Sunday, August 8, 2010

Last mile or first mile ?

Physical-layer access technologies with limited range are usually called last mile technologies. More specifically, we usually use the expression last mile when considering xDSL, that enables several Mbps to be transported over several kilometers, or a fiber optic link or PON, that enable hundreds or even thousands of Mbps to be transported over tens of kilometers.

In the year 2000 the IEEE started talking about “Ethernet in the First Mile” (EFM). In 2001 the EFM task force (802.3ah) was created, that developed extensions to Ethernet that are now incorporated into 802.3 as clauses 56 through 67. These extensions include:

  • a VDSL physical medium called 10PASS-TS (clause 62)
  • an SHDSL physical medium called 2BASE-TL (clause 63)
  • a new inverse multiplexing method (different from LAG, sometimes referred to as EFM bonding) called PME aggregation (subclause 61.2.2)
  • a 100Mbps 10 km point-point 2-fiber medium called 100BASE-LX10 and a single-fiber one called 100BASE-BX10 (clause 58)
  • a Gbps 10 km 2-point-point 2-fiber medium called 1000BASE-LX10 and a single-fiber one called 1000BASE-BX10 (clause 59)
  • a Gbps point-multipoint single-fiber medium with 10 km range called 1000BASE-PX10, and one with a 20 km range called 1000BASE-PX20 (clause 60)
  • logic for the EPON Ethernet Passive Optical Network (clause 64)
  • OAM features (clause 57)

The EFM task force closed down in 2004, and thus it is no longer accurate to say “EFM bonding” or “EFM OAM”. Yet the expression “first mile” remains in use. Is there a difference between the “last mile” and the “first mile”?

I was not there when the IEEE came up with the nomenclature, but I feel that I understand the idea behind it. The term “last mile” was invented by core network engineers. For someone who lives in the WAN, the short-range link that reaches the end-user is justifiably called the “last mile”. On the other hand, the IEEE 802 standards committee takes a LAN-centric point of view. For someone who lives in the LAN, the technology that provides the first link to the outside world is understandably called the “first mile”.

For those of us who live in the access network it doesn’t matter whether you call it first or last mile, we call it home.