Introduction and comparison of EPON and GPON

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, G, advocated by the FSAN organization (full service access network). 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 encapsulation.

GPON (Gigabit-CapablePON) was proposed by the FSAN organization in September 2002, and the ITU adopted G in March 2003. 984. 1 and G. 984. 2 agreement. G. 984.1 The overall characteristics of the GPON access system are specified.G. 984. 2 specifies the physical distribution related sublayer of the ODN (Optical Distribution Network) of GPON.In June 2004, the ITU passed G again. 984. 3, which specifies the requirements for the Transmission Convergence (TC) layer.

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 of 1.25Gbps, using 8b/10b line coding, and the actual rate is 1Gbps.

GPON supports multiple speed grades and can support uplink and downlink asymmetric speeds, 2.5Gbps or 1.25Gbps downstream, and 1.25Gbps or 622Mbps uplink. According to the actual demand, the uplink and downlink rates are determined, and the corresponding optical modules are selected to increase the optical device speed price ratio.

This conclusion: GPON is better than EPON.

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, technical EPON systems can also achieve higher split ratios, such as 1:64, 1:128, EPON control protocol can support more ONUs.The road ratio is mainly limited by the performance specifications of the optical module, and the large split ratio will cause the optical module cost to rise significantly. In addition, the PON insertion loss is 15 to 18 dB, and the large split ratio reduces 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.

　QOS(Quality of Service)

EPON adds a 64-byte MPCP(multi point control protocol) to the MAC header Ethernet header.MPCP controls 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-IDs.T-CONT is divided into five types of services. Different types of T-CONTs have different bandwidth allocation modes, which can meet different QoS requirements of different service flows for delay, jitter, and packet loss rate.T-CONT Type 1 is characterized by a fixed-bandwidth fixed time slot, corresponding to a fixed-bandwidth (Fixed) allocation, suitable for delay-sensitive services, such as voice services. Type 2 is characterized by a fixed bandwidth but an indeterminate time slot. The corresponding guaranteed bandwidth (Assured) allocation is suitable for fixed bandwidth services that do not require high jitter, such as video on demand services. Type 3 is characterized by minimum bandwidth guarantee and dynamic sharing of redundant bandwidth, and has the constraint of maximum bandwidth, corresponding to non-assured bandwidth (Non-Assured) allocation, suitable for services with service guarantee requirements and large burst traffic. Such as downloading business.Type 4 is characterized by BestEffort, no bandwidth guarantee, suitable for services with low latency and jitter requirements, such as WEB browsing service. Type 5 is a combination type, after allocating guaranteed and non-guaranteed bandwidth, additional The bandwidth requirements are allocated as best as possible.

Conclusion: GPON is better than EPON

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

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.

Figure 2: Comparison of GPON and EPON protocol stacks

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 that only supports GFP encapsulation is available. The lite device came into being, removing ATM from the protocol stack to reduce costs.

GFP is a generic link layer procedure for multiple services, defined by the ITU as G. 7041.A small number of modifications were made to GFP in GPON, and PortID was 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 for directly supporting TDM services, 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.

Conclusion

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 a 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. For customers with low cost sensitivity, QoS and security requirements, EPON has become the dominant factor.