Ethernet in the First Mile reuses copper

One key carrier challenge in metropolitan networks today is bringing broadband access to customers not currently served by fiber. An emerging standard, Ethernet in the First Mile, holds the promise of making copper a viable alternative to fiber in the enterprise and metropolitan loop.

There are two approaches to delivering Ethernet over copper: Encapsulate Ethernet traffic into either ATM cells or High-Level Data Link Control (HDLC) frames. In the first approach (RFC 2684), Ethernet traffic is encapsulated over ATM Adaptation Layer 5 into fixed-length cells. Packets are segmented into 48-byte increments to fit ATM cell payload, and 5 bytes of header are added, creating a stiff "cell tax" for large datastreams.

HDLC, on the other hand, transmits the entire Ethernet stream in one frame, with only 4 bytes of header and 2 bytes of cyclic redundancy check for error checking. This is one of the proposed methods of the EFM IEEE 802.3ah working group, which promises point-to-point Ethernet of 10M bit/sec for up to 2,460 feet over copper.

Inverse multiplexing adds reach

To increase speed, reliability and distance of EFM over last-mile copper, some equipment vendors use software-based inverse multiplexing and transmit the protocol over multiple copper pairs. Now a new programmable silicon technology provides inverse multiplexing in hardware, bringing added speeds and reliability to local-loop transmission.

This new chipset technology can transmit symmetric 10M bit/sec EFM to distances of 4,500 feet over a single copper pair, and can attain higher bandwidth with more pairs. For example, using four copper pairs, this technology can inverse-multiplex EFM to speeds of up to 100M bit/sec for 1,600 feet (or up to 5,000 feet with more pairs). Because inverse multiplexing happens at Layer 1, it's automatic; equipment vendors see only one channel.

Traditional Ethernet services, such as 10Base-T and 100Base-T Fast Ethernet, are limited by the quality of the wiring, requiring multiple pairs and Category 3 and Category 5 for transmission distances of only 300 feet. This situation is exacerbated in the first mile, where outside plant quality can be affected by weather, spectral interference and other issues. Silicon-based EFM relieves equipment vendors from dealing with loop problems, providing on-chip quality of service (QoS).

On-chip QoS automatically adapts to the conditions of the loop seamlessly. Thus, with on-chip QoS, EFM can operate on any wiring that exists today and can deliver Ethernet services reliably over longer reach.

For example, when using four copper pairs to transmit Ethernet at 100M bit/sec, the chipset monitors the pairs to ensure the total aggregate bandwidth remains consistent. If one pair out of four goes down, the chipset can automatically throttle the speeds of the remaining three pairs and maintain the link (albeit at less than 100M bit/sec). Equally important, the four loops can be in different binder groups, can have different lengths, and can have different quality characteristics. The chipset automatically adapts to the conditions of the loops available to it.