According to the ITU-R, the planning objectives are defined for the radio-relay link, ie the radio circuit, and not for the individual hop. In order to compare the predicted performance and unavailability with the planning objectives, the hops have to be combined to radio links.
The link consists of a number of tandem connected hops. The minimum number of hops in a link is 1 hop.


Radio-relay link layout
In principal, the link layout is a copy of the network layout. On that layout, the hops included in the link have to be set by clicking the hop (ie the interconnection line between two sites) - see the below figure. The link shown below consists of 4 hops connecting Rancher St with City.

This operation has to be carried out for each link, ie each link has its own layout chart.

The program keeps count of that the link is build up of only tandem-connected hops. Also cable lines may be included in a link.


Planning objectives - ITU error performance and unavailability definitions
The operator can either select its own planning objectives or the ITU-R recommended ones. Until 1999, the error performance was defined by ITU Recommendation G.821 and the unavailability by Rec. F.557. Due to unsatisfying correspondence between definition and measured error performance, the error performance was redefined in Rec. G.826, involving also a redefinition of the unavailability in Rec. G.827.

Since the end of 1999, methods are available to predict the error performance with reference to G.826.
However, as a great number of networks have been designed applying G.821 and F.557, operators may wish to utilize these recommendations for some longer time, eg when existing networks shall be extended.

RLTool feature:

• RLTool allows to run the performance and unavailability calculations with either own planning objectives, or to select ITU-R planning objectives according to G.821 and F.557, or G.826 and G.827.

ITU Rec. G.821 and F.557
Three performance-planning objectives are defined:

• the severely errored seconds - SES
• the degraded minutes - DM
• the residual bit-error ratio - RBER

They refer to the average worst month.
The planning objective for the unavailability refers to the average year.
The numerical values for these planning objectives are dependent on whether the link belongs to

• the high grade portion of the network
• the medium grade portion, or to
• the local grade portion.

As this affiliation can differ between different links, it has to be selected individually for each link.
The planning objectives are expressed in percentage of time, during which the planning objectives may be exceeded. Consequently, the predicted figures are also expressed in %.


ITU Rec. G.826 and G.827
Again three performance-planning objectives are defined:

• the severely errored seconds - SES
• the errored seconds - ES
• the background block errors - BBE

They, again, refer to the average worst month.
The planning objective for the unavailability refers to the average year.
The numerical values for these planning objectives are dependent on whether the link belongs to

• the international portion of the network
• the long-haul portion of the national network
• the short-haul portion of the national network, or to
• the access network.

As this affiliation can differ between different links, it has to be selected individually for each link.
The planning objectives are expressed as the ratio between the number of errored bits related to the number of bits conveyed during the observation period. Consequently, the predicted figures are also expressed as a ratio.

RLTool feature:

• RLTool has all the numerical values for the ITU-R planning objectives in its internal database. By activating one of the alternatives stated above, the corresponding data will apply for the link concerned.
  Only for the custom-defined planning objectives, the numerical figures have to be inserted by the operator.
  

Performance evaluation
RLTool combines the performance data from the individual hops to the overall performance of the circuit and processes the data in accordance with the selected ITU-R rec-ommendations. In case the performance for the two directions of a hop are different, the worse performance of the two will be considered.

If a cable line is included in the circuit, its performance is also included in the overall performance.
The results of the performance evaluation are summarised in a link report, one for each link.


Unavailability evaluation
The unavailability is defined for the average year. Only three unavailability events are predictable:

• unavailability due to rain
• unavailability due to multipath propagation
• hardware faults.

However, all causes for unavailability have  to be considered, as long as they are unintentional. To consider unpredictable events, such as human factor etc, a part of the unavailability objective should be reserved for these events.

RLTool feature:

• RLTool allows to reserve a selectable margin of the unavailability objective for unpredictable events. The default figure is 20 %.

Unavailability due to rain
The calculation is carried out acc. to section 2.6.5. The figures from the individual hops are added together and shown in the link report.


Unavailability due to multipath propagation
The calculation is carried out acc. to section 2.6.4. The figures from the individual hops are added together and shown in the link report.


Hardware unavailability
To predict this figure, the following data have to be known:

• the system configuration
• failure rate (failures per time unit) - the sum of the failure rates for the individual tandem connected units in the system
• mean-time-to-repair (MTTR) for the single structure, in the same time unit as the failure rate

and additionally for duplicated structures:

• failure rate (failures per time unit) for the (non-duplicated) splitting and switching device(s) proper
• mean-time-to-repair for the (non-duplicated) splitting and switching device(s) proper, in the same time unit as the failure rate

It should be noted, that the MTTR for the single structure differs from that for the splitting and switching devices.
The hardware unavailability includes also that from cable lines.

Comments
It should be observed, that for the hardware unavailability both the figures for the go and the return direction have to be added, while the rain and multipath unavailability counts only once, as these affect both directions simultaneously. This is taken care of by RLTool.

RLTool feature:

• The equipment failure rates can be obtained from a database or given by the operator. For the MTTR figures, editable default values are stated.