Time Slot Availability: Blocking or Nonblocking | PBX Switch Network Issues

The terms blocking and nonblocking have been used previously. In PBX terminology, blocking is defined as being denied access to any segment of the internal switch network because there is no available talk slot or communications channel to complete a call connection. Blocked calls are characterized by busy signals. Nonblocking switch network access means that an attempt to access the internal switch network will always be successful because there is a sufficient number of talk slots or communications channels to support simultaneous call attempts by every configured station user in the system.

Blocked calls due to unavailable trunk carrier circuits are not included as part of this discussion because the issue being addressed is blocking and nonblocking access to, and connections across, the internal PBX switch network.

There are several connection points in the overall PBX system and switch network design that can cause a call attempt to be blocked (Figure 1):

Figure 1: Traffic handling red flags: blocking points.

1. Port circuit card

2. Local TDM bus

3. Highway bus

4. Switch network interfaces

5. Center stage switch

Although it may seem strange that a call can be blocked at the port circuit card level, the number of physical communications devices supported by a port card can be greater than the number of communications channels supported by the desktop. For example, an ISDN BRI port circuit card that conforms to passive bus standards can support up to eight BRI telephones, but only two can be active simultaneously, because the BRI desktop communications link is limited to two bearer channels.

Scenarios also exist where a digital station card can support more desktop communications devices than available desktop communications channels. For example, a Siemens optiSet digital telephone equipped with two adapter modules can be connected concurrently to a second desktop optiSet digital telephone and a desktop analog telephone, with all three communications devices being supported by a single communications link to the Hicom 300H PBX and sharing the wall interface jack, inside telephony wiring, and port circuit card interface. Like the BRI port interface circuit, the optiSet interface can support only two active bearer communications channels, which means that one of the three desktop devices can be blocked from accessing the system.

Several recently introduced IP station cards supporting LAN-connected desktop telephones may also block call attempts because the number of physical telephones supported by the card can be greater than the number of local TDM bus connections supported by the card. For example, the Avaya Definity Media Processing Board for IP Telephony can support 96 IP telephones but can support between 32 and 64 connections to the local TDM bus based on the audio coder standard used for IP:TDM/PCM protocol conversion.

The local TDM bus is usually the most likely switch network element to be the cause of a blocked call. Although a greater number of current PBX systems are designed with a nonblocking switch network architec- ture, a good percentage of installed and new systems must be traffic engineered because the number of port circuit interfaces is greater than the maximum number of time slots on the local TDM for connecting the call. For example, the local 32-Mbps TDM bus supporting an Avaya Definity port network cabinet can support a maximum of 483 active ports, although the cabinet can physically support several times this number of ports. Avaya typically recommends installing 800 stations per port network cabinet for customers with moderate traffic requirements. It is unlikely that all 800 station users will attempt to place a call at the same time, but if they do only 483 time slots are available, and quite a few station users will hear a busy signal when they make their call attempt. Similarly, a Nortel Meridian 1 Superloop can support 120 active ports at full TDM bus utilization but is typically configured to support at least 200 station users. If properly traffic engineered, based on station user traffic requirements, call blocking should be minimal, but it can occur.

Most Highway buses provide nonblocking switch connections between local TDM buses and have sufficient communications channels for nonblocking access to the center stage switch complex. However, the bandwidth of the Highway bus may be less than the total bandwidth of the local TDM buses it supports and a call may be blocked if based on traffic conditions. Almost all current switch network interfaces and center stage switch complexes are also designed for nonblocking access and transmission, but exceptions do exist. For example, until Nortel recently upgraded the Meridian 1 Option 81C Sub Group Assembly module (a center stage switch complex) with a fiber optic ring design, it was possible, if not highly probable, that calls between switch network groups within the center stage could be blocked.