What Is the Difference Between EPON and GPON?

With the prevalence of FTTH (Fiber To The Home)in the access networks, PON (Passive Optical Network) naturally plays a key role in it. To further keep pace with the demands, EPON (Ethernet Passive Optical Network )and GPON(Gigabit Passive Optical Network) emerges from PON and become popular versions of it. Maybe one is confused about the difference between EPON and GPON.

What’s the GPON?

GPON tech is the latest-generation broadband passive optical access tech based on ITU-TG.984.x standard, which is with high bandwidth, high efficiency, wide coverage, rich user interface and so on. It’s considered as an ideal tech to realize broadband access network services.

GPON uses an IP-based protocol and either ATM or GEM (GPON encapsulation method) encoding. Data rates up to 2.5Gbps are specified and it is very flexible in what types of traffic it carries. GPON enables “triple play” (voice-data-video) and is the basis of most planned FTTP (Fiber to the Premises) applications in the near future.

Simultaneously, it also applies optical wavelength division multiplexing (WDM) so that a single fiber can be used for both downstream and upstream data. A laser on a wavelength (λ) of 1490nm transmits downstream data. Upstream data are transmitted on a wavelength of 1310 nm. If TV is being distributed, a wavelength of 1550nm is used.

Moreover, It’s with 1.25Gbit/s or 2.5Gbit/s downstream or upstream bandwidths scalable from 155Mbit/s to 2.5Gbit/s. GPON does not support multicast services, which makes support for IP video more bandwidth-consuming.

As the main forces of optical network access, EPON and EPON have their own advantages for competition, as well as compensate for each other. For the difference between EPON and GPON, it will be shown in the following EPON and GPON comparison.

What’s the EPON?

As the name implies, EPON tech is a version of PON tech based on Ethernet, offering point-to-multipoint network access with lower installation and maintenance costs. EPON tech is standardized by IEEE802.3EFM group and endowed with a standard, IEEE802.3ah. In this standard, the PON tech is integrated with Ethernet tech and a new physical layer specification applied in the EPON system and extended Ethernet data link layer protocol to realize the TDM access of the Ethernet frame in the PON with point-to-multipoint architecture.

In the physical layer, single fiber WDM Technology (downstream wavelength is 1490 nm, the upstream wavelength is 1310 nm) are specified for application in IEEE 802.3-2005to realize bidirectional transmission via a single fiber. Meanwhile, the two PON optical interfaces, the 1000 BASE-PX-10 U/D and 1000 BASE-PX-20 U/D, are defined to respectively support the maximum transmission distances,10 km, and 20 km. In the physical coding sublayer, the EPON system inherits the original standard of Gigabit  Ethernet, adopts 8B/10B line coding and standardized upstream and downstream symmetric 1Gbit/s data rate (line rate is 1.25 Gbit/s).

EPON and GPON Comparison: What Is the Difference?

In split ratio, GPON is neck and neck with EPON. Split ratio mainly limited by the performance index of optical modules means the quantity of ONU user port in one OLT interface. The standard split ratio of EPON is 1: While the split ratio of GPON includes these types: 1:32; 1:64; 1:128. Actually, the split ratio of EPON can also reach higher, such as 1:64; 1:128. And the control protocol of EPON can support more ONUs.

The high split ratio will bring about a sharp rising in costs. Although GPON is able to provide multiple options, and it has few benefits in costs. In addition, the insertion loss of PON tech is 15~18dB, and the higher split ratio will shorten the transmission distance. When the split ratios are 1:16 and 1:32, the maximum physical distance of GPON can respectively reach 20km and 10km, which is the same as that of EPON.

In the cost, the EPON is more cost-effective than GPON. Generally speaking, the cost of GPON or EPON deployment consists of that of OLT, ONU/ONT, and passive optical components. An ODN is combined with fiber cable, cabinet, an optical splitter, connector and so on. For the same amounts of users, the cost for the fiber and cabinet with EPON is similar to that of GPON. The cost of OLT and ONT is decided by the ASIC (Application Specific Integrated Circuit) and optical transceiver modules.

The GPON chipsets available in the market are mostly based on FPGA (Field Programmable Gate Array), which is more expensive than the EPON MAC (Media Access Control) layer ASIC. There are only several chipset vendors who can provide GPON chipsets, and it is likely that the price of GPON equipment cannot be reduced rapidly. The optical module of GPON is also more expensive than EPON. When GPON reaches deployment stage, the estimated cost of a GPON OLT is 1.5 to 2 times higher than that of an EPON OLT, and the estimated cost of a GPON ONT will be 1.2 to 1.5 times higher than that of an EPON ONT.

In data rate, GPON is higher than EPON. EPON uses standard 802.3 Ethernet data frames: IEEE 802.3 standard, ratified as 802.3ah-2004 for 1.25 Gbps (1.0 Gbps prior to 8B/10B coding) and IEEE 802.3av standard for 10Gbps (10G-EPON). The upstream and downstream data rate of EPON is symmetrical.

While GPON supports various bit rate options using the same protocol, including asymmetrical data rate of 622 Mbps in both downstream and upstream, a symmetrical data rate of 1.25Gbps in both streams, as well as a data rate of 2.5Gbps in downstream and a data rate of 1.25Gbps in upstream. 2.5Gbps of downstream bandwidth and 1.25Gbps of upstream bandwidth are the data rates supported by typical GPON systems. Thus, one can decide the upstream and downstream data rate depending on the requirements and then choose the corresponding optical transceiver modules. It is more flexible than EPON.

In OMA(Operation Administration and Maintenance), GPON has more advantages than EPON. EPON does not take OMA into consideration, and just simply defines the remote failure indication for ONT, loopback and link monitoring. On the contrary, GPON defines the PLOAM(Physical Layer OAM) in the physical layer and OMCI(ONT Management and Control Interface) in the high-level layer.

GPON has OAM management in multi-layers. PLOAM is applied to realize data encryption, state inspection, error code monitoring. OMCI is applied to manage the higher-level layer services, such as ONU functional parameters, the types, and quantity of T-CONT, QoS parameters, information of appliance for deployment and performance statistics, to implement the OLT management for the ONT deployment, fault diagnosis, performance, and safety.

In QoS(Quality of Service), GPON is superior to EPON. Ethernet protocol has no inherent QoS capability. On account that a PON system is not viable without QoS, most vendors provide it by using VLAN (Virtual Local Area Network) tags. Without automatic provisioning of VLAN tags, to a large extent, they are manually provisioned. GPON is integrated with QoS to make it better than EPON, due to that EPON QoS is with high cost relative to GPON.

Conclusion

Although both GPON and EPON have their own features and advantages, and to some extent, they compensate for each other. In performance, GPON is superior to EPON but inferior to EPON in costs. GPON is coming up from behind. For the future broadband access market, the co-existence and complementation principle should be kept rather than one replacing another one. In some certain sense, GPON seems to be more suitable for clients having higher requirements for bandwidth, services, QoS, security and ATM tech.

Author: Cindy

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