Teletraffic engineering/Teletraffic grading

Summary
Grading is the limiting of the number of outlets that in incoming call (into an exchange) can be switched to. Grading only occurs in exchanges setup for Limited Availability.

Background
In order to understand the concept of Grading some background is required. Grading is utilized in PSTNPSTN] networks, which are circuit-switched networks. For large PSTN networks (exchanges) the question of availability becomes an issue.

Availability in the context of TeleTraffic Engineering is the ability of an exchange to switch incoming calls (or lines) onto outgoing lines. This lead to the notion of all outgoing lines being available for switching (Full Availability) or a limit on which in coming lines may be switched to which outgoing lines (Limited Availability). Exchanges are limited due to older electromechanical switches not being able to switch large numbers of lines, greatly increasing the expense of making large exchanges fully available.

Definition
Grading is the technique of limiting the available outgoing lines that the incoming lines can switch to in an exchange. This technique was once a major statistical endeavor undertaken for almost all exchanges; advancements in technology however have started to make the use of grading obsolete, due to modern technology allowing for all exchanges to be fully available. The simple examples following demonstrate the concept of grading.

Example 1:
Figure 1 shows a simplified exchange in a fully available configuration. Note that incoming lines A-D can connect to all four of the outgoing lines 1-4.

Figure 1

In Figure 2 however this is not the case; only three of the four outgoing lines is available to each of the four incoming lines. For example, incoming line A can access outgoing lines 1-3, but if those lines are occupied by incoming lines B-

Example 2:
An alternative method a looking at the concept of grading is shown in Figure 3 below. In the example there are 8 inlets and 15 outlets. The circuits are divided into grading groups, made up of 8 individuals, 4 pairs, 2, quads and 1 common. It should be note that multiple calls can come in on a particular inlet, but due to the switching of the calls, only one outlet per call may be used. If an incoming call on one of the 8 inlets comes in and the individual is occupied, then the call is passed to the right (to the specified pair) unless that outlet is itself occupied. Incoming calls can be passed to the right 4 times (including the original individual), resulting in a grading of 4.