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.

Time Slot Access and Segmentation | PBX Switch Network Issues

Some switch network designs are based on universal port access to the local switching network; that is all ports in a carrier shelf or cabinet can use the full bandwidth capacity of the local TDM bus. For example, any port interface circuit housed in a Definity PPN cabinet can be assigned any talk slot on the local 32-Mbps TDM bus regardless of its port circuit card slot location. A five-carrier shelf PPN cabinet has 100 port card slots, and the local TDM bus supports every port interface circuit card in the cabinet. The Definity TDM bus is said to be universally accessible to all PPN port circuit terminations. The 512 time slot (483 talk slots) TDM bus supports the traffic needs of hundreds of system ports in the cabinet.

A switch network design is said to be segmented if the local switching network is based on segmented TDM buses supporting a single port carrier shelf or cabinet. For example, the Siemens Hicom 300H LTU carrier connects to the center stage switch complex via a 32-Mbps Highway bus. The local switching network consists of two 16-Mbps segmented local TDM buses, with each local TDM bus supporting different port card slots. Although the total TDM bandwidth at the LTU carrier shelf level is 32 Mbps, the 512 time slots are divided equally between both halves of the shelf (eight port card slots per half). If the segmented TDM bus supporting port card slots 1 to 8 fails, available time slots on the second operational TDM bus are not accessible to port circuit interfaces on the port card housed in slots 1 to 8. The LTU carrier shelf is said to be based on a segmented TDM bus design. This is the downside of a segmented TDM bus design when compared with a universally accessible design. The upside is that the Siemens system can be traffic engineered to a greater degree. The local TDM bus supports only eight port card slots, a fraction of the number the Definity local TDM is required to support (Figure 1).

Figure 1: Segmented bus design.

The segmented TDM bus design of the F9600 was described earlier. Each port carrier shelf is supported by a 16-Mbps Highway bus that segments into eight 2-Mbps local TDM buses, with each bus supporting two port card slots. Minimizing the number of port card slots supported by a TDM bus is not always a good design objective because there may be less flexibility when configuring the port circuit cards. The F9600 backplane used to access the local TDM bus can support only 32 connections, which is also the maximum number of total port circuit terminations allowed for the two adjacent port cards. If 16 port circuit cards are installed, there is no problem, but problems may occur when a higherdensity digital trunk card is installed. A 24-port T1-carrier interface card will limit the flexibility in configuring the adjacent port card slot that shares the 32 connections to the 2-Mbps local TDM bus (32 time/talk slots). Only an eight-port card can be housed in the second port card slot if the configuration rules are followed. If a 16-port card was installed and only eight telephones were installed, thereby limiting the number of configured ports to 32 (24 + 8), the system configuration guidelines would still prohibit such an installation. The Fujitsu system was programmed for nonblocking switch network access only, and more ports than fixed TDM bus connections/time slots are not allowed. The TDM bus segmentation design can limit port configuration flexibility, if the number of backplane connections per TDM bus is relatively small.

Switch Network Redundancy | PBX Switch Network Issues

Switch network redundancy is a design criterion that minimizes switch network downtime for one station user, a few station users, or all station users in the system. The term redundancy is often confused with the term duplication. A switch network design incorporated with duplicated elements is said to be redundant, but a redundant switch network may not necessarily have any duplicated elements that might prevent downtime for some or all station users. Duplication is the highest form of redundancy, but it is not the only type of redundancy, particularly in PBX switch network architectures, as we will shortly see.

If a customer wants a redundant switch network design that is based on duplication of critical design elements, the checklist of duplicated elements may include any or all of the following:

1. Center stage switch complex

2. Local TDM buses

3. Highway buses

4. Switch network interface (including embedded TSI)

5. Intercabinet cabling

Duplication of the center stage switch complex is a vital redundant switch network requirement in a centralized design topology because all calls are connected through this switch network element. Center stage switch complex errors or failure affect every call in the PBX system. Center stage switch problems may be slightly less important in a dispersed design topology, but it would still be highly desirable to have duplication of the switch network element because it is needed to con- nect all calls between local switch networks. A few PBX systems have a fully duplicated center stage switch complex as a standard design feature, such as the Nortel Networks SL-100, a modified version of the supplier’s DMS-100 central office switching system. It is more common that the duplicated center stage switch complex is available as an option, although some intermediate/large PBX systems do not offer it as a standard or optional design element. The Siemens Hicom 300H is available in two models: the large line size Model 80 has an optional duplicated center stage switch complex and the smaller Model 30 does not offer it as standard or optional.

Loss of the local TDM bus will negatively affect the communications capabilities of all ports to which it connects. Redundancy of the local TDM bus can vary between different PBX systems based on the definition of redundancy. The Fujitsu F9600 XL has a fully duplicated local switching network design, including duplication of the local TDM buses. The Avaya Definity PBXs have a redundant local TDM bus design: the local 32-Mbps TDM bus (512 time slots) supporting all of the communications needs within a Port Network cabinet is based on two independent TDM buses, each with a 16-Mbps bandwidth (256 time slots), but operating as a single TDM bus from the viewpoint of the port interface circuit cards. If one of the two 16-Mbps TDM buses fails, all system ports can still connect to the remaining TDM bus. The Siemens Hicom 300H offers a similar redundant design concept for its TDM bus architecture. Two 8-Mbps TDM buses support eight port interface circuit card slots (one half of an LTU carrier shelf), each operating independently and accessible by any of the eight port interface cards. In these Avaya and Siemens models, loss of one TDM bus will place a heavier traffic load on the remaining TDM bus and may increase the number of blocked call attempts due to the reduced number of available time slots. The major difference between the two designs, however, is that a Definity 32 Mbps TDM bus can support a five-carrier shelf cabinet with several hundred stations and associated trunk circuits, and the Siemens 16-Mbps TDM bus design supports only eight port card slots (nominally 192 ports). Failure of a Definity TDM bus segment will have greater traffic handling consequences than failure of a Siemens TDM bus segment. Siemens offers switch network redundancy at a more local level than does Avaya.

Nortel Networks has claimed that the multiple Superloop design in its intermediate/large Meridian 1 models is a form of redundancy because loss of a single Superloop affects only a limited number of the ports in a cabinet stack. The term limited, however, can be misleading because a Superloop can support up to 32 port card slots, and each port card slot can support 24 digital telephones. If strategic system ports, such as attendant consoles or trunk circuits, are affected by loss of a Superloop, the redundancy level of the design might not be acceptable.

Highway buses may be fully duplicated, or loss of a TDM bus segment comprising the Highway bus may not affect the remaining bus segments (although traffic handling capacity will be reduced). Highway buses are used for connections between local TDM buses and to provide communications paths to the center stage switch complex. Loss of a Highway bus can be significant in a centralized switch network design.

Switch network interfaces are printed circuit boards connecting local switch networks to each other or the center stage switch complex. It is an electronic switch network design element that can fail and affect communications traffic between port cabinets. A duplicated switch network interface typically links the local switching network element, such as a TDM bus, to a high-speed fiber optic cable communications link. Duplicated center stage switch complex designs usually have duplicated switch network interfaces and duplicated cabling links for intercabinet communications connections.