Abstract With the higher demand for broadband access, the gradually decreasing cost of fiber-to-the-home, and the rapid development of fiber-optic access technology, fiber-to-the-home broadband access will gradually mature. In this paper, on the basis of a detailed analysis of the advantages, disadvantages and applications of various fiber-optic access technologies for fiber-to-the-home, it is proposed that GEPON andGPONare the two most promising fiber-optic access technologies for fiber-to-the-home in the future, and thekey technologies of GEPON, which are more mature in terms of the current standards and equipment,are introduced.

Keywords: FTTH , MSTP, point-to-point Ethernet, GEPON, GPON.

1、Background

Firstly, as users put forward higher and higher requirements for broadband access, the existing broadband access methods, such as ADSL andLANaccess,have become increasingly unable to meet the needs of usersdue to the existence ofproblems such asshort transmission distance, limited access bandwidth, low security, andno good guarantee ofQoS.Secondly, the rapid development of optical access technology, from active optical access technology(PDH,SDH,MSTP, point-to-point Ethernet system)toPONpassive optical access technology(APON,BPON,GPON,EPON,GEPON).Finally, due to the cost of the optical fiber itself, thecost ofoptical transceiver modules,OLTandONUequipment, as well as the existing fiber-to-the-home supporting costs continue to decline, so that the current realization of fiber-to-the-home equipment costs and line costs than before a substantial decline. Therefore, the fiber-to-the-home access method is gradually maturing, and is now gradually becoming a hot spot in the domestic and international communications industry. In the near future will become an important means of user access. However, among the many current fiber optic access technologies, which fiber optic access technology is more suitable forthe large-scale development ofFTTH?

2 , active fiber optic access technology

2.1 PDH

PDHtechnology with its maturity in the field of optical access has been widely used, its security, reliability and highQoSguarantee performance, so that for some time it is still an important fiber optic access technology for telecommunications operators. However, traditionalPDHtechnology in access applications inevitably has certain limitations, mainly reflected in:

( 1 ) lack of unified network management;

( 2 ) Lack of network organization capability;

( 3 ) Poor protection capability for services;

( 4 )Lack of effective bearing means for the rapidly growingIPdata services;

( 5 ) Lack of flexibility in expansion and upgrading;

( 6 ) Single interface, equipment cascade, more external cable connections, more failure points, bringing difficulties to maintenance.

PDHoptical access technology is mainly applied to point-to-point small-capacity private line enterprise users.

2.2 SDH

SDH , which is more widely used in the fiber optic access of enterprise and public utility customers at present,has the following obvious advantages compared withPDH:

( 1 ) Uniform bit rate, uniform interface standards, easy interconnection between equipment;

( 2 ) Network management capability is greatly enhanced;

( 3 ) Self-healing protection function.

The main disadvantage ofSDHis that it is designed for the transmission ofTDMinformation. It lacksthe functions required toprocessinformation other than traditional voice informationbased onTDMtechnology and is not suitable for transmittingATMand Ethernet services other thanTDM.

SDHoptical access technology is mainly used for point-to-point high-capacity private line enterprise subscribers, inter-office or point-of-presence(POP)communications.

2.3 MSTP

Based on SDH ,MSTPsimultaneously realizesTDM,ATM, Ethernet and other service access, processing and transmission, and provides unified network management, with the following advantages:

( 1 ) Providing multiple physical interfaces to meet the rapid access of new services. While ensuring compatibility with traditionalTDMservices, it can provide flexible access to multiple services. Typical services mainly include:IP,ATM,SDH,FR.

( 2 ) As it is based on the existingSDHtransmission network, it can be well compatible with the existing technology and ensure the existing investment.

( 3) MSTP adoptsVCvirtual cascade technology, which effectively utilizes the bandwidth and realizes smaller granular bandwidth management.

( 4) MSTP adoptsLCAStechnology, which ensures that the number of virtual cascades can be dynamically adjusted without interrupting the data flow.

( 5) MSTP technology supports mesh, tree, star, multi-ring cutover and other networking methods, which improves network scalability and facilitates flexible and efficient configuration of the system environment.

( 6 )High reliability of transmission and automatic protection and recoveryMSTPinheritsthe protection characteristics ofSDH, less than50msof automatic protection and recovery, to ensure that users are satisfied with the service.

The disadvantages of MSTPare mainly:

( 1 ) Lower bandwidth utilization;

( 2 ) Limited maximum provided bandwidth;

( 3 ) Mainly realizes Layer 2 functions, as well as simpler Layer 3 functions;

( 4 ) Insufficient ability to provide services flexibly.

( 5 ) The occupation of more optical fiber

The applications of MSTPare mainly located in the inter-office or inter-sink communication and the point-to-point communication of large enterprise and public utility users.

2.4 Point-to-Point Ethernet System

Point-to-point Ethernet system is the most direct Ethernet fiber access technology. Each subscriberis directly connected to a subscriber optical interface of the local Ethernet switchthrough onefiber/pair. In the point-to-point Ethernet system approach, the expanded EthernetOAMprotocol allows remote management of the subscriber-side equipment through the local switch, thus providing carrier-grade operable and manageable Ethernet access.

2.4.1 Advantages

( 1 ) High access bandwidth and easy network upgrade;

( 2 ) Simple network hierarchy, seamless connection between access network and user Ethernet;

( 3 ) Ethernet switches are placed in the building, district or bureau server room, and the connection between bureau and subscriber is directly through optical fiber, so the whole access network structure is simple;

( 4 ) High service turn-up rate and fast investment recovery;

( 5 ) Remote management of user-side equipment is possible through the bureau switch, and line detection and fault localization can be easily carried out at the bureau end, which reduces the difficulty of maintenance.

2.4.2 Disadvantages

( 1 ) Re-laying of fiber optic lines is required;

( 2 ) Each user occupiesone/pair of optical fibers, and the number of optical fibers is large, making construction more difficult;

( 3 ) Because the inherent mechanism of Ethernet technology does not provide end-to-end packet delay, packet loss rate and bandwidth control, it is difficult to ensure the quality of service for real-time services, and itis more difficult to provideTDMservices;

( 4 ) The maintenance cost is very high;

( 5 ) Lack of security mechanism guarantee;

2.4.3 Application occasions

When the users are very dense, the space demand and cost of the server room increases rapidly, thus it is not suitable for high density user areas, and is more suitable for decentralized user access.

3 , passive optical fiber access technology

Passive optical network (PON ), refers to theOLT (optical line terminal)andONU (optical network unit)between the optical distribution network(ODN)without any active electronic equipment. Its typical topology is a point-to-multipoint star structure(shown inFigure1). There is no need for node equipment at the optical branch point, only a simple passive optical splitter needs to be installed, so it has the advantages of saving fiber optic cable resources, sharing bandwidth resources, saving investment in server room, high security, low comprehensive network construction cost, low maintenance cost and high reliability.

Figure 1 Passive optical network(PON)topology diagram

Disadvantages of PONfiber access technology:

( 1 ) The initial investment cost is too high;

( 2 ) Its topology makes the user does not have the protection function or the protection cost is too high.

PONfiber-optic access technology applications are mainly suitable for decentralized small businesses and residential users, especially those user areas are more dispersed, and each area users are relatively concentrated in a small area of dense user areas.

At present, PON-basedfiber access technology hasfivekinds ofAPON,BPON,GPON,EPONandGEPON, etc. SinceAPONandBPONare based onATM, whichis not the direction of development, and its rate is limited, the equipment is complex, and it can't satisfy the user's high-bandwidth and low-cost requirements, soAPONandBPONare not the direction of development. This paper mainly introducesEPON/GEPONandGPONfiber access technology.

3.1 EPON/GEPON

EPON/GEPONisan Ethernet-basedPONtechnologyproposed bythe IEEE, has formed the standard802.3ah, the standard is mainly driven by equipment vendors.

3.1.1 Advantages

( 1 ) Elimination of theATMlayer, reducing equipment complexity and implementation difficulties, thereby reducing costs.

( 2 ) Higher rate, upstream and downstream bandwidth up to1Gbit/s.

( 3 ) Mature standards and equipment.

3.1.2 Disadvantages

( 1 ) Difficult to support services other than Ethernet, especiallyTDMservices that require real-time performance.

( 2 ) Low transmission efficiency, due to line coding, bearer layer, transmission aggregation layer, service adaptation efficiency, etc., making the transmission efficiency is very low, onlyhalf of GPON.

3.1.3 Applications

Mainly located in the provision of data service access for users, mainly commercial users and individual users.

3.2 GPON

GPONisaPONtechnologybased onATMandGFPproposed byITU-T, which has formed the standardsG.984.1andG.984.2. The standard is mainly driven by operators, so it has a more thoughtful consideration of operational interests, with a higher rate of up to2.4Gbit/s; a common mapping format, which can be adapted to any new or old service; rich operation, management, maintenance andConfiguration(OAM&P)features; high transmission efficiency for all kinds of services, even forTDMservices can be transmitted efficiently and without overhead.

3.2.1 Advantages

( 1 ) Higher rate of2.5 Gbits/s;

( 2 ) Higher transmission efficiency;

( 3 ) Able to carryTDMservices efficiently.

3.2.2 Disadvantages

( 1 ) Hardware realization is difficult and costly;

( 2 ) Less mature equipment and standards.

3.2.3 Applications

It is mainly targeted at providing data and voice services and private line access for high-end users, mainly large customers and commercial users.

4 .GEPONKey Issues and Technologies

4.1 Ranging and synchronization

Ranging technology isa key issue in the TDMA scheme. It is essentially a synchronization problem of uplink signals. Due to thedifferent fiber paths ofeachONUfrom theOLTand theinconsistency ofeachONUcomponent,the loop delay betweenthe OLTand eachONUis different, and due to changes in ambient temperature and aging of the device, the loop delay will also change continuously. Therefore, ranging techniques must be introduced to compensate for the delay differences caused by the above reasons to ensure thatthe signals fromdifferentONUscanbe accurately multiplexed together at theOLTby time slots to avoidsignal collisionsatthe OLTdue to unsynchronization between uplink time slots.The synchronization technique used in theGEPONis the Absolute Time Scale(ATS)technique, which includesthe insertion and extraction of the ATS, and so on.

The OLThas a local clock counter that counts time particles. Whenthe OLTsendsanMPCPframe, it inserts the value of the local clock counter, i.e., the absolute clock, into its timestamp field.There is also a local clock counter in theONU. This counter also counts time particles. However.wheneverthe ONUreceivesanMPCPframefromthe OLT. it refreshes the value of its local clock counter with the new timestamp value carried by the frame. Whenthe ONUsendsanMPCPframe. it also has to map the value of its own clock counter into the timestamp field.the OLTwillcheck the timestamp of thereceivedONUs.The timestamp ranging method is tocalculatethe difference betweenthe received timestamp value andthe local clock ofthe OLTto get theRTTvalue ofthe ONUthrough thetransmission ofthe timestampbetweenthe OLTandthe ONU.The OLTshould carry out rangingas long as it receivesthe MPCPframefrom theONU.The principle of ranging by using the timestamp method isshown in Figure2.

Figure 2 GEPONranging technique

In the figure, TR is thetotal response time ofONU,TDOWNis the downlink transmission delay,TUPis the uplink transmission delay, andTWAITis thewaiting time between whenONUreceivesthe MPCPmessagefromOLT(generallyGATEmessage)and the start of the sending window.OLTsends anMPCPframetoONUwhenthe local time isTO, and it carries a timestamp value ofTS=TO.After a transmission delay ofTDOWNtime, thisMPCPframe arrives atthe ONU.the ONUupdates the value of the local time counter totoand then waits.Afterwaiting forthe TWAITtime, thisONU's transmit window starts and it sends the data andthe MPCPframe andinsertsthe value of the local clock counter, t1, intothe time scale field of theMPCPframe.theMPCPframesent bythe ONUarrives atthe OLTaftera propagation delay of theTUPtime.

4.2 Burst Transmission and Reception

Like allPONsusing TDMA technology, theproblem of burst transmission and reception of uplink signals is faced inGEPON.The unequal distances ofdifferentONUstothe OLTandthe different strengths of the optical signals emitted by the optical modules ofeachONUcausethe signal power received by the receiver ofthe OLTto be different in each time slot, which leads toeasy misjudgment by the OLT.

In order to solvethe problem of OLT misjudgment, there are two methods, one is to requirethe ONUto dynamically adjust the luminous power, and the other is to requirethe OLTto dynamically adjust the judgment level, asshown inFig.3.The former methodputs higher requirements onthe ONUand has a higher cost, while the latter method is simple to realize and has a lower cost, and most of the latter methods are currently used.

Fig. 3 GEPONburst reception technique

4.3 Security Issues

Since GEPON downlink is a shared network, user security is alsoan issue of greater concern inGEPON.Asshown inFigure4, in order to ensure the security of user data, two main methods are used at present, one is toassign a uniqueLLIDto eachONU, and the other is toencrypt the user databy usingAES128encryption technology, wherethe ONUwill generate a new key at regular intervals and send it tothe OLT, whichwillconvert the key generated by the ONUinto a real encrypted pattern according to a certain algorithm, and encrypt the downlink data stream is encrypted.

Figure 4 GEPONsecurity issues

4.4 QoSissues andDBAtechnology

Due to the high burstiness of data flow in Ethernet, if a static bandwidth allocation scheme is used, low bandwidth utilization or unfair bandwidth allocation will occur. Therefore, to improve the bandwidth utilization,a dynamic bandwidth allocation mechanism is used in GEPON .The benefit ofadoptingDBAis that it not only improves bandwidth utilization, but also adopts bandwidth scheduling algorithms to ensureQoSfor certain high-priority services.DBAensures that the service level contracts(including minimum bandwidth, maximum bandwidth, and latency sensitivity)signed by subscribersare fairly enforced.The prioritization of services and users can be specified when signingSLAs. High-priority services or users are given priority access to network resources. While ensuring that the minimum bandwidth is guaranteed for all services or users, the remaining bandwidth can be dynamically allocated for use by more emergent services.

4.5 MPCPTechnology

MPCPis a multipoint control protocol that specifiesthe control mechanism betweenOLTsandONUs, asshown in Figure5, with the following functions:

Figure 5 GEPON MPCPtechnology

( 1 ) Controls the network start-up import process, i.e.,the registration process of ONUs.

( 2) Assigns bandwidth to the end station(ONU)

( 3 ) Ask forbandwidth requestsfrom the end station(ONU)