AT&T 7400 series, 7500 series, 7100 series, 2500 series, 7200 series, 7300 series manual ETN Trunks

5-2ELECTRONIC TANDEM NETWORK (ETN) THROUGH THE ETN AND PNA PACKAGES

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The characteristics of a hierarchical network (see figure 5-1)are:

∙Each tandem switch has an assigned level (upper and lower).

∙Each lower level switch connects to an upper level switch.

∙Upper level switches are interconnected.

∙A routing plan that prevents circular call routing.

The characteristics of a symmetrical network are:

∙Tandem switches are of an equal level.

∙All routes (trunk groups) are high usage.

∙Circular call routing is prevented by limited tandem-to-tandem routing to the first several routes.

In figure 5-1,the hierarchical ranking of switches is shown. The ranking allows an orderly routing of on- net access calls. The switching portion of the network is represented in figure 5-1by the upper-level and lower-level tandem switches connected by intermachine tie trunks. The tandem switches can accept voice/data calls from any connected point and pass the calls to another connected point. The upper-level switches handle heavy call volumes and the lower-level switches handle lighter volumes. Lower-level switches can be administered to overflow to upper-level switches.

ETN TRUNKS

The trunks that connect the switches within the private network are named according to function. That is, names like intermachine (intertandem), access, bypass access, and off-net denote functional (type of routing) rather than hardware differences. (See the Trunking section of chapter 1 and administrative details.) As its name implies, an intermachine tie trunk interconnects two tandem switches. It can be 1- way incoming, 1-way outgoing, or 2-way. Intermachine tie trunks are further classified as primary high- usage, intermediate high-usage, or final, depending on how blocked calls overflow from one trunk group to another. How calls overflow is administered by designating routing patterns and trunk group preferences in AAR.

The routing characteristics for intermachine tie trunks are as follows:

∙Primary high-usage (PHU) intermachine tie trunks — These trunks serve first-choice traffic only. That is, they do not receive any overflow traffic, but calls that they cannot handle may overflow to intermediate or final trunk groups.

∙Intermediate high-usage (IHU) intermachine tie trunks — These trunks serve first-choice traffic and receive overflow traffic from primary high-usage trunk groups. They direct any overflow traffic to final trunk groups.

∙Final intermachine tie trunks — These trunks accept first-choice traffic and also receive overflow traffic from primary and intermediate high-usage trunk groups. They do not redirect calls that they cannot handle to any other trunk group. Depending on how the trunk is administered, when a call is blocked, the caller receives busy or intercept tone or the call is placed in queue.

Refer to figure 5-2for the types of overflow routing.