Trunk Interfaces
These are in the physical form of specialized circuit boards which serve to interface the PBX switch to trunks that in turn, connect it to other switches. Trunks can be one-way outgoing, one-way incoming or two-way (combination trunks). Often a PBX will employ all three variations in order to serve various, specific applications, to maximize system performance, and to ensure a minimum acceptable level of both incoming and outgoing network access. ISDN (Integrated Services Digital Network) offers dynamic call direction on a channel-by-channel basis that is much more flexible therefore, more cost-effective. ISDN will be discussed in detail in Chapter 8.
Trunks may be single-channel or multichannel. High-capacity, multichannel trunks support multiple conversations. Examples include E/T-carrier and ISDN Primary Rate Interface (PRI), also known as Primary Rate Access (PRA). Trunks of the same directional type that serve the same purpose are organized into trunk groups. The PBX will hunt for available trunks within a trunk group, based on a pre-determined, user-definable hunt sequence. Trunk interfaces provide access to the following specific types of trunks:
- Central Office Exchange
- trunks connect the PBX to the LEC exchange providing access to the local calling area and all other areas served by the local telephone company. They provide access to IXCs for long distance calling where the user organization has not made special arrangements for direct IXC access.
- Interexchange
- trunks provide direct access to an IXC, bypassing the LEC. Such trunks generally are intended only for long distance network access to geographic areas outside the LECs local calling area.
- Foreign Exchange (FX or FEX)
- trunks connect directly to a foreign central office exchange. They are used for cost-effective access to and from a distant geographic area where there exists a high concentration of traffic. For example, a user organization with a high concentration of traffic between its Dallas, Texas offices and the area immediately surrounding Denton, Texas might choose to avoid long distance calling charges by installing a FX trunk. That trunk is priced on a flat-rate basis, regardless of the level of outgoing traffic. In this fashion, the user organization avoids long distance charges to that area. Callers from the Denton area dial a local Denton telephone number that is associated with that FX trunk, similarly avoiding long distance. As a side benefit, the organization in Dallas creates the illusion of having established a local presence in Denton.
- Direct Inward Dialing (DID)
- trunks are for incoming traffic only. A special signaling and control arrangement exists between the PBX and CO. The CO passes the dialed station number of the destination address across the trunk to the PBX. With that information, an intelligent PBX with the proper software can route the call directly to the station, without the intervention of an attendant. DID numbers are rented from the service provider in groups (e.g., 50, 100, and 250). Each station is assigned a DID number that roughly corresponds to the internal station number. As a call is placed to a DID number, the CO recognizes it, and connects the call over a special DID trunk, passing the DID number in advance of the call. The PBX recognizes the DID number and processes the call automatically.
- TIE
- trunks connect directly to remote PBXs within the user networkin effect tying them together. Through the use of ARS/LCR software, the system routes calls between offices over a leased-line TIE trunk, avoiding toll charges in the process. Coordinated, abbreviated dialing plans, programmed within each PBX, cause the various PBX systems to interact on a networked basis, appearing to the user as though they were a single system. TIE trunks are sometimes referred to as nailed-up circuits, because they once were literally nailed-up on the central office wall and specially tagged in order to distinguish them from typical shared circuits.
- WATS (Wide Area Telecommunications Service)
- trunks allow for discounted long distance access in defined service areas.
- INWATS
- trunks serve INcoming WATS calls (1-800 and 1-888 in the United States and 0-800 and 0-500 in other countries). The costs are incurred by the called party, at a discounted cost per minute.
- Direct Inward System Access (DISA)
- trunks allow access (generally toll-free) to the PBX system and, subsequently, to connected resources (e-mail servers, voice processors, computer systems, and outgoing toll trunks) through the entry of authorization codes. While DID trunks are useful to authorized users, they often are the target of hackers, who seek access to the same resources. Therefore, the integrity of the organization, its toll network, various networked systems, and the resident databases can be compromised. DISA trunks should be avoided, and, where in place, they should be disconnected unless extreme security measures are in place.
Station Interfaces
Station interfaces are in the form of printed circuit boards which can support multiple stations of the same general type through multiple ports on a single interface card. For instance, analog line cards with analog ports support analog sets, in which case the signal is digitized within the PBX system. Digital line cards support digital terminal equipment, such as digital telephones and computer workstations.
Terminal Equipment
Because PBXs are designed primarily to support voice traffic, the user terminal equipment generally is in the form of a telephone set. Data terminals are often supported, as well.
Telephone Sets
PBXs support both dial and tone, single-line sets of a generic nature. They also support highly-functional programmable electronic sets that offer access to a number of system features. Although such sets are proprietary to the system manufacturer and are considerably more expensive, they let the station user invoke system features through depressing a single designated button. Proprietary electronic sets generally are digital, with the voice signal being digitized at the set, rather than at the station interface card in the PBX cabinet.
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