FP7-217014 Deliverable D22 Access Architecture Definition Document June 2010 ReDeSign 217014 Research for Development of Future Interactive Generations of Hybrid Fiber Coax Networks Information for Publication: Version: 1.0 Status: Public (PU)
Executive summary This deliverable, called Access Architecture Definition Document, makes a summary of the research results that have been achieved in the ReDeSign project on the next generation HFC research. During the 2,5 year project all aspects of an evolution of the cable network has been analyzed and reported in different deliverables. First the requirements of the evolution of the HFC network, called Next Generation HFC, have been defined based on a questionnaire that has been answered by a lot of European cable operators and by interaction with cable operators on MSO forum meetings. A cable network makes use of two different transmission media, fiber and coax. For both technologies a detailed analysis has been performed during the project to define the most optimal technology for the realization of the next generation HFC network. Possible topologies for the creation of the NG-HFC network have been studied and discussed. In case of a shared medium, such as an HFC network, also the MAC protocol is rather important to enable efficient transmission and utilization of the network. Different MAC protocols used in the world to manage shared medium networks have been analyzed and compared. Operators do not only consider technology and performance for choosing a migration path towards the future, but also cost is a driving element. Within this project a cost analysis has been performed on existing HFC networks, and compared this with existing GPON FttH technology. In this deliverable all these aspects are summarized, for a more detailed analysis the reader is referred to previous deliverables of the project. Based on all the work that has been performed in the context of the ReDeSign project on evolutions of the HFC network, there were guidelines developed that can be given to cable operators. Assessments have proven that there is a need for the definition of a Next Generation HFC network. Upgrading the current HFC network with techniques as node splitting, analogue switch off or other proposed and used techniques will prolong the lifetime of the current HFC network, but will not take away the need for a next generation of the HFC network. The proposed solution for the realization of a NG-HFC is different in case of brownfield deployments or in case of greenfield deployments. For the greenfield deployments there is no question that Fiber to the Home (FttH) or the Building (FttB) is the optimal way forward, both on the short and long term. Activation of the fiber could happen with RFoG or TDM-PON (EPON, GPON) technologies. In a brownfield the situation is different, the deployed coax is such a big asset that it should be leveraged as long as possible. The potential capacity of the coax is also of an order that from a physical layer point of view it can compete with fiber. The point is that the available bandwidth over this coax network should be activated in the most cost and power efficient way, taking also into account possible density issues in the hub or Central Office. In this deliverables three options have been put forward, all based on bringing fiber closer to the user, combined with coax transmission for the last/first part of the network. For European cable operators, which have in most cases already a rather good bidirectional triple play ready HFC network in place, an evolution of the current DOCSIS approach is most attractive. The architecture that is put forward in Europe for the realization of the NG-HFC network is based in a network element called µ-cmts combined with a deep fiber topology. This is a new network element, located at the position of the fiber node, deep in the network, that takes care of the full coax transmission between this node and the cable modem at home. The proposed solution is fully compatible with currently used analogue and digital TV distribution concepts and technologies, and solves at the same time the density issues in the hub or other locations, while increasing strongly the potential bandwidth for the user. Transmission between the Central Office/hub and the µ-cmts is standard Ethernet/IP transmission. In order to make this solution the most cost and power efficient solution possible, a new evolution of DOCSIS will be needed. Within this deliverable some suggestions for DOCSIS standardization are given. A proposal based on the Ethernet concept, only supporting essential features and making nice-to-have features optional, would downscale the complexity of DOCSIS. 2 Access Architecture Definition Document
In summary a proposal for the realization of a NG-HFC network has been described in this deliverable, taking into account both brownfield and greenfield deployments. Necessary technologies have been analyzed and described. Migration paths towards this NG-HFC network are indicated. A final remark that should be given is that the technologies developed within the ReDeSign project have a place in the migration towards or in the definition of the NG-HFC network. The new standard for coax transmission, DVB-C2, will increase strongly the capacity of the coax network. In brownfield deployments still coax will be used as transmission medium in the NG- HFC network, so DVB-C2 is the best candidate for the physical layer of the coax transmission of the NG-HFC network, certainly in case DOCSIS or an evolution of DOCSIS is still used as MAC layer. The new amplifier technology as demonstrated in the project has a higher output power over a large band (+3dB till 1GHz). One of the possible migration paths between an HFC network and a NG-HFC network is the increase of the downstream frequency band till 1GHz. Practical problems with respacing of amplifiers makes this upgrade technique in reality not very attractive. The introduction of this new type of amplifier can prevent the need for respacing, making this option of upgrading the HFC network much more attractive. 3 Access Architecture Definition Document
Contents Executive summary...2 Contents...4 List of figures...6 List of tables...6 1 Introduction...7 2 Requirements of the NG-HFC network...8 3 Existing HFC networks...8 3.1 The Coax Network...10 3.1.1 Topology...11 3.1.2 Technology...12 3.2 The Fiber network...13 3.2.1 Topology...14 3.2.2 Technology...15 3.2.3 EPON...16 3.2.4 WDM-PON...16 3.2.5 RFoG or RF-PON...16 4 Next Generation HFC networks...17 4.1 Upgrades from existing HFC techniques...18 4.2 More disruptive scenario s...19 4.3 NG-HFC MAC layer functionality...19 4.3.1 Traffic Management...20 4.3.2 Privacy and security...20 4.3.3 Digital efficiency...21 4.3.4 OSS services...21 4.3.5 General Considerations...22 4.4 Greenfield networks...22 5 The optimal NG-HFC network...23 5.1 NG-HFC topologies...23 5.2 NG-HFC technologies...24 5.2.1 Fiber technology...24 5.2.2 Coax technology...25 5.2.3 MAC technology...28 5.3 Cost evaluation...29 5.3.1 Cost comparison of HFC and FttH...29 5.3.2 Cost of FttLA...32 5.4 Proposals for NG-HFC architectures...34 4 Access Architecture Definition Document
5.4.1 Brownfield...34 5.4.2 Greenfield...35 6 Conclusions...35 Annex 1: List of abbreviations...37 Annex 2: References...39 5 Access Architecture Definition Document
List of figures figure 1: generic figure of a HFC network...8 figure 2: Tree and Branch topology...10 figure 3: Hybrid topology...10 figure 4: Star topology...10 figure 5: Three possible FttH topologies...14 figure 6: Topology of a Next Generation HFC network...17 figure 7: Three most probable architectures for the realization of the NG-HFC network....27 figure 8: Total investment versus time for an HFC network and a GPON FttH network, with average cost for digging included...31 figure 9: Total investment versus time for an HFC network and a GPON FttH network, digging cost not included...31 figure 10: Topologies for a FttLA approach: high density...32 figure 11: Topologies for a FttLA approach: low to medium density...33 figure 12: Different FttLA topologies as used for the cost calculations...34 List of tables table 1: Design parameters and principles currently deployed by operators...9 table 2: European coaxial network parameters, as been reported in deliverable D06...11 table 3: Comparison of different upgrade technologies...26 table 4: Comparison of the different MAC layers with the NG-HFC MAC....29 table 5: Results of a cost analysis of the FttLA approach...34 6 Access Architecture Definition Document
1 Introduction This deliverable D22 summarizes the work carried out by work package 6 of the project. The focus of WP6 is on the definition of a next generation HFC network, that should support cable operators to be competitive also in the future when new competition is coming from operators utilizing fiber networks. As explained in other deliverables and demonstrated by the ReDeSign project, the cable network, including the coax segments, have a large potential bandwidth. By defining new physical layer technologies the net bandwidth available to the cable operator can still strongly be increased. The project has demonstrated this by its collaboration in the DVB-C2 standardization process, improving the bandwidth over a coax segment considerably. The main question is how the network should evolve and should look like in the future in order to activate the bandwidth in the most cost, power and density efficient way. Therefore the ReDeSign project has analyzed the technologies used in HFC networks (fiber technology and coax technology) and reported the results of that study in previous deliverables. Topologies for a future evolution of the cable network have been defined, this network was called Next Generation HFC network or short NG-HFC network. In order to make optimal use of the cable infrastructure not only the physical layer technologies (coax, fiber) are important but also the method to activate the technologies, the way information is transported over the network. This is organized by the MAC-layer of the network. For digital interactive services this MAC- layer in HFC networks is currently DOCSIS. The project alsohas studied on the functionalities in the MAC-layer, different existing MAC layers have been compared and guidelines for the evolution towards the NG-MAC layer used in NG-HFC networks have been formulated. In this deliverable all above mentioned aspects, equally important for the definition of an efficient NG-HFC network, are brought together. First the requirements for a Next Generation HFC network are recapitulated. These requirements are coming from cable operators and are obtained by the project by sending out a questionnaire during the first year of the project. The results of this questionnaire have been discussed in detail in deliverable D03 and D04. Out of these results the requirements have been extracted, as reported in deliverable D09. A summary of these conclusions is given in this deliverable. The existing HFC networks are the main asset of a cable operator. The ReDeSign project has studied the European cable networks and has described these networks and its communalities and differences at length in deliverable D06. Two types of transmission techniques are used for the creation of an HFC network, coax and fiber. Within the project both techniques have been analyzed, and all different existing technologies have been compared with each other. In chapter 3 a summary is presented on the existing HFC networks, as on the coax and fiber transmission state-of-the-art technologies. A next generation HFC network should be a natural evolution of an existing HFC network, still based on fiber and coax, but should also embark on the technical evolution of the technologies in order to make it future proof. Within chapter 4 all aspects of the NG-HFC network are summarized, based on detailed studies performed during the project and reported in previous deliverables. Both topology and technology aspects are mentioned. Fiber technology as coax technology but also possible MAC technologies have been analyzed. Based on all the knowledge build up during the project lifetime clear guidelines can be formulated on how a European cable operator could evolve his network towards a NG-HFC network. Chapter 5 will give guidelines based on the analysis reported on in chapter 4, for the different aspects of the NG-HFC network. A preferred way forward in brownfield and greenfield deployments is described, based on availability and capability of technology but also from a cost analysis point of view. Finally conclusions are formulated in chapter 6. 7 Access Architecture Definition Document
2 Requirements of the NG-HFC network The requirements for a next generation HFC network have been analyzed based on the input from cable operators that has been received via the questionnaire, as created and distributed by the ReDeSign project. The questionnaire and its results has been described in earlier deliverables of the project (see deliverable D03, D04 and D07). In D09 this input is translated into requirements for the NG-HFC network. In this deliverable only the conclusions are listed, for more information the reader is referred to the previous deliverables. Requirements for a future network will come from different angles. Part of the requirements comes from the services an MSO wants to offer to his customers. A detailed analysis has been made for this in deliverable D07. Second part of the requirements for a next generation network comes from competition. Any network that has to be defined needs to be better than that of the competition. In case a MSO can offer all services a subscriber wants, but a competitor can offer more, subscribers will move to that service provider. As such the next generation HFC network has to beat the offering of competitors network by that timeframe. Finally MSOs will not start from scratch building a next generation network. The existing network should be leveraged as much as possible and a clear migration path from the existing network towards the next generation HFC network needs to be defined. As such this also put some requirements on the future HFC network. In order to facilitate this work, reference architectures summarizing the existing variations in networks as documented in deliverable D06 will be used. As discussed above the requirements for a next generation network come from different angles. To conclude the requirements for a next generation HFC network can be summarize as follows: At first the bandwidth offered by the network to any user should be increased, both offering more unicast interactive services and also more broadcast digital services. Analogue distribution of radio and TV will remain important but will occupy less total bandwidth of the available spectrum in the future. In the future it is expected that not only the total raw bandwidth offering will be important, but also the QoS in order to bring services with a high quality of experience. The second important set of requirements has to do with cost. The future network should realize a competitive offering at a very low cost. As such leveraging from technology also used in other areas is a plus Finally the future network should not fully break with the existing HFC concepts and network, but should leverage as much as possible the investments already made, and define clear migration paths. 3 Existing HFC networks figure 1: generic figure of a HFC network 8 Access Architecture Definition Document
figure 1 shows the generic figure of a HFC network. Basically an HFC network exists of a fiber part and an coax part with an optical node between the physical networks. In deliverable D06 of the ReDeSign project a study has been made for the dimensions of the European HFC networks. A part of this result is shown in table 1. Parameter Average value Remarks Homes Passed per Head End 239 k 2 groups can be distinguished, a group with relative on average small amount (between 10k and 100k) and a Homes Passed per Head End maximum Homes Passed per primary hub on average Homes Passed per primary hub maximum Homes Passed per optical node on average Homes Passed per optical node maximum Optical node served from a primary hub by the same fiber on average Homes Passed per first coaxial distribution point on average Homes passed per end amplifier on average group with a large amount (100k 1000k) 327 k 2 groups can be distinguished, one between [10k 300k] and one between [300k 1000k], on average the maximum is around 2 to 3 times the average value, but exceptions exist in both direction (max = average, 1 head-end for the whole network, 10, lot of different networks distributed over the network) 37k Most of the operators are in the range of [10k- 50k] 103 k Most of the operators give values between [10k 100k], but some give figures above 250k, impacting heavily the average value. 1074 Again 2 groups can be distinguished, one between 100 and 1000 and one between 1000 and 3000, on average 2300 Most of the values are between [1500 3500] 20 For most cases the maximum value is between [2-5] times the average value, with a few exceptions in both directions 407 23 The maximum varies between [1,3 3] times the average value table 1: Design parameters and principles currently deployed by operators The fiber part of the network exists of typical network elements that should be taken into account while designing the NG. The Head End (HE) or Central Office (CO): this is the location were the cable operator has installed the main equipment for delivering the services. For instance will this be the location were the TV signals from satellite enters the network of the operator. In a centralized DOC- SIS network this will be the location were the CMTSs are placed. Distinguished characteristics of a HE is that the amount of Homes Passed (HP) it serves are very large, on average within Europe 239.000 Homes Passed, see also table 1. The Local Centre (LC) is often a smaller subset of the HE and bigger than the hub. A Local Centre doesn t have to be within the network of the cable operator it is an optional location. For the reference network it will not be taken into account, but for an operator it is a possible location to place equipment for the NG-HFC network. The hub is the next segmentation of the network. In General a hub is an acclimatized location it services on average 37.000 Homes Passed. Same as with the Local Centre, a Hub is not general designed for all networks. 9 Access Architecture Definition Document
General remark, in this deliverable CO and hub will both be used but can be interchanged. While in reality in most cases a hub will be smaller than a CO, this might also be different. Both are acclimatized locations in the network. The last element is typically for a HFC network, it is the optical node, which is the demarcation point in the network between fiber and coax. This is the point where the information from a optical medium translates into a electrical medium (and vice versa). Typically the optical node is installed in non-acclimatized street cabinets. On average it services around 1074 HP, see table 1. From the optical node towards the customer the HFC network is existing of a coax part with electrical amplifiers. Most coax networks exist of one or more group-amplifiers and at the end of the coax line, before the multitap an end-amplifier. Again the network is segmented more and more, see table 1 for the typical amounts of Homes Passed that a element services. More details about the coax network will be described in paragraph 3.1. 3.1 The Coax Network As explained in the introduction the HFC networks exists besides a fiber part also of a coax network. When looking at the current designs, as being used by the European cable operators, see table 1 and table 2, it is obvious that a cable operator has a large amount of coax and allocated equipment in it s network. Therefore a upgrade from an existing network to the NG-HFC network to comply to the future demands of bandwidth and associated services will be the most challenging job for an operator. Before designing the NG-HFC network, the Re- DeSign project had to study the average European network to define it s reference model on which a NG-HFC network could be build. Based on this study, see Deliverable D06, The following paragraph will shortly describe the existing coax topologies and the technologies that currently are available for use in a coax environment. figure 2: Tree and Branch topology figure 3: Hybrid topology figure 4: Star topology 10 Access Architecture Definition Document
3.1.1 Topology As shown in deliverable D06, the coax part of the network has basically three varieties within the European cable networks. It can be a tree and branch, see figure 2, a star, see figure 4 or even a hybrid topology, see figure 3. Not only the type of topology and the length between the branches varies but also the way that the coax networks are build even when they share the same topology, see table 2. E.g. the amount of cascades amplifiers vary from 1 till even networks with 15 amplifiers in a branch! But there are, for this deliverable, a couple of important characteristics of the coax network. The most important similarity is that in all networks coax is the transport medium that connects the customer to the cable operators network. In principle the capacity of a single coax cable is unlimited but the attenuation due to the skin-effect will limited it s use in relation to it s maximum reach. The higher the frequency of the used signal the higher the attenuation of the signal will be and thus limits it s maximum distance. Until now all networks and most of the associated electrical equipment are designed on a maximum frequency use of 862MHz. Based on this maximum frequency, the type of coax cable with its associated attenuation and the delivery of broadcast signals like analogue TV and radio the original HFC networks were designed. Originally the HFC network was designed for broadcast services like TV and radio distribution. About 15 years back operators started to roll out interactive services for which they needed a return path. Because of all legacy equipment and services operators decided to use the frequencies below 87MHz and thus the return channels or upstream band was created, nowadays often from 20 till 60MHz. Summarizing can be stated that the coax has in principle unlimited capacity, but that the capacity is limited through: o The use of limited frequency band for the upstream and downstream o The attenuation per length of the coax cable Parameter Tree-and-branch Hybrid Star Occurrence 47% 29% 18% # Cascaded distribution amplifiers on average [1-15] [2] [1-2] # Cascaded distribution amplifiers maximum [1-40]] [3-4] [2-4] # Trunks per distribution amplifier on average [1-4] [2-3] [2-12] # Trunks per distribution amplifier maximum [1-4] [2-4] [3-18] Distribution branch length on average [0,1-5] km [0,3-13] km [0,2-3] km Distribution branch length maximum [0,4-28] km [0,5-16] km [0,4-5] km # Cascaded trunk amplifiers on average [4-15] [2-4] # Cascaded trunk amplifiers maximum [3-40] [4-11] # End branches per trunk amplifier on average [2-4] [2-3] # End branches per trunk amplifier maximum [2-4] [2-4] Trunk length on average [0,2-1,5] km [0,6-8] km Trunk length maximum [0,5 12] km [1,2-12] km # End amplifiers per end branch on average [1-6] [3-4] [2] # End amplifiers per end branch maximum [2-39] [4-10] [1-3] End branch length on average [0,1-1,5] km [0,1-0,4] km [0,2-3] km End branch length maximum [0,1-1] km [0,2-0,6] km [0,3-5] km table 2: European coaxial network parameters, as been reported in deliverable D06 11 Access Architecture Definition Document
3.1.2 Technology Besides the coax network itself, the used techniques also have to be analyzed, to determine the migration path to the NG-HFC network. Basically all coax transmission technologies can be divided into two basic technologies: First are the analogue transmission technologies, these are legacy techniques, like FM, PAL, SECAM, used to deliver radio and television signals. Within a standard 8MHz bandwidth channel only one analogue TV-station can be broadcasted to the customers. Second are the digital transmission technologies, these techniques are mostly based on Quadrature Amplitude Modulation (QAM) and are used for transmitting digital information. There are currently two basic transmission techniques used for broadcasting digital signals: o DVB-C, used for transmitting digitalized TV stations over cable networks. The digital information is MPEG-2 encoded. The used modulation order within DVB-C are 64QAM and 256QAM which offers a total 38 to 51Mb/s bandwidth within a 8MHz channel. DVB-C is a broadcast (downstream) technique and doesn t support bidirectional communication. o EuroDOCSIS, is used for transmitting data bidirectional, and used for Internet and telephony services. The Euro in EuroDOCSIS stands for a European version of DOC- SIS. The main difference is the adaptation of European channel widths (8MHz) and spectrum use. EuroDOCSIS exists of a several versions, 1.0, 1.1, 2.0 and 3.0. From a transmission point of view 2.0 and 3.0 are very important. DOCSIS2.0 offered higher modulation, thus more capacity, in the upstream, towards the Central Office. While DOCSIS3.0 supports channel bonding to offer higher download and upload speeds. In the downstream, towards the subscribers, there is no difference between all the DOCSIS versions. the maximum modulation rate for all versions is 256QAM and offers a 51Mb/s capacity in a 8MHz channel. DOCSIS also uses MPEG-2 transport framing to transmit the data. But there are also new digital techniques, these are not (yet) implemented in a coax access network: o DVB-C2, as a part of the ReDeSign project in Work Package 5 a new transmission technology has be developed for transmitting digital information over the cable network: DVB-C2 has the possibility to use OFDM to increase the spectral efficiency. Also due to the use of new FEC coding schemes, DVB-C2 allows higher modulation rates like 1024QAM and 4096QAM which offers within a 8MHz Channel a bit rate of 70 resp. 84Mbps. Ethernet over Coax (EoC) is a general name used to describe all techniques that can be used for transmission of information over a coax access networks, different from DOCSIS. The names come from the fact that all these techniques have in common that the layer 2 protocol that is supported is Ethernet. The physical transmission of the information over the coax is different for the different approaches. In the previous deliverable a number of techniques have been analyzed, that are used in the home environment for transmission of information over coax networks, but the reader should be aware that the Ethernet over Coax family is broader than only the techniques mentioned above (MoCA, G.hn, Home-PNA). Different vendors bring solutions on the market that realize coax transmission based on a lot of concepts, e.g. WiFi over coax, ultrawideband over coax, powerline over coax are other examples of this group. It has to be mentioned that none of these solutions have been standardized yet for access networks, which does not mean that this is only theoretical. A number of propriety solutions are available on the market and some of them are being trialed by cable operators in the field. A number of solutions, based on existing standards, are listed here: 12 Access Architecture Definition Document
o Standard baseband IEEE802.3 Ethernet, at this moment for coax there are two standards defined 10Base2 and 10Base5 both with a maximum speed of 10Mbps. These do not meet the current demand of at least of 100Mb/s per customer. There is no standardization yet for baseband Ethernet 100Mb/s or 1Gb/s over coax. o MoCA, Multimedia over Coax Alliance, is an open standard for delivering data over the unused spectrum of coax cable. MoCA uses a 50MHz channel above 862MHz and has a maximum throughput up to 270Mb/s. MoCA can coexist with cable TV, DVB-C and DOCSIS. The frequency range within MoCA operates is 800-1600MHz o ITU-T G.hn / HomeGrid, the ITU-T G.hn or HomeGrid is a standard being developed by the ITU-T. The G.hn standard is based on the MoCa technology. The main difference is that besides 50MHz bandwidth it also supports 100MHz bandwidth and allows a theoretical 762Mb/s Ethernet throughput. G.hn can operate in the frequency range from 300 2500MHz. Because G.hn / HomeGrid is partly based on MoCA it can coexist with MoCa on the same medium. ITU-T G.hn / HomeGrid is standardized, while MoCA is based on corporation of members of the MoCA alliance. o HomePNA 3.1, HomePNA 3.1 uses the frequency range of 4 to 52MHz and can overcome greater distances due to low loss caused by the skin effect at high frequencies. Home PNA was originally development for delivering services over copper / telephony cables, version 3.1 includes also coax as a transmission medium. HomePNA 3.1 also includes guaranteed QoS. HomePNA is also standardized by the ITU-T. The theoretical throughput is up to 320Mb/s. Because of the used spectrum, HomePNA cannot coexist with (Euro)DOCSIS on the same coax network. Because of the limited maximum speed of 10Mb/s Ethernet is supposed not to be a suitable solution for use in the access network of the cable operator. The other three above mentioned home solutions have possibilities for use in the HFC network. HomePNA is on a small scale already been used in Hotel and Fiber to the Building solutions. On the other hand MoCA is used in the field for in-home transmission. The G.hn / HomeGrid solutions looks like the most universal and standardized solution and has the highest theoretical throughput which is from a cable operator point of view more interesting. To use one of the solutions in the HFC network the expectation will be that the solution needs deeper fiber in the HFC network, until the end amplifier. At this location a Multi Dwelling Unit (MDU) is needed to offer the baseband Ethernet, MoCA, G.hn or HomePNA. If one of the standards can be redesigned to serve more devices (2000HP according to the reference HFC network, but with node split this will be reduced to 600 or 256 or even less in a FttLA approach) and offering more theoretical capacity it should be even theoretical possible to place this MDU in a optical node. 3.2 The Fiber network The fiber part of the HFC network is the part between the Central Office or Hub and the optical node. In the optical node the optical transmission is terminated and coax transmission is started. The fiber network of existing HFC networks is mainly based on a point to point topology, sometimes a point to multipoint topology from a fiber point of view is used, but in that case each optical node has its own set of wavelengths over the fiber, implementing a point to point architecture between the hub and the optical node. Node segmentation increases the demand on the fiber part of the network. This increase is solved by adding more fibers in the network or by using WDM technology over the fiber. In case of WDM technology more than one wavelength is used for transmission between hub and fiber node (e.g. one per segment). 13 Access Architecture Definition Document
In case an operator is planning to extend his fiber network (e.g. for the implementation of a deep fiber architecture) he can choose from three basic topologies and associated technologies. This section will summarize the topologies and technologies that can be used for the realization of the fiber part of the HFC and next generation HFC network. A Fiber to the Home (FttH) terminology is used in this section, but the topologies and technologies are also applicable for other architectures such as fiber to the node (FttN), fiber to the last amplifier (FttLA) or fiber to the building (FttB). For more information on this topic the reader is referred to deliverable D12, here a summary of the conclusions is given. 3.2.1 Topology Basically there are three optical solutions that are generally used when deploying fiber in a greenfield FttH situation, see figure 5. Central Office Access loop Home Active Ethernet IP Ethernet switch Ethernet switch Point -to- Point IP More concentrated More distributed Ethernet switch Splicing PON IP Optical splitter PON OLT figure 5: Three possible FttH topologies 1. Active Ethernet (AE) is an Ethernet FttH architecture employing a fiber-feeding approach that supports larger deployments than P2P. AE offers long reach to a high number of customers from one centralized local exchange. With AE, fewer fibers pass through smaller ducts with greater consolidation in the CO. Its topology is similar to that of FttN; fiber extends to an aggregation node (e.g. the fiber node in an HFC network). On the downside, operational expenditures (OPEX) are typically high given the active equipment that sits vulnerable in the OSP, requiring care and maintenance. 2. Point-to-Point (P2P) is an Ethernet FttH architecture similar in structure to a twistedpair cable phone network; a separate, dedicated fiber for each home exists in the CO. On the positive side, this results in a completely passive OSP and a solution based on mainstream Ethernet technology. Weaknesses are related mainly to the massive amount of fibers in the feeder section, at least one fiber for each subscriber and the large CO, with high power consumption. 14 Access Architecture Definition Document
3. PON is a fully optical FttH architecture option that offers the best of all worlds. Like AE, it aggregates users in the OSP, resulting in a CO without a mess of fibers; like P2P, it employs a passive OSP device (the optical splitter). The splitter is extremely stable (it virtually never fails) and requires no cooling or powering. PON offers long reach and supports deployments on any scale. It is highly reliable and requires minimal maintenance, offering an important OPEX advantage. For these reasons, it is best suited to mass-market deployments. When assessing the merits of the various architectures for FttH deployment, scalability requirements are perhaps the most important consideration. AE, P2P and PON all differ in: the amount of fiber required the amount of CO-based hardware to be deployed the amount of power consumed at the CO/Hub, Access Loop and Home reach from CO passive or active OSP None of the technologies is better than the other in all cases, so careful analysis has to be made case by case which of the technologies to deploy. 3.2.2 Technology See deliverable D12 for a more detailed description of the used fiber technologies described below. A summary of the technologies most attractive for cable operators is given here. 3.2.2.1 Active Ethernet Active Ethernet is, as the name says, based on Ethernet switching techniques. In the access loop an aggregation switch is placed. All subscribers are point-to-point connected to this switch and there connection is being trunked to the main switch in the CO. The technique is based on the Ethernet 802.3 standards. The maximum distances and data capacity for AE is depending on the availability of the standard Ethernet products. This makes this technique rather cheap and easy to implement, compared to PON techniques. The main drawback is that the operator has active equipment in the field that consumes energy and is more service depended than an optical fiber or a passive splitter / combiner. An Active Ethernet technology is less suited for the broadcast transmission of analogue or digital television to all users on a separate wavelength. This drawback makes this technology less attractive for cable operators. 3.2.2.2 GPON Gigabit PON (GPON) was defined by the Full Services Access Network (FSAN), a group of operators and specified in ITU-T Recommendation G.984. GPON technology supports integrated audio, data and video services, providing 2,5Gb/s-1,25Gb/s to/from a maximum of 64 users. GPON provides a reach of 20km with a 28dB optical budget, which can be extended to 30km if the splitting ratio is limited to 1:16. Typically, an OLT-system can cope with up to approximately 3600 subscribers, based on a maximum of 64 subscribers per GPON connection. 3 wavelengths are defined on the PON system, 1 wavelength for DS digital transmission (1490nm), 1 wavelength for US digital transmission (1310nm) and 1 wavelength for DS broadcast RF transmission (1550nm). GPON uses a Time Division Multiplexing (TDM) based scheme. In the downstream direction, data are sent to all ONTs connected with the PON OLT-interface. In the ONT, only data destined to that ONT are filtered out based on the portid sent along with the data. In the downstream direction the MAC protocol controls the assignment of transmission windows for each ONT. In this way also bandwidth allocation and prioritization is performed. In the upstream 15 Access Architecture Definition Document
TDMA is used as scheme to share the bandwidth between the subscribers. The scheme is rather comparable to the DOCSIS methodology; a central manager allocates the bandwidth over the users and distribute grants to the user to indicate which user can transmit traffic in which timeslot. The length of the timeslots allocated to a certain user is variable. 3.2.3 EPON Ethernet PON (EPON) is a PON technology developed by the IEEE and specified in IEEE802.3ah. It provides 1Gb/s symmetrical to/from users (proprietary 2,5Gb/s solutions are available on the market) and a reach of 20km with a 29dB optical budget. EPON uses the same fiber for both up- and downstream traffic with different wavelengths: 1490nm for downstream and 1310nm for upstream. The Multi-Point Control Protocol (MPCP) performs the bandwidth assignment, bandwidth polling, auto-discovery and ranging. Like GPON, EPON uses a TDM based scheme. Frames that are broadcasted downstream are extracted in the Optical Network Unit (ONU) (similar to the ONT of GPON) based on the logical link ID in the preamble. Upstream bandwidth is assigned by means of GATE messages sent by the OLT. 3.2.4 WDM-PON In a pure Wavelength Division Multiplexing PON separate wavelengths run over the same shared fiber infrastructure, where in the last fiber part each wavelength is directed to the right per home subscriber fiber. Dense Wavelength Division Multiplexing (DWDM) requires accurate lasers with little mode partition noise (MPN). These types of lasers are 25 to 50 times more expensive than the simpler laser types. Two of these lasers are needed per customer for PON applications, making optical components a significant fraction of the overall capital expense. o CWDM, the relatively small numbers of available CWDM channels (a maximum of 5 downstream/upstream channels) in the band of interest, together with the fact that CWDM is not compatible with TDM (difference in wavelength range used), makes CWDM unattractive for WDM PON deployments. o DWDM, in contrast with CWDM a relatively large number of channels can be packed into the allocated bands using DWDM. But this density comes with the increased cost associated with tight manufacturing tolerances of lasers as well as with stabilizing the laser within the narrower channel width. To reduce costs wider channel spacing can be employed (3.2 or 1.6nm) which can still result in many useable channels per 20nm window. However the costs of these optics is still substantially higher than for GPON optics. 3.2.5 RFoG or RF-PON Radio Frequency over Glass (RFoG) or Radio Frequency PON (RF-PON) is a technology that enables transmission of radio signals over fiber. Since the radio frequencies are the same as used on HFC networks, there is a high degree of compatibility between an RFoG system and equipment already deployed in the field by cable operators, such as the CMTS, OSS/BSS systems and CPE equipment. To interwork with an existing cable modem only a relatively simple Optical-Electrical converter is needed between the fiber network and the coax network in the customer premises. RFoG can be put in as standalone transmission medium or as an overlay system combined with EPON or GPON. In the last scenario WDM equipment multiplexes the downstream signals of RFoG and e.g. GPON on the fiber, which is possible as both transmission systems use different wavelength for up and downstream. In downstream direction services are broadcasted through the RFoG network just as they are in traditional HFC-networks. In upstream direction the CMTS controls the cable modem in exactly the same way as it does in an HFC-network. The cable modem, in its turn, deter- 16 Access Architecture Definition Document
mines when the RFoG transmits. Extremely responsive lasers react on the signal from the cable modem. The lasers are only on when the cable modem is transmitting. One of the advantages of RFoG is that it introduces very low noise in both upstream and downstream, this makes it possible to use higher modulation schemes enabling higher bit rates, or to bridge longer distances between the central office and the customer premises. Another advantage of RFoG over HFC networks is that fiber is less sensitive to humidity, temperature, lightning (EMC) than copper based networks. Compared to GPON or EPON systems, rolling out of RFoG technology is much easier and less expensive since a lot of equipment can be reused and operational processes can be unchanged, there will be no or very limited impact on the OSS and BSS systems. The drawback of the solution is that it is still based on the CMTS as generator/receiver for the digital information, as such the density and power / cost issues of the CMTS remain fully valid. With this solution the bandwidth of the fiber is not exploited as it could be, no differentiation can be made between a subscriber connected to the RFoG system compared to the HFC network. This draw back will certainly be important in case the operator has competition from an other FttH operator. Most likely RFoG or RF-PON will be used by a Cable operator at the beginning of the transition phase to a full fiber network. This way it offers the Cable operator at the start of implementing FttH to use the known systems like DOCSIS and DVB-C and the corresponding OSS and BSS. When the percentage of FttH customers grows, after a certain penetration degree the Cable operator can decide to introduce a full FttH solution including the corresponding OSS and BSS. With this scenario the investment costs per customer are lower. 4 Next Generation HFC networks Cable operators are basically standing before two kind of networks transitions to a NG-HFC network, see figure 6 Brownfield deeper Fiber Hub Hub Greenfield xpon figure 6: Topology of a Next Generation HFC network 1. Brownfield: In a brownfield case the cable operator has already an HFC network in place and the operator wants to have a transition in the most cost and technical effi- 17 Access Architecture Definition Document
cient way from his current network to a next generation HFC network, capable of offering more bandwidth. The cost modeling studies performed in deliverable D16 shows that the coax network is a strong added value and that it should be leveraged as much as possible, especially when digging costs are taken into account. 2. Greenfield: In a greenfield case there is not yet a network in place and the cable operator is completely free to design the NG-HFC network from scratch. In a greenfield situation the digging costs of fiber and coax should not be taken into account, because these costs have to be made anyhow. Fiber to the Home deployment is much more economical and future proof and therefore the correct way to go forward, as was concluded in deliverable D16. Given the fact that in a lot of European countries there is already a cable network in place with a good coverage, brownfield upgrades are important. Therefore the focus of this section is mainly on the brownfield situation, on how the operator can migrate to the NG-HFC network in the most cost and technology most efficient manner. The greenfield situation is discussed at the end of this chapter. 4.1 Upgrades from existing HFC techniques Deliverable D16 shows a summary of possible techniques that an operator can use in it s transition to the NG-HFC network. The first type of upgrades are derived nowadays from existing solutions but refined for NG usage: o o o Classical fiber node split, This architecture is based on dividing the Homes Passed behind an optical node over two or more mini nodes. All mini nodes have the same spectrum of the first node, but less homes passed have to share this capacity so more bandwidth is available per individual customer. The node split exists of implementing two or more nodes at the original node location. a node split does need more fiber capacity for connecting the optical nodes with the head end and vice versa. If there is no fiber capacity between these locations a WDM solution can be involved. This will increase the cost per homes passed. For future use the optical nodes should become dense to fit in the street cabinets and at the same time have less power consumption. Extension to 1GHz, Nowadays most HFC networks and all of its equipment and even the coax cables are designed for use till a maximum frequency of 862MHz. In theory the coax is not bound by the frequency of 862MHz but allows frequencies above. An extension to 1GHz can deliver extra capacity: 1000GHz 862MHz = 138MHz extra bandwidth. If this bandwidth is divided by 8MHz channels it supplies 17 extra channels, when using 256QAM it offers around 850Mb/s. But when using 4096QAM it can offer 1.4Gb/s. For future use this means that the HFC equipment should support frequencies till 1GHz. The biggest disadvantage of the Extension to 1GHz is that the networks and the equipment were not designed (in the past) to go above the 860MHz. Extension to 1GHz means that attenuation will be much higher in the upper spectrum than originally calculated, and thus an operator needs amplifiers with a usability to go till 1GHz and a higher gain. Full digital no FM, Full digital means that there will be no analogue signals (like PAL) in the complete spectrum of the cable operator. For future use this means that the HFC network can be redesigned for use with only digital signals, but also that the customer needs DA converters and/or STBs to use his legacy analogue equipment with the full digital network of the operator. 18 Access Architecture Definition Document
4.2 More disruptive scenario s Besides extending the already existing methods for increasing capacity in the network, the ReDeSign project also analyzed potential techniques to deliver interactive services. Again see deliverable D16 for detailed information. The main challenge for defining the NG-HFC equipment for interactive services is based on current costs and equipment, the operator will face huge cumulative investments costs and needs a lot of space in the CO or Hub for installing the equipment. For NG-HFC networks investment cost reduction and denser equipment is eminent to use the HFC network in the most efficient and valuable way. Based on current techniques, suitable to implement in a HFC network ReDeSign has done studies on the following possible NG-HFC solutions. o o o µ-cmts approach, The idea with the µ-cmts solution is to place small rugged CMTSs at the position of the optical nodes. This µ-cmts will serve one or more branches connected to the node. Because it is a CMTS it will contain the MAC functionality. The µ-cmts approach leaves the possibility to coexist with legacy services, but as the demand for capacity rises, eventually an operator can decide to go to full IP based on DOCSIS. µ-edge QAM approach, This solutions is based on the M-CMTS principle. It exists of one main Core CMTS based at a Head End or CO and Edge QAMs placed remotely at the position of the optical node. The Core CMTS will perform all the DOCSIS functionalities while the Tx- and Rx Edge QAM will perform the transmitting and receiving of the DOCSIS signals and the translation to Gigabit Ethernet. Like the µ-cmts the evolution of the network will start with a couple of Edge QAMs in the node, coexisting with legacy services and techniques. In the end there is no technical need to phase out digital and/or analogue services, but this solution offers the possibility to go to full digital. Because of the use of universal Edge QAMs in the node it is still possible to bypass the CMTS for digital TV services. Ethernet over Coax (EoC) solutions based on Access-MoCA / -G.hn / -Home PNA, This option is based on techniques that are used for IP connection within the home network. These techniques, if possible, can be extended for use in the access network of the operator. For the global description there is no difference between these three techniques, but the technique itself will determine if it s possible to extent the technique for use in the access network. Also the cost of certain techniques will influence the feasibility of using a certain technique. 4.3 NG-HFC MAC layer functionality In the previous paragraphs an overview has been given of different techniques, each with it s own (Medium Access Control) MAC protocol used to manage a shared medium. The reader is referred to deliverable D18 1 for detailed information and comparison of the different MAC technologies that are nowadays available to manage a shared medium infrastructure. In this chapter, based on the lessons learned from existing MACs a summary is given of what a MAC for a NG-HFC network should look like. Based on above information the technical requirements for a NG-HFC MAC solution will be specified on a basis of current techniques. The NG-HFC-MAC solution will not be specified in detail but a global description of the desired functionalities per technical aspect will be given: 1 In D18 the WiMAX MAC layer was also analysed. But because WiMAX is a wireless protocol, derived from DOCSIS and possible become obsolete, it will not be discussed in this deliverable. 19 Access Architecture Definition Document
4.3.1 Traffic Management Even in the NG-HFC MAC layer, traffic management will be the most important functionality of the MAC-layer. The amount of users in a shared segment of the HFC network may become smaller and smaller during the years, but the NG-HFC network will still be based on a shared medium based on broadcast technologies in the downstream and unicast technologies in the upstream. Because of the way the medium is build the NG-HFC network will always need a contention mode or period for new modems to notify themselves to the network. It will be a centralized master who will plan and grant access to the shard medium. Probably the allocation of bandwidth will be TDMA and FDMA based. S-CDMA still has to prove itself in the HFC network. The way to address the physical layer on FDMA can change due to use of channel bonding and/or different channel widths (e.g. as specified in DVB-C2). When the NG-HFC network doesn t have to support the legacy TV-channels anymore the channel width can become wider and thus decreasing the need for (complex) bandwidth techniques like channel bonding. When the total available amount of capacity is limited or has to be shared with many clients, the demand for Quality of Service (QoS) levels for different services will be more dominant than in networks were capacity is no issue at all. In these networks there will be less need to implement more advanced QoS levels, as long the service provider established the minimal requirements, like latency and jitter for the most critical services even on a Best Effort base. This will mean that the access to the medium and inherent to this the traffic management will remain an important functionality of the MAC layer. Especially when (future) applications or functionalities will become more critical with regards to bandwidth, timing, latency, jitter etc. Thus delivering the right Quality of Service will become necessary to optimally support these future services. Especially when the total amount of network capacity has to be taken into account, which is the case in a (NG-)HFC network that is upgraded following the demand curve. The NG-MAC layer should support at least 2 QoS classes, one QoS class with guarantees on transmission parameters and a Best Effort QoS class. The NG-MAC layer may support more QoS classes, but this should be balanced with the implementation complexity, manageability and usability of the QoS class by the cable operator. Looking to the described MAC protocols in this deliverable, it is clear that the MACs have each a different approach for the support of traffic management. Ethernet on the one hand has a very limited management of bandwidth and QoS, while on the other hand DOCSIS is fully optimized in terms of traffic management including an extensive support for QoS. Home oriented protocols like MoCA typically have lower support for QoS. Comparing PON MACs a strong difference is seen regarding bandwidth management and QoS, GPON is rather bandwidth efficient and has an extensive standardized QoS support while EPON supports 8 QoS classes and is less efficient. 4.3.2 Privacy and security Because of the shared character of the NG-HFC network, privacy and security will remain important. The NG-MAC layer should support data encryption based on AES (existing standard for encryption in access networks), support a secured way for exchanging encryption key information and perform checks on system integrity. Admission control is also a necessity in a modern access network. The MAC layer should take care of a security check in case new modems want to connect to the network. DOCSIS and MoCA support encryption based on DES which is considered nowadays to be insecure. GPON, G.hn and also DOCSIS support AES encryption which is the successor of DES. EPON does not have a standardized encryption, different vendors offer a propriety solution. Ethernet itself does not specify encryption at a MAC layer. All MACs that support AES encryption do support key management protocols for exchanging encryption keys and admission to the network. With MoCA the support for admission is ra- 20 Access Architecture Definition Document