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National Fiber Optic Backbone Design Considerations

04 April, 2014

Source: ICT Africa

 
Jabulani Dhliwayo

The unprecedented demand for communication services in Africa is putting a tremendous amount of strain on national backbone networks. It is for this reason that Africa is experiencing a stampede in the development of national backbone networks in almost all countries, from Cape to Cairo. So far, more than 30Mkm of fiber strands have been installed and the installed base is growing rapidly by the day. But it is one thing to saturate the continent with optical fiber and quite another to deploy fiber optic networks that are meticulously designed to optimize quality of service (QoS) and long term cost efficiency.

In a number of cases, African network developers have relied on equipment vendors to support the design of their networks. Unfortunately, the objectives of equipment vendors and that of network developers/owners are not always aligned. It is imperative that every fiber network operator has a team of network engineers with the necessary network design knowledge to influence how their networks are developed. ICT Africa will conduct detailed discussions on network design considerations with African network engineers and managers during our fiber optic communications training workshops. A brief summary of some of the network design aspects to be discussed is summarized in this article.

Network Route
The first consideration in implementing a national backbone network is the network route, the path along which the cable network will be deployed. A route survey should be planned to identify any aspects of the route that will impact network deployment, such as highways, bridges, hills and rocks. This will help identify all the rights of way (ROW) required and lead to an accurate estimate of the trenching cost if the cable is to be deployed underground. A number of networks in Africa end up costing substantially higher than initially budgeted for because of oversight on trenching requirements, such as the high cost of destroying rocks in certain parts of the route.

An important consideration is whether the ROW will be granted to build the network along the route. All authorities that should grant the ROW along the route should be identified and approached. In some countries, there may be more than one government department claiming the oversight of ICT infrastructure. All such departments should be approached if unnecessary delays are to be avoided. Some fiber optic projects in Africa have been delayed by up to ten years, primarily because the granting of ROW was delayed. It is also important that government departments talk to each other to ensure that no other departments are planning projects that could lead to the destruction of cable in future. It is ironic that one government that purports to advocate for the rapid development of ICTs and fines its operators heavily for poor QoS is the same government that is responsible for digging out and destroying operator fiber cables during road construction. This is obviously a result of poor coordination among all involved.

When mapping the route, engineers should identify where traffic will be dropped and/or added to the network at commissioning and in future. This will ultimately determine where equipment such as add/drop multiplexers, amplifiers and regenerators will be housed.

The availability of power along the route can be a major constraint in the development backbone networks in Africa. If the national grid does not supply power where equipment will be housed, alternative energy sources such as solar, diesel or wind power should be planned for. Routes may include long stretches of uninhabited jungles or deserts such as in the Democratic Republic of Congo and Algeria, respectively. Careful consideration should be made on where equipment sites should be built in these environments and whether they can be accessed for maintenance, especially during severe weather conditions such as thunderstorms and sand storms. Such consideration will also have an impact on the transmission equipment, amplifiers and fiber optic types to be used.

Redundancy should be considered during the route planning stage of the network. Many users in Africa experience network down time whenever there is a cable cut along a network route. Network designs should provide for an alternative path to temporarily route traffic when there is a cable cut.

Network traffic consideration
Network engineers and managers need a way to properly estimate near term and long term network capacity. The growth in demand for mobile services in Africa is a good indicator of the potential for significant changes in network traffic patterns and demand over time. There are traffic theories available for network designers to make sound assumptions about future bandwidth demand based on past traffic patterns. The traffic engineering process should identify current and future needs of potential customers to be serviced - such as mobile subscribers, government departments, educational institutions and businesses.

Choosing Transmission Equipment
A thorough understanding of the traffic demand and patterns and type of customers to be serviced will help the design engineer to properly determine the type and minimum specifications of the equipment to be used. Some criteria for selecting transmission equipment include the following:

Bandwidth requirements
Equipment exists from basic SONET/SDH equipment capable of transmitting as low as STM1 (155Mbps) to high data rate OTN systems capable of transmitting at 100Gbps per wavelength channel. Designers are encouraged to consider modular transmission systems capable of multiple services, including carrier Ethernet, SONET/SDH and OTN with line cards of varying data rates – from 1Gbps to 100Gbps or higher. With such systems, the service provider can initially operate at low data rates and upgrade to higher data rates in future by simply purchasing higher data rate line cards without the need to reinvest in a new platform.

Span length
In 10Gbps systems, inline erbium doped fiber amplifiers (EDFAs) are necessary to boost the system power after every 80-100km distances (or shorter distances in higher data rate systems). The span is the distance between the inline amplifiers. For moderately longer spans, say 150km, a combination of low attenuation fiber and booster amplifiers may be necessary to overcome such spans. Some systems come equipped with booster amplifiers to overcome such distances. Distributed Raman amplifiers, deployed end to end, may also be used to extend the span length.

Network reach
As signals propagate through erbium doped fiber amplifiers, they accumulate random noise in the form of amplified spontaneous emission (ASE). After a number of amplifiers or spans, accumulated ASE becomes too high and the optical signal to noise ratio (OSNR) diminishes requiring a signal regenerator or repeater to reshape and retransmit the signal. Regeneration results in high capital expenses for the equipment as well as operational expenses for installation, space, and power at the regeneration location. A competent design engineer should be able to review the combination of low attenuation fiber and relevant systems to extend the potential system reach. The use of regenerators should be included as a last resort. If used, the number of regenerators should be kept to a minimum.

Fiber optic selection
Attenuation, chromatic dispersion, polarization mode dispersion (PMD) and nonlinear impairments are the key attributes to consider in the design of long haul networks. High dispersion causes signal pulses to overlap after propagating over certain distances. Dispersion limits the distance a signal can propagate before a dispersion compensating module is required. Attenuation reduces system power and the distance signals can propagate before an inline amplifier is used to boost power. PMD leads to signal distortion in high data rates of 10Gbps and beyond. PMD is difficult to compensate for and the best way to deal with it is to select fiber with low PMD. Nonlinear impairments such as four-wave-mixing degrade OSNR in DWDM systems and can be mitigated by the use of large effective area fiber with small amounts of dispersion.

In some network designs, pairs of EDFAs or dual stage amplifiers are used with a dispersion compensating fiber module inserted in the middle. The second amplifier boosts the signal after it goes through the dispersion compensating module whose attenuation is high. The use of dual stage amplifiers leads to an increase in ASE and lower OSNR and ultimately reduces the system reach.

Two main fiber types used in terrestrial long haul networks are standard single mode fiber which complies with the ITU-G.652 standard and non-zero dispersion shifted (NZDS) fiber (G.655 or G.656 compliant fiber). G.652 fiber typically has lower attenuation, with some manufacturers specifying attenuation as low as 0.17dB/km. On the other hand, G.655/6 fiber has low dispersion at the important 1550nm transmission window, ~6ps/(nm.km) compared with ~17ps/(nm.km) for G.652 fiber.

NZDS fiber can boost OSNR by enabling the use of fewer dispersion compensators and dual stage amplifiers and therefore extend system reach before the need for regeneration. On the other hand, low attenuation fiber can improve system OSNR by lowering the overall network attenuation. For higher transmission at 40Gbps and 100Gbps with coherent systems, chromatic dispersion is compensated for electronically in the transmission systems. The G.652 fiber will become the fiber of choice for these systems.

The network designer should carefully consider the network configuration, current and future bandwidth requirements before selecting a fiber. Many designers deploy hybrid cable, with both standard single mode fiber and NZDS fiber so that they have the flexibility to switch from one fiber to another when requirements change. If the network is also used for access, such as connecting buildings at the 1310nm window, G.652 can be used for the access application while NZDSF is used for long reach at 1550nm. NZDSF is not optimized for transmission at 1310nm where the attenuation is not specified and could be high.

Once the type of fiber is selected, the designer has to be mindful of factors that may change the performance of some fibers over time. This includes phenomenon such as post hydrogen aging attenuation, post heat aging attenuation, temperature dependency of attenuation and fiber reliability. Your fiber cable manufacturer should guarantee that the fiber performance will not degrade after installation due to environmental factors. A company in Kenya has reported fiber that goes dark at about midday in hot weather and operates normally at other times. This is likely due to high temperature dependence of attenuation.

Fiber optic cable
The choice of cable depends on the deployment scenario. For deployment over power lines, the network designer should choose from optical ground wire cable (OPGW), all-dielectric self-supporting
cable (ADSS) or skywrap solution. The designer should be well informed about cable installation conditions that can degrade fiber performance such as increased PMD due to wind or current induced cable oscillations or galloping and sage induced stress.

In underground installations mini or micro cable for installation in ducts are becoming a trend. The designer should select cable from a manufacturer who can guarantee minimal cable induced attenuation due to micro-bending and other factors.

Summary
The list of key considerations for terrestrial long haul network design is not exhaustive but presents some of the most important aspects of network design. Any operator or fiber infrastructure company should have engineers on their employ who have a thorough grasp of these issues. We encourage questions or comments for further discussion either on the ICT Africa site or during our workshops.


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