Specifications

190 IBM Voice Over Frame Relay Perform Guide
PBXs can handle as many as 20,000 telephones. Indeed, a PBX of this size
bears a close resemblance to a modern central office switch used by the phone
companies and PTTs in the public switched network.
The traffic handling capacity of a switch is measured in terms of Busy Hour Call
Completions (BHCCs) and Busy Hour Call Attempts (BHCAs). The BHCA is
approximately three times the value of BHCC, with a range of 20,000 to 30,000.
An additional rating classifies the switch as blocking or non-blocking. A
non-blocking switch ensures that a connection is always possible between any
two idle ports.
A few of the PBXs that are in use today include Lucent’s Definity ECS, Nortel’s
Meridian, Alcatel’s 4400, Siemens’ Hicom, NEC’s NEAX, ROLM’s CBX,
Ericsson’s MDX, and Harris’ B200.
C.3 Key Telephone Systems
Key Telephone Systems (KTSs) are smaller office automation systems but they
are similar in functionality to a PBX. In fact, the distinction between a PBX and a
key system is becoming somewhat arbitrary. Historically, a key system was an
analog device that really didn’t have a switching fabric per se. However, key
systems employ microprocessors and TDM backplanes with a hardware
architecture that is very similar to that of the PBX.
A key system often uses proprietary multi-key telephones ,which provide a
dedicated key for each function such as line selection, call pickup, transfer, hold,
forward and so forth (hence the name key telephone system.) With these
systems, the telephone itself (along with the user) has most of the intelligence in
the system. For example, if you need an outside line (a trunk to the PSTN), you
select a line button (key) on the telephone. Based on the button you pressed, the
key system connects you to the corresponding trunk port. You have to determine
yourself if the line is busy or not by looking at the indicator on the phone for that
line. This is in contrast to the PBX, where the user dials a number and the PBX
determines whether there is an available trunk port and automatically selects one
for you.
The phones themselves use proprietary signaling between themselves and the
key system, using one or more separate pairs of wires between each phone and
the key system.
Another of the distinguishing characteristics of a key system today is the lack of
support for some of the advanced features of their PBX counterparts such as Call
Detail Recording (CDR). However, many small and medium offices that do not
need these sophisticated features deploy the KTS which serves general needs.
C.4 Interfaces
This section provides information on some of the various ways of interfacing with
telephony devices such as telephones, FAX machines, PBXs, and Key Systems
analog interfaces.
There are three main types of analog interfaces:
• Foreign Exchange Subscriber (FXS)