Analysis: Advantages and disadvantages of EPON and GPON

August 27, 2021

EPON and GPON have their own advantages. GPON is better than EPON in terms of performance indicators. However, EPON has the advantage of time and cost. GPON is catching up. Looking forward to the future broadband access market may not be the replacement, it should be complementary. For bandwidth, multi-service, high QoS and security requirements, and ATM technology as a backbone customer, GPON will be more suitable.

What is PON? Broadband access technology is surging, and it is destined to become a battlefield where smoke will never dissipate. At present, the domestic mainstream is still ADSL technology, but more and more equipment manufacturers and operators have turned their attention to optical network access technology.

Copper prices continue to rise, cable prices continue to decline, and the growing demand for IPTV and video game services is driving the growth of FTTH. The beautiful prospect of replacing the copper cable and the wired coaxial cable by the optical cable, the telephone, the cable TV, and the broadband data triple play becomes clear.

Figure 1: PON topology

PON (Passive OpTIcal Network) passive optical network is the main technology to realize FTTH fiber to the home, providing point-to-multipoint fiber access, as shown in Figure 1, it is the OLT (optical line terminal) and user side of the office side. The ONU (Optical Network Unit) and the ODN (Optical Distribution Network) are composed. Generally, the downlink adopts the TDM broadcast mode and the uplink adopts the TDMA (Time Division Multiple Access) mode to form a point-to-multipoint tree topology. The biggest highlight of PON as optical access technology is “passive”. The ODN does not contain any active electronic devices and electronic power supplies. All of them are composed of passive components such as splitters, which have low management and operation costs.

PON development history

The PON technology research originated in 1995. In October 1998, the ITU adopted the ATM-based PON technology standard advocated by the FSAN organization (full service access network) - G.983. Also known as BPON (BroadbandPON). The rate is 155Mbps and can optionally support 622Mbps.

EFMA (EtherneTIn the First Mile Alliance) introduced the concept of Ethernet-PON (EPON) at the end of 2000 with a transmission rate of 1 Gbps and a link layer based on a simple Ethernet package.

GPON (Gigabit-CapablePON) was proposed by the FSAN organization in September 2002. In March 2003, the ITU adopted the G.984.1 and G.984.2 protocols. G.984.1 specifies the overall characteristics of the GPON access system; G.984.2 specifies the ODN (OpTIcal DistribuTIon Network) physical medium correlation sublayer of GPON; in June 2004, the ITU passed G.984.3, which The requirements for the Transmission Convergence (TC) layer are specified.

Comparison of EPON and GPON products

EPON and GPON are the two main members of the optical network access, each with its own merits, competing with each other, complementing each other and learning from each other. The following compares them in various aspects:

Rate

EPON provides fixed uplink and downlink 1.25Gbps, 8b/10b line coding, and the actual rate is 1Gbps.

GPON supports multiple speed grades, can support uplink and downlink asymmetric speed, downlink 2.5Gbps or 1.25Gbps, uplink 1.25Gbps or 622Mbps, determine the uplink and downlink speed according to actual needs, select the corresponding optical module, and improve the optical device speed price ratio.

This conclusion: GPON is better than EPON.

2. Split ratio

The split ratio is how many ONUs (users) are carried by one OLT port (office).

The EPON standard defines a split ratio of 1:32.

The GPON standard defines the split ratio to the following 1:32; 1:64; 1:128

In fact, the technical EPON system can also achieve higher split ratio, such as 1:64, 1:128, EPON control protocol can support more ONU. The split ratio is mainly limited by the performance index of the optical module. The split ratio will cause the cost of the optical module to rise sharply. In addition, the PON insertion loss is 15 to 18 dB, and the large split ratio will reduce the transmission distance; too much user sharing bandwidth is also the cost of the large split ratio.

This conclusion: GPON provides multiple selectivity, but the cost consideration is not obvious. The maximum physical distance that the GPON system can support. When the optical split ratio is 1:16, the maximum physical distance of 20km should be supported. When the optical split ratio is 1:32, the maximum physical distance of 10km should be supported. EPON is the same, this conclusion: equal.

3.QOS (Quality of Service)

EPON adds a 64-byte MPCP multipoint control protocol to the MAC header Ethernet header.

(multipointcontrolprotocol), MPCP controls the access to P2MP point-to-multipoint topology through messages, state machines, and timers to implement DBA dynamic bandwidth allocation. The MPCP involves the allocation of ONU transmission time slots, automatic discovery and joining of ONUs, and reporting of congestion to higher layers to dynamically allocate bandwidth. MPCP provides basic support for the P2MP topology. However, the protocol does not classify the service priorities. All services randomly compete for bandwidth. GPON has a more complete DBA and excellent QoS service capabilities.

GPON divides the service bandwidth allocation method into four types. The highest priority is fixed (Fixed), Assured, Non-Assured, and BestEffort. The DBA further defines a traffic container (T-CONT) as an uplink traffic scheduling unit, and each T-CONT is identified by an Alloc-ID. Each T-CONT can contain one or more GEMPort-ID.T-CONTs are divided into five service types. Different types of T-CONTs have different bandwidth allocation modes, which can meet different traffic flows delay, jitter, and loss. Different QoS requirements such as packet rate. T-CONT type 1 is characterized by fixed-bandwidth fixed time slots, corresponding to fixed-bandwidth (Fixed) allocation, suitable for delay-sensitive services, such as voice services; type 2 is characterized by fixed bandwidth but time slot uncertainty, corresponding guaranteed bandwidth Assured allocation, suitable for fixed bandwidth services with low jitter requirements, such as video on demand services; type 3 is characterized by minimum bandwidth guarantee and dynamic sharing of redundant bandwidth, and maximum bandwidth constraint, corresponding to non-guaranteed bandwidth (Non -Assured), suitable for services with service guarantee requirements and large bursts of traffic, such as download services; type 4 is characterized by BestEffort, no bandwidth guarantee, suitable for delay and jitter requirements The service, such as the WEB browsing service; type 5 is the combination type, after allocating the guaranteed and non-guaranteed bandwidth, the extra bandwidth requirement is allocated as best as possible.

Conclusion: GPON is better than EPON

4. Operate and maintain OAM

EPON does not have too much consideration for OAM, but simply defines ONT remote fault indication, loopback and link monitoring, and is optional support.

GPON defines PLOAM (PhysicalLayerOAM) at the physical layer, and OMCI (ONTManagementandControlInterface) is defined at the upper layer to perform OAM management at multiple levels. PLOAM is used to implement data encryption, status detection, and error monitoring. The OMCI channel protocol is used to manage the services defined by the upper layer, including the function parameter set of the ONU, the type and quantity of the T-CONT service, the QoS parameters, the request configuration information and the performance statistics, and automatically notify the running events of the system to implement the configuration of the OLT to the ONT. Management of fault diagnosis, performance and safety.

Conclusion: GPON is better than EPON

5. Link layer encapsulation and multi-service support

As shown in Figure 2, EPON follows a simple Ethernet data format, but adds a 64-byte MPCP point-to-multipoint control protocol to the Ethernet header to implement bandwidth allocation, bandwidth round-robin, and automatic discovery in the EPON system. Ranging and other work. There is not much research on the support of services other than data services (such as TDM synchronization services). Many EPON vendors have developed some non-standard products to solve this problem, but they are not ideal and it is difficult to meet the carrier-class QoS requirements.

GPON is based on the completely new transport convergence (TC) layer, which can complete the adaptation of high-level diversity services. As shown in Figure 2, it defines ATM encapsulation and GFP encapsulation (general framing protocol). You can choose both. One is for business encapsulation. In view of the current popularity of ATM applications, a GPON.lite device that only supports GFP encapsulation has emerged, which removes ATM from the protocol stack to reduce costs.

GFP is a general-purpose link layer procedure for multiple services. The ITU defines G.7041.GPON with a small number of modifications to GFP. PortID is introduced at the head of the GFP frame to support multi-port multiplexing. A Frag (Fragment) segmentation indication is also introduced to increase the effective bandwidth of the system. And it only supports the data processing mode for variable length data and does not support the data transparent processing mode for data blocks. GPON has powerful multi-service carrying capacity. GPON's TC layer is essentially synchronous, using standard 8 kHz (125 μm) fixed-length frames, which allows GPON to support end-to-end timing and other quasi-synchronous services, especially to support TDM services directly, the so-called NativeTDM. GPON has "natural" support for TDM services.

This conclusion: The TC layer supporting GPON for multi-service is stronger than the MPCP of EPON.