Increasingly competing with copper as the infrastructure for access networks, fiber is making rapid headway in the world's leading technology-adopter markets. With passive-optical-networking (PON) technology gaining popularity, two point-to-multipoint standards Ethernet passive optical networking (EPON) and ATM-based broadband passive optical networking (BPON) are both in active deployment. Meanwhile, industry observers view ATM-based Gigabit passive optical networking (GPON) as the eventual successor to BPON, anticipating that mass deployment is at least two years away. GPON defines a completely new protocol designed to support multiple services in their native formats.
Often seen as a "replacement" technology for traditional broadband solutions such as DSL or cable-modem, PON, in its various flavors, promises bandwidths of up to one gigabit and beyond. ATM-based passive optical networking (APON), and subsequently BPON, got an early start, with the International Telecommunication Union's (ITU) ratification of the G.983 standard. In January 2003, ITU ratified GPON, but it has not yet reached the deployment stage.
Meanwhile, in June 2004, the IEEE ratified EPON as the IEEE802.3ah standard. Since then, it has been rapidly adopted in Japan. EPON is also gaining momentum with carriers in China, Korea, and Taiwan.
While GPON promoters argue that the ITU standard is approaching maturity faster than the IEEE EPON standard, EPON advocates cite the recent emergence of the IEEE standard, deployments of EPON underway, and announced deployment plans by carriers as strong evidence of EPON's acceptance. Additionally, EPON partisans note that most data begins and ends its life as IP/Ethernet traffic, and they ask the question, why interpose still another protocol encapsulation?
As you can see from above, the debate over EPON and GPON runs deep. In this article, we'll provide a practical comparison of the two technologies. Let's start by looking at the key differences between the two technologies and examine the strengths of each protocol.
GPON and EPON Differences
Perhaps the most dramatic distinction between the two protocols is a marked difference in architectural approach. GPON provides three Layer 2 networks: ATM for voice, Ethernet for data, and proprietary encapsulation for voice. EPON, on the other hand, employs a single Layer 2 network that uses IP to carry data, voice, and video.
A multiprotocol transport solution supports the GPON structure (Figure 1). Using ATM technology, virtual circuits are provisioned for different types of services sent from a central office location primarily to business end users. This type of transport provides high-quality service, but involves significant overhead because virtual circuits need to be provisioned for each type of service. Additionally, GPON equipment requires multiple protocol conversions, segmentation and reassembly (SAR), virtual channel (VC) termination and point-to-point protocol (PPP).
Figure 1: Diagram showing a typical GPON network.
EPON provides seamless connectivity for any type of IP-based or other "packetized" "communications" (Figure 2). Since Ethernet devices are ubiquitous from the home network all the way through to regional, national and worldwide backbone networks, implementation of EPONs can be highly cost-effective. Furthermore, based on continuing advances in the transfer rate of Ethernet-based transport now up to 10 Gigabit Ethernet EPON service levels for customers are scalable from T1 (1.5 Mbit/s) up through 1 Gbit/s.
Figure 2: Diagram showing a typical EPON network.
Comparisons and Contrasts
Clearly, there are some distinct differences between EPON and GPON at Layer 2. However, these aren't the only differences between the technologies. Designers will also find differences in terms of bandwidth, reach, efficiency, per-subscriber costs, and management. Let's look at each of these elements in more detail.
1. Usable Bandwidth
Bandwidth guarantees vary between the two protocols: GPON promises 1.25-Gbit/s or 2.5-Gbit/s downstream, and upstream bandwidths scalable from 155 Mbit/s to 2.5 Gbit/s. EPON delivers 1-Gbit/s symmetrical bandwidth. EPON's Gigabit Ethernet service actually constitutes 1 Gbit/s of bandwidth for data and 250 Mbit/s of bandwidth for encoding. The approach of EPON, as part of the Gigabit Ethernet standard, parallels that of Fast Ethernet, which also uses 25 percent for encoding.
GPON's 1.25-Gbit service specifies a usable bandwidth of 1.25 Gbit/s, with no requirement for encoding. Will the additional 250 Mbit/s promised by GPON promoters stand as a clear advantage for GPON? The answer may lie not in the sheer bandwidth comparisons, but in the practicality of 1.25-Gbit uplinks.
Gigabit Ethernet interfaces to the aggregation switch, central office, and metro are currently the cost-effective way to aggregate 1-Gbit ports for transport. With no cost-effective switches for 1.25 Gbit available, the added bandwidth promised by GPON, although measurable, could come at a significant premium over the price of EPON equipment. In other words, the low-cost uplink for the foreseeable future is likely to be Gigabit Ethernet, which is the exact bit rate of EPON. In that light, GPON's "added" bandwidth may not prove advantageous for carriers.
With either protocol, the practical limitation to reach comes from the optical-link budget. With the reach of both protocols currently specified at approximately 20 kilometers, the difference in split rates the number of optical network units (ONUs) supported by one optical line terminal (OLT) is a point of differentiation.
GPON promises to support up to 128 ONUs. With the EPON standard, there is no limit on the number of ONUs. Depending on the laser diode amplitude, when using low-cost optics, EPON can typically deliver 32 ONUs per OLT, or 64 with forward error correction (FEC).
3. Per-subscriber costs
The use of EPON allows carriers to eliminate complex and expensive ATM and Sonet elements and to simplify their networks, thereby lowering costs to subscribers. Currently, EPON equipment costs are approximately 10 percent of the costs of GPON equipment, and EPON equipment is rapidly becoming cost-competitive with VDSL.
4. Efficiencies of Each Standard
With both PON protocols, a fixed overhead is added to convey user data in the form of a packet. In EPONs, data transmission occurs in variable-length packets of up to 1518 bytes according to the IEEE 802.3 protocol for Ethernet. In ATM-based PONs, including GPONs, data transmission occurs in fixed-length 53-byte cells (with 48-byte payload and 5-byte overhead) as specified by the ATM protocol. This format makes it inefficient for GPONs to carry traffic formatted according to IP, which calls for data to be segmented into variable-length packets of up to 65,535 bytes.
For GPONs to carry IP traffic, the packets must be broken into the requisite 48-byte segments with a 5-byte header for each. This process is time-consuming and complicated and adds cost to the central-office OLTs as well as the customer premise-based ONUs. Moreover, 5 bytes of bandwidth are wasted for every 48-byte segment, creating an onerous overhead that is commonly referred to as the "ATM cell tax". (This is the case with GPON's ATM encapsulation mode. In its other encapsulation mode, called GEM, the ATM cell tax does not apply.)
By contrast, using variable-length packets, Ethernet was made for carrying IP traffic and can significantly reduce the overhead relative to ATM. One study shows that when considering trimode packet size distribution, Ethernet packet encapsulation overhead was 7.42 percent, while ATM packet encapsulation overhead was 13.22 percent.1
In addition, since Ethernet frames contain a vastly higher ratio of data to overhead than GPON, that high utilization can be reached while using low-cost optics. The more precise timing required with GPON results in more expensive optics. High-precision optics are mandatory as part of the GPON standard.
5. Management systems
EPON requires a single management system, versus three management systems for the three Layer 2 protocols in GPON, which means EPON results in a significantly lower total cost of ownership. EPON also does not require multiprotocol conversions, and the result is a lower cost of silicon.
GPON does not support multicast services, which makes support for IP video more bandwidth-consuming.
6. Support for CATV Overlay
Both protocols support a cable television (CATV) overlay, which meets requirements for a high-speed downstream video service. EPON wavelengths are 1490 nanometers downstream and 1310 nanometers upstream, leaving the 1550-nanometer wavelength for a CATV overlay similar to the wavelengths for BPON and GPON.
With GPON, encryption is part of the ITU standard. However, GPON encryption is downstream only.
EPON, on the other hand, uses an AES-based mechanism, which is supported by multiple silicon vendors and deployed in the field. Furthermore, EPON encryption is both downstream and upstream.
8. Network Protection
Both protocols provide vendor-specific and carrier-specific protection. This includes support for vendor-specific and carrier-specific operations, administration and maintenance (OAM).
While pundits are lining up in opposite corners of the ring, it's still unclear whether EPON or GPON will prevail or if each will take its own share in a burgeoning market. One thing is clear: fiber deployments will continue expanding, and at the expense of copper, as consumer demands for "triple-play" (video, voice and data) grow.
About the Author
Onn Haran is the chief technology officer at Passavé. Onn holds a B.Sc. (cum laude) from the Technion, Israel Institute of Technology in Haifa, and a M.Sc. in Electrical Engineering from Tel-Aviv University. He can be reached at firstname.lastname@example.org.