Legacy PBX Switch Network Design

The most fundamental function of a PBX system is to support switched connections between peripheral endpoints. Stations users are accustomed to picking up their handset, hearing the dial tone, dialing a telephone number, and being connected to the called party. The possibility always exists that the station user receives a busy signal when the dialing process is completed. The most probable reason for a busy signal is that the called party is off-hook and engaged in another call. Infrequently, all telephone company trunk circuits are busy, and the station user hears an announcement to call again at a later time. A busy signal also may be received when all PBX trunk circuits are in use or internal switch network resources are not available. The PBX station user cannot control the availability of the called party or the availability of PSTN trunking facilities but can minimize the probability of busy signals due to blocked access to the internal switch network or local trunk circuits, if the PBX system is properly configured and engineered to meet the expected traffic demands of the customer. PBX traffic analysis and engineering tools are used to achieve acceptable customer service standards for internal switched connections and off-premises trunk calls.

Nonblocking/Blocking PBX Systems

PBX systems can be classified into two switch network design categories based on traffic engineering requirements: nonblocking and blocking. A PBX system is said to be nonblocking where no switch network traffic engineering is required because there will always will be sufficient switch network resources (local TDM bus talk slots, Highway bus communications channels, switch network interfaces, and center stage switch connections) to satisfy worse-case customer traffic demands at maximum system port capacity. Worse-case traffic demand occurs when all equipped ports are simultaneously active; that is, transmitting and receiving across the internal switch network. Although this would be a very unlikely customer situation, because PBXs are never configured at maximum port capacity and the probability is almost infinitesimal that all ports require simultaneous access to the internal switch network for communications applications, the assumption is used to define a nonblocking system.

Station users may have nonblocking access to the internal switch network but receive a busy signal when attempting to place an off-premises trunk call. The PBX system is still classified as a nonblocking PBX sys- tem because the term does not apply to access to trunk circuits or other external peripherals that may have limited port capacity, i.e., VMS. Trunk traffic engineering is an independent discipline that will be discussed later in this chapter.

A PBX system is said to be blocking if traffic engineering is required, at maximum port capacity, to satisfy worse-case traffic demand situations. For example, a small/intermediate line size PBX system based on a switch network design consisting of one 16-Mbps (256 talk slots) TDM bus can appear to be nonblocking to customers with requirements of 100 station users and 30 trunk circuits because the total number of potentially active ports is smaller than the total number of available talk slots. If the total port requirements of the customer were to grow beyond 256 ports, e.g., 240 stations and 60 trunk circuits, some ports might be denied access (blocked) to the switch network because the number of active ports may be larger than the number of available talk slots. A PBX system with a blocking switch network design can operationally function as a nonblocking system if two conditions are satisfied:

1. The system is traffic engineered

2. There are sufficient switch network resources to satisfy actual customer traffic requirements

The typical PBX system is usually installed and configured with a number of equipped ports with significantly less than the maximum port capacity. The switch network resources of a blocking PBX system are usually sufficient to provide nonblocking access to the equipped system ports, but as customer port requirements approach maximum port capacity, the probability of blocking increases. The probability of blocked access to the switch network is based on the potential number of active ports (communications sources) and switch network resources required to connect a call.

The most important switch network resource determining the probability of a blocked call placed by a station port is the number of available talks slots on the local TDM bus. The local TDM bus is the most likely switch network element to have insufficient resources; talk slots, because most (if not all) PBX systems are based on switch network designs with sufficient Highway bus traffic capacity to support access to the center stage switch or connect local TDM buses. Most PBXs also are designed with a nonblocking center stage switching system complex, even if local TDM bus traffic capacity is limited. The Definity G3r is a good example of a PBX system that requires traffic engineered local TDM buses, although the Highway bus/center stage switch complex used to link the local TDM buses supports nonblocked access across the internal switch network. If customer traffic requirements are light to moderate, a G3r local TDM bus (483 talk slots) can adequately support about 800 user stations. A Nortel Networks Meridian 1 Option 81C is typically configured with about 200 stations supported by a single Superloop (120 talk slots). Although the number of equipped ports is larger than the number of available talk slots, the two blocking PBX system designs are usually sufficient to support typical station user traffic demand. Customers with heavy traffic requirements would need to traffic engineer the Definity G3r/Meridian 1 Option 81C because the number of local TDM bus talk slots is smaller than the maximum number of ports, and the number of blocked calls could increase to an unacceptable level.

PBX Grade of Service (GoS)

PBX systems with blocking switch network designs are traffic engineered by the vendor when they are configured and installed based on customer traffic requirements. Customer traffic requirements are based on two parameters: required GoS level and expected traffic load. PBX GoS may be simply defined as the acceptable percentage of calls during a peak calling period that must be completed (connected) by the PBX switch network. Calls that are not completed, because the PBX switch network cannot provide the connection between the originating and destination endpoints, are known as blocked calls. The traditional method of stating a customer GoS level for a PBX system is to use the acceptable level of blocked calls instead of completed calls. The PBX’s GoS level is stated with the symbol P, representing a Poisson distribution, although it is more commonly referred to as probability. For example, P(0.01) represents the probability that one call in 100 will be blocked. This is the same as saying that 99 percent of calls will be completed. P(0.01) is the most common GOS level used for PBX traffic analysis and engineering, although customers with more stringent traffic requirements may require a GoS level of one blocked call in 1,000, or P(0.001).

The GoS level is applied during the peak call period, which is typically 1 hour. In traffic engineering analysis, this peak call period is known as the Busy Hour. The Busy Hour for most PBX customers usually occurs during the mid-morning or mid-afternoon hours, although the exact time of day will differ from customer to customer. The GoS at Busy Hour, a worse-case traffic situation, is a unit of measurement indicating the probability that a call will be blocked during peak traffic demand. There are numerous methods used to find the Busy Hour. A common method is to take the 10 busiest traffic days of the year, sum the traffic on an hourly time basis, and then derive the average traffic per hour.

If a customer does not have access to traffic data over a long period, there is a simple method to estimate Busy Hour traffic load based on daily traffic load. Busy Hour traffic for a typical 8-hour business operation is usually 15 to 17 percent of the total daily traffic. Traffic usually builds up from the early morning to mid-morning, declines as lunch hour approaches, builds up again after lunch hour to mid-afternoon, and then declines toward the end of the business day. Traffic during off-business hours is usually very light, but a 24/7 business is likely to have very different traffic patterns from a business keeping traditional 9 to 5 hours.

The Busy Hour analysis must take into account seasonal variations in customer PBX traffic demand, such as the pre-Christmas holiday period. Although the average hourly PBX traffic load may be significantly less than the Busy Hour, and early Monday morning traffic is usually less than Wednesday mid-afternoon traffic, the worse-case situation is used for traffic engineering purposes. PBX switch network resources cannot be increased and decreased for fluctuations in traffic during the day, week, month, or year.