PON (Passive Optical Network) passive optical network is the main technology for realizing FTTH fiber-to-the-home, providing point-to-multipoint fiber access. It consists of ONU (Optical Network Unit) and ODN (Optical Distribution Network).Generally, the downlink adopts TDM broadcast mode, and the uplink adopts TDMA (time division multiple access) mode, forming a point-to-multipoint tree topology structure.The biggest highlight of PON as an optical access technology is "passive". ODN does not contain any active electronic devices and electronic power supplies. It is all composed of passive devices such as optical splitters (Splitter), and the management, maintenance and operation costs are low.The research of PON technology originated in 1995. In October 1998, ITU passed 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 the rate of 622Mbps can be selected.EFMA (Ethernetin the First Mile Alliance) proposed the concept of Ethernet-PON (EPON) at the end of 2000, with a transmission rate of 1Gbps, and the link layer is based on simple Ethernet encapsulation.GPON (Gigabit-CapablePON) was proposed by FSAN in September 2002, and ITU passed G in March 2003.984.1 and G.984.2 Protocol.G.984.1 specifies the general characteristics of the GPON access system; G.984.2 stipulates the physical medium-related sublayer of GPON's ODN (Optical Distribution Network); in June 2004, ITU passed G again.984.3, which specifies the relevant requirements for the Transmission Convergence (TC) layer.EPON provides fixed uplink and downlink 1.25Gbps, adopts 8b/10b line coding, and the actual rate is 1Gbps.GPON supports a variety of rate grades, and can support asymmetric rates of upstream and downstream, downstream 2.5Gbps or 1.25Gbps, upstream 1.25Gbps or 622Mbps, according to actual needs to determine the upstream and downstream rates, select the corresponding optical module, improve the rate of the optical device price ratio.In fact, technically, the EPON system can also achieve a higher split ratio, such as 1:64, 1:128, and the EPON control protocol can support more ONUs.The split ratio is mainly limited by the performance indicators of the optical module. A large split ratio will cause a significant increase in the cost of the optical module; in addition, the PON insertion loss is 15-18dB, and a large split ratio will reduce the transmission distance; too many users share Bandwidth is also the price of a large split ratio.This conclusion: GPON provides multiple selectivity, but the advantage is not obvious in terms of cost. Maximum transmission distance The maximum physical distance that the GPON system can support, when the optical splitting ratio is 1:16, it should support the maximum physical distance of 20km; when When the optical splitting ratio is 1:32, the maximum physical distance of 10km should be supported.EPON is the same, this conclusion: equal.(multipointcontrolprotocol), MPCP controls access to P2MP point-to-multipoint topology through messages, state machines and timers to achieve DBA dynamic bandwidth allocation.The contents involved in MPCP include the allocation of the ONU's sending time slot, the automatic discovery and joining of the ONU, and the congestion situation reported to the upper layer in order to dynamically allocate the bandwidth.MPCP provides basic support for the P2MP topology, but the protocol does not classify the priority of services. All services compete for bandwidth randomly. GPON has a more complete DBA and excellent QoS service capabilities.GPON divides the service bandwidth allocation method into 4 types, the priority from high to low is fixed bandwidth (Fixed), guaranteed bandwidth (Assured), non-guaranteed bandwidth (Non-Assured) and best effort bandwidth (BestEffort).The DBA also defines a service container (trafficcontainer, T-CONT) as an uplink traffic scheduling unit, and each T-CONT is identified by an Alloc-ID.Each T-CONT may contain one or more GEMPort-IDs.T-CONT is divided into 5 service types. Different types of T-CONT have different bandwidth allocation methods, which can meet different QoS requirements of different service flows such as delay, jitter, and packet loss rate.T-CONT type 1 is characterized by fixed bandwidth and fixed time slot, which corresponds to fixed bandwidth (Fixed) allocation and is suitable for services that are sensitive to delays, such as voice services; type 2 is characterized by fixed bandwidth but uncertain time slots, corresponding to guaranteed bandwidth. (Assured) allocation, suitable for fixed bandwidth services that do not require high jitter, such as video-on-demand services; type 3 is characterized by the minimum bandwidth guarantee and the ability to dynamically share the surplus bandwidth, and has the constraint of the maximum bandwidth, corresponding to non-guaranteed bandwidth (Non -Assured) allocation, suitable for services with service guarantee requirements and large burst traffic, such as download services; type 4 is characterized by best effort (Best Effort), no bandwidth guarantee, suitable for low latency and jitter requirements services, such as WEB browsing services; type 5 is a combined type, after allocating guaranteed and non-guaranteed bandwidth, additional bandwidth requirements are allocated as best as possible.GPON defines PLOAM (PhysicalLayerOAM) at the physical layer, and OMCI (ONTManagementandControlInterface) is defined at the upper layer, and OAM management is performed at multiple levels.PLOAM is used to implement functions such as data encryption, status detection, and bit error monitoring.The OMCI channel protocol is used to manage the services defined by the upper layer, including the functional parameter set of the ONU, the type and quantity of T-CONT services, QoS parameters, request configuration information and performance statistics, automatically notify the system operation events, and realize the OLT's configuration of the ONT, Troubleshooting, performance and security management.As shown in Figure 2, EPON follows the simple Ethernet data format, but adds a 64-byte MPCP point-to-multipoint control protocol to the Ethernet header to realize bandwidth allocation, bandwidth polling, and automatic discovery in the EPON system. ranging, etc.There is not much research on the support of services other than data services (such as TDM synchronization services). Many EPON manufacturers 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 a completely new Transmission Convergence (TC) layer. This sublayer can complete the adaptation of high-level diversified services. As shown in Figure 2, ATM encapsulation and GFP encapsulation (general framing protocol) are defined, and both can be selected. One for business encapsulation.In view of the current ATM application is not popular, so a GPON that only supports GFP encapsulation.The lite device came into being, which removed ATM from the protocol stack to reduce costs.GFP is a general link layer procedure applicable to multiple services, defined as G by ITU.7041.A small amount of modification has been made to GFP in GPON, and PortID is introduced in the head of the GFP frame to support multi-port multiplexing; Frag (Fragment) segment indication is also introduced to improve the effective bandwidth of the system.And it only supports the data processing mode oriented to variable-length data but not the data transparent processing mode oriented to data blocks. GPON has a strong multi-service bearing capacity.The TC layer of GPON is essentially synchronous, using a standard 8kHz (125μm) fixed-length frame, which enables GPON to support end-to-end timing and other quasi-synchronous services, especially TDM services, which is the so-called NativeTDM, GPON has "natural" support for TDM services.EPON and GPON have their own strengths and weaknesses, and GPON is better than EPON in terms of performance indicators, but EPON has advantages in terms of time and cost, and GPON is catching up. Looking forward to the future broadband access market, it may not be one who replaces the other, but should coexist and complement each other.For customers with high bandwidth, multi-service, high QoS and security requirements and ATM technology as the backbone network, GPON will be more suitable.For the customer group that is cost-sensitive, QoS, and security requirements are not high, EPON has become the dominant.Copyright © 2005-2022 51CTO.COM Beijing ICP Certificate 060544 All rights reserved. Please do not reprint without permission