Fundamentals of PBX Private Networks

The basic elements of a PBX private network are PBX switch nodes and tie trunks. Tie trunks are telecommunications channels that directly connect two PBXs. The first analog transmission tie trunks were known as E&M interface signaling trunks. The term E&M originally comes from the works earth (earth grounding) and magnet (electromagnetic generated tones). The letters E&M have also become known as “ear” and “mouth” or “receive” and “transmit.” E&M supervision signaling (on-hook/off-hook signaling) is used for a variety of networking operations, but the most basic function is to pass address signaling (called party number) between two endpoint PBXs in the private network. The most basic private network consists of two PBXs and at least one E&M tie trunk.

During the mid-1980s, E&M signaling was incorporated into T1 circuit digital private line services. In the early 1990s, ISDN PRI services were the primary trunking services used for private network tie trunk operations. By the end of the decade, IP-based trunk services could support traditional E&M signaling capabilities for inter-PBX networking requirements.

The first large private networks consisting or three or more PBX systems were known as tandem tie trunk networks (TTTNs). Each PBX was assigned a location code, and each station was assigned a private network extension number. TTTNs were based on a nonhierarchical network of tie trunks. A station user at the originating switch would dial the location code of the destination switch and wait to receive another dial tone signal before dialing the desired extension number. The request for dial tone was carried from switch to switch over E&M tie trunks. The private lines also were used to provide the dial tone signal back to the calling station user.

The first smart ETNs were designed in the late 1970s. An ETN was an enhanced version of a TTTN. It was based on a hierarchy of tie trunks and PBX switching nodes, with multiple call routes between network endpoints. A major innovation was an automatic alternate routing (AAR) program that selected a call route based on the number dialed and the most economic (or preferred) route available at the time the call was placed. The tandem switch node in the early ETNs was equipped with routing tables for determining the best route for on-net calls, had the capability of modifying outpulsed digits (for rerouting and directing calls), and could allow or deny call routing privileges to certain station users. Switch nodes in the ETN one level below the tandem switches are known as main switches. Main switches have trunk circuits for local CO switching access, but all network calls must be routed through a directly linked tandem node. Switches working behind the main nodes are known as satellites and tributaries. These switches are equipped with tie trunks to the main node only. They lack local CO trunk services and attendant operator positions. CO and private network access is through the main switch for all off-premises incoming and outgoing calls.

PBXs with ETN options support the important features described in the next sections.

Uniform Dial Plan (UDP)

A UDP provides for a common multidigit (usually four or five) dial plan that can be shared across a group of networked PBXs for interswitch and intraswitch dialing. The UDP includes a network location code, comparable to a CO code, and a multidigit extension number. The UDP extension number is not necessarily the same number as the station directory number. The network location code is often designated as the RNX; the extension number as XXXX. The network location code determines how the call is routed. Whenever a UDP is used to route a call, the number it outputs is in the form of RNX-XXXX. This must be taken into account so that the correct digit deletion or insertion can be specified within the routing pattern, so that the receiving switch receives digits in the format it expects. The UDP software program automatically translates the RNX- XXXX number to the directory number associated with the called station for digit analysis and routing by the destination PBX system.

Automatic Alternate Routing (AAR)

AAR provides alternate routing choices for on-net calls carried over the private network. Based on a routing table designed by the customer, the AAR software automatically selects the most preferred (usually the least expensive) route over the hierarchical tie trunk network. If the first route in the program table is not available, another route may be selected if the station user calling privileges warrant a more costly route based on the individual’s FRL. Large PBX systems can support several hundred different routing patterns (originating and terminating nodal pairs), with more than a dozen different call routes per routing pattern. The AAR routing patterns are shared with the ARS feature.

AAR also provides for digit modification to allow on-net calls to route over nonprivate PSTN trunk facilities when an on-net call route is not available. Calls rerouted off the network require digit modification to translate an RNX-XXXX number to a direct distance dialing number for national or international calling. On-net calls that are routed off-net are then controlled by the ARS feature of the PBX system.

FRLs and Network Class of Service (NCOS)

FRL and NCOS are features that provide for multiple levels of restriction for users of the AAR and ARS features. FRLs and NCOS allow a certain call type to a specific station user and deny the call to another user. For example, some station users may be allowed to place calls only to private network nodes and not off-net long distance toll calls. A call type that is highly restrictive is international direct distance dialing. FRLs and NCOS are transparent to the station user and are assigned and programmed by the system administrator. Each system facility, station and trunk, is assigned an FRL. Whether or not a call is allowed is based on the originating caller’s FRL and the availability of idle trunk circuits within an assigned trunk group required to route the call. If the station user’s FRL is equal to or greater than the trunk group FRL, calls may be routed using a trunk circuit in the trunk group. If the station user’s FRL is less than the trunk group FRL, the call is denied. Most intermediate/large PBX systems support at least eight FRLs (0 to 7).

The NCOS level of the originating station user determines which call routes can be used to complete the call for a specific routing pattern while the call route is being established across the network. NCOS is also known as traveling class mark (TCM) because the assigned restriction level of the originating station user is embedded within the voice communications signal as the call is routed across the network. If trunk facilities at one tandem switch node are busy and an alternate route (more expensive) is available, the NCOS/TCM level determines whether or not the station user is allowed access to a different call route. A call can be blocked or denied anywhere in the network. Another private network feature, advanced routing (look ahead routing) can be used to avoid possible blocked calls at tandem switch nodes.

Automatic Circuit Assurance (ACA)

ACA is a feature that helps customers identify trunk errors and problems, particularly for private tie trunk circuits. The PBX system maintains a record of individual trunk performance relative to short and long holding-time calls. Holding time is the period from trunk access to trunk release. A system administrator defines short and long holding-time limits. When a possible trunk circuit failure is detected, the system automatically initiates a referral call to an attendant, station user with a display-based voice terminal, or system manager with a CRT monitor. Based on system measurements of holding times per call, referral calls may be placed to attendant, station, or system manager positions. The display information identifies the call as an ACA call, identifies the trunk group access code and the trunk group member number, and shows the reason for the referral (short or long holding time). The ACA feature provides better telecommunications service through the early detection of faulty trunk circuits and possibly reduces out-of-service time.

Virtual Private Networks (VPNs)

A VPN is based on a custom switched telecommunications service tariffed by an interexchange carrier, which permits a customer to establish a communications path between two stations using a UDP. PBX systems are linked to the carrier’s CO facilities using private tie trunks or through dial-up facilities using special access codes. The network facili- ties are shared as part of the PSTN. The key benefit of a VPN is a significant reduction in private line services between networked PBX systems. The first voice communications VPN service was AT&T’s SDN. SDN was designed to expand the scope of private network solutions to customers who could not justify dedicated private line services in support of their private networking requirements. Other similar services were soon available from AT&T’s competitors. VPN telecommunications services, from exchange carriers such as AT&T, MCI Worldcom, and Sprint, simplify private networking applications for business customers because:

1. The backbone private network carrier facilities are maintained and managed by the service provider.

2. The UDP and AAR/ARS databases are centralized in the service provider service control points.

3. Less training and specialized communications equipment is required to design, implement, operate, and maintain a private network.

4. Call accounting records and billing information are provided by the service provider for all on-net and off-net calls.

Most customers with private network requirements use a combination of traditional ETN tie trunk links (analog, mostly digital) and virtual network services for their on-net and off-net calling requirements. Private networks using VPN services can enjoy the following benefits:

• Flexible system and station user port capacity

• Integrated voice/data communications

• System compatibility across different PBX platforms

• Cost effective usage- and distance-sensitive pricing

• Porting of private network numbering plans

• National/international service transparency

• PSTN reliability and QoS standards

• Customized report options

• Centralized network management capabilities

• Secure access via screening provisions and enforced use of authorization codes

There are several options available for PBX access to the carrier network, including local exchange service access and special services access. Local exchange access is usually reserved for very low-traffic volume locations that cannot justify a special access service. Local exchange access is provided through a switched connection from a local exchange carrier’s equal access end office. A customer using this access method may presubscribe to the VPN carrier code, or individual station users can dial the carrier code.

Special access arrangements provide direct access to the VPN provider network facilities:

• PSTN analog trunk circuit facilities

• PSTN digital trunk circuit facilities

• Customer-provided access (local bypass)

Digital trunk and customer-provided access may support multiple exchange carrier services, not only VPN service.

The VPN service functions similarly to private line ETNs for on- and off-net calls. Basic call processing features include:

• Seven-digit dialing (national and international calls)

• Advanced numbering plan (flexible multidigit numbering plans)

• Private network interface to ETNs

• Flexible routing when conditions warrant

• Network intercept announcements for call completion procedures

• Network remote access from off-net locations

VPN call management features include:

• Authorization codes

• Originating call screening—Grouping of callers with same calling privileges

• Feature screening—Specifies calling privileges for each screened caller group

• Access line grouping call screening—Call restriction by specific off-net telephone numbers

• Partitioned database management—Partitioning of VPN locations into multiple autonomous network groups with direct distance dialing and private networks

Exchange carrier VPNs originated as a voice-focused service. During the past few years VPN services have evolved to support voice and data communications networking applications. Most of today’s VPN offerings are focused on packet switched services to support customer WAN data communications requirements, but can be used for voice networking needs. As VPN has evolved, PBX networking has evolved with the emergence of ToIP technology and the trend toward VoIP trunking.