Good backhaul designs have protection mechanisms that provide reliable services. Sequel to large transport capacity this proposed backhaul design intends to carry; failures will cause huge losses of data/information and will affect large number of users. On the part of the operator, huge fortune will be lost during the downtime. To enhance reliability and survivability of this design, one of the protection options as specified by ITU-TG.983.1 in (Cedric Lam, 2007) must be adopted. The specified standard is basically duplicating the fibre links and/ or the components for protection. There are five basic elements that can be protected within the PON network. They are;
i, OLT equipment
ii, Fibre F1 (from the OLT to the splitter) iii, Passive Optical splitter/ WDM coupler iv, Fibre F2 (from the splitter to ONU) v, ONU equipment
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The choice of which to protect is influenced by many factors like service level agreement, cost implications, competition among the operators, etc. However, there are four protection architectures suggested by ITU-TG983.1 when a tree topology (points to multipoint) is adopted. Figures 2.28 -2.31 show the different protection architectures with different levels of protection.
In Figure 2.28 only the fibre from OLT to splitter is protected. If there is any fibre cut on the active line, the transmission will automatically be switched to the protection line (fibre). In this protection architecture, failures from other components like OLT, splitter, fibre length F2, and the ONU will result in downtime. In a paper presented by (Abhishek Dixit et al, 2016), this class of protection is graded 1/5 representing 20% protection.
Figure 2.28: Fibre (F1) from OLT to Splitter Protected
Figure 2.29 has two optical transceivers and with duplication of F1 (fibre from OLT to splitter), this represent 40% protection according to (Abhishek Dixit et al, 2016), Failures from either OLT or F1 will not cause downtime to the network, unless in a rear case where the two go down at same time.
Active Line
Protection Line
ONU 1
ONU N OLT
Splitter
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Figure 2.29: OLT, F1 Splitter port Protected
Figure 2.30 shows architecture with protections on OLT, F1, some ONUs transceiver, some F2 and optical splitter, representing about 80% protection. This type of protection will no doubt give better and more reliable network than those in figures 2.28 and 2.29 but will definitely be more expensive as most components are duplicated.
Figure 2.30- OLT, F1 Splitter, some F2, some ONU Protected
In Figure 2.31, the optical transceivers at both OLT and ONU are doubled, F1 and F2 and the splitter are also doubled. The total network is protected representing 100% protection.
Failures at any segment of the network will not cause downtime unless if there is natural disaster, causing more than one component to collapse at the same time. This type of
Active Line
Protection Line
ONU 1
ONU N OLT with
protection
Splitter 2xN
Active Line
Protection Line
ONU 1
ONU N OLT with
protection
2 Splitters with
configurations
Some ONUs with protection
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protection would have been ideal for all networks, but considering the cost implications, not too many operators will afford to build it. In the events were the operators put up such formidable protection, the services may go high thereby defeating the initial aims of PON (Cost effectiveness).
Figure 2.31: All System Protection
In this design, after many considerations such as cost, complexity and reliability of the network, the work has chosen to employ the protection offered in figure 2.29. To capture the market in any business, the operators must consider the cost of rendering the service to its customers. Protection architecture in figure 2.29 will help to bring down the cost of services and at same time provide quality and reliable services needed as optical network equipment like splitters are passive and may not easily develop faults. Coming to the side of ONUs, since Mobile networks like LTE have handover facilities, it may take care of some faults beyond the splitter, as mobiles within the affected eNB will switch over to close by eNBs to continue their conversation.
Active Line
Protection Line
ONU 1
ONU N
OLT with protection
2 Splitters
ONUs with protection
52 2.6 SUMMARY OF REVIEWED WORKS
In search of backhaul link that supports quality of service (QoS), unlimited capacity, unlimited range, high reliability and as well cost effective, a good number of researchers have done tremendous works on improving backhaul networks especially for 4G LTE networks.
(i) Many researchers pointed out that microwave transmission links encourage speed of rollout especially in green field areas, however noted that limited capacity, poor weather effects, limited range as well as scarce spectrum were its biggest drawbacks as a quality backhaul for 4G LTE networks
(ii) Researchers also discovered that not only microwave links that are affected by poor weather, limited range and poor capacity, it was noted however from their numerous researches that all radio wave links and all “line of sight” links like WiMAX, mm-wave, Satellite, etc used as backhaul were also having technical some limitations like limited capacity, low range, etc .
(iii) In furtherance, researchers revealed that in all wired backhaul, only fibre optics has all technical attributes like unlimited capacity, unlimited range, reliability, scalability required for a good backhaul system. Regrettably, point to point fibre optics deployments tend to be a bit expensive and hence, slow speed of rollout.
(iv) Researchers also revealed that Passive Optical Networks PON, which is fibre Optics, reduces the cost of fibre deployments by a reasonable percentage, above 70% depending on the split ratio chosen when used in access network.
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