With the increasing demand for transmission volume and diversity of service and personalization, the capacity and functional diversity of the access network need to be improved urgently. In recent years, more and more device suppliers and network providers put their focus on fiber optic network access technology, so that FTTH (fiber to the home) network will undoubtedly become the future trend of accessing to the network. Passive optical network (PON) is the main technology of FTTH which provide point-to-multipoint network access service with a way of low installation and maintenance cost. Now, the mainstream PON includes Ethernet passive optical network (EPON) and Gigabit passive optical network (GPON). EPON and GPON each has its own merits and have complementary advantages, this tutorial will analyze the difference between them.

Background Knowledge

PON began to spring up in the mid-90s of the 20th century, one PON network system are usually made up of one optical line terminal (OLT) which located in central office and multiple optical network units (ONU) which near the end users. In addition, optical splitter also plays an important role in the PON network system, and it enabled multiple users to share a fiber.

 

Early in the 21st century, in order to develop Gigabit network solutions to provide Ethernet and IP transport services, the Institute of Electrical and Electronics Engineers (IEEE, Institute of Electrical and Electronics Engineers) and the International Telecommunication Union Telecommunication Union (ITU-T, International Telecommunication Union-Telecommunication Standardization Sector offers EPON and GPON solutions. EPON and GPON are similar in PON systems, optical distribution networks, wavelength planning and applications, but because of the different services, they support their own characteristics, and the process of running are different. PON is an all-Ethernet solution that fully complies with the features and performance of Ethernet protocols. GPON utilizes the Synchronous Optical Network (SONET) and Synchronous Digital hierarchy (SDH) Generic Framing Protocol (GFP) for Ethernet transmission.

 

EPON is an Ethernet-based PON technology, which adopts PON technology in the physical layer, uses Ethernet protocol in the data uplink layer, and utilizes PON topology to achieve Ethernet access. The infrastructure of EPON and GPON are the same, but the data protocol of them are totally different. EPON are very common in Asia.

 

GPON makes use of IP-based protocol and asynchronous transfer mode (ATM) or GPON encapsulation mode (GEM) code. The transmission rate of GPON can reach 2.5Gbps that can provide enough bandwidth to meet future demand for bandwidth. Using GPON can also offer synchronous transmission for voice, data and video, which is the basic technology of the fiber to the premises (FTTP) application.

The figure below shows the development history of PON

EPON vs GPON

As two different PON technology, EPON and GPON are basically the same in the infrastructure, but their protocols and applications are different, they each has its own advantages. Here, we are going to compare EPON and GPON from different aspects.

Data rate

EPON uses Ethernet data frame accord with IEEE802.3 standard, which IEEE802.3ah standard adopts 8B / 10B line coding and standard up and downlink symmetrical 1Gbps data rate to provide 1.25Gbps line rate, IEEE802.3av standard to provide 10Gbps line rate. The up and downlink data rate of EPON is symmetrical.

 

GPON supports various levels of data rate, and also supports the symmetry rate or the asymmetric rate of up and downlink. For example, the rate of up and downlink is 622Mbps, , the rate of up and downlink is 1.25 Gbps or the uplink rate is 1.25Gbps, among them, the downlink rate is 2.5Gbps and the uplink rate is 1.25Gbps is the main rate supported by GPON network system. Thus we can choose the proper up/downlink rate based on the specific needs when using GPON network system, and buy the appropriate rate of the optical module. From this point of view, GPON is more flexible than EPON.

Conclusion: From the data rate of view, GPON is better than EPON.

Splitting Ratio

Splitting ratio refers to the number of users that a PON network system can support, usually it can be 1: 32, 1: 16, 1: 64 or even 1: 128.

 

The standard definition splitting ratio of EPON is 1:32. In fact, EPON technology can also achieve higher splitting ratio, such as 1:64 and 1:128, network providers determine the specific splitting ratio according to the bandwidth and service they support. While the standard definition splitting ratio of GPON has 1:32, 1:64 and1:128. View from this aspect, GPON is capable of providing multiple standards splitting ratio.

 

Considered from cost perspective, GPON have not advantages at all. Splitting ratio is mainly affected by the performance of the optical module, large splitting ratio will let the cost of optical modules to rise significantly. Moreover, large splitting ratio will reduce the transmission distance. For instance, the transmission distance is up of 20km when the splitting ratio is 1:16, while the transmission distance only reaches 10km when is 1:32. Not only that, the large splitting ratio will cause too many users to share the bandwidth.

Conclusion: View at splitting ratio angle, EPON and GPON about the same.

Link layer and access service

EPON follows a simple Ethernet data format, greatly simplified the process of link layer and related management. EPON enables offer multiple businesses on the Ethernet, while GPON must be based on GEM frames and ATM cells if want to provide the same business as EPON. Firstly, TDM and Ethernet frames form GEM frames. GEM frames are similar to GFP frames (a common link layer protocol for multiple services). Secondly, ATM and GEM frames are encapsulated together as GTC frames (as shown in the following figure).

(EPON vs GPON layering)

Comparison of EPON and GPON link layer

GEM is a method to encapsulate data on GPON, it can implement simple and efficient package for a variety of data, will process the data fragment with variable or fixed length for unified adaptation, provide port reuse function, and offer the same connection-oriented communication as ATM.

 

Compare with GPON, EPON seems to be more simple and direct. The two package ways of GPON and ATM support will not play much role in the pure Ethernet transmission.

 

However, EPON is only suitable for providing the service of data transmission in the access network, while GPON supports three in one service. EPON is a kind of all-Ethernet solution that fully comply with the characteristics and performance of the Ethernet protocol. And GPON takes use of synchronous optical network (SONET) / synchronous digital hierarchy (SDH) technology and generic framing protocol to transmit Ethernet.

Conclusion: From link layer point of view, the link layer of EPON is easier, but GPON have a better performance on service.

Quality of Service (QoS)

Ethernet protocol itself have not the ability to handle QoS, thus the concept of virtual local area network (VLAN) was proposed that allowing EPON to provide services to users. VLANs can be made by modifying the priority identification of the received frame to lay the foundation for QOS, but the costs of VLAN is stay at very high level because is achieved by manpower. While GPON itself has an excellent QoS performance.

(EPON vs GPON QoS Architecture)

Conclusion: View at QoS angle, GPON is better than EPON.

OAM (operation, administration and maintenance

GPON use three parts to control and manage the information: Ont Management and Control Interface (OMCI), Embedded OAM and Physical Layer OAM. The functions of these three parts as below:

Control channel Format Functions
OMCI Ethernet or ATM Used to manage the services defined by the upper layer, including the achievable feature set of the ONU, the type and number of the T-CONT services, and the negotiation of the QoS parameters.
Embedded OAM IP overhead Uplink bandwidth authorization, key switch and DBA information report
Physical Layer OAM ATM Transmit all the information that is not transmitted through the OAM channel in the physical layer and TC layer

On the contrary, EPON does not take too much consideration for OAM, but combines with simple network management protocol based on IEEE802.3ah standard to quickly find invalid link, position malfunction and ensure network quality.

Conclusion: Considered from OAM, GPON is superior to EPON.  

Cost

The cost of EPON or GPON network systems is primarily affected by the cost of OLT, ONU / ONT and other optical passive assemblies.

 

An optical distribution network includes optical cable, distribution box, optical splitter, optical connector etc. If the number of users of EPON are equal to GPON’s, so the cost of optical cable and distribution box of EPON network system is similar to GPON. While the cost of OLT and ONT is determined by the cost of Application Specific Integrated Circuit (ASIC) and optical module. The most of GPON chipsets on the market are manufactured by Field Programmable Gate Array (FPGA) technology, but EPON uses the specific integrated circuits of media access control layer, which is much cheaper than GPON. Moreover, the chipset of GPON just provided by several suppliers, so that the prices of GPON equipment are unlikely to drop significantly. Not only that, the optical modules used in GPON network system are more expensive than EPON used. According to the statistics, the cost of laying a GPON OLT network is 1.5 to 2 times higher than the cost of laying the EPON OLT, while the cost of laying the GPON ONT is 1.2 to 1.5 times higher than the cost of laying the EPON ONT.

Conclusion: in terms of cost, EPON is much more cost-effective than GPON. 

Summary

From what has been discussed above, EPON and GPON each has merits and demerits, GPON has a better performance than EPON, but EPON occupy the bigger advantages both in time and cost. So far, EPON is still the mainstream of PON network technology, but GPON is also catching up. Looking forward to the future of broadband access market, the problem between EPON and GPON is not replacement, but should be coexistent. GPON is more suitable for the users who has multi-service needs, QoS and security requirements and ATM technology as a backbone network, and EPON is welcomed by users who has cost requirements, and don’t ask too high for QoS and security. As a professional optical communication supplier, Cozlink provides various high quality optical network devices and accessories, please feel free to contact us if you have any interests.

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