Frequently Asked Questions

EFM Copper Related Questions


1. What is “bonding” or “PME Aggregation”?

“Bonding” or “PME Aggregation” (formerly also known as “Loop Aggregation” or “PMI Aggregation”) is a technique where multiple 2BASE-TL or 10PASS-TS lines are combined to create a single logical link with a higher bandwidth. At the transmitter, individual Ethernet frames are broken into fragments that are transmitted over the different physical links. At the receiver, these fragments are recombined into the original Ethernet frames.

“PME Aggregation” must not be confused with “Link Aggregation” (a.k.a. 802.3ad), which also creates a single high-bandwidth logical link from multiple physical links. Link Aggregation operates over a set of identical, full-duplex links. It works by assigning Ethernet frames belonging to different “conversations” to different physical links. Link Aggregation does not perform any fragmentation.

IEEE Std 802.3ah was the first Ethernet bonding standard, chronologically. ITU-T Recommendation G.998.2 and American National Standard T1.427.02 are generalizations of that standard to all xDSL flavors. Clause 6 of G.998.2 and T1.427.02 points out the differences between these standards and IEEE Std 802.3ah. The differences are all architectural; the underlying functionality is the same, except that EFM has only one bearer channel.


2. Is 10PASS-TS limited to 10 Mb/s? Is 2BASE-TL limited to 2 Mb/s?

No! The source of this very common misconception is the fact that people confuse the objectives of the standard with the actual specifications. The objective for 10PASS-TS was to provide 10 Mb/s at 750 m. The objective for 2BASE-TL was to provide 2 Mb/s at 2700 m. Based on those objectives, a VDSL-based PHY was specified as 10PASS-TS, and a SHDSL-based PHY was specified as 2BASE-TL. Both PHYs offer bitrates that vary with the used spectrum, the length of the line, the amount of noise, and the interference. Both PHYs offer bitrates that go way beyond the original objective (up to 100 Mb/s downstream for 10PASS-TS, and up to 5.696 Mb/s for 2BASE-TL).


3. Is EFM Copper the same thing as Ethernet-over-DSL?

No. The EFM Copper PHYs (10PASS-TS and 2BASE-TL) are based on existing xDSL standards (VDSL and SHDSL, respectively), and built according to a model which can also be used to transport Ethernet over any standardized xDSL flavor (e.g., ADSL, ADSL2, SHDSL, enhanced SHDSL, VDSL). However, only PHYs complying to the full specification of either 10PASS-TS or 2BASE-TL may be called “EFM Copper”.


4. Does EFM Copper use HDLC encapsulation?

No. The ITU-T's VDSL specification (ITU-T Recommendation G.993.1) specifies the use of HDLC for packet transport (the corresponding sublayer is called the “Packet Transfer Mode Transmission Convergence” sublayer or PTM-TC). The use of this same PTM-TC was also proposed for EFM Copper, but the Task Force eventually selected a new encapsulation scheme, known as “64/65-octet encapsulation”, which is both more efficient and more deterministic than HDLC.

Future xDSL flavors will probably use an enhanced variant of “64/65-octet encapsulation” which also allows transmission of packets shorter than 64 bytes, and a suspend/resume mechanism. This enhanced variant is currently under definition in the ITU-T, in the framework of the ADSL2 project.

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