Introduction to EPON and GPON
EPON and GPON are variations of Passive Optical Network (PON). A PON system typically comprises an Optical Line Terminal (OLT) located at the central office (CO) of the service provider, along with multiple Optical Network Units (ONUs) situated near end users. Optical splitters are employed to enable a single optical fiber to serve multiple premises. The aim of EPON and GPON, developed by the Institute of Electrical and Electronics Engineers (IEEE) and the International Telecommunication Union-Telecommunication Standardization Sector (ITU-T) respectively, is to provide Gigabit-rate solutions for delivering Ethernet and IP services.
EPON, which is based on the IEEE standard for Ethernet in the First Mile, capitalizes on the capabilities, compatibility, and performance of the Ethernet protocol. It facilitates packet-based transmission at speeds of 1 Gbps and 10 Gbps. On the other hand, GPON utilizes SONET/SDH technologies and employs the Generic Framing Protocol (GFP) for Ethernet transport. It operates using an IP-based protocol and can utilize either ATM or the GPON Encapsulation Method (GEM) encoding. GPON enables the provision of “triple play” services encompassing voice, data, and video, and it serves as the foundation for most planned Fiber-to-the-Premises (FTTP) applications in the foreseeable future.
Although EPON and GPON share general concepts and architectures such as PON operation, Optical Distribution Network (ODN) framework, wavelength plan, and application scenarios, their operational aspects, data protocols, supported features, and services differ significantly.
EPON vs GPON Comparison
EPON and GPON have their own advantages and drawbacks, making them complementary and competitive in certain aspects. The following comparison aims to clarify the differences between EPON and GPON.
EPON is defined by the IEEE 802.3 standard, with a ratified specification of 802.3ah-2004 for 1.25 Gbps (1.0 Gbps prior to 8B/10B coding), and the IEEE 802.3av standard for 10Gbps (10G-EPON). The upstream and downstream data rates of EPON are symmetrical.
GPON supports various bit rate options using the same protocol, including symmetrical data rates of 622 Mbps in both downstream and upstream, symmetrical data rates of 1.25 Gbps in both directions, as well as a data rate of 2.5 Gbps in downstream and 1.25 Gbps in upstream. Users can select the upstream and downstream data rates based on their requirements, making GPON more flexible compared to EPON.
Split ratios refer to the number of users intended to be served by a given PON. Typically, this is 32, optionally 16, 64, or even 128. The split ratio can be influenced by the performance of the optical transceiver modules. A higher split ratio significantly increases the cost of optical transceiver modules and reduces the transmission distance. For example, with a split ratio of 1:16, the maximum transmission distance can be 20 km. However, with a split ratio of 1:32, the maximum distance is reduced to 10 km. EPON and GPON are the same in this aspect.
EPON generally supports a minimum of 32 (i.e., 1:32), and it does not limit the split ratio as 1:64 and 1:128 are also available. Service providers can define the split ratio according to the services and bandwidth they want to support. In contrast, GPON defines an upper limit on the split ratio, supporting up to 128 users, but typically it is 64. Common split ratios for GPON include 1:32, 1:64, or 1:128. While GPON offers a range of split ratios, it does not provide a significant advantage in terms of cost consideration. EPON can deploy cheaper optics at the Optical Network Unit (ONU) as it does not need to reach a split ratio of 128.
Layering & Access Service:
The layering model and associated management services are mapped over Ethernet (directly or via IP) in EPON. In GPON, two layers of encapsulation are required to achieve the same result. TDM and Ethernet frames are first wrapped into GTC Encapsulation Method (GEM) frames, which have a format similar to GFP (Generic Frame Procedure derived from ITU G.7401). Then, both ATM and GEM frames are encapsulated into GTC frames, which are transported over the PON. This can be seen in Figure 2.
EPON provides a simpler and more straightforward solution compared to GPON. The support of ATM and the double encapsulation in GPON do not offer significant benefits over a pure Ethernet transport scheme. In terms of access service, EPON is suitable for data-only services, while GPON is suitable for triple-play services. EPON is a native Ethernet solution that leverages the Ethernet protocol, while GPON utilizes SONET/SDH and Generic Framing Protocol (GFP) to transport Ethernet. Therefore, in terms of layering comparison, EPON is better than GPON, whereas for service provision, GPON has an advantage.
QoS (Quality of Service):
The Ethernet protocol does not inherently support Quality of Service (QoS). Since a PON system requires QoS, most vendors enable it in EPON by using VLAN (Virtual Local Area Network) tags. While this solves the QoS issue, it increases the overall costs. VLAN tags are often provisioned manually since there is no automatic provisioning. GPON, on the other hand, has integrated QoS handling, making it better than EPON in terms of QoS. Figure 3 illustrates this difference.
OAM (Operation Administration and Maintenance)
In GPON, there are three different types of control messages: OMCI (ONT Management and Control Interface), OAM (Operations, Administration, and Maintenance), and PLOAM (Physical Layer OAM). Each of these control messages serves a specific purpose, as described below:
1. OMCI: OMCI messages in GPON are transmitted over the Ethernet or ATM control channel. OMCI is responsible for the provisioning and management of ONT (Optical Network Terminal) services, defining the layers above the GTC (G-PON Transmission Convergence). It facilitates the configuration, activation, and control of ONTs through the Element Management System (EMS).
2. Embedded OAM: Embedded OAM messages are carried in the header overhead of GPON frames. These messages are used for various purposes such as bandwidth granting, encryption key switching, and Dynamic Bandwidth Allocation (DBA). Embedded OAM allows for efficient management and monitoring of the GPON network.
3. PLOAM: PLOAM messages in GPON are transmitted over the ATM control channel. PLOAM is responsible for auto discovery and the management of all other Physical Medium Dependent (PMD) and GTC management information. PLOAM messages are directed to specific ONTs or can be broadcasted to multiple ONTs or the entire PON.
In EPON, the control messages differ from GPON. EPON uses IEEE 802.3ah OAM messages for provisioning, fault isolation, and performance monitoring. These OAM messages are used in conjunction with SNMP (Simple Network Management Protocol) sets and gets through the IETF (Internet Engineering Task Force) and MIBs (Management Information Bases). Additionally, EPON uses MPCP (Multi-Point Control Protocol) GATEs/REPORTs for bandwidth granting.
The deployment cost of GPON or EPON relies on the OLT, ONU/ONT, and passive optical components. An Optical Distribution Network (ODN) comprises fiber cable, cabinet, optical splitter, connector, and so on. For an equivalent number of users, the expense for fiber and cabinet in EPON is comparable to that in GPON. The cost of OLT and ONT is determined by the Application Specific Integrated Circuit (ASIC) and optical transceiver modules. The majority of GPON chipsets available in the market are predominantly based on Field Programmable Gate Array (FPGA), which is more costly than the ASIC used in the Media Access Control (MAC) layer of EPON. Additionally, the optical module of GPON is pricier than that of EPON. During the deployment phase of GPON, the estimated cost of a GPON OLT is 1.5 to 2 times higher than that of an EPON OLT, and the projected cost of a GPON ONT will be 1.2 to 1.5 times higher than that of an EPON ONT.
EPON and GPON have their respective advantages and disadvantages. In terms of performance, GPON outperforms EPON, but EPON has advantages in terms of deployment time and cost. Currently, EPON remains the mainstream technology, while GPON is catching up. In the broadband access market, it is likely that both technologies will coexist and complement each other. GPON is ideal for users with multi-service requirements, high QoS and security needs, and an ATM backbone network. On the other hand, EPON may be more suitable for users who prioritize costs and have fewer security requirements.