Comparison of GPON and EPON Technologies

On this page:

• 1. Standardization and Evolution
• 2. Protocol Structure
  • EPON Protocol
  • GPON Protocol
• 3. Framing and Service Adaptation
• 4. Basic Operational Principles
  • EPON Downstream Transmission
  • GPON Downstream Transmission
• 5. Service Hierarchy
• 6. Bandwidth Allocation & Dynamic Bandwidth Assignment (DBA)
• 7. ONT Authentication & Activation
• 8. Security & Encryption
• 9. Bandwidth & Efficiency
• 10. Ethernet Service Support
  • EPON Ethernet Functions
  • GPON Ethernet Limitations
• 11. Conclusion
• 12. Which One to Choose?

Passive Optical Network (PON) technology has been available since the 1990s. Its significant development phase occurred in the early 2000s, focusing on achieving gigabit data transmission speeds to support Ethernet and IP-based services. Two distinct solutions emerged from the standardization efforts of IEEE and ITU-I: EPON and GPON. While both technologies share fundamental PON concepts such as optical distribution network (ODN) structure, wavelength plans, and applications, they differ significantly in their operational principles, supported services, and protocols.

Key Milestones in PON Standardization

Standardization and Evolution

Over the past 15 years, IEEE and ITU-I have been actively involved in standardizing PON technologies.

• EPON and 10GEPON fall under the latest IEEE-approved standards.
• GPON is defined by the ITU-I standard.

Initially, APON and BPON were introduced in the early-to-mid 90s, using ATM framing for data transmission. However, with the rise of Internet and Intranet traffic, ATM-based BPON networks proved inefficient due to the dominance of variable-length IP traffic in access networks.

This inefficiency led to the development of EPON, leveraging Gigabit Ethernet (GbE) to support Quality of Service (QoS), VLAN, prioritization, and OAM while ensuring cost-effective integration with Ethernet equipment. Meanwhile, GPON was introduced after FSAN group companies (Quantum Bridge, etc.) failed to gain IEEE 802.3ah approval for a unified gigabit ATM and Ethernet PON solution, prompting them to continue their work under ITU.

Although GPON and EPON share fundamental principles from the G.983 GPON standard, including PON operation, ODN structure, and wavelength plans, their approaches to data transmission differ significantly.

GPON Layer Hierarchy

EPON Layer Hierarchy

Comparison of GPON and EPON Protocols

Protocol Structure

EPON Protocol

EPON is based on IEEE 802.3 Ethernet and modified to support point-to-multipoint (P2MP) communication. Ethernet traffic is transported natively, with all Ethernet functions fully supported.

GPON Protocol

GPON is a transport-layer protocol, where Ethernet services are adapted to OLT and ONT interfaces and transmitted using an independent synchronous framing structure.

Framing and Service Adaptation

• GPON's Transmission Convergence (GTC) layer converts service interfaces (e.g., Ethernet) into a unified, service-independent structure.
• Ethernet frames are encapsulated into GEM frames (GTC encapsulation method), following a GFP-like format (ITU G.7401).
• These GEM frames are further encapsulated into SONET/SDH-like GTC frames, transmitted synchronously every 125 µs over the PON network.

In contrast, EPON transmits Ethernet frames natively, without additional adaptation or encapsulation, preserving the efficiency of Ethernet-based transmission.

Downstream GTC Frames

Basic Operational Principles

EPON Downstream Transmission

EPON follows the IEEE Ethernet MAC architecture. Downstream transmission operates similarly to GbE MAC, broadcasting all Ethernet traffic through passive optical splitters to ONT endpoints.

• All data is transmitted to all ONU units, with ONU addressing determining the destination data.
• Upstream bandwidth is managed using TDMA (Time-Division Multiple Access), where the OLT assigns time slots to each ONT dynamically using a Dynamic Bandwidth Allocation (DBA) algorithm.

EPON Downstream Direction

EPON Upstream Direction

GPON Downstream Transmission

GPON functions as a transport-layer network with SONET framing and GFP encapsulation.

• 125 µs time slots are used for TDM (Time-Division Multiplexing) to distribute bandwidth among up to 32 ONTs.
• Synchronization is necessary, so idle symbols are inserted to accommodate asynchronous Ethernet traffic.

Service Hierarchy

Since PON networks are inherently P2MP, OLTs must be able to identify and communicate with each ONT uniquely.

• EPON uses Logical Link Identifiers (LLID) for addressing ONTs, supplemented by VLAN_ID for service differentiation.
• GPON employs Traffic Containers (T-CONTs) to establish virtual point-to-point connections between OLT and ONT.
• Within a T-CONT, multiple Port IDs further define individual ONT ports.

Service Hierarchy

Bandwidth Allocation & Dynamic Bandwidth Assignment (DBA)

EPON Medium Access Control

GPON Medium Access Control

Both technologies support DBA, dynamically adjusting upstream bandwidth based on real-time ONT traffic requirements.

ONT Authentication & Activation

• GPON authenticates ONTs via serial numbers (SN) using two methods:
  • Method A: Pre-registered SN in OLT.
  • Method B: Automatic SN detection and assignment of ONT-ID.
• EPON generally uses MAC addresses instead of serial numbers for ONT identification, but some vendors implement SN-based authentication.

Security & Encryption

• Both EPON and GPON support AES-128 encryption.
• GPON key exchange occurs through PLOAM messages, while EPON encryption is managed via VLAN or IEEE 802.3ah OAM messages.

Bandwidth & Efficiency

GPON vendors often highlight its higher theoretical speed:

However, protocol overhead affects actual efficiency:

• EPON upstream efficiency: 90.33% - 97.08% of GbE performance.
• EPON downstream efficiency: 97.13% - 98.92% of GbE performance.
• GPON GEM mode efficiency: ~95% of bandwidth usage.

EPON can also operate in Turbo Mode, doubling downstream speed to 2.5 Gbps, making its bandwidth performance comparable to GPON.

Ethernet Service Support

EPON Ethernet Functions

As an IEEE Ethernet standard, EPON natively supports all Ethernet functions, including:

• VLAN tagging
• QoS prioritization
• OAM monitoring

GPON Ethernet Limitations

• No native Ethernet switching at OLT → Requires an external Ethernet switch for VLAN tunneling and transparent LAN services (TLS).
• Lacks default Ethernet bridging, requiring an external Ethernet switch for standard Ethernet bridging.

Conclusion

Both EPON and GPON were developed to enable gigabit-speed Ethernet and IP transport over PON networks. However, their approaches differ:

• EPON extends Ethernet functionality for P2MP architecture, leveraging its seamless integration with existing Ethernet networks.
• GPON follows SONET/SDH principles, using GFP frames for efficient Ethernet transport in large-scale carrier networks.

Which One to Choose?