LAN forms basis of all commercial,research,and data communication networks.Recently an increase in applications demand significantly higher band width. Ethernet is an easy to understand and extremely cost-effective technology. For these reasons, 98% of local area network (LAN) connections are now Ethernet based.
To meet this,high speed LAN types have been developed ,including a number of variations of basic ethernet LANs. CSMA/CD(carrier sense multiple access with collision detection) is used widely in wired LANs as a MAC(Medium Acess Control) method. Transmission medium is coaxial cable (ether) up to 2.5km long,with repeaters at every 500meters.Upto 256 machines could be attached to the system via transceivers screwed onto the cable.The system ran at 2.94Mbps
Optical Ethernet is the technology that extends Ethernet beyond the local-area network (LAN) and into metropolitan-area networks (MANs) and wide-area networks (WANs). While Ethernet LANs are almost exclusively used within the enterprise, optical Ethernet technology can be used as a service provider offering. Key components of Optical Ethernet are the abilities to segregate traffic of different users and to deliver the particular service level each user purchases
They combine the flexibility, simplicity and cost effectiveness of Ethernet with the reliability, speed and reach of optics to allow users to extend their LAN environment across the MAN and WAN.
Optical repeaters were part of the first Ethernet standard back in the early 1980s. Today, optical Ethernet advances promise to take Ethernet transport to levels undreamed of back then and not even feasible using copper technologies today. Thanks to advances in optical Ethernet, this most common (and most standard) of LAN technologies will soon be the most common (and most standard) of WAN technologies. The history and potential of optical Ethernet technology is explained , focusing specifically on its impact on service-provider networks and services.
Ethernet was defined as an open standard in the early 1980s by a consortium comprised of Digital Equipment Corp., Intel, and XEROX; the resulting standard was called the DIX Ethernet Spec. The goal was to promote a relatively high- performance and low-cost LAN implementation using digital (baseband) signaling on a shared coaxial cable.
In 1983, the IEEE 802 Local-Area Network/ Metropolitan - Area Network Standards Committee (LMSC) released the 802.3 standard for Ethernet—a shared medium for LANs using a distributed media access control (MAC) mechanism called carrier sense multiple access with collision detection(CSMA/CD).
Ethernet has proven to be a highly flexible standard. It has evolved to include point-to-point signaling, full-duplex (unshared) links, and very-high-speed networks. Several Ethernet characteristics have persisted, the most important of these being the framing format itself .
An Ethernet frame specifies a minimum and maximum packet size, a “protocol-type” field (expanded by 802.3 to function sometimes as a packet-length field), and both a destination and a source MAC address. The address fields have been particularly valuable, as the globally unique MAC addresses (each 48 bits long) have enabled the creation of new devices (Ethernet switches) that have simplified the development of large Ethernet networks.
Optical links have been a part of Ethernet standards since the early 1980s. The needs and potential of optical transport, including point-to-point and full-duplex links, have driven some of the evolution of Ethernet. Optical Ethernet technologies are providing the longest spans and greatest speeds used in LANs today, and they will undoubtedly, continue to do so in the future.
The figure shown below reveals the frame format of ethernet .
The First Optical Ethernet Repeaters
The first Ethernet standard included a provision for a single 2-km optical repeater in an Ethernet LAN that was expected to be used to link different buildings in a large LAN. As parts of a shared LAN, these links were not only half- duplex—they also propagated collisions (the signals used to limit access to a single sender at a time). Their spans were limited by the maximum delay that could be allowed on an Ethernet LAN and still detect collisions. They were true Layer-1 repeaters.
Campus Optical Ethernet
The advent of the Ethernet “bridge,” now commonly called an Ethernet “switch,” changed the game. The purpose of an Ethernet bridge is to connect two different Ethernet LANs (the name “switch” evolved to denote interconnecting more than two). This occurs at the MAC layer, Layer 2 of the 7-layer OSI protocol model, and there are two important features involved. First, not all traffic on either end is transported — only traffic destined for the “other” LAN. Second, collisions (and collision-detection signals) are not transported; each side is its own Layer-1 LAN. Together, these features not only improve network performance by isolating LAN segments but also greatly increase the maximum size of an Ethernet LAN.
The Ethernet bridge enabled large LANs to be deployed because a network of campus “bridges” could interconnect all of the building LANs. Instead of forming a simple “star” network, these were implemented as meshes, with multiple connections to each LAN. This required the development of the spanning-tree protocol (802.1D), which works by disabling redundant paths and which implements a form of path protection for the LAN.
People quickly realized that if both ends of an optical link terminated on a bridge port, then the normal limits on the size of an Ethernet LAN segment no longer applied. The optical link could be operated as full-duplex, thereby doubling the bandwidth of the link. And with only one transmitter on a LAN segment, there could be no collisions; thus, the need to limit the size of the span for collision detection vanished, allowing an optical Ethernet segment to span as far as the lasers could reach.
In the early days, this still meant only a few kilometers because light emitting diodes and multimode fibers were used; but this was still enough to enable large campuses to be fully connected. The loss of collisions as a flow-control mechanism required the development of a new protocol, 802.3x, to handle that need.
Optical Fast Ethernet
In 1995, a new standard emerged for 100–megabits-per-second (Mbps) Ethernet transmission (or “Fast Ethernet”) over Category-5 (Cat-5) unshielded twisted pair (UTP) copper cable. Actually, several standards were proposed and implemented, but only one gained significant acceptance—that was 100BASE–TX using two Cat-5 UTPs at distances up to 100 meters.
Following close on the heels of the “copper” Fast Ethernet standards development was the optical side. The first standards to emerge were adapted from fiber distributed data interface (FDDI) technology. The transceiver design and encoding technique were the same, simplifying the work of the standards committee and ensuring that the standardized technology would actually work. While there were some differences, considerable effort was expended to make sure that FDDI transceivers could be readily adapted to optical Fast Ethernet.
As on the copper side, several standards were ratified on the optical side. The first standard was for medium-range multimode fiber transmission at 1310 nm (100BASE–FX), based upon the FDDI standards. This provided for a normal range of about 2 km—adequate for most campus environments. 100BASE–FX was part of the original 802.3u Fast Ethernet specification back in 1994. The second optical fast-Ethernet standard was 100BASE-SX, ratified in June of 2000 (a full six years later).
This standard enabled backward compatibility with 10BASE–FL by using the same 850 nm wavelength and an auto-negotiation protocol.
There was no single-mode fiber standard for optical Fast Ethernet transmission (and there is still none at the time of this writing, nor is one expected). This lack of a formal standard has not stopped equipment manufacturers from implementing longhaul (10 km – 100 km) fast-Ethernet links, and in practice they are likely to be interoperable, at least when operating at the same wavelength. They are available both at 1310 nm (the wavelength used in 100BASE-FX, FDDI, and SONET equipment) and 1550 nm (the wavelength used in wavelength division multiplexing [WDM] systems). This de facto compatibility has resulted from the evolution of Ethernet devices, which originally separated the digital logic from the analog logic.
This distinction was formalized in the Fast Ethernet standard as the media independent interface (MII). Today, the chip sets that handle Ethernet are generally independent of the chips that handle the media, be they copper, fiber, or potentially something else. Moreover, the fiber-optic drivers themselves don’t know (or care) if the fiber is multimode or single-mode, what the type of connector is, or what wavelength is being used. The bottom line is that the approach used by Ethernet component manufacturers has led to a great deal of flexibility and interoperability, often transcending the standards that were created for that very purpose.
Optical Gigabit Ethernet
The 802.3z gigabit Ethernet standard describes multiple optical specification. 1000BASE–SX describes short wavelength (850 nm) transmission using multimode fiber with a maximum range of 550 meters on new fiber, or 220 meters on older fiber (with poorer dispersion characteristics). 1000BASE–LX describes long-wavelength (1310 nm) transmission using either multimode fiber (with a range of 550 meters) or single-mode fiber (with a range of 5000 meters). As before, the standards committee took full advantage of existing technology and “borrowed” the transceivers and encoding formats of fiber channel. Specifically, the FC–0 fiber driver/receiver was copied, along with the FC–1 serializer/ deserializer. The 8B/10B encoding format was used, which specifies the framing and clock-recovery mechanism. The significant changes were the signaling rate, which was increased to 1.25 gigabits per second (Gbps) from fiber channel’s 1.06 Gbps, and the frame content and size, which are the same as previous Ethernet implementations.
Much of the early gigabit Ethernet standards work was in specifying half-duplex operation in a collision detection environment equivalent to the shared LANs of ordinary and Fast Ethernet. However, that work was effectively wasted—all commercial Gigabit Ethernet implementations today are point-to-point, full- duplex links.
Just as in Fast Ethernet, the range of commercially available optical Gigabit Ethernet interfaces exceeds the limits outlined by IEEE 802.3z. There are devices available that operate at 1550 nm and devices that operate at much greater distances than 5km. In fact, 150km spans are possible without repeaters or amplifiers. Again, this was enabled by the separation of Ethernet control logic from media control logic, which has now been formalized in a new way. It is not the formal definition of an MII (called gigabit MII [GMII] in the Gigabit Ethernet standards), but rather a new standard for a GigaBit Interface Converter (GBIC).
Originally specified for fiber channel, the GBIC standards have evolved to support Gigabit Ethernet, and these modules have become the de facto standard for gigabit Ethernet interfaces. They provide hot-swappable modules that can be installed to support LAN, campus area network (CAN), MAN, and even WAN transport interchangeably, as needed. And they are available from a large number of suppliers, keeping prices competitive and capabilities expanding.
The GBIC revolution is one of the best examples of industry consortia creating a new market by consensus, yielding a net increase in the revenues of all the participants. The modules themselves, by virtue of being small and plugging into a standardized slot, challenged the transceiver manufacturers, who responded
with technological innovations and features beyond all reasonable expectations. The chip manufacturers contributed readily available chip sets that support these GBIC modules, simplifying and speeding up the task of the hardware engineers.
Optical Ethernet Today
Optical Ethernet systems are evolving beyond mere “optical links” that interconnect isolated LANs. Rather, they are becoming systems in themselves, providing scale and functionality that is simply not feasible with copper-based Ethernet, including those linked by routers.
Today, few optical Ethernet links are implemented within a computer room or small building. But there are exceptions for electrically noisy environments, highly secure transmissions , and ground isolation. And even in a small building, it is easier to run fiber-optic conduit than electrical wires because there are fewer issues with building codes. Traffic segregation is accomplished by using the IEEE 802.1pQ virtual LAN (VLAN) standard. This standard lets Optical Ethernet networks mark each user's traffic with a VLAN tag as it enters the network and then use this tag to keep each user's traffic separate as it crosses the network. Of course, 802.1pQ was designed for enterprise networks and the number of possible VLAN tags is too low. Work is under way in IEEE to extend this number from 4,096 to approximately 16 million.
This situation is likely to change, as very-short-reach optics support much higher speeds than copper does. It is important to note that gigabit copper links are limited to about 30 meters and that the next generation of Ethernet at 10 Gigabits would drop this already inadequate distance dramatically.
Today, virtually all Ethernet links greater than 200 meters are implemented optically. The CAN is dominated by multimode fiber, although most CANs are really multiple LANs interconnected by routers that use optical links. This situation is changing, as the scale and functional capabilities of Ethernet switches increase.
Ethernet 10BASE–T hubs, once dominant, no longer offer a sufficiently lower cost to justify their use. More and more LANs are being implemented with Ethernet switches, providing separate switch ports to every node on the LAN.
Traditionally, these Ethernet switches aggregate traffic into a high-speed uplink port (once Ethernet, then Fast Ethernet, and now Gigabit Ethernet), which feeds a router that itself interconnects the LANs and provides WAN access. But today, when the cost of a router is weighed against the cost of a GBIC module, it loses every time.
This trend is being accelerated by the proliferation of virtual LAN (VLAN)–capable Ethernet switches and by the development of even larger and more capable routers that understand and route to VLAN IDs in the now larger LAN.
Optical Ethernet in the MAN is a relatively recent development. Gigabit optical Ethernet has the capacity to provide direct Ethernet services as a carrier offering, with service switches that limit actual delivered bandwidth as needed. Multiple vendors now offer direct Ethernet services to subscribers, with only a few core routers linking those subscribers to the outside world (the Internet).
But “Ethernet services” is not the leading reason to implement an Ethernet MAN today. Rather, the desire to reduce the number of routers in the network is becoming the most compelling reason to use a Layer-2 technology (Ethernet) in the metro area. The reason is simple—every router in a path, many of which are unnecessary, adds delay to the
packet transport. In an ideal world, routers provide a buffer between management domains, which are generally companies or network providers.
They provide a place to control access, provide security (via firewalls) and manage addresses. But within a management domain, layers of routers generate excess complexity and require large staffs of “router guys.” They also bypass a primary tenet of many routing protocols—that every router should have a direct link to every other router that it knows about.
Based fundamentally on Ethernet, Optical Ethernet MANs let carriers deliver standard, well-known 10/100M bit/sec or 1G bit/sec Ethernet interfaces - the same as those used to easily connect office networks today.
Instead of a SONET ring, the metropolitan backbone for Optical Ethernet networks will be based on the 10G bit/sec Ethernet standard from the IEEE working group 802.3ae.
Optical technologies enable Ethernet networks to extend over much greater distances than campus Ethernet nets. Running over single-mode fiber, Optical Ethernet lets links in the network range from 3 to more than 6 miles in the case of 1310-nm wavelength technology, and up to 43.4 miles for 1550-nm wavelength technology.
Ethernet transport has not yet taken off in the long-haul network, but this is expected to change as 10-Gigabit Ethernet interfaces become available. Some of those are expected to operate at SONET OC–192 speeds and at the distances needed for long-haul networks. The distance limitations are not a serious concern because most long-haul networks use dense
wavelength division multiplexing (DWDM) systems to combine multiple circuits over a single fiber, each on its own wavelength, and these DWDM systems provide the long-haul capability themselves. Still, wide-area Ethernet networks are expected to be implemented, the main reasons being speed, cost, and simplicity.
Consider a nationwide 10-gigabit IP SONET–style ring implemented as OC–192 packet-over–SONET (POS) links between a dozen cities. Each city would need a large router with two relatively expensive POS ports, and the average packet would traverse half a dozen routers as it crossed the network. In the event of a link failure, the routers would spend a significant span of time converging on a new set of routing tables to bypass the failure.
Now consider the same network with each city containing an Ethernet switch with two 10–Gigabit Ethernet switch ports and a 1-gigabit port connected to the local router. Total costs here would be much lower because 10–Gigabit Ethernet switch ports are expected to be much less expensive than the equivalent router ports, and 1-gigabit router ports are relatively inexpensive.
Today, several vendors have products that aggregate and transport Ethernet traffic at 10-gigabit speeds suitable for the WAN. These are proprietary, requiring matching devices from the same vendor at each end— normally not a problem in a full-duplex, point-to-point
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Ethernet over Fiber is primarily deployed in a point-to-point or mesh network topology, and delivers packet services over dark fiber typically in the Metro Area Network (MAN).
Ethernet over RPR solution is through next-generation SONET products that enable simplification of network provisioning, elimination of SONET bandwidth waste, improvement of bandwidth efficiency, and link-layer protection.
Ethernet over SONET/SDH solutions deliver high-QoS Ethernet Private Line services across an arbitrary multi-domain SONET/SDH network, addressing established service providers who want to develop maximum return on network infrastructure assets.
Ethernet over Coarse/Dense Wave Division Multiplexing (WDM) is particularly effective in high-bandwidth, extreme-performance scenarios
Optical Ethernet Switches
A true “all-optical network” is not likely to happen in the Ethernet space in the foreseeable future. The reason for this is that there are no technologies that enable packet switching at the optical layer today. However, purely optical Ethernet switches have already been demonstrated, in the sense that all of the Ethernet interface ports are optical (while the internal switching remains electrical).
This is today’s technology optimized for carriers selling Ethernet services. When a carrier’s customers are spread over metropolitan distances (10 km – 50 km), then the lowest-cost service-provider network is Ethernet with all of the device ports themselves being optical.
Businesses are requesting native Ethernet services to interconnect their facilities into VLANs. This trend will be accelerated by the fiber-to-the-consumer move, as the lowest-cost optical service will once again be Ethernet. So the bulk of early fiber-to-the-home (FTTH), and possibly the majority of fiber-to-the-business (FTTB) services, may well be optical Ethernet (at least as viewed by the carrier).
The latest GBIC innovation is a mini–GBIC, also known as the small form-factor pluggable multisource agreement (SFP MSA) module. The mini–GBIC is only about half the size, effectively doubling the available capacity that can be designed into the face of an equipment shelf. New designs are quickly taking advantage of this space savings.
Resilient Packet Rings (RPR)
While not formally a part of Ethernet, the IEEE 802.17 committee is creating a standard for packet transport over fiber-optic rings. Current directions include using Ethernet framing in SONET–style rings. The goal of RPR is simple: to define a high-performance, high-availability optical transport suitable for carrier networks in metropolitan service areas. Note that it is easier to implement a fast and robust link-failure recovery in a ring topology than in a mesh topology; this is because in a ring, the alternate route is always known.
The IEEE 802.17 committee does not view RPR as Ethernet, and indeed there is no intent that an IEEE 802.3 device could be directly connected to an 802.17 interface. To a degree, a metro Ethernet is a competing mesh (not ring) technology with relatively slow spanning-tree failover, instead of SONET–style, fast (50 ms) failure recovery. Still, RPR implementations are likely to become a popular transport mechanism for Ethernet packets, especially for telecommunications service providers
10-Gigabit Ethernet Proposed Standards
The 10GEA (10-Gigabit Ethernet Alliance) is an industry consortium of about 100 members working to promote the acceptance and success of 10-gigabit Ethernet. This group is not the same as the IEEE 802.3ae standards committee, which is working on a set of proposed standards for 10-Gigabit Ethernet. The manufacturers are not passively waiting on a standard, as multiple vendors expect to announce 10-Gigabit Ethernet products during 2001, in advance of the standards ratification expected in March 2002. Of course, each of these vendors hope that their products will be one of the standards, and they promise that they will conform once those standard interfaces are defined.
10-Gigabit Ethernet May Be Optical Only
here are real challenges to making electrical signals carry a significant distance at those rates, and the implementations may simply be more expensive than their more capable optical cousins (meaning that they simply won’t happen).
One of the proposed standards uses very-short-reach optics, to be implemented as parallel data streams over a fiber-optic ribbon containing 12 multimode fibers. This was proposed as a low-cost method to interconnect devices in a room.
A second proposed standard uses a very compact package (about 1" x 0.75" x 3") containing a coarse WDM device, four receivers, and four lasers operating approximately 25 nm apart in wavelengths near 1300 nm. Each transmitter/receiver pair operates at 3.125 gigabaud (data stream at 2.5 Gbps). The proposed device has very aggressive engineering challenges and a very aggressive target-price point.
A third proposed standard is a serial interface using 64B/66B encoding (instead of the 8B/10B used in gigabit Ethernet), a data stream of 10.000 Gbps, and a resulting clock rate of 10.3 Gbps. This is a favorite of the Ethernet “purists,” who like the simplicity of just moving the decimal place; moreover, the resulting optical data stream may be directly interoperable with some of today’s DWDM systems.
A fourth proposed standard is a SONET OC–192 compatible stream, which is therefore clocked at 9.953 Gbps. The disadvantage is that it is not a pure 10 times 100 megabit Ethernet. There may also be disadvantages of cost—SONET is not the least expensive transport method. The advantages, however, are that it would be guaranteed to interoperate with all of the OC–192 SONET devices, including the networks of all of the major telephone companies, and all of the OC–192 compatible DWDM systems. And the reality is that 9.953 Gbps is close enough to 10.000 that no “real” applications are likely to notice the difference.
Ethernet is (by a huge margin) the most successful networking technology ever. After all, 99 percent of all TCP/IP packets (including Internet traffic) traverse an Ethernet somewhere (and more likely five or six Ethernet LANs in the World Wide Web). And the future is even brighter! The Radiance Optical Ethernet System is a new breed of optical access and connectivity that provides a secure, yet manageable, Ethernet demarcation point between the service provider and business user
Optical Ethernet to the Consumer
FTTB is a reality for the Fortune 500 today and will be a reality for 95 percent of non–small office/home office (SOHO) businesses within just a few years. A carrier that runs fiber to a business enables the delivery of all of today’s communications services and, likely, all of tomorrow’s. And Ethernet services are the least expensive services that can be provided over that fiber today.
FTTH is already in trial deployments in several communities. Several studies have been completed suggesting that building an FTTH network is no more expensive than a full-scale DSL buildout or a two-way cable television (CATV) upgrade, costing in the neighborhood of $1,000 per home passed. The potential capacity of an FTTH network is orders of magnitude greater than DSL or CATV, so it is clear that for any new buildout, laying fiber is the logical choice. And just as in FTTB, Ethernet is the least expensive technology providing optical-network access today.
It seems clear that Ethernet services will be coming soon to businesses near you— and to residences soon after. The only questions are how soon, how fast, and how expensive? In many metropolitan areas, some service providers already provide Ethernet services by the megabit of capacity: you can buy from 1 to 100 megabits, as needed
A few service providers already sell Fast Ethernet (100 megabit) Internet access for $1,000 per month—a truly compelling price when compared to the Old World SONET– or ATM–based services.
Still, is Fast Ethernet fast enough? Of course, that depends upon the business, but the cost of laying FTTB dominates the capital expenditures, and today’s Fast Ethernet infrastructure can be readily upgraded tomorrow. The switching cost and Internet-access cost are still much higher for Gigabit speeds, but those costs are rapidly declining. For Ethernet services between locations for a business.
Gigabit Ethernet may already be needed to support file servers, backup servers, and other intranet applications. After all, most workstations installed today come with fast Ethernet built-in, implying that the switching and office-services infrastructure should be significantly faster to avoid bottlenecking.
Residential applications are more limited. 10-Megabit Ethernet should be adequate for Web browsing for the near future, and the bandwidth needed for telephony is negligible. The most bandwidth-hungry application recognized today is TV, especially premium services such as video-on-demand.
A single channel or movie at broadcast quality requires about 4 Mbps; DVD quality requires only about 9 or 10 Mbps; and even HDTV will probably only require 20 Mbps. Of course, these figures are per-channel-viewed, and the industry should plan on an average capacity of perhaps two channels per household at a time. Still, even the most aggressive of these numbers imply that a single Fast Ethernet service to each home (not shared) is more than adequate for the services we know of today.
Optical Ethernet Area Networks
Large optical Ethernet networks are changing the definition of the LAN. “Local” might even be “global.” The original barriers in an Ethernet LAN (3-km span, 1023 nodes, 1 optical repeater) have long since been vanquished. Today, the practical limits are more because of the need to terminate broadcast traffic or to provide security between management domains, or because of today’s limits to the number of MAC addresses that an Ethernet switch might support.
VLANs already start to address these issues; and larger VLAN–enabled switches in the future are at least likely to control the problems and isolate them for a really large router to handle.
The practical limits to the size of an optical Ethernet are not geographic; rather, they involve bandwidth, node count, and overlying protocol (broadcast traffic, routing-table size, etc.). As VLAN and other Ethernet services become more common, we may even see large corporate networks simplify into a single, optically connected Ethernet LAN, with only a few large routers providing the necessary functions of security, address management, and interdomain routing.
The largest optical Ethernet networks are likely to belong to carriers, interconnecting all of their points of presence (POPs). Even today’s optical Ethernet technology could be used to create a nationwide (or even global), very- high-speed optical Ethernet that can be used to interconnect the large core routers in each POP. Such a network would have fewer router hops, faster link- failure recovery, and lower cost than today’s “normal” network of OC–192 POS routers.
Beyond 10 Gigabits
Just as the growth of 10-Megabit Ethernet led to the need for 100-Megabit Fast Ethernet, and just as the growth of Fast Ethernet led to the need for Gigabit Ethernet, the growth of Gigabit Ethernet is now driving the market to 10-Gigabit Ethernet. This trend is not likely to stop anytime soon. Servers—whether Web servers, file servers, e-commerce servers, or others—must have greater bandwidth than the customers they serve, otherwise, those customers will feel frustrated with inadequate performance and possibly go elsewhere for service. The best example is Web servers.
If the average Web browser is using a 56k modem, a server on a T1 line can simultaneously handle approximately 30 customers. But the broadband movement has already started, and millions of consumers are now accessing the Internet from DSL and cable networks. These consumers access the Internet at speeds up to 10-Megabit Ethernet (typical connectivity for a cable-modem service), and for them, a service provider limited to a T1 line is already unacceptably slow.
This requirement to always be faster than your customer (and by a factor of the number of simultaneous accesses) drives the need for T3 (45 megabits) or even greater speeds today and will drive the need for Fast Ethernet and Gigabit Ethernet tomorrow.
Some carriers are already deploying gigabit optical-Ethernet services today. They may limit their customers to a few megabits per second, but the links are gigabit- capable; and someday the fees for gigabit-scale Ethernet services will be affordable. Then, even a 10-Gigabit–Ethernet transport will be inadequate. Engineers are already dreaming of the next step, and arguing over what speed it will be. 40-gigabit speeds (SONET OC–768) have already been demonstrated, so that is a possible Ethernet target. But the Ethernet “purists” insist that the only logical next step is to move the decimal point one more time—to 100 gigabits.
In the meantime, the protocols and techniques for bandwidth sharing over parallel links exist, work well, and are used in thousands of sites. It is a simple step to run parallel optical-Ethernet trunks, each on a separate wavelength, all multiplexed over a single fiber pair using DWDM technology.
In this way, a point-to-point Ethernet link could have scores of 10-gigabit channels, with an aggregate Ethernet bandwidth of perhaps 400 gigabits, today! Using recently announced DWDM capacity of 160 wavelengths, 1600-gigabit-per-second links could be implemented. So in a sense, Terabit Ethernet is already available—of course, this kind of network requires very large Ethernet switches at the ends of that fiber.
The limits on optical-Ethernet bandwidth may just be the limits of fiber-optic bandwidth— perhaps 25 Terabits per second for the available spectrum on today’s fiber, which is still well beyond the capabilities of today’s lasers and electronics. Still, extrapolating from recent trends gets us to that level in only 5 or 10 years.
The advantages of optical Ethernet:
· Ethernet is 10 times less expensive than the SONET technology being used today.
· Ethernet is a simple and widely understood technology.
· Ethernet is the best technology for carrying IP traffic - Ethernet and IP have grown up together.
· Optical Ethernet networks can easily handle the needs of both data and circuit-switched or voice applications. Circuit traffic requires only modest bandwidth,
4. Computer Networks - Tannenbaum A