EPON vs GPON Standard

EPON, based on Ethernet technology, is compliant with the IEEE 802.3ah (mid-2004) Ethernet in the First Mile standard that is now merged into the IEEE Standard 802.3–2005. It is a solution for the “first mile” optical access network. While GPON, or Gigabit PON, is expected to prevail as a leading optical access technology and to eliminate the bandwidth bottleneck in the last mile. Its requirements were set to force by the Full-Service Access Network (FASN) group, which was later adopted by ITU-T as the G.984.x standards.

EPON vs GPON Data Rate

In EPON, both downstream and upstream line rates are 1.25 Gbps, but due to the 8B/10B line encoding, the bit rate for data transmission is 1 Gbps. GPON, on the other hand, supports an asymmetrical data rate of 1.25 Gbps in both streams, as well as a data rate of 2.5 Gbps in downstream and a data rate of 1.25 Gbps in upstream. Hence GPON is better than EPON in this aspect. The following table has a brief comparison of GPON and EPON technology in the PON upstream and downstream bandwidth, bandwidth efficiency and the transmission.

EPON vs GPON Layering

Besides the above characteristics, perhaps the most striking distinction between the two protocols is a marked difference in the architectural approach, especially in layering. The image below will help you to figure it out.

In EPON, Ethernet frames are carried in their native format on the PON, which greatly simplifies the layering model and the associated management. EPON employs a single layer that uses IP (Internet Protocol) to carry data, voice, and video.

GPON, on the other hand, supports two layers of encapsulation. First, TDM (Time Division Multiplexing) and Ethernet frames are wrapped into GEM (GPON Encapsulation Mode) frames, which have a GFP-like format (derived from Generic Frame Procedure ITU G.7401). Secondly, ATM (Asynchronous Transfer Mode) and GEM frames are both encapsulated into GTC (GPON Transmission Convergence) frames that are finally transported over the PON.

The main purpose of the GEM frame is to provide a frame-oriented service, as an alternative to ATM, in order to efficiently accommodate Ethernet and TDM frames. With GEM, all traffic is mapped across the GPON network using a variant of SONET/SDH GFP. GEM supports a native transport of voice, video, and data without an added ATM or IP encapsulation layer. That’s why GPON supports downstream rates as high as 2.5 Gbps and upstream rates from 155 Mbps to 2.5 Gbps. It is much faster than EPON. However, EPON clearly offers a much simpler and more straightforward solution than GPON. The support of ATM and the double encapsulation of GPON serve no real benefit over a pure Ethernet transport scheme.

Cost Comparison

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.

Summary

It is hard to say GPON is better than EPON, or vice verse. They each have their merits and demerits. When it comes to certain IP/Ethernet services, EPON is more suitable and cost-effective. While GPON has its own advantages in higher bandwidth, faster transmission rate and supporting triple-play services. Until now, EPON is still the mainstream of PON, especially in the Asian countries, but GPON is expanding quickly lately.

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