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With this “Top Down” approach the main traffic flows are accommodated into a route network, which is independent of the existing sectorisation.  Subsequent and suitable sectorisation must be developed to support the network, including the accommodation of all relevant traffic flows. A consequence of the above approach will be a re-organisation of sectors, involving at sector boundaries a delegation of ATS where necessary.  At this initial proposal development stage the network requirements take precedence over sectorisation.
However, it must be recognised that because of the two-way relationship it may not be possible to develop an operationally viable and efficient sectorisation.  As pointed out above, sector capacity is the crucial element in the whole ATM system.  Route structure, although one of the main factors, is only one of the elements which determine sector capacity. Therefore, in those instances where the lack of adequate sector capacity may be a significant constraint on the ATM system, and whenever a proposed improvement in route alignment leads to a complication of the sector’s organisation, resulting in an unacceptable reduction in capacity, then both the route alignment and sector configuration should be re-examined.  Because of this two-way dependency between airspace sectorisation and route network, it is essential that both are addressed immediately after the initial proposal development stage and throughout the planning process this relationship is always taken into consideration.
Summary of method/rationale:
.Step
 1: Route Network initial proposal

.Step
 2: Examination of sectorisation viability

.Step
 3: Harmonisation of outcome of step 1 and step 2

Edition: 2.0 Released Issue Annex 4B – Page 4-19
1.3 Airspace Structure:  Options to enhance ATM capacity at the sector level
Air traffic control is currently based on sector structures. Sectorisation is the means of subdividing the totality of control tasks into manageable portions, at which throughput and capacity can be quantified.  ECAC airspace has currently in excess of 400 sectors distributed in more than 50 ACCs. Capacity is a theoretical indicator of traffic loads, which can safely be handled by a sector team, rather than the loading they are currently subject to.
The main constraints on ATM capacity are airspace limitations and controller workload. The classic method to overcome these constraints is to provide more sectors.  By either resizing or providing additional sectors, one can reduce the airspace volume, the number of routes/crossing points (conflicts) and the number of aircraft on the frequency at any time. This results in a reduction of workload and a corresponding increase in capacity, while maintaining at the same time a balanced co-ordination workload (e.g. through the use of improved/ automated co-ordination procedures).
The sub-division of sectors is a finite strategy and a point is reached, when the benefit of further reduction is outweighed by other factors (especially in the core area).  Furthermore, the increase of capacity is not proportional to the number of sectors available (law of diminishing returns).

Therefore, the efforts to handle traffic growth have to be focused on a more efficient method, which is to increase sector productivity and consequently capacity. This can be achieved by reducing the complexity of the airspace structure, resulting not only in a more balanced distribution of traffic within different sectors, but also in a redistribution of workload. The redistribution should be made on a sector to sector balanced basis.
Note: Whichever method is used will entail a cost in either human and technical resources or non-optimum route/flight profiles
Page 4-20 – Annex 4B Released Issue Edition: 2.0

1.3.1 Option 1: Additional Sectors
The provision of additional sectors is the classic method of increasing capacity. Although scope still exists for this in most of the ECAC airspace outside the core area or in the upper layers (vertical split), this is not always the most efficient method.  Furthermore, in the core area the introduction of additional sectors is not always possible because:
　
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