RETURN Voice Networking


The Private Branch Exchange (PBX)

A PBX is a business-oriented phone system, designed to provide efficient voice communications between an organization’s users. The word “Private” indicates its use in the private sector. “Branch” indicates a remote subsystem, while “Exchange” indicates switching functionality.

PBXs are also referred to as Private Automatic Branch Exchanges (PABX), or Electronic Private Automatic Branch Exchanges (EPABX).

For most business communications, calls are placed between users in the same organization. While PSTN communications are viable, users do not need simultaneous access to the phone system at the same time. In fact, most users will use the phone for about five minutes or less each hour. Therefore, PBX systems provide switching of in-house (e.g. on-net) calls, and “pool” outside line resources (PSTN lines).

Within the domestic PBX marketplace, AT&T enjoys the most sales, while Northern Telecom, Inc. (NTI) and Rolm are not far behind:

     AT&T Dimension Series
     AT&T System 75
     AT&T System 85
     AT&T Definity 75/85 (unites the two systems)

     Northern Telecom SL-1
     Northern Telecom SL-100

     Rolm CBX II
     Rolm CBX II 9000AE
     Rolm/IBM 9750
     Rolm 9752
     Rolm Redwood


Some other frequently used PBXs include the Siemens Saturn series, the NEC NEAX 2400 series, and the Mitel SX-2000 series.

Centrex Services

Centrex was originally introduced in the early 1960s to large customers, but has migrated to targeting low and mid-range customers as well. Centrex, which stands for “Central Exchange”, can basically be described as LEC-provided PBX services.

Centrex services are heavily marketed by the LECs and are called a variety of different names. Usually, Centrex services are implemented on switches that otherwise would have be discarded like the 1/ESS or the retrofitted 1A/ESS systems. However, Centrex services are available on digital switches, LEC dependent.

The advantage in using Centrex services is that the Centrex services can be leased, therefore the purchase of PBX equipment is not required.

A Centrex switch can also be equipped with an EPSCS (Enhanced Private Switched Communications Services) package to facilitate trunk level interconnection to other EPSCS-equipped Centrex switches, or a PBX. These trunks are known as Intermachine Trunks (IMTs) and may be provided by the carrier or the customer.

Customers with heavy data transport requirements, and transient voice requirements can transport Centrex IMTs over their data backbone. When using voice compression, Private Network transport of Centrex IMTs can be very economical:

Observation indicates that these types of Centrex configurations have been (and are) successfully deployed within banking industry customer networks. Presumably, buildings go up, move, and otherwise change constantly, while the need for data processing transactions is always present.

PBX Networking

The following subsections discuss Switched Services Networks and Electronic Tandem Networks.

Switched Services Networks (SSN)

SSNs are commmunications packages designed by public carriers for large customer networks. To be classified as a SSN, a LEC switch, and private circuits provided by an IC must interconnect with multiple PBX systems.

There are three basic SSNs:

 - Tandem Tie Trunk Networks (TTTN)
 - Common Control Switching Arrangements (CCSA)
 - Enhanced Private Switched Communications Services (EPSCS)


Tandem Tie Trunk Networks (TTTN)

The forerunner of all SSNs, TTTNs usually are comprised of a conglomeration of PBXs, with each PBX capable of providing basic tandem trunk operation.

TTTN customers must be alert to the fact that connections traversing several tandem switches may incur problems with routing and dialing. Therefore, most TTTNs do not allow for more than three tandem PBXs.

Common Control Switching Arrangements (CCSA)

As the second generation of Switched Services Networks, these CCSAs can provide uniform numbering plans. Typically, these plans consist of an “access code” plus seven digit network subscriber phone numbers. These phone numbers are usually unrelated to those assigned in the public DDD (Direct Distance Dialing) network.

CCSAs usually transmitted address information in a “senderized” fashion, meaning that dialed numbers are “stored” by the switch, then output when a trunk circuit is selected.

There is only limited off-net routing capabilities, subject to LEC switch capabilities.

Enhanced Private Switched Communications Services (EPSCS)

As the third generation of SSNs, EPSCS routinely provides off-net calling. Since these networks are totally electronic, information regarding usage, cost allocation, and facility management can be obtained in almost real-time.

EPSCS networks typically provide four-wire services between subscribers, while retaining the basic architecture of CCSA networks.

Electronic Tandem Networks (ETN)

Electronic Tandem Networks (ETN) consist of privately owned trunk and switching facilities. One important aspect of an ETN is a uniform numbering plan. Each switch has a specific location code, similar to the 3-digit CO codes in the PSTN. Within each ETN switch, a unique number is used to identify each connected station or telephone.

ETNs provide automatic routing of private network traffic and can take advantage of least-cost routing schemes. Additionally, ETN switches can define calling privileges for specific terminals or groups of terminals.

Most ETN switches can also support Satellite PBX systems. This is usually done through special switch trunks that make certain attendant and station features transparent to the network. These Satellite PBX system functions are usually performed through HDLC-based communications between the Remote and Host PBXs.

ETN switches can interface with EPSCS or CCSA networks. In fact, most customer networks are migrating towards ETN architectures.

PBX Circuits

The following subsections provide further detail about these types of PBX circuits:

   - Central Office Trunks
   - Foreign Exchange Circuits
   - Tie Trunks/Lines
   - OPX Lines
   - Satellite Tie Trunks


C.O. Trunks

       2-Wire Trunks
       DID Trunks
       WATS Trunks
       800-Service Trunks
       Long-Distance Trunks


2-Wire Central Office Trunks

The most common 2-Wire C.O. trunks used in PBX networks are of the Ground Start variety. These trunks may be restricted such that operation may be outgoing only, incoming only, or both. In the case of outgoing calls (known as DOD – Direct Outward Dialing), operation may additionally be restricted to local calling only, versus DDD (Direct Distance Dialing). In the case of incoming calls, most calls are routed to an attendant or, sometimes, a message/answering system.

Most of these trunks are assigned the same phone number within the PSTN, thus operate in a “hunt group” fashion.

With these types of trunks, the RINGBACK tone is generated from the LEC switch that interconnects to the PBX.

DID C.O. Trunks

These trunks allow for Direct Inward Dialing from the PSTN. In order to accomplish this, the PSTN switch (LEC or IC), must transmit the number of the called party to the PBX for routing.

These trunks are often implemented using existing 2-Wire circuits, and typically use Ground Start signaling. However, implementations also can occur with 4-Wire circuits using E&M signaling or through digital access (DS-1).

DID trunks facilitate only INCOMING CALLS from the PSTN.

DID arrangments usually involve having to reserve a block of PSTN telephone numbers from the IC or LEC.

These types of trunk circuits usually employ either “Wink Start” or “Delay Dial” signaling schemes, so that the network knows that the PBX is ready to accept the incoming address supervision. THE PBX MUST RETURN ANSWER SUPERVISION TO THE NETWORK, for billing purposes. With DID trunks, a RINGBACK/BUSY tone is generated from the PBX.

WATS C.O. Trunks

Sometimes referred to as “OUTWATS”, Wide Area Telephone Service (WATS) trunks are used for outgoing calls and take advantage of interconnection to the IC.

These trunks require that the PBX output digits to the PSTN switch for routing. The network will usually provide Answer and Disconnect Supervision.

These trunks can operate using Ground or Loop Start signaling as well as E&M signaling.

WATS trunks do not have addressable PSTN telephone numbers. Therefore, these trunks are typically implemented in a “hunt group” fashion within the PBX.

RINGBACK/BUSY is generated from the remote switch/PBX.

800-Service C.O. Trunks

800-Service trunks have been referred to in the past as “INWATS” service trunks. These trunks are used for incoming calls and take advantage of interconnection to the IC.

With these trunks, the PSTN does not output digits to the PBX. Therefore, incoming calls on these trunk numbers are routed to specific individuals or groups by the PBX. These trunks may utilize Loop Reverse Battery or E&M signaling.

Generally, these trunks are implemented in a “hunt group” fashion.

RINGBACK/BUSY tones are usually generated by the PSTN switch that is interconnected to the PBX.

Long Distance Trunks

These types of trunks interconnect to Inter exchange Carriers (IC) to allow both incoming and outgoing long-distance calling.

Generally, these types of trunks are implemented in a “hunt group” fashion and incoming calls are routed to an attendant or message system.

RINGBACK/BUSY tones are usually generated by the IC’s PSTN switch that is interconnected to the PBX.

These trunks will usually operate on 2-Wire Ground Start circuits, or 2-Wire/4-Wire E&M circuits. If access to the PSTN switch is digital (DS-1), then conventional robbed-bit signaling is used.

Foreign Exchange (FX) Circuits

Foreign Exchange (FX) circuits may consist of either stations or trunks.

FX Station (FXS)

A Foreign Exchange line is a special line that is run from a local telephone to a remote CO/LEC switch or PBX switch. In this case, the local telephone is assigned a number on a remote switch; and for all inbound and outbound calls, the telephone acts just like it was connected to the remote switch.

Generally, a Foreign Exchange Station application will use 2-Wire circuits with LOOP START signaling. Stations (telephones) that can support GROUND START signaling are not readily available.

RINGBACK/BUSY tones are provided by the connected C.O. switch.

FX Trunks (FT)

A Foreign Exchange Trunk (FT) operates in a similar fashion as a 2-Wire FXS circuits, except that a local trunk provides inter connectivity to a remote switch. For outbound calls, local PBX users dial an access code (“9”, etc.), draw dial tone from the remote switch, then output digits to the remote switch for routing. Incoming calls are either barred, or are routed to a PBX attendant or message system.

FX TRUNKS NORMALLY UTILIZE 2-WIRE GROUND START SIGNALING. Since a TRUNK circuit is involved, there is a possibility that CALL TRANSFERRING, or CALL FORWARDING techniques can be deployed. These types of activities require that FAR-END DISCONNECT supervision be provided, such that the PBX trunk can be released. THE LACK OF FAR-END DISCONNECT SUPERVISION WITH LOOP START SIGNALING NORMALLY DISCOURAGES THE USE OF LOOP START ON TRUNK CIRCUITS:

In the example illustrated above, loop start trunks exist between PBXs A, B, and C. A call is placed from PBX A to PBX B, then transferred (or forwarded) to PBX C. At this point, a problem exists! When the users at PBX C and PBX A conclude business and hang-up, their local PBXs (A and C) will disconnect and release the trunk. BUT, SINCE LOOP START SIGNALING DOES NOT PROVIDE FAR-END DISCONNECT SUPERVISION, PBX B IS NOT MADE AWARE OF THE DISCONNECTION. THE RESULT: TWO “HUNG” PBX TRUNKS AT PBX B!

Tie Trunks/Lines

Tie trunks interconnect PBX switches within a customer’s network. These circuits are usually heavily used (HU – High Usage) and normally support both incoming and outgoing calls.

Tie trunks are typically 4-Wire E&M type circuits, but are now often interconnects through DS-1 facilities with robbed-bit signaling. The use of 2-Wire trunks is allowable, and will typically use Ground Start signaling.

Tie trunks typically connect both trunks and station lines through a network connection.

Tie lines are considered the same as tie trunks; and are, according to most reference materials. HOWEVER, THERE ARE SOME CUSTOMERS AND ORGANIZATIONS THAT MAKE A DISTINCTION BETWEEN TIE TRUNKS AND TIE LINES:

  Tie lines only connect lines throughout a network connection, whereas tie
  trunks can interconnect both trunks and lines.  Where routing of a call is
  totally automatic in tie trunk operation, routing of a call over tie lines
  is under control of the user.

  In tie line operation, a user dials an access code (e.g. "9", etc.) to gain
  access to the desired remote PBX.  The remote PBX usually supplies another
  dial tone, at which the user enter the necessary address for the called
  party at that PBX.  Tie lines are actually tie trunks that use "Tone Start"
  Start-Dial Supervision, and "cut-through" operation.


PBX Tie Trunk Hierarchy

Private Branch Exchange networks also tend to operate within specific hierarchies, as illustrated in the following diagram:

OPX Lines

In Off-Premise Extensions (OPX) applications, the telephone is placed at a distance (off-premise) from the PBX location. In many cases, no special consideration needs to be placed on these applications. Most PBXs support OPXs and rate the maximum distance from the PBX in terms of resistance (ohms). This is called the Maximum OPX Loop Limit.

However, should the distance be such that direct PBX support cannot be provided, OPXs can be supported through the use of FXS (Foreign Exchange Station) circuits.

Satellite Tie Trunks

Satellite tie trunks are ideal for use in “campus” environments and allow for functional transparency between Remote and Host PBXs.

To provide transparency, special signaling schemes are used on these S/TTs. Usually, HDLC protocol-based signaling methods are employed. These special channels are normally transported by DS-1 system, using DS0 #24. But, some PBXs (e.g. AT&T SYS/85) can provide this channel through a standard digital interface (V.35/RS-422, etc). The following table lists some of these systems:

AT&T Distributed Communications System

Worthy of mention is the AT&T DCS signaling method, since its does require some special care in multiplexer implementations. At the DS-1 level, the PBXs utilize DS0 24 for Common Channel Signaling (CCS), of which there are two types:

 - Bit Oriented Signaling (BOS)
 - Message Oriented Signaling (MOS)

   MOS is used when the PBX is ISDN-equipped.

   BOS seems to more frequently implemented than MOS.  When BOS is used, DS0
   24 contains a special multiplexed pattern that contains the A, AB, or ABCD
   signaling bits for each channel.  Conceptually, this is similar to (but not
   the same as) to G.732S systems using Channel Associated Signaling (CAS).


System 75 and 85 PBX systems utilize a special HDLC protocol between them. This capability can be provided by Data Communications Interface Units (DCIUs). It is this special feature that allows “feature transparency” for the Remote Satellite PBX systems. Each DCIU supports interconnection of up to eight switches, but DCS messages can be alternate routed across a maximum of two paths. These constraints allow a maximum PBX network of 63 switch points with a “busy hour” call capacity of about 10,000 attempts.

Now, in addition to the DS0 #24 signaling channel, an additional data port, the DCS channel, can be utilized on DS0 #1 for the DS-1 interface. This port operates at 9600 BPS and uses a DS0-A (byte repetition) format. This DS0 should be defined for DATA and “A-Bit” signaling. OOS operation should be defined to force the A-Bit to a Zero (“A-OOS = SPACE”).

All interconnected voice trunks (DS0s 2-23) should be defined as necessary. The big restriction here is ensure that the DS0s are setup to transport the A and B signaling bits and that the the OOS signaling condition on these DS0s be defined for “Mark” (Seizure).

On DS0s 1-23 (including the DCS channel), the PBX system will attempt to verify that DS0s are available for service by “Winking” the remote end, then waiting for a return “Wink”. This “Wink/Wink” process consists of an On-Hook/Off-Hook/On-Hook transition of the signaling bits, in the standard wink duration range of 140 – 290 mS.

PBX Feature Descriptions

A description of PBX features is provided in the following subsections.

PBX System Features

LINE RANGE         | Minimum and Maximum numbers of stations supported.
TRUNK RANGE        | Minimum and Maximum numbers of trunks supported.
TRUNK GROUPS       | A trunk group is comprised of trunks that support the
                   | same function.  This feature indicates the maximum
                   | allowable number of trunk groups within the PBX.
LINE EXPANSION     | Number of lines per interface card.
TRUNK EXPANSION    | Number of trunks per interface card.
ATTENDANT CONSOLES | Number of Attendant Consoles supported.


PBX Traffic Loading Features

SYSTEM NONBLOCKING | The maximum number of active lines that the switch can
                   | process while being truly nonblocking.
SIMULTANEOUS CALLS | The maximum number of simultaneous calls that the switch
                   | can effectively process.
CCS/hr @ P.01 GOS  | Centum Call Second (CCS) per Hour at P.01 Grade Of
                   | Service (GOS).  The Centum Call Second is frequently used
                   | to represent telephone traffic load.  P.01 Grade Of
                   | Service indicates that the probability of blocking is 1
                   | in 100.
                   | CCS is derived by multiplying the number of calls per
                   | hour by the average call duration (in seconds), then
                   | dividing the result by 100.
SWITCH TYPE        | Whether the switch is Digital or Analog.  Most PBXs sold
                   | today are digital.
INTERNAL SWITCHING | This parameter describes whether the PBX switches the
                   | path in a 2-Wire or 4-Wire fashion.
MATRIX TYPE        | Determines whether Space-Division switching (physical
                   | connectivity) or Time-Division (timeslot
                   | cross-connections) switching is used.
DIGITAL MODULATION | The type of digitization scheme that is used internally
USED               | by the PBX.  Also, the sampling rate of the signal should
                   | be noted.


PBX Trunking Features

TRUNK TYPES        | Describes PBX support for CO/DID/TIE/FX/etc trunks.
OPX LOOP LIMIT     | Expressed in resistance (ohms), this represents the
                   | Maximum distance between the PBX and OPX station.
DIGITAL CAPABILITY | Determines if the switch can support digital transmission
                   | facilities (T1/E1).


PBX Intra-Networking Features

NETWORK NODES      | The number of systems that can comprise a Satellite PBX
SUPPORTED          | network (e.g. DCS environment).
PORTS PER NODE     | This represents the maximum number of physical
                   | terminations for line and trunk terminations.
SHARED TENANT      | Many PBXs can be "split in-half" such that two distinctly
PARTITIONS         | separate customers can be served by the same PBX;
                   | providing separate PBX attendant and feature support.


PBX Inter-Networking Features

REMOTE SWITCHES    | This represents the maximum number of remote PBXs that
                   | can be supported in a network.
PRIVATE NETWORK    | Host/Satellite/End-Point role that the PBX plays in a
ROLE               | Private Network.
NETWORK PLANS      | This defines the types of networking plans supported
                   | (TTTN, CCSA, EPSCS, ETN) by the PBX.
ISDN COMPATIBILITY | This defines the support the PBX provides for ISDN
                   | functionality.
SIGNALING SYSTEM   | This defines the digital signaling system used between
SUPPORT            | PBXs (SS7, proprietary).
UNIFORM DIALING    | This indicates whether or not the PBX can support a
                   | uniform dialing plan.
DIGIT TRANSLATION  | This operation is typically employed when PBX automatic
                   | route selection (ARS) is used.  This allows the PBX to
                   | modify digits from dialed numbers, depending upon routing
                   | requirements.
ALTERNATE ROUTING  | This determines if the PBX can place calls over a
                   | secondary path if the primary path is not available.
LOOK-AHEAD ROUTING | This describes PBX operation where the PBX checks for
                   | network busy conditions before attempting to route the
                   | call.
CENTRALIZED        | This indicates that a single group of attendants can
ATTENDANT          | process calls from various REMOTE PBXs.
TRAVELING CLASS    | When Automatic Routing or Uniform Numbering operations
MARK               | are employed on a trunk, the calling party's Class Of
                   | Service can be sent over the trunk.  Routing facilities
                   | can then determine the required facilities, based upon
                   | the calling party's priveleges.
TIME-OF-DAY        | Determines if the PBX Automatic Route Selection can
ROUTING            | route calls that take advantage of a carrier's discount
                   | periods.
FEATURE            | Used to describe system feature availability throughout
TRANSPARENCY       | the entire network.
OPX CALL           | Describes whether or not the PBX can route a call to a
FORWARDING         | phone not associated with the PBX.
                   | Direct Inward System Access (DISA) allows outside callers
DISA AVAILABILITY  | to dial directly into a PBX and gain complete access to
                   | the PBX and its facilities.  DISA can be a major factor
                   | in "toll fraud" problems.


PBX Station Features

TELEPHONE TYPES    | Most PBXs support industry-standard Type 500 (Rotary) and
                   | Type 2500 (DTMF) telephones.  Also look for Proprietary
                   | stations offered by the PBX vendor.
TELEPHONE LINES    | This determines how many lines can be supported by the
                   | PBX vendor's Proprietary telephone stations.
NUMBER OF FEATURES | This indicates the available "feature buttons" on the
                   | telephone station.
NUMBER OF WIRE     | This indicates how many wire pairs are required to
PAIRS              | support specific Proprietary telephone stations.
TRANSMISSION       | This indicates the maximum distance an On-Premise
DISTANCE           | station may be from the PBX (in feet).
TRANSMISSION       | This indicates whether the station uses Analog or
FORMAT             | Digital transmission characteristics.
SPEAKERPHONE       | This indicates "hands-free", talk-and-listen operation
                   | through a built-in speaker and microphone in the
                   | telephone station.
DISPLAY            | This describes the display capabilities of the telephone
CAPABILITIES       | station.  Many stations can provide information
                   | pertaining to the Calling Number, Dialed Number, and
                   | data messaging functions.


PBX Console Features

CONSOLE REQUIRED   | This indicates whether or not a central attendant
                   | function is required by the PBX.
CONSOLE WIRING     | This identifies the wiring requirements for the PBX
REQUIREMENTS       | console, including the maximum distance the console may
                   | be located from the PBX.
DISPLAY TYPE       | This indicates the type of display used on the console's
                   | data display (e.g. LED, LCD, CRT) and the number of
                   | characters displayed.
PER-CALL DISPLAY   | This indicates the information displayed about each call
                   | processed by an attendant (e.g. Trunk/Line Class Of
                   | Service, Inbound Trunk/Line ID, Outbound Trunk/Line ID,
                   | etc.).
BUSY LAMP FIELD    | This determines if an optional or standard panel is
                   | provided to the attendant to provide a visual indication
                   | of the busy/idle status of individual stations.
DIRECT STATION     | This determines if single pushbuttons are provided to
SELECTION          | select any associated telephone station.  This DSS
                   | capability can simplify attendant procedures.