Reality Check: Flavors of PONs

This article describes the different types of passive optical networks (PONs) and what they do. A PON is an optical network from the headend all the way to the home. It implies a shared network, usually with passive optical splitting in the field. Several standards for PONs exist.

We can classify PONs by the sponsoring organization: International Telecommunication Union, Institute of Electrical and Electronics Engineers or Society of Cable Telecommunications Engineers.

The ITU was the first organization to take up PON standardization with its APON (asynchronous transfer mode, ATM, PON) in the mid ’90s. It specified 622 Mbps downstream data on 1,550 nm and 155 Mbps upstream on 1,310 nm. There was no provision for broadcast wavelengths.

Subsequently, ITU developed the BPON (broadband PON) standard, which was the APON standard with downstream data moved to 1,490 nm to make room for a broadcast wavelength on 1,550 nm. This is the standard that Verizon has used to date, though they have announced a switch to the GPON (Gigabit PON) standard at some point in the future. These wavelengths are now used by all standards.

GPON is the latest standard in the ITU series. It has provision for native transport of ATM, time division multiplexing (TDM) (DS-1/E-1), and Ethernet. But the entire standard is very complicated as a result. Thus, it languished for a year or two until people decided there was no reason to implement the ATM portion, since Ethernet is taking over the marketplace. Many systems don’t implement the TDM portion, either, since there are now standards for transporting TDM over Ethernet. Thus, GPON effectively becomes an Ethernet transport standard, with the most common form operating at 2.488 Gbps downstream and 1.244 Gbps upstream (including overhead).

The second entity to enter the market is the IEEE with its GE-PON (Gigabit Ethernet PON, also known as EPON, Ethernet PON). The current standard is widely used in Southeast Asia and has a number of users in North America and Europe, as well as elsewhere. The current version operates at 1.25 Gbps in both directions, but because of the encoding methods used, the payload speed is a bit less than 1 Gbps in both directions.

Compared with GPON, GE-PON tends to be a little bit less self-contained. That is, some things specified for GPON are not specified internally for GE-PON. These include encryption, though in practice, certain modes of advanced encryption standard (AES) are used in both GE-PON and in GPON. New forms of both GPON and GE-PON are currently being developed, which are intended to expand the standards’ speed to 10 Gbps downstream and either 1 or 10 Gbps upstream.

The most recent entrant to the PON fray is the SCTE, which is now in the process of writing its RF over glass (RFoG) standard. It is essentially HFC with a node size of one, with the node located on the side of the customer premises. A wavelength of 1,550 nm is used for downstream, and a couple of different options are under discussion for upstream wavelengths. Unlike the other standards, RFoG uses DOCSIS for data transport and thus is limited to DOCSIS speeds. You can, of course, use DOCSIS 3.0. It is likely that DOCSIS upstream will work better (faster) with RFoG than with HFC because of lack of noise buildup.

Since the upstream must handle RF modulated carriers from both DOCSIS modems and set-top control systems, some coordination issues are currently under discussion. It is not too late to join the RFoG standardization effort and make your voice heard.

Jim Farmer is CTO of Wave7 Optics. Reach him at [email protected].