GigE goldmine? Gigabit Ethernet over copper offers old technology new��and potentially lucrative��potential - Broadband Access

Two strands of networking--IP and Ethernet--are coming together and continuing a relentless march to become the unifying communications technology for the 21st century.

IP has become the dominant WAN networking protocol, and IP infrastructure has evolved to offer businesses new networking opportunities via VPNs. Ethernet was created to carry high-speed backbone data within LANs. Because LANs are linked by fiber, Ethernet today can transmit IP at ultrahigh-speed GigE and 10 GigE rates.

GigE and 10 GigE have piqued the imagination of service providers worldwide as a potential high-speed transport method for IP data services in the fiber-based metro area network and WAN. GigE offers a number of advantages. Distance limitations are virtually nonexistent over fiber, so data can travel on a single port segment without bridging or routing. No WAN protocol translation is required between the end points of the link. Ethernet ports are only one-tenth the cost of SONET ports.

Efforts are underway to make GigE more suitable for use in carrier networks. MPLS technology, for example, ensures end-to-end QoS and overcomes virtual LAN (VLAN) limits in large networks, RPR (Resilient Packet Ring) will allow Ethernet networks to offer diverse routing with recovery times inside the SONET 50 ms threshold, thus paving the way for a packet-based voice and data network to replace SONET standards. Carriers are also working to remove barriers to GigE in the backbone, such as the need for new operations and management methodologies for Ethernet and the ability to provide bandwidth on demand.

The Fiber Barrier

With GigE beginning to appear in the metro, many enterprises with GigE LANs expected they would be able to receive end-to-end GigE connections. Unfortunately, that expectation is unrealistic: GigE requires fiber for deployment, and fiber is not universally available. RHK estimates that fiber serves only 11 percent of businesses today. The cost of speculatively trenching fiber to every potential customer is prohibitive, and GigE service could probably not justify the cost. Most end users do not need--and cannot afford--a guaranteed 1-Gbps connection delivered via fiber to their buildings. As long as GigE remains just another fiber-dependent service, its promise will not be fulfilled.

Nevertheless, service providers recognize GigE could cure end users' broadband service woes. What's needed is an inexpensive on-ramp long enough to reach from commercial buildings to the existing fiber plant and that allows users to select--and pay for--bandwidth from 1 Mbps to 1000 Mbps on-demand. Carriers have tried to revitalize the existing copper loop plant for high-speed service delivery with ISDN, DSL, IMA (Inverse Multiplexing over ATM), and VDSL (very-high-data-rate DSL). However, achieving fiber-like performance requires not just speed but also a host of other characteristics. Problems such as crosstalk, interference, high error rates and distance barriers have historically limited the potential of copper for delivering ultra-high-speed services.

A new class of high-speed transport systems is emerging that radically boosts the speed, range and reliability of existing copper local loop facilities. These systems achieve the fiber-like performance and quality required by GigE while eliminating the enormous costs and lead times associated with stretching fiber to the curb.

The most advanced new solutions employ variable-rate symmetrical DSL modems and new multiplexing technologies to bond multiple copper pairs to deliver high-speed, very reliable broadband services at distances in excess of 12,000 feet, the typical GSA (carrier serving area). These new technologies handle adaptive FEC (forward error correction) as well as adaptive interleaving and adaptive spatial division. Adaptive interleaving spreads the transmitted bits in time, while spatially interleaving the data--distributing it among several lines--gives better immunity to noise bursts.

Since modems do not normally operate at equal rates and BERs (bit error rates), use of these interleaving technologies results in sending variable quantities of data on each modem per unit of time. This approach takes full advantage of the maximum potential of each copper pair and thus provides greater overall system throughput. It also provides great advantages over inverse multiplexing technologies such as IMA, MFR and MLPPP, which typically bond pairs using a round-robin approach for data distribution. With these technologies, the slowest line dictates the pace of the entire system, additional capacity on faster lines is ignored, and delay is high, making it unsuitable for voice and video traffic.

Six Prerequisites for GigE

To attain fiber-like speeds over copper, and to make GigE services available quickly to end users, any GigE solution must offer the following six prerequisites:

* Automatic Line Pre-Qualification. The solution must be able to test copper lines automatically to find out if they are suited to carry GigE service. Today, lines and bridges have to be physically tested, an expensive and time-consuming process. Eliminating these procedures enables first-day deployment and lets carriers start realizing revenues immediately.

Furthermore, the solution should be able to configure itself to run services over lines at the optimal rate, even if not every line is perfect. Unlike DS3, Ethernet is a best effort technology; it is acceptable for a 10-Mbps service to fluctuate from, for example, 9.5 Mbps to 12 Mbps. Even when throughput fluctuates, the new high-speed transport systems offer fiber-like quality.

* Immunity to Internal and External Disturbances. Any solution needs to ensure service availability and QoS even in extreme interference environments, including impulse noise; RFI emissions; adjacent T1, E1 or ISDN services; and other alien xDSL disturbers. The best new systems employ multiplexing schemes that give high immunity to both external disturbances such as lightning as well as to internal disturbances in the wire bundle. Immunity to interference is an important factor in achieving a BER exceeding the fiber threshold, achieving levels of [10.sup.-10] or better.

* FEC. In many xDSL concentrators, every modem uses spatial RS (Reed Solomon) coding as an FEC scheme and encapsulates within it TCM (Trellis Coded Modulation) for internal error correction. However, this coding scheme was abandoned during the development of the HDSL2 standard, mainly due to the delay incorporated in long code words running at a Ti rate. The latest GigE solutions centralize FEC so it encapsulates all the modems and constructs code words at the aggregated rate of the entire system. By adding only nominal amounts of redundant data to the transmitted data, FEC allows recovery of encoded data in a noisy channel while reducing delay to a minimum. FEC also provides advantages related to space diversity.

* Crosstalk Management. Crosstalk management and cancellation are crucial to extending the reach of modems beyond today's 6,000 to 7,000 feet as well as to dramatically improving throughput. Crosstalk is the primary reason for xDSL performance degradation between modems and the major culprit in BER performance of [10.sup.-4] over copper. The best new high-speed transport systems manage how modems operate on the various pairs of wires to reduce crosstalk interference to a minimum. In the future, active crosstalk cancellation mechanisms will further improve performance. A combination of crosstalk cancellation and FEC guarantees reliability and BERs of [10.sup.-10] or better as well as maximum rate and reach for the system.

* Cut-Line Immunity. Existing xDSL services auto-reset on a sporadic basis, causing data outages for minutes at a time. Since premium services such as DS3/E3 cannot tolerate such outages, a GigE solution thus must include a cut-line resiliency feature. A system that employs spatial RS FEC protection encapsulation can recover the lost data at one or more failed pairs with no disruption to the overall data stream.

* Traffic Prioritization. The ability to prioritize traffic is important in the event of a line cut that reduces bandwidth. If there is more than one traffic flow on the bandwidth, a traffic prioritization scheme ensures that services such as lifeline telephone are not interrupted.

While these prerequisites ensure a consistently high-performing system, they're not sufficient for market acceptance. Any copper-based GigE solution must also comply with all industry standards, including NEBS, and have OSMINE certification.

It's a GigE Universe

The introduction of new high-speed transport systems for copper infrastructure will make end-to-end GigE transport of IP a reality, connecting GigE LANs to the GigE core. With a reach of 18,000 feet or more, the systems enable carriers to provide GigE service to any customer within a CSA served by a CO or remote terminal. Because they employ existing copper, carriers can deploy the systems within hours, not the months required by fiber. Carriers can immediately start offering ultra-high-speed services that cost far less than services available over fiber. End users can get, and pay for, only the level of service they need-and they'll receive a QoS never before available over copper.

David Perry is the director of marketing for Actelis Networks

([email protected]).

COPYRIGHT 2003 Horizon House Publications, Inc.
COPYRIGHT 2003 Gale Group