Centrex
With all of the potential problems associated with telephone lines, a customer cannot afford to run their own business, and also be in the telephone business. When buying another a PBX system, for example, that's exactly what they're getting into. They own the hardware and they're responsible for keeping it running. PBX customer's have to deal with the headaches of managing and maintaining a PBX. The Centrex is basically an "offsite PBX" managed by a vendor. The difference is that a Centrex is an entire suite of hardware and software that is capable of managing a large number of customer's. Therefore, functionally, a Centrex is like a room full of PBX units.
The software that runs the Centrex System resides in a Central Office or CO.
Since customers aren't locked into expensive equipment they can take advantage of new technologies and features as they become available.
Centrex features match or improve on PBX features for most customer needs.
Secondly, the vendor takes care of all the maintenance and repairs for a Centrex
System, usually before the customer notices any difficulty. When PBX systems break down customers are responsible for their own maintenance.
Centrex Features
Centrex Architecture Components
Common Block - the Common Block is a block of memory in the Central Office dedicated to a specific subscriber and programmed by complex translations for that subscriber's particular service. The Common Block controls :
Centrex service offers subscribers access to the public switched telephone network, or PSTN, by using either Network Access Registers (NARs), or Line
Rated Usage.
Network Access Register - software that provids blocked or engineered
access to the PSTN. NARs are provisioned as one or as a combination of In-Only, Out-Only,
and Two-Way trunks. Calls made within the Centrex System are not blocked, thus freeing up NARs
for access to the PSTN.
Line Rated Usage - eliminates the need for NARs by providing unlimited access to the network. Unlike most other telephone systems, Centrex provides each station line with simultaneous Network Access.
MFT (Miscellaneous Facility Terminations) - enables a subscriber to connect private networks to a Centrex System. The subscriber can then
direct communications traffic over the private network rather than the PSTN.
Private networks can include facilities connecting to another switch type such as a PBX. They also could connect to another Centrex Common Block or even
to an InterExchange Carrier's network. The types of facilities can vary but could include: Foreign Exhange lines, Analog or Digital Private Lines or Wide
Area Telecommunications Services known as WATS lines.
Station Lines - connect the Central Office with the subscriber location. Each line has a separate 10-digit telephone number that allows the user to make and receive phone calls from inside and outside the Centrex System.
CPE (Customer Provided Equipment) - the telecommunication equipment owned and maintained by the subscriber. Both analog and ISDN connectivity is offered. The CPE can include standard telephone sets, key systems, PBXs, fax machines, answering machines, data modems, or voicemail systems. Almost any type of telephone set or system works with any Centrex System. This means that a customer can have a PBX and still purchase a Centrex service (they can expand the feature set of their PBX by doing this).
Analog CPE
Customers can choose several types of CPE to connect to their Centrex station lines. The following options are particularly well-suited to analog Centrex service:
Switch hook flash - Switch hook flash happens when the user depresses and releases the telephone switch hook. This sends a signal to the common block that starts many Centrex functions. Switch hook flash must happen within a designated range of time in order to send its signal. This range is 200 to 1500 milliseconds, or six-tenths to 1½ seconds. The common block doesn't recognize a switch hook flash of less than 0.2 seconds in duration. If a flash lasts more than 1.5 seconds, the common block disconnects the call. Switch hook flash timing is critical to the operation of Centrex System. The operation of the switch hook is manual on standard telephones such as zero button sets. It only takes the user a little practice to learn how to send the right amount of flash. Some telephones are equipped with a tab flash or recall button, which allows the user to flash without using the switch hook.
Electronically controlled flash is available on electronic sets and most key systems. It is important to set the flash timing on these systems within a range the common block will recognize. Consult a CPE manufacturer for assistance in setting electronic flash timing.
Terminal type ("terminal" is the phone) - any existing analog CPE that provides switch hook flash will work with the Centrex System. The ability to support enhanced features and capabilities makes certain units better choices for a Centrex System. Electronic key systems and Electronic Business Sets are good examples of CPE that compliments the features and functions of the system. Most customers find electronic sets easier to use than traditional telephone systems. Electronic telephone systems allow the user to access features simply by pressing buttons on the set, unlike traditional sets, which force the user to remember and dial codes to access the system. Analog Terminal types are classified as either proprietary or non-proprietary electronic telephone sets.
Proprietary Terminal Sets - manufactured by the same company that makes the switch with which they work. The Northern Telecom DMS 100 has proprietary terminal equipment designed to work with its switch. These sets are designed to work with Centrex offerings. Proprietary DMS100 EBS (Electronic Business Sets) - Proprietary sets for the DMS 100 are called Electronic Business Sets, or EBSs. The RCMAC can program Centrex features onto EBS buttons. They can assign more than one telephone number onto each EBS buttton. Many EBSs provide additional features that aren't available to basic Centrex station lines. While EBSs may be the most desirable CPE for your system, additional factors (such as cable length between the site and the Centrex) will determine whether this choice is feasible. NOTE: the cabling must be "unloaded" for connection of EBS to the Central Office. Unloaded cable pairs serve only one telephone line. However, if there are subscriber loop carrier systems between the Central Office and subscriber premises, unloaded cable pairs can't be used.
Non-proprietary Terminal Sets - these work with any office and are interchangeable.
Modems and Fax machines - you should also think about your data transmission needs. Modems and fax machines are compatible with the Centrex System.
ISDN (Integrated Services Digital Network) CPE
Centrex ISDN CPE provides the standard for digital communication including voice and data transmission. Customers using
Centrex ISDN CPE have a choice between using 2B+S and 2B+D station lines. This is the standard international ISDN configuration: two B channels and one D or S channel. Centrex ISDN service is supported
by 5ESS and DMS100 only.
Advantages of ISDN over Analog
Speed - One of the primary benefits of Centrex System ISDN service is that each B channel can transmit circuit switched data, both directions, at speeds of up to 64 kilobits per second, a significant gain over traditional analog modem service which can download at approx 48 kbps but can only upload at 33 kbps.
BRI actually has two 64k B channels and one 16k D channel (2B + D). A process called B channel bonding allows users to send one data call over both B channels resulting in an even faster transmission rate of 128 kilobits per second. This does take two ISDN calls. Only Non-EKTS terminals allow simultaneous use of both B channels for data. Both Non-EKTS and EKTS allow a customer to simultaneously use one B channel for voice and one B channel for data.
In addition, digital voice CPE can provide multiple call appearances. Analog Centrex users must have Basic Station Lines or BSL's assigned to each telephone number. The ISDN terminal set permits multiple telephone numbers per BSL.
Another adcantage is that ISDN offers "Multipoint capability", which allows more efficient CPE use by using up to eight terminals per line. This allows cost savings from reduced number of lines
ISDN Centrex Configuration groups
Specific to individual switches. The Centrex ISDN offers 12 standard configuration groups. More than one customer can use the same configuration group. Customers can also request custom configuration groups, which incurs an additional charge. A variety of terminals work with configuration groups. Several manufacturers make equipment that supports particular groups. If a subscriber is using a 5ESS switch, and needs a configuration group, it is important to know what standard configuration will work with the equipment.
ISDN Components
NT1 - NT1 is a hardware device that converts a two-wire interface into a four-wire interface at the customer's site from the Central Office. It requires the power supply to operate.
TE1 - an ISDN compatible device such as an ISDN telephone or data terminal that is capable of digitizing analog, voice or data. It connects to the terminal site of the NT1 device.
TE2 - an analog device such as a telephone. TE2 is not compatible with ISDN and requires a conversion device to translate signals.
TA (Terminal Adapter) - a device to convert an analog TE2 to digital ISDN. This conversion device is called a Terminal Adapter or TA. They serve either a voice or data terminal.
ISDN Reference Points
U Reference Point - the interface between the Central Office loop and the subscriber premises. Wiring from the U Reference to the NT1 is customer provided. It provides the link between the CPE and the network. The U Reference point combined with a local loop becomes the digital subscriber line or DSL that connects to the NT1. The NT1 is customer provided. Digital Subscriber Line is 2B+D or 2B+D. There are two ways to locate the U Reference point:
There are trade offs between the two methods. In some cases it may be more cost effective to purchase additional desktop equipment for each user.
In other cases it might be more beneficial to rewire the office to serve users from one central power supply and battery backup. In those instances where
there are no clear financial benefits to either method you should consider
issues related to power failure. Customers without battery backup may be more susceptible to service
interruptions from power outages.
S/T interface - or Buff - a four-wire connection between the TE1 and the NT1 or between a Terminal Adapter and an NT1. An NT1 can connect to as many as eight devices via an S/T interface. The S/T interface is on the user side of the NT1.
R Interface - provides the connection between the non-ISDN device or TE2 and an ISDN Terminal Adapter. The R interface is between TE2 and TA.
ISDN Terminal Types
There are three functional types of Centrex ISDN terminals.
Examples of voice only types are fax machines and digital telephones. Data only types would include desktop PCs and point of sale devices. Both voice and data are found in video teleconferencing equipment.
Smart Terminals (for ISDN)
Circuit switched voice terminals are called logical terminal or Smart telephones. They are designed specifically for ISDN. These smart terminals work with a Centrex ISDN and are not switch dependent. All Centrex ISDN terminals in the 5ess are identified by the letters a or c and in the DMS100 they are identified as Non-EKTS, EKTS or EKTS.CACH
There are two terminal types which this section will introduce. The two terminal types are:
5ESS (Erricson Switch) - Configuration groups work with voice terminals designed for the 5ESS switch. Configuration groups allow feature access buttons on the phone sets so users don't have to memorize access codes. Configuration groups tell the switch which features are assigned to which buttons. Complex translations build the feature group into the switch and assign it a configuration group number.
For example a customer may request a specific feature assigned to 12 buttons. This group of features is configuration group CTP21221. The customer may then want to modify this group for a different terminal by replacing one feature. The resulting new configuration might be configuration group 21223.
DMS100 (Nortel Switch) - Two National Types of DMS100. National Centrex served from a DMS100 works specifically with National ISDN2 (IN-2) technology. The Central Office programs the features onto buttons, therefore they are pre-programmed when they reach the customer. In addition, it is helpful to know that the DMS100 supports a maximum of two circuit switched terminals.
DMS100 Centrex ISDN terminals include Non-EKTS Terminals and EKTS Terminals.
EKTS (Electronic Key Terminal Service) Terminals - Terminal EKTS is an ISDN compatible telephone making it TE1 equipment that doesn't require Terminal Adapter. EKTS stands for Electronic Key Terminal Service. Specific types of terminals provide access to multiple directory numbers and permit sharing of these numbers across multiple standard EKTS terminals. There are two types of EKTS terminals. Type C in 5ESS. EKTS CACH, which allows multiple appearances of DN on one terminal, which eventually have shared, call appearances.
EKTS.BASIC is relatively new and allows only one appearance of each DN on each terminal. In the DMS100 the two terminals are functionally the same, however they use different set up messages. EKTS terminals have the same features as Non-EKTS; however, they also allow shared directory numbers, multiple directory numbers and bridging and manual exclusion.
Non-EKTS Terminals - "type A" in the DMS100 is a terminal with a primary directory number. It can control multiple incoming calls. A variety of hardware can be classified as Non-EKTS. Terminal Non-EKTS delivers flexible call offering, which allows terminals to have multiple call appearances and conference call, dropped, and transfer programmed onto buttons. The customer can choose which buttons will correspond to these features. The Centrex provider often has predefined configuration groups for these flexible feature buttons. Terminal Non-EKTS normally terminates two primary directory numbers that may simulate multiple call appearances. The user can receive an additional call while on the same directory number. This is similar to call waiting. Non-EKTS is also used for video. Usually uses B channel bonding transmitting video data on both B channels.
Video
Most video equipment is categorized as Terminal Type A in the 5ESS. In the DMS100 it is categorized as Non-EKTS. Centrex System ISDN offers three types of video conferencing:
Computer based desktop systems
Video phone systems
Portable systems.
Desktop video conferencing allows such things as real time sharing of applications and file transferring. The CPE required for desktop video conferencing includes personal computers, ISDN boards for ISDN connectivity, video cameras and video capture cards and software.
Video conferencing can also be done with video telephones that resemble small TV sets with built in handsets and speaker phones and portable systems that look like televisions but have ISDN coded circuitry.
Computer Data (non-EKTS Terminals)
The digital capability provided by the Centrex System ISDN extends the scope of CPE to include computer equipment, individual PCs, data routers and LAN inter-connective devices. You can use your individual PC to send data. PCs are usually TE2 or non-ISDN non-compatible equipment, therefore they require Terminal Adapters in order to transmit data. Data routers let users communicate over many data networks. They can multiplex the signals from ISDN devices and handle switching and network management. LAN interconnect devices include routers and network termination devices that handle tasks like multiplexing and signal translation.
CPE Backup
One of the primary benefits of Analog CPE is that it works when commercial
power fails. Central Offices are equipped with batteries and generators that provide continuous power.
However, all electronic systems get electrical power from a commercial
power source. So, although the Central Office can still function during a power outage, ISDN and EBS telephones won't unless they are connected to
some type of alternate power supply. Only the non-EBS phones will work without power.
To help address this problem, it is recommended that customers equip their telephone systems with emergency battery back-up power units. Battery back-up provides electricity to the system for a limited period if commercial power fails.
Another options is to equip the Centrex System with some non-electronic analog telephone sets on site. In case of power loss these telephones will still function. Many customers keep some single line sets for this purpose. This option is less expensive than UPS.
