Metro Ethernet Switching Solution for Service Providers

White Paper

Ethernet in the First Mile

    Setting the Standard for Fast Broadband Access

by Bruce Tolley

Introduction

The "first mile" is the critical connection between business and residential users and the public network. Homes and businesses today have access to substantial bandwidth in their own LANs, whether they are 802.3 Ethernet, Home Phone Networking Alliance (PNA), or 802.11 wireless LANs. These homes and businesses want to connect to the gigabits and terabits of bandwidth in the metropolitan-area networks but are offered submegabit services that are "thin straws" for interconnect links. Ethernet is well-positioned to capture the first mile as the Layer 2 protocol of choice that offers many megabits of bandwidth. In almost all businesses and in many homes, the Ethernet RJ-45 connector is today the access point to the public network. This white paper presents an overview of Ethernet-in-the-first-mile (EFM) technology and communicates the strategic direction of Cisco Systems regarding the recently established IEEE 802.3ah standards effort:

EFM and the Cisco Metro Solutions Portfolio

Metro access with EFM technologies is just one component of the Cisco metro solutions portfolio. Cisco delivers industry-leading metro solutions that enable service providers to grow quickly and profitably with the most comprehensive multilayer service portfolio. This service portfolio is enabled through the industry's most complete modular system, which allows service providers deployment flexibility with operational consistency across all metro environments. The Cisco modular system is the only one to enable the metro with both 10 Gigabit Ethernet and 10 gigabits-per-second (Gbps) Synchronous Optical Network (SONET)/Synchronous Digital Hierarchy (SDH) OC-192. These benefits are achieved while delivering an unparalleled high-availability transport and data infrastructure. Figure 1 shows a comprehensive view of the metro landscape from products to technologies to systems and services to service provider and customer segments.

Customer Issues in the First Mile/Last Mile

The Ethernet industry recently christened this market opportunity the "first mile" to emphasize the need to put end customers first whether they are business customers buying T1/T3 services or residential customers seeking to upgrade from a 128-kbps DSL link. The first mile is an important business opportunity because of the mismatch between current solutions and customer needs. Subscribers have networks capable of supporting tens of megabits of bandwidth. The metro core supports gigabits of bandwidth. But the links between the subscriber and the metro run at submegabit or even kilobit rates.

The first mile is defined as the infrastructure that connects the business or residential subscriber to the carrier's or service provider's central office. Also referred to as the subscriber access network or the local loop, the first mile is the neighborhood communications infrastructure that connects the subscriber to the central office or point of presence (POP). The central office is also known as the local exchange or headend. The central office is the building where switching and routing equipment interconnect data to the metropolitan and core networks. The central office is the carrier's on-ramp to the national high-speed packet and circuit-switched networks.

The subscriber is at the other end of the first mile and might be sitting in a single-family residence, a campus, or a business. In a Public Switched Telephone Network (PSTN), the subscriber is the ultimate user or customer of a communications service. Subscribers can include individuals, activities, and organizations. Subscribers use end stations that are connected to a central office and are usually subject to a tariff.

Sometimes, the first mile is located within a building as an in-building distribution network connected through a router or Layer 3 switch to a service provider via some high-speed connection. Such buildings are called MxUs and include apartments or multidwelling units (MDUs), office buildings or multitenant units (MTUs), and hotels or multihospitality units (MHUs).

The first mile consists of diverse media. With unshielded twisted pair (UTP), various grades exist from untwisted copper pulled in the 19th century to one or more pairs of voice-grade-or-better copper. Coaxial cabling also exists in the last mile deployed for all copper cable television (CATV) or multiservice operator (MSO) networks or for hybrid fiber-coaxial (HFC) networks. The fiber, for the most part, is single mode, although multimode fiber is occasionally deployed in access networks of short reach in Europe and Asia. And in wireless applications and free space optics, the medium is air.

A variety of protocols exist in the last mile: DSL, cable modems, ISDN, satellite, broadband wireless, and so on. DSL, cable modems, and ISDN all offer data rates slower than 10-Mbps full-duplex Ethernet. Satellite solutions are download-only. The role of broadband wireless is still to be determined. Free space lasers seem ideal for business applications where fiber cannot reach the building, in other words, metro fiber extensions. Both Fast Ethernet (100 Mbps) and Gigabit Ethernet have been successfully deployed for dark-fiber business applications and are beginning to be evaluated and deployed for trial use for fiber-to-the-home applications.

To address the problem of the "thin straw" and other customer issues in the first mile, Cisco Systems has helped start a new IEEE 802.3 Ethernet standards effort to develop standards-based Ethernet first mile solutions. Cisco has proposed that the task force work on both copper and optical solutions for subscriber access in the first mile. As a leader in the IEEE 802.3 Ethernet standards process, Cisco is working with other systems vendors, as well as with other Ethernet industry participants, such as optical component suppliers, to define the protocols and interfaces to support subscriber access over various topologies in the first mile.

Ethernet in the First Mile

All Internet traffic today begins and ends as IP and Ethernet. By developing Ethernet as the network transport technology of choice in the first mile, network designers can build networks with IP and Ethernet and avoid the cost and complexity of protocol conversion. Ethernet is the lowest-cost, highest-volume networking technology. Ethernet solutions in the first mile enable designers of hardware systems to use the installed base of 300 million Ethernet ports and merchant industry of chipsets and optics. Because Ethernet is familiar technology with a large installed base, the development of Ethernet in the first mile will enable network managers to take advantage of their investments in the installed equipment, network management and analysis tools, and information technology staff expertise. Ethernet also supports all services, data, voice and video, and all media types, copper and fiber.

Ethernet is cost-effective in a first-mile network. By removing protocol layers and the associated network elements at the edge of the last mile, the use of Ethernet lowers equipment costs, lowers operating costs, lowers complexity, and simplifies the architecture. More important, the design philosophy of the Ethernet industry promotes high-volume manufacturing and low-cost design. Because the whole industry—from chipset vendors and optical components manufacturers to systems vendors—participates in the standards process, the 802.3 Ethernet interfaces are very well-defined and can be implemented with available technology. This Ethernet standards process enables intense market competition at every stage of the value chain, and that lowers the costs of components and subsystems and lowers the costs of the systems available to customers, service providers, and other network managers. This collaborative process means that most of the complexity of the encoding schemes and electronic circuitry becomes embedded in merchant silicon, which lowers costs and increases competition. The contribution of Ethernet to technical innovation, competition, and increasingly lower costs to final end users is evident in the traditional Ethernet cost curves witnessed with Ethernet, Fast Ethernet, and most recently Gigabit Ethernet technology.

IEEE 802.3 Ethernet Standards Process for EFM

In November 2000, the IEEE 802.3 Ethernet Working Group formed an Ethernet in the First Mile Study Group. Working with more than 100 companies and 200 engineers, the study group has converged on objectives to work on point-to-point Ethernet on optical fiber, Ethernet passive optical networks on point-to-multipoint networks, and a physical layer for copper in the local loop, and Operations, Administration and Maintenance (OAM) for Ethernet subscriber access links. In July 2001, the IEEE officially approved the study group's project authorization request to create a standards project and created the IEEE 802.3ah EFM Task Force to develop technology and standards with the goals listed in Table 1.

Table 1   IEEE 802.3 Ethernet-in-the-First-Mile Task Force Objectives

Source: IEEE 802.3ah EFM Task Force

The Ethernet layer diagram (Figure 2) shows how various topologies can be supported with physical layers defined by the standards process.

Ethernet over Point-to-Point Copper (EoVDSL)

Because almost all the installed media in the outside cabling plant in the local loop is voice-grade, single-pair copper and most of the installed media inside buildings is Category 1, 2, or 3 copper, there is a strong desire for the IEEE 802.3ah EFM Task Force to support point-to-point topologies on copper and define a physical layer for this voice-grade copper. Specifically, the EFM Task Force's goal is to specify a physical layer for single-pair, nonloaded, voice-grade copper that reaches distances of at least 2,500 feet and speeds of 10 Mbps.

Cisco expects the task force to adopt Ethernet over very-high-bit-rate digital subscriber line (VDSL). That means using an Ethernet Media Access Control (MAC) layer on top of an ANSI VDSL physical layer (PHY). This kind of adoption of a proven physical layer is consistent with the philosophy and practice of the Ethernet industry to borrow existing PHYs. For example, Fast Ethernet (IEEE 802.3u) borrowed portions of the ANSI Fiber Distributed Data Interface (FDDI) physical layer and Gigabit Ethernet (IEEE 802.3z) borrowed the ANSI Fibre Channel physical layer. Table 2 summarizes the rates and reach of ANSI VDSL.

Table 2   Very-High-Bit-Rate Digital Subscriber Line (VDSL)

Source: Cisco Systems

The Ethernet-over-VDSL (EoVDSL) technology is an ideal solution for delivering 5-15 Mbps performance over existing Category 1, 2, and 3 cabling. Cisco Systems is already shipping products branded Cisco Long Reach Ethernet using this type of technology. With 10-Mbps Ethernet performance that reaches up to 5,000 feet or about 1.5 km, EoVDSL enables simultaneous voice, video, and data applications, such as high-speed Internet access, video streaming, and IP telephony. EoVDSL technology enables network designers to build high-performance access networks for multiunit building (MxU) and enterprise campus environments. MxU buildings include hotels, residential multidwelling units (MDUs), and commercial multitenant units (MTUs). Enterprise campuses include manufacturing sites, educational campuses, and hospitals. Data rates in these environments are suitable for multiple channels of video service in addition to voice and data applications. Ethernet over VDSL is also ideal for network access to residential customers from curbside distribution boxes or nodes. Both of these infrastructure topologies, when combined with EFM optical solutions allow end users to be coupled to the next-generation metropolitan networks cost-effectively.

Ethernet over Point-to-Point Optical Fiber

Advantages of a Point-to-Point Topology

The Gigabit Ethernet over point-to-point optical fiber topologies delivers cost-effective, high-performance broadband access to single-family homes and businesses. Fiber is the future-proof medium for delivery of today's and tomorrow's voice, video, and data applications such as high-speed Internet access, video streaming, and IP telephony. The advantages of optical fiber include high bandwidth, long runs up to 10 km, and the capability of being run in the same conduits as power and gas. For service providers considering new build-outs, fiber is the logical choice.

Historically, a major barrier to delivery of fiber to the home or fiber to the curb has been the inability to achieve a low absolute cost and an appropriate price-to-performance ratio. Optical Ethernet over point-to-point fiber will be able to make use of the high-volume, low-cost advantages of 1000BASE-X transceivers, which have been shipping in volume since the end of 1997. Not only will point-to-point solutions leverage the 1000BASE-LX cost curves; they will also deliver bandwidth to subscribers up to 1000 Mbps.

Ethernet is ideal for community networks that are deployed over point-to-point fiber topologies. Because the fiber runs all the way to the subscriber, it is possible to provide homes or businesses with a full gigabit of bandwidth. Service providers can enable new revenue alternatives by using a Layer 3 function known as rate limiting. With rate limiting in combination with service-level agreements, a 1000 Mbps physical link could be used to provision a 10, 100, or 200-Mbps service. Point-to-point gigabit networks offer incredible flexibility and scalability for the future. When the business case exists to deliver not just high-speed Internet access but other services such as voice and video, point-to-point Ethernet over optical fiber is an excellent solution.

Gigabit Ethernet over point-to-point optical fiber provides enough bandwidth to ensure a very long life span for the network infrastructure; the optical fiber infrastructure may be amortized over 20 or more years. Optical EFM therefore lowers the annual costs of transporting services while at the same time increasing the bandwidth available to deliver multiple revenue-generating services.

Gigabit Ethernet Technology

Table 3 lists the basic technology building blocks of building optical Ethernet point-to-point networks. Fundamentally, the network is a star-wired Gigabit Ethernet network using active 1000BASE-LX electronics on full-duplex links. To reduce the costs of fiber deployment in the link between the business or house and the distribution or access switch, Cisco has proposed to the EFM Task Force that a standard be developed for single-fiber, single-mode fiber transceivers. Cisco has also helped lead the effort to standardize extended-temperature-range optics to enable service providers to locate the optical network unit (ONU) outside the residence at the demarcation point between the optical network and the business or home network. These optics will be specified to operate at ranges from -40 to 85° C.

Table 3   Standards Supporting Point-to-Point Ethernet in the First Mile

Optical Point-to-Point Architecture

Figure 3 shows the architecture of a point-to-point network over optical fiber. In the case of Ethernet to the home, residences can be connected with the access or aggregation switches located on pedestals in the neighborhoods inside environmental enclosures, lifeline 911 voice services can be supported via battery backup service inside the enclosures. Connectivity to the core switches is accomplished with single or dual Gigabit Ethernet or 10 Gigabit Ethernet single-mode fiber links.

Ethernet over Point-to-Multipoint Optical Fiber (EPON)

EPON Benefits

While Ethernet over point-to-point optical fiber offers the highest bandwidth at reasonable cost, Ethernet over point-to-multipoint optical fiber offers relatively high bandwidth at a low cost. Depending on the optical split ratio, Ethernet over passive optical network (EPON) can support subscriber bandwidths of 30 Mbps and, with service-level agreements, an EPON could enable bursting of 100 Mbps or more.

EPON offers multiple economic advantages. The aggregation device, called the optical line terminator (OLT), supports 16 to 32 subscribers per port by means of a passive optical splitter. Thus, the Ethernet PON minimizes the number of fibers that need to be managed in the service provider's point of presence or central office, minimizes the number of central office transceivers, and reduces the rack space required in the central office, compared with a point-to-point topology. EPON also enables a low-cost fiber infrastructure by reducing the number of fibers in a trunk fiber. This economic benefit is significant. Finally, the EPON topology reduces maintenance costs by removing the need for electrical power and active electronics in the field although the diagnostic and troubleshooting overhead is increased. In addition, passive optical splitters have no need for curbside batteries or environmentally protected enclosures.

Table 4   New Residential Cost Comparision of a Typical Buried Suburban Housing Development

Source: N. Huffman, IntelligentCities 2001 Conference Proceedings, April 2001

For new construction, the installation of fiber to the home is more than competitive with copper-based digital loop carrier deployment (Table 4).

Finally, like the other complementary EFM solutions, EPON allows one IP/Ethernet network for all services—voice, video, and data—at speeds up to 60 Mbps.

Standardizing EPON within the IEEE 802.3 Architecture

While a passive optical network (PON) is a new media, the 802.3 working group over time has developed new physical layers (PHYs) and minimal augmentations to the Ethernet MAC protocol to support new applications and new media. For example, in recent years the IEEE 802.3 Working Group has developed the WAN physical layer to use the SONET infrastructure for the transport of native Ethernet across OC-192 networks, the 1000BASE-T physical layer to support 1,000 Mbps operation on four pairs of Category 5 cabling, and 802.3x flow control to manage access to full-duplex media. While the EFM Task Force is still defining its technical approach for EPON, to date the task force has discussed at least three different approaches that support PON media. These approaches maintain the simplicity of the Ethernet MAC and are interoperable with other Ethernet interfaces. The three approaches are extensions to 802.3x flow control, control packets, and enhancements to the PHY itself. Figure 4 shows how the laser control function may be implemented within the MAC layer, which makes EPON consistent with the IEEE 802.3 architectural model.

To review the EPON concept, it will use standard Ethernet frames and standard Ethernet MAC and PHY chipsets. The OLT broadcasts frames to the ONUs. Each ONU transmits upstream based on grants issued by the OLT. Thus, downstream the network operates much the same as an Ethernet broadcast network. The OLT regulates the amount of upstream bandwidth given to each ONU. Quality of service (QoS) and class of service (CoS) can be supported across the network through the use of priority tags and queuing mechanisms inside the OLT. By using service-level agreements, individual subscribers can be allowed bursts at very high bit rates.

PON Topology

Within the larger PON architecture, the optical distribution network is composed of the fiber distribution plant and the passive optical splitter. Figure 5 shows the components of the optical distribution network. Comparing Figure 3 with Figure 5 makes clear that the major difference between the two solutions is that EPON has no active Ethernet or optical equipment in the distribution or access layer in the neighborhood.

Certain physical characteristics of EPON will be standardized by the IEEE 802.3 Task Force. The IEEE 802.3 EFM effort is proposing a transmission medium that consists of one single-mode fiber. Bidirectional transmission could be accomplished by use of a wavelength division multiplexing (WDM) technique on a single fiber or some other means. In the one fiber system, the forward path and return path share the fiber by using different wavelengths or frequencies. Various bidirectional schemes have been proposed. For example, in one scheme borrowed from full service access network (FSAN) specification wavelengths, the downstream wavelength is nominally 1550 nanometers and the upstream wavelength is nominally 1310 nanometers. The distance between the OLT and ONU can be 10 kilometers or more depending on split ratio and optical link budgets.

The ONU provides the necessary functionality to connect the service provider-owned fiber to the media in the residence. The ONU is assumed to be on the outside of the residence. The ONU may be attached to a more intelligent residential gateway or Ethernet switch in the house (Figure 5). While the scope of the IEEE 802.3 EFM standards effort will include defining the interface and demarcation point between the optical network and the home network, the network inside the home will be outside the scope of the EFM Task Force.

Multiple devices in the home can be connected to a single Ethernet port from the home to the carrier. The ONT is responsible for media conversion from the optical to the copper network or other media in the home (Figure 5). The home, for example, may use phone wire, coaxial cable, or even a wireless connection to connect to televisions and personal computers. The connection between the ONT and the home network can be Category 5 or better data-grade copper cabling. Inside the home, the home-area network can be IEEE 802.3 10/100 Ethernet, IEEE 1394, IEEE 802.11, or Home PNA. While clearly outside the scope of the IEEE 802.3 EFM effort, because of the complicated nature of the interface functions, a component called the residential gateway or home network gateway could sit inside the home and decouple the purely optical functions of an ONU from high-layer protocol functions.

Table 6 shows that the same Ethernet switching platform can support point-to-point topologies as well as EPON. Table 5 also shows that point-to-point topologies support more bandwidth per subscriber but a smaller number of subscribers. Although the EPON requires more complex optics and the OLT must support the PON protocol, the greater complexity and greater cost can be supported at the headend in the central office, and the cost of each headend port can be amortized across 16 or 32 subscribers.

Table 5   Comparing Optical Ethernet in the First Mile Solutions

Supporting the Installed Base of Voice-Grade Copper

Despite the advances in DSL and, more recently, VDSL technology, some skeptics argue that voice-grade copper cabling is not up to the job of residential broadband. One of the arguments is that copper cabling was not designed for high-speed transport, and, even if it were, there are too many impairments owing to the age of the installed copper. In some parts of the United States, the cabling is decades old and diminished by corrosion and poor insulation. Nevertheless, all indications are that marrying an Ethernet MAC with a VDSL PHY can definitely support in-building installations of voice-grade cabling and most likely many cases of outside plant copper cabling. In real network deployments, copper and fiber are likely not only to coexist but also to be used to build hybrid networks.

Ethernet over point-to-point copper is probably the best fit for established neighborhoods, business parks, and MxUs because it can reuse the existing first mile of voice-grade, twisted-pair copper cable. For new residential developments and business applications, Ethernet over point-to-multipoint optical fiber will be the best fit because of its high bandwidth and long potential service life. For high-end commercial customers, Ethernet over point-to-point fiber may provide the best solution because it can scale to meet future bandwidth demands. Also, when the distance between the central office and the subscriber exceeds a mile, either point-to-point or point-to-multipoint optical fiber can be used as the interconnect technology, extending the reach of the Ethernet over point-to-point copper solution.

Management of Ethernet in the First Mile

The first mile is an entirely new application space for Ethernet. Because in the first mile the end user or customer is not employed by or associated with the provider of the Ethernet service and the Ethernet service provider may not own or control the medium over which the service is deployed, a whole new class of Ethernet management is required. Unlike traditional Ethernet, the first mile includes concepts of "headend" and "remote end." Therefore, it is critical that the headend equipment monitor vital aspects of the physical link up to a physical or logical demarcation point between the service provider's network and the customer's network.

The EFM Task Force agreed that the standard shall provide features that support management of this demarcation point, including such functions as remote failure indication, remote loopback, and link monitoring. There is substantial agreement in the task force that Ethernet needs additional management capabilities to succeed as an access technology. Such additional features are targeted at managing the physical layer itself. While specific proposals are still being developed, such proposals should possess such attributes as being transparent to the existing Ethernet MAC, causing no change to the existing Ethernet frame or data rate, and providing a broad set of OAM capabilities, including failover, a message channel, and performance monitoring. All service provider customers need remote OAM to manage subscriber access networks. The OAM effort will be one common work item across all three of the topologies by the EFM Task Force.

Conclusions

The use of IP over Ethernet in subscriber access applications eliminates unnecessary network layers. The elimination of network layers reduces the number of network elements in a network, and that reduces equipment costs, operational costs, and complexity. At the edge of the first mile, simpler architectures are always easier to manage. In the first mile, native Ethernet on copper or fiber will offer significant cost-performance advantage over competing technologies. These IP/Ethernet networks will of course coexist with time-division multiplexing (TDM) and SONET/SDH services. For example, for business customers, T1 and fractional T1 might be provisioned over Ethernet on optical fiber. Also, in many cases the service provider might backhaul data, voice, and video to a SONET/SDH network.

Metro access for business and residential subscribers with EFM technologies is one critical component of the larger metro solutions portfolio that Cisco Systems offers. The three EFM topologies being defined by IEEE 802.3ah will complement each other. Ethernet over VDSL on copper is the best fit for established neighborhoods, business parks, and MxUs because it can reuse the existing voice-grade, twisted-pair copper cable. For new residential developments and many business applications, Ethernet over PON will be the best fit because of its high bandwidth and long potential service life. For high-end commercial customers, Ethernet over point-to-point fiber may provide the best solution because it can scale to meet future bandwidth demands. Service providers will build hybrid networks especially when the distance between the central office and the subscriber exceeds a mile. In fiber-to-the-curb and fiber-to-the-cabinet applications, point-to-point optical fiber or EPON can be used as the interconnect technology to the central office, extending the reach of the Ethernet-over-VDSL solution.

References