Cost vs. Redundancy in FTTH access networks: A case study of a Danish village

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1 Cost vs. Redundancy in FTTH access networks: A case study of a Danish village Gustav Helgi Haraldsson, Jens Myrup Pedersen Center for TeleInFrastruktur, CTIF - Aalborg University, Denmark ghha3@kom.aau.dk and jens@control.aau.dk Abstract This paper studies the feasibility of offering redundancy within the access network by planning an access network using the ear topology. This proposed network is compared to a traditional tree structured access network. The basic idea of the ear topology is to offer two individual fibers from separate distribution nodes to Network termination points. The cost of the ear topology is kept down by reusing trenches making extra digging minimal. The results show however that the ear topology with the home-run method is not suitable compared to the tree topology due to the extra fibers needed. Further work could apply the PON architecture to the ear topology, reducing the amount of fibers. Index Terms Redundancy, Network Planning, The Ear Topology and The Tree Topology. I. INTRODUCTION Communication over the Internet is growing fast, followed by the recognized need for still more bandwidth[1]. With the Internet market today keeping the costs down and focusing on increasing the bandwidth, reliability and sustainability is left behind[2]. This will cause problems in the future as more services are being introduced and the need for reliability and sustainability within access networks becomes apparent. Much study has gone into different types of fiber topologies offering redundancy, reliability and sustainability [3][4], but most of these have aimed at the higher layers of networks. The access network of the Fiber To The Home (FTTH) is mostly left behind and planned with a tree topology which offers no redundancy [2]. As different services are moving to one general purpose network the need for redundancy within the access network needs to be addressed. With the cost of fibers down, offering redundancies to the end user is becoming a more practical thing to achieve. Though the problems of the access network reliability have been recognized not much work has been done towards a solution. In some cases redundancies have been planned to vital end users like hospitals, police and larger enterprises by connecting these to the higher network layers where redundancy is available[5]. So what would be needed is a topology designed to be used in an access network environment. A recent study[2] addresses this issue using "The Ear Topology". This paper will describe a case study, where this topology is practically planned and the amount of digging and fiber needed calculated. All other variables needed in a construction of an access network will also be calculated to view the total cost of it. Those results will then be compared to the old traditional method of tree topology access networks. II. BACKGROUND AND TERMINOLOGY This section will explain the used terminology, levels of service, and the background of the networks used. Finally guidelines are put forth to maintain consistency throughout the planning of access networks. A. Network Architecture Terminology A suitable network architecture terminology has been defined by the Swedish ICT commission[6]. The network architecture is set up of 3 network layers and nodes: Main network: This is the highest layer in the network (backbone). It has a number of main nodes (MN) which connects the distribution network to the rest of the world. The main network is not included in this study. Distribution network: This consists of distribution lines which inter-connect the distribution nodes(dn). The DN s connect the access network to the main network so the location of the DN s affects the planning of the access network. Access network: This consists of Network Termination points (NT s) and lines connected to a DN. NT s can be e.g. households and business. Each NT can withhold more end-users, but this paper will only view each NT as one connection. As the paper focuses on the access network most of the planning and study will be done to this layer. B. Level of service The Swedish ICT commission constructed guidelines for planning fiber networks and introduced different layers to represent an IT-infrastructure[6]. The levels from ducting to transmission are central for this study: Ducting level: This level holds the ducts and pipes for the fiber cables. It is the level that needs the most planning, because of the long lifetime (at least 3-4 years) and the cost of digging making it difficult and expensive to change later on. The plan should utilize the digging effectively in order to circumvent later excavation and digging. This also involves placing enough pipes so further extension is possible without having to excavate. Cable Level: This level holds the cables and air blown fibers. The expected lifetime is the same as for the pipes. To support expandability the planning of fibres should incorporate easily replaceable fibres.

2 Transmission level: This is where multiple logical connection are mapped onto one physical connection. This is where the switching equipment is placed. The lifetime of the equipment used depends largely on the technologies used. Changes in this level are cheaper than in the lower levels. C. Networks Background Information Knowledge of the distribution network is required, as planning an access network involves connecting NT s to the DN s. Distribution ducts placement is also a factor, as the reuse of the trenches will help bring the cost down. The creation of the ear topology consisted of having a Double Ring (DR) as a distribution network topology. The ear topology is designed to connect NT s to two different DN s, thus giving the redundancy. Fig. 1 shows the principle of the ear topology. The idea Maximum amount a SP can hold are 96 micro-ducts but 192 splicings. Districts are not to exceed 7 NT s in the planning phase, in order to be expandable later on. The maximum possible blowing distance is 1 meters, from DN to SP s, and from SP s to NT s[7]. Digging The planning must incorporate all possible connections. Fusion splicing is used as a splicing method. Digging over roads is can only be done where needed and depending on circumstances, excessive amount of road digging should be avoided. Depending on the cost, digging over roads for, DN to SP ducts (main duct) is done by digging a trench or blow pipes under the road. Depending on the cost, digging over roads for, SP to NT ducts (NT ducts) is done by removing the asphalt or blow pipes under the road. Planning incorporate all possible shortcuts to districts and NT s. Trenches are to be reused where possible, depending on circumstances. Scenario specific guidelines will be given later in the paper. Fig. 1. The new proposed ear topology. is to offer each NT two physically independent fibers, which would be led from two different DN s. As the ear topology will share the trenches the extra digging required is minimal, but still avoid single points of failure. D. General Guidelines for Planning an Access Network In order to be consistent with methods and materials used when planning, a list of guidelines where created which we will give a rough overview of: Pipes, Ducts and fiber: Type of cable used is duct cable. Loose tube fiber system is used. The placement of the fiber are done using the blowing method. Enough pipes are placed for possible expansions. Splicing points (SP s) are placed in NT districts 1 to connect NT s to the main duct from the DN. Street boxes are used as a splicing points. Pipes from DN to SP s are at least 5mm PVC pipes. Pipes from SP s to NT s are a large enough PVC tube to hold 96 individual fiber micro-ducts. Enough available space is in a SP to allow expansion of a district. 1 To simplify the planning, NT s close together are grouped into districts III. THE NETWORKS SCENARIOS The two scenarios are planned manually and compared to each other where Scenario A exhibits the traditional method of planning an access network with no redundancy, while scenario B exhibits the new ear topology. Both scenarios use double ring distribution network as upper layer network. A. The scenarios Due to low cost and ease of implementation today s access networks are based mainly on the tree topology. However it offers no redundancy. Comparing fig. 1-2 shows that minimal amount of extra digging is needed for the ear topology when sharing trenches. The main difference would be the amount of fibers needed, but with the cost of fibers down this shows promising potentials. B. Scenario specific guidelines In Section II-D, general guidelines for planning an access network were given. Additional guidelines apply depending on the scenarios as follows: Digging: Digging for scenario A should be along one side of the road if no connection is needed on the other side. Digging for scenario B should be along one side of the road where possible, however that is seldom possible due to the design of the ear topology. Digging over dead end roads for scenario A should only be done at the start, to reduce fiber need.

3 A. Distribution network The distribution network was not planned per say, since the role it plays in this paper is mainly location of its nodes and ducts, so two DN s were strategically placed in the north and the south end of the town. A distribution duct was then placed through the town connecting the two nodes. Only placements of nodes and ducts within the town of Vester Hassing are considered relevant. Fig. 2. The traditional tree topology. Digging over dead end roads for scenario B should be done at the start and end, to reduce fiber need. Fibers from different DN s in scenario B can never re-use trenches in the same direction, as that avoids the redundancy. IV. EVALUATION OF THE NETWORK PLANNING Evaluation parameters were chosen so that scenarios can be compared and a conclusion drawn on if the ear topology is a suitable topology to plan today s access networks. Each scenario will be evaluated according to these parameters : Amount of fiber in main ducts: This is where 96 fiber cables are blown from the DN to the SP s. Amount of fiber for SP s to NT s: As the scenarios uses home-run, each NT receives its own fiber(s),making it necessary to evaluate it separately from the main duct fibers. Amount of digging and reuse of trenches for each scenario: The digging for the main duct and splicing point to NT duct was done by planning trenches alongside the road network of Vester Hassing. This parameter views the length of trenches and the amount of reusable trenches in the scenarios. Amount of road crossing for each scenario: As road crossing can be done in multiple ways they are not accounted as digging but rather as a unit. This means that depending on the method used the cost can vary, thus giving reason to count each road crossing on its own. Total cost of scenario: Here the total cost of each scenario is presented. The evaluation parameters mentioned before cover the variables which are the most expensive. There are also smaller variables included in the planning which are also taken into account when estimating the total costs. V. NETWORK PLANNING METHOD The network planning was done with already available Geographic Information Systems (GIS) data of Vester Hassing in the municipality of Hals. The planning of the access networks is done by the means of thehome-run method[5][7], where the NT s will each get an individual fiber. B. Access network The access network was planned, using a step-wise approach. As there are many scattered agriculture NT s in the vicinity a line had to be drawn in what the access network should cover. From that it was gathered that within the boundaries lied 83 NT s. Districts were created where NT s are grouped together, and within these, splicing points were placed to simplify the planning. The districts created can not hold more than 96 NT s as this is the maximum amount the splicing points can withhold. In order to be able to extend the network without unnecessary excavation each district holds no more than 7 NT s. In order to be consistent, districts are not changed between scenarios, while the locations of the SP s will change. C. Scenario A For the tree topology the districts have been dealt between the two DN in order to keep the length of the fibers down. Main ducts holding 96 fibers for each SP are spread to every SP. By doing so larger fiber cables can be blown in one session to each district keeping the cost down. From the SP to the NT s the access network is planned according to the guidelines and is planned in the cheapest way possible. Fig. 3 shows Scenario A. D. Scenario B As the ear topology calls for fiber from each DN, two SP s are now placed within each district. Each DN connects like in scenario A, however the placement of the main duct needs to be carefully planned, as it can not share a trench with each other in the same direction. Since main ducts are used to get fibers to splicing points, the ear topology is only applied from the SP s to the NT s. This however will not affect the redundancy issue. Fig. 4 shows Scenario B. Because the ear topology travels from one splicing point towards the other in opposite direction, streets with many dead end roads can cause problems. As the ear topology is constructed the fiber will travel up a dead end road and down again, doing so will make the blowing distance and fiber needed from an splicing point to an NT too large. Instead of only crossing the road in the end, fibers are also crossed in the start. With this, only the needed fiber is placed within the road. Depending on circumstances, roads can be crossed in order to shorten the distance to a number of NT s. This should be done with precautions as crossing the same road many times can be problematic from a practical point of view.

4 Double Ring Distribution nodes Splicing points Main duct SP to NT duct Distribution line Fig. 3. Scenario A. Double Ring Distribution nodes Splicing points Main duct SP to NT duct Distribution line Fig. 4. Scenario B. VI. RESULTS The results show that compared to the tree topology, cost of the ear topology is doubled due to the amount of fibers and blowing it. The ear topology however shows some potential as one of the major cost factor is kept down, namely the digging. A. Scenarios Scenario A and B was planned according to the guidelines mentioned in section II-D and section III-B. Tables I and II show the breakdown cost of scenario A, and Tables III and IV shows the breakdown cost of scenario B. The total cost of scenario A is 599,211 Euros and the total cost of scenario B is 1,152,741 Euros.. These results compared to the other scenario can be viewed in figure 5. B. Interpreting and comparing the results As the paper focused on comparing the traditional tree topology access network to the new ear topology, the results need to be viewed against each other.

5 Fibers 7, 756 [m] , 447 Blowing of fibers 7, 756 [m] Digging 2, 245 [m] , 22 Road crossings 15 [u] , 44 Pipes 3, 294 [m] 1.3 4, 282 Handling of pipes 3, 294 [m] 1.4 3, 426 Wells [u] 65 TABLE I BREAKDOWN OF MAIN DUCT COST VARIABLES IN SCENARIO A Euros Fibers 169, 534 [m].52 88, 158 Blowing of fibers 169, 534 [m].78 13, 224 Digging 17, 395 [m] , 362 Road crossings 122 [u] , 419 Pipes 2, 264 [m] , 343 Handling of pipes 2, 264 [m] , 75 Terminations 83 [u] 39 32,37 Splicing 83 [u] ,975 Street boxes 14 [u] 65 9, 1 TABLE II BREAKDOWN OF SP TO NT COST VARIABLES IN SCENARIO A. Fibers 21, 759 [m] , 748 Blowing of fibers 21, 759 [m].78 1, 697 Digging 3, 724 [m] , 94 Road crossings 3 [u] , 88 Pipes 6, 527 [m] 1.3 8, 485 Handling of pipes 6, 527 [m] 1.4 6, 788 Wells 2 [u] TABLE III BREAKDOWN OF MAIN DUCT COST VARIABLES IN SCENARIO B. Fibers 532, 269 [m] , 78 Blowing of fibers 532, 269 [m].78 41, 517 Digging 22, 258 [m] , 225 Road crossings 194 [u] , 158 Pipes 42, 842 [m] , 695 Handling of pipes 42, 842 [m] , 556 Terminations 166 [u] 39 64, 74 Splicing 166 [u] , 95 Street boxes 28 [u] 65 18, 2 TABLE IV BREAKDOWN OF SP TO NT COST VARIABLES IN SCENARIO B. Scenario A compared to Scenario B: Large increase in use of fibers is shown between the two scenarios. This is due to the ear method offering two sets of fibers and placement of splicing points are not as ideal as for the tree topology. The amount of extra fibers used, is the main reason for scenario B costing 1,152,741 Euros or 92,4% more then scenario A. The cost of extra digging is only increased by 98,935 Euros but fibers and blowing fibers is increased by 316,39 Euros in scenario B. VII. CONCLUSION The paper concludes that the new ear topology shows some potential, as one of the main cost factor, digging, 1. Fig. 5. Fiber 5. Digging 5. Scenario A Scenario B Re-use of trenches 2. Total Cost Fiber, digging, reuse of trenches and total cost graphs. is kept down by its design. The amount of fiber needed is far greater than initially expected. Despite that the cost is kept down by minimizing the extra digging, the fiber and blowing of fiber costs bring the total cost between the scenarios to almost double. This paper can thus conclude that with the planning method used, the use of home-run fiber and the placement of splicing points in particular, the ear topology is not a feasible solution in offering redundancy within the access network. VIII. FURTHER WORK Since the ear topology is new, there are no rules on how to plan the topology and practically every aspect of the planning is wide open. This makes it interesting to create other case studies where the ear topology is planned with different planning methods e.g. different placement of splicing points. Further work on this topology could study how the costs can be reduced with the use of Passive Optical Networks (PON), reducing the amount of fiber and thus the cost. REFERENCES [1] M. Farmer, Internet history, 22, 7. Dec 25. [Online]. Available: /msc/intro/history.shtml [2] J. M. Pedersen, T. M. Riaz, T. P. Knudsen, and O. B. Madsen, Designing broadband access network with triple redundancies, in Proceedings of the Fifth International Workshop on Design of Reliable Networks (DRCN 25). Ischia, Italy: DRCN, October 25, pp [3] J. M. Pedersen, Structural quality of service analysis in large scale networks, Ph.D. dissertation, Aalborg University, April 25. [4] S. Bujnowski, B. Dubalski, and A. Zabludowski, The evaluation of transmission ability of 3rd degree chordal rings with the use of adjecent matrix, in Proceedings of the 7th INFORMS Telecommunications Conference. Miami, USA: INFORMS, March 24. [5] T. M. Riaz, F. Fjermestad, P. Helle, G. Martin and P. Veltcheva, Strategy and Development plan for the IT Infrastructure in the municipality of Hals, January 24, student report. [6] The Swedish ICT Commission, General guide to a future-proof IT infrastructure, April 25. [7] S. Nilsson-Gistvik, Ericsson Network Technologies AB., Optical fiber theory for communication networks, second edition, 22.