As part of its sweeping potential in the access network several years ago, the IEEE 802.3ah Ethernet in the First Mile (EFM) standards group defined two technologies for delivering Ethernet over plain old telephone lines: 2BASE-TL and 10PASS-TS. These technologies offer higher bandwidth and higher quality services than existing T1/E1 and xDSL solutions, delivering the simplicity and flexibility of Ethernet, while still maintaining full spectral compatibility within the existing network. And since these standards were created, their capabilities and benefits have been accepted by carriers and standards organizations across the globe.

2BASE-TL and 10PASS-TS are expanding the copper access network. For those carriers offering Ethernet services over optical or SONET/SDH infrastructures, these Mid-Band Ethernet technologies make Ethernet services available to the vast majority of customers that do not have access to fiber. Instead of Ethernet services being limited by fiber availability to less than 10% of potential business sites, the services are now available to almost any subscriber location. With distance potential beyond 20 Kft (6 Km), 2BASE-TL can reach almost any business subscriber, providing a universal multi-megabit on-ramp to a Metro Ethernet network.

Adhering to the EFM standards is just the first requirement for providing a Mid-Band Ethernet solution. Additional capabilities are needed to reach the full potential of 2BASE-TL services, including full service edge functionality, with QoS, carrier grade reliability, and a low initial cost. The next few pages dive into the details of the EFM standards, showing how they work, and how they can help to increase revenues, decrease costs, and move toward a simplified network infrastructure.

The Ethernet in the First Mile Project (IEEE 802.3ah)
The demand for Ethernet in the carrier network was realized within the IEEE 802.3 Ethernet standards body, which created the 802.3ah Ethernet in the First Mile (EFM) task force. This task force developed enhancements to existing Ethernet standards targeted to address carrier concerns about deploying Ethernet in the access network.

The EFM task force developed two new Ethernet physical layers to address two very different and very important segments of the market. The first technology, 2BASE-TL, is a long-reach Ethernet-over-copper technology focused on high-bandwidth symmetric services for business customers from central offices or remote terminals. 2BASE-TL is the natural upgrade and replacement for today’s T1/E1 and G.shdsl services.

The second technology, 10PASS-TS, is a short-reach high-bandwidth asymmetric technology targeted for in-building or FTTC deployments, permitting some increased bandwidth options for residential or business access.

Both 2BASE-TL and 10PASS-TS enable native Ethernet frames to move across existing voice-grade copper pairs in carrier access networks.

What’s in a Name?
The IEEE 802.3ah copper pair standards are each named in the traditional Ethernet fashion with 2BASE-TL and 10PASS-TS. As with all Ethernet interfaces, the naming of the technology tells you a lot about the technology itself. Figure 1 dissects one of these names for its true meaning.  

The first digit gives the speed of the interface. In the case of 2BASE-TL and 10PASS-TS, this is considered the nominal speed of the interface as the interface itself can run at different speeds depending on the conditions of the environment. 2BASE-TL is targeted to deliver at least 2 Mbps symmetric operation per pair in a long-reach nominal noise environment, while 10PASS-TS is targeted to deliver at least 10 Mbps per pair in a very short-reach nominal noise environment. 

The second token is BASE or PASS, indicating whether the system is baseband or passband. 10PASS-TS is a passband system in that it can run on copper pairs that have a baseband POTS service (e.g., traditional POTS voice with data on the same line). 2BASE-TL is baseband in that it cannot run on the same line as POTS, but can carry VoIP. Both systems are run over twisted copper pair (hence the T). Finally, 2BASE-TL is a long reach service, while 10PASS-TS is a short reach service (hence the L and S). And there you have the parsing of the IEEE 802.3ah nomenclature!

The EFM Family
The EFM technologies cover the full spectra of copper access deployment possibilities, from short-reach to long, from business to residential. When looking at rate/ reach possibilities, 2BASE-TL and 10PASS-TS combine to blanket the chart as shown in Figure 2. 

With 10PASS-TS, EFM can offer very high rates on very short loops, reaching as high as 100 Mbps in asymmetric mode, and 50 Mbps in a more symmetric mode. 10PASS-TS also supports a bonded multi-pair application for additional bandwidth and reliability. It is targeted at 5 Kft (1500 m) and below in terms of Customer Serving Area (CSA) distances, so the reach is limited.

Additionally, although 10PASS-TS has more symmetric modes of operation, the reach of the symmetric capabilities is more limited than the asymmetric reach. For these reasons, 10PASS-TS targets true Triple Play residential services, with asymmetric, very high bandwidth service potential. 

In contrast to 10PASS-TS, 2BASE-TL is focused on delivering symmetric services to business customers from central offices and remote terminals. It supports service delivery out to CSA distances (9-12 Kft, 2,700-3,600 m) and beyond. With a maximum symmetric rate of 5.7 Mbps per pair, 2BASE-TL delivers high-bandwidth Ethernet services over just a single pair. However, EFM also defined a multi-pair bonding technique whereby up to 32-pair can be grouped into a single Ethernet port, thus increasing the bandwidth and resiliency of the subscriber connection. Businesses can now be reached with symmetric services of 10 Mbps and higher, on as few as two pairs of copper, as 2BASE-TL continues to be rolled out as the next-generation replacement for traditional T1/E1 services.

Mid-Band Ethernet and VDSL2
VDSL2 did not exist when the IEEE 802.3ah was being developed. In fact, VDSL2 wasn’t standardized until a year after the IEEE 802.3ah standard was ratified, long after the technical decisions were made. If VDSL2 were ready one year earlier, it would have been the underlying technology for 10PASS-TS. 

VDSL2 offers significant improvements over the original VDSL on which 10PASS-TS was based. In fact, because of the benefits of VDSL2 compared with VDSL, and its timing in coming out shortly after IEEE 802.3ah was finished, true 10PASS-TS implementations are unlikely to ever be developed. 

However, as mentioned earlier, the techniques of IEEE 802.3ah copper access, including efficient transport and dynamic bonding protocols, have been carried over into other ITU and ANSI standards. One can therefore use VDSL2, instead of VDSL, as the basis for a 10PASS-TS replacement, creating a new and improved short-reach Ethernet-over-copper technology. 

Note, however, that a VDSL2-based 10PASS-TS implementation still suffers from the same limitations as a VDSL-based implementation, just not quite as much. For example, VDSL2 still has extremely limited reach, and even more so when deployed in a more symmetric manner. Figure 3 shows the upstream line rate performance of ADSL, VDSL2, and 2BASE-TL in a typical noise environment. 

Figure 3 assumes 26 AWG cable and 24 disturbers. The 2BASE-TL numbers assume a worst case 24-disturber model (all self disturbers), while the VDSL2 and ADSL numbers are not worst case (12 self and 12 HDSL disturbers).

As Figure 3 shows, VDSL2 (and ADSL) upstream performance cannot support high-speed symmetric business applications with anywhere near the capabilities of 2BASE-TL. So although VDSL2 is a dramatic technology for Triple Play residential services, it has a difficult time for business applications that have a hard requirement for symmetric capability. 

The performance of VDSL2 suffers even more when classic T1/E1 disturbers are present in the environment, as was the case of ADSL as discussed earlier. As these classic business technologies are still very common to binders that serve enterprise locations, VDSL2 performance will commonly be much less than the diagram predicts. 

For reaches longer than the 6 Kft shown in Figure 3, the relative performances of VDSL2 and 2BASE-TL are even more dramatically different. VDSL2 bandwidth drops to nearly zero very quickly, while 2BASE-TL provides a graceful decrease in bandwidth and can deliver services to 20 Kft and beyond. Therefore not only is the symmetric performance of 2BASE-TL superior under almost all conditions, the reach of the technology, and therefore the size of the addressable market, is significantly higher with 2BASE-TL than with VDSL2.

The size of the addressable market, and the importance of the ubiquity of the service, is something that cannot be underestimated. The distance distribution of subscribers from the central office differs from carrier to carrier, and from region to region. However, some tendencies remain constant:

• Enterprise customers are generally much closer to central offices than residential customers.

• The vast majority of enterprise customers (70-90%) are within CSA distance (12 Kft) of the central office.

• Less than half of enterprise customers are within the 5 Kft or so reach of VDSL2.

• There is some non-zero percentage of customers that remain at very long distances (18 Kft or beyond).

 
  So although the exact distance distribution may differ depending on the geography, what is a constant is that 2BASE-TL can serve the vast majority of customers with a symmetric 10 Mbps service, while VDSL2 cannot reach even half of the customers with a much lower service offering. 

Additionally, 2BASE-TL is a technology that can be regenerated in the outside plant to provide almost unlimited reach. Because 2BASE-TL is based on the well-established standards of SHDSL and Enhanced SHDSL, and these standards are designed to allow regeneration in the outside plant, 2BASE-TL repeaters can be deployed to reach almost anywhere. VDSL2, as a very high speed short-reach technology, cannot offer the possibility of universal reach because it is not regenerated in any standard fashion. 

Note, however, that VDSL2 and 2BASE-TL are still very complementary technologies, just as was originally true with 10PASS-TS and 2BASE-TL. The two technologies target different reach segments, different market segments, and different symmetries. And when loops are very short (less than the 2 Kft lower limit showed in the diagram), VDSL2 can deliver even more bandwidth on a single pair. VDSL2 thus provides a more optimal technology for very short-reach applications, and 2BASE-TL and VDSL2 can be used to offer complementary services across enterprise and residential applications.