users at distances to 10 or 20 km from the headend, depending on the type of fiber (see Fig. 3). Each subscriber will have its own time slot for upstream transmission, so now two signals overlap, an approach called time-division multiple access. Coarse WDM will allow upstream and downstream transmission over a single fiber. Upstream transmission will be in the 1300-nm window, where sources are cheap; downstream will be at 1490 nm, leaving the 1550-nm band open so broadcast video can be added separately.
The Ethernet outlook
Ethernet has already found small niches in metro and access networks. Bill St. Arnaud, senior director of advanced networks for the Canadian Canarie consortium (www.canarie.ca) has developed a system that links schools and homes by sending Gigabit Ethernet over leased dark fibers. Cogent Communications (www.cogentco.com) delivers 100 Mbit/s Fast Ethernet services to subscribers over dedicated fibers. Yet the penetration of these systems remains limited.
Developers are optimistic that they can leverage the efficiency and low-cost of mass-produced Ethernet terminals to spread Ethernet into many more metro and access systems. Nearly a billion Ethernet ports have been shipped, Frazier says, and the economies of scale mean that ATM ports now used in these systems cost 6 to 10 times more than Ethernet ports operating at the same bandwidth. Ethernet would be a natural for broadband transmission, because it's already used for computer interfaces, but not inside DSL or cable modem networks.
These visions are not new; similar proposals emerged during the telecom bubble. Yet virtually all carriers stayed resolutely with circuit switching to maintain compatibility with their existing networks. New standards will build better transitional bridges by giving Ethernet systems the functions that carriers want in a form compatible with their existing systems. Carriers including SBC and BellSouth are among the sponsors of the Metro Ethernet Forum. The big question is how well the new systems will meet carriers' evolving needs for metro and access equipment.
ACKNOWLEDGMENTS
Thanks to Howard Frazier, Ethernet in the First Mile Task force; Craig Easley, Ethernet in the First Mile Alliance; and Nan Chen, Metro Ethernet Forum.