Public Networking: Automatic Route Selection (ARS)

All station user calls, direct distance dialed and private network, are routed to trunk groups that have access to exchange carrier facilities terminating in a central office. The many types of port circuit interfaces and trunk facilities. The software feature controlling access to the trunk groups is ARS. PBX ARS features and functions support control and routing of calls over public network carrier facilities and across private networks. ARS is used to select among various types of trunks:

1. Local CO, analog or digital

2. Foreign exchange (FX)

3. Wide area telecommunications service (WATS)

4. Tie line (private line); analog and/or digital

5. ISDN PRI service circuits

When the ARS software program routes calls to public network carrier facilities, it is referred to as off-net routing; when calls are routed to private network carrier facilities, it is referred to as on-net routing. Off-net calls are based on the public network dialing format and terminate in the public network. On-net calls are based on a private network dialing format and usually terminate within the private network, although some calls may be routed off the private network and onto public network facilities, if the called station is not configured as part of the network. The private networking feature used to describe the latter routing option is known as tail end hop off (TEHO). On-net routing programs support tie-trunk protocols, proprietary private network protocols, and industry-standard private network protocols.

The ARS feature is activated when a station user dials the access code for an off-premises call requiring a trunk circuit, followed by the telephone number to be called. Public network calls in North America are usually dialed beginning with the digit 9, which alerts the common control complex that an outside line (trunk) is required to place the call. Calls placed over traditional private tandem networks are usually dialed with a customer-selected digit, such as 8, to distinguish the call from public network calls. Customers with traditional private tandem networks create a unique numbering plan for on-net calls. An intelligent private network call is placed by dialing a station directory number within the network that matches the local directory number of the dialed station. Intelligent private networks do not require a separate private network numbering plan because the entire network of PBXs operates as a single homogeneous network for most system features and functions, including the uniform dialing plan. Small network configurations may be supported using a four- or five-digit numbering plan; larger networks may require more dialed digits for on-net calls because a limited digit dialing plan cannot support the number of station users.

ARS routing table rules determine whether the call is routed off- or on-net. For off-net calls the ARS program determines which trunk group the call is routed to for network access. PBX system administrators rank trunk groups from lowest to highest cost to ensure that calls are sent over the least costly route available to the caller, based on customer’s class of service or restriction level. Public network route selection is based on data in the dial plan databases. The ARS software analyzes and compares the dialed digits with the digit string patterns in the ARS dial plan. If there is no database match, the call is blocked. Blocked calls may be the result of call restriction levels for individual callers. It is common today for system administrators to block many outgoing 900 calls, or direct-dialed international calls, except for select station users. Restriction features may limit an individual station user’s dial capabilities to internal calls or calls to very select off-net locations, such as local exchange calls.

If there is a database match, the system may request an account code before continuing, or the ARS feature can immediately define the call route and determine which digits should be sent over the network while the call processing software seizes an available trunk according to the station user’s network class of service level. If all trunk circuits are busy for the lowest-cost trunk group, the call may be routed to the next highest-level trunk group based on call costs, or the call may be queued until a trunk circuit is available in the originally selected trunk group.

The trunk circuit seized by the PBX system is determined by analysis of the ARS routing table and associated trunk tariff database. The ARS routing table is made of routing patterns that map to call routes for the dialed CO location code. CO codes are defined by specific country and city locations. For each dialed CO code, there may be one or more routing patterns, and each routing pattern may have one or more call routes. More than one pattern of dialed digits can translate to the same routing pattern. The facility restrictions level (FRL) feature determines access to a select call route within a routing pattern. For any particular call route, multiple trunk groups may be used to handle the call, if the customer subscribes to more than one exchange carrier service. The lowest-cost route is usually programmed as the preferred route if the trunk circuits are available.

The most common ARS feature capabilities are:

1. Area code/office code restriction (toll restriction)

2. Alternate route selection (route advance)

3. Time-of-day routing

4. Day-of-week routing

5. Trunk queuing

6. Digit analysis and manipulation

7. Call screening