Achieving goal performance parameters with ACM

The benefits of ACM utilization in microwave communications have been already discussed in my previous post. It was mentioned that this technique would allow prioritizing the traffic and making high-priority data more resistant to varying signal propagation conditions. The means of achieving the desirable link performance parameters will be covered in present article.

The initial phase of link planning procedure starts with defining the goal parameters such as fade margin and annual availability. Then the path budget calculation is performed to evaluate the expected link behavior for each of the modulation states and capacity values.

The capacity is usually defined by SLA (service-level agreement) negotiation between a service provider and a client. The goal performance parameters can differ depending on the nature of provided service. Some mission-critical applications such as telecom core network or broadcasting STL (studio-transmitter link) can demand up to 35dB of fade margin and 99.999% of availability. In turn, for WISP operators these requirements could be relaxed.

In most cases, the requirements are applied to modulation state providing the Committed Information Rate (CIR) which is guaranteed by SLA. Everything that is above can be considered as “extra” or best effort capacity. Having QoS as the prioritizing mechanism it becomes possible to define which type of traffic will be transmitted as critical with CIR throughput and which is going to be considered as “extra” (please refer to fig.1).

Given that ACM algorithm is going to be used, it becomes necessary to calculate link performance for all possible modulation states. Thus fade margin/availability requirements should be defined not only for CIR but also for the maximum ACM state that could be achieved with a certain degree of probability in perfect propagation conditions.

Fig. 1. Availability for different levels of traffic priority

The table containing SAF Tehnika recommendations regarding ACM calculations has been provided in the previous post. These recommendations should be perceived as the goal performance parameters for highest modulation state that is for maximum throughput a customer is planning to achieve. CIR throughput, in turn, should be obtained with higher confidence. Thus stricter requirements should be applied. The default SAF Tehnika recommendations for SLA-guaranteed bandwidth are provided in table below:

There are two main groups of customers depending on the application:

• General network group consisting of spur links, or last mile access links. This category mainly includes ISP customers.
• Mission critical group containing broadcasting STL, telecom core network, public safety and other mission-critical applications.

Running link budget calculation for ACM in professional link planning tool, complete table of results is obtained with fade margin and availability predicted for each state. Please refer to fig.2 for an example of such table taken from Pathloss 5.1 software. Core parameters: fade margin and annual availability are highlighted in red.

Fig.2. Results of ACM link budget calculation in Pathloss 5.1 software.

Please note that ACM calculation can provide results for complete set of modulations also for those which do not satisfy any of the above-mentioned requirements. Additional capacity achieved in these states is the bonus customer gets without taking the risk as hitless ACM switching is used.

Summarizing all written above it is possible to conclude that customer`s requirements should be always used as main criteria for link planner. The default recommended parameters (if customer fully relies on link designers) have to be applied taking into account customer`s group as well as some path specifics (e.g. higher fade margin may be necessary if deep signal fading is expected). It is important to separately define fade margin and availability requirements for all levels of throughput starting with CIR and then for the extra bandwidth available.