Pulse Code Modulation | Fundamentals of PBX Circuit Switching

PCM is a sampling technique for digitizing the analog voice-originated audio signals. PCM samples the original analog signal 8,000 times a second. This is more commonly referred to as 8-KHz sampling. The sampling rate used to code voice audio signals is based on the frequency range of the original signal. To accurately represent an analog signal in digital format, it is necessary to use a sampling rate twice the maximum analog signal frequency, a calculation based on the Shannon theorem. The maximum frequency of human voice is about 3.1 KHz. This frequency was rounded up to 4 KHz for ease of engineering design, resulting in an 8-KHz (2 × 4 KHz) sampling rate for digitizing voice audio signals. An 8-KHz sampling rate translates into a one sample every 125 microseconds (8 KHz^–1; Figure 1).

Figure 1: PCM encoding.

Each digital sample is represented by an 8-bit word (2⁸ = 256 sample levels) that measures the amplitude of the signal. The amplitude of the signal is based on the power (expressed in units of voltage) of the electri- cal signal generated by the telephone transmitter/receiver in the handset. This signaling technique has become known as Digital Signaling 0 (DS0), or 64-Kbps (8 bits × 8 KHz) channel transmission format. The term DS0 was defined based on the Digital Signaling 1 (DS1) format used to describe a digital T1-carrier communications circuit supporting 24 64-Kbps communications channels.

The PCM samples generated from each communications system port are transmitted onto the TDM bus in a continuously rotating sequence based on the time slot assignments given to each port circuit interface (see below). Only a single PCM word sample is transmitted at a time; that is the entire electrical transmission line is reserved for use by only one port circuit for transmission of its sample signal. The PBX processing system monitors each port circuit’s transmission time assignment in the rotating sequence, controls when the sample is transmitted, and coordinates transmission of the sample between the originating and destination endpoints.

There are two standards for coding the signal sample level. The Mu-Law standard is used in North America and Japan, and the A-Law standard is used in most other countries throughout the world, although each uses the 64-Kbps transmission format. For this reason, PBX systems must be designed and programmed for different geographic markets. Using firmware downloads, system vendors and customers can program their PBX systems to support the local PCM standard. The early digital PBX systems used different hardware equipment based on the location of the installation.

To summarize the fundamentals of PCM:

1. 4-KHz analog voice signals are sampled 8,000 times per second (8-KHz sampling rate)

2. Each sample produces an 8-bit word number (e.g., 11100010)

3. 8-bit samples are transmitted onto the TDM bus at a 64-Kbps transmission rate

4. The samples from each port circuit are transmitted in a continuously rotating sequence

TDM Bus Bandwidth and Capacity

Bandwidth is the amount of data that can be transmitted in a fixed period. For digital transmission, bandwidth is expressed in bits per second; for analog transmission, bandwidth is expressed in cycles per second (Hertz). The bandwidth of an 8-bit PBX TDM bus is determined by the internal switching system clock rate used to create time slots for each channel’s transmission. The faster the clock rate, the more digitized samples per second can be transmitted over the TDM bus. The clock functions merely as a counter; the faster it “counts,” the more sampled digital signals within a fixed period (usually defined as 1 second) can be transmitted over the TDM bus. For example, an 8-bit TDM bus operating at 2.048 MHz has a bandwidth of 16 Mbps. If you double the clock rate (double the operating frequency), the bandwidth capacity doubles.

If the operating frequency of the TDM bus is not provided but the number of time slots is known, the bandwidth of a TDM bus can be calculated by multiplying the number of time slots (as determined by the system clock rate) by 64 Kbps (the number of transmitted bits per communications channel). A TDM bus segmented into 32 time slots has a transmission bandwidth of 2.048 Mbps (32 × 64 Kbps ). A system with a faster clock rate that is capable of segmenting the TDM bus into 512 time slots would have a bandwidth of 32.64 Mbps (512 time slots × 64 Kbps). It is usually awkward to refer to the TDM bus bandwidth by the exact transmission capacity, so it is common to see the TDM bus bandwidth written, and referred to, as a 32-Mbps TDM bus. The most common PBX TDM bus bandwidths are usually based on exponential multiples of 2 Mbps (2ⁿ Mbps): 2 Mbps, 8 Mbps, 16 Mbps, or 32 Mbps.

Not all of the time slot segments on a TDM bus are designed to handle communications traffic. Most PBX system TDM buses reserve a few time slots for the transmission of control signaling across the internal system processing elements. For example, a control signal time slot is used to alert the main system control complex that a telephone has gone off-hook. The signal is passed from the telephone instrument to the port circuit card and across the internal processing/switching transmission network via the local TDM bus. A variation of this design is to dedicate an entire TDM bus for control signaling. Examples of PBX systems using a dedicated signaling bus for system port control are the Siemens Hicom 300H and Hitachi HCX 5000 products. When a single bus is used for communications and processing functions, the control signaling time slots are not available for port communications requirements and should not be considered in the analysis of the system’s traffic handling capabilities. Time slots that can be used for real-time communications applications are sometimes referred to as talk slots. The total number of talk slots and control signaling slots per TDM bus are equal to the number of time slots (Figure 2).

Figure 2: TDM transmission bus.

The number of available talk slots limits the number of active PBX ports that can be simultaneously supported by a single, common TDM bus. An active PBX port is simply defined as a port that is transmitting and receiving real-time communications signals—on-line. A port may be customer premises equipment working behind the PBX system, such as a telephone, or an off-premises trunk circuit. For example, a 2.048-Mbps TDM bus with 30 talk slots can support 30 active communications ports (telephones, modems, facsimile terminals, trunk circuits, voice mail ports, etc.).

Port-to-Port Communications over a Single TDM Bus

When a station port is about to become active, the PBX processing system will assign that port a talk slot on the local TDM bus connected to the port’s circuit interface card. For the remainder of the call, the port will use the designated talk slot to transmit its digitized voice communications signals across the internal circuit switched network. The port receiving the call may be another station or a port interface connected to a trunk circuit. If the originating station port places an internal system call to another station, the PBX processing system will assign the destination port a designated talk slot on its interface circuit card TDM bus. If the originating station port is making an off-premises call requiring a trunk circuit connection, the processing system will assign the trunk circuit port a talk slot on the TDM bus supporting its interface circuit card. The same process takes place for incoming trunk calls to user stations: the trunk interface port circuit is assigned a talk slot, as is the destination station interface port circuit. The circuit switching system will use the two designated talk slots to connect the two ports together to transmit and receive communications signals for the duration of the call. The two talks slots will work in tandem for talking and listening between ports, with each port physically linked to both talk slots.

The number of talk slots required per call will depend on two conditions:

1. The number of connected ports per call

2. The number of TDM bus segments required for port connections

A two-party conversation between PBX ports interfacing with the same TDM bus will require two talk slots, but multiparty conference calls will require as many talk slots as conference parties. For example, a four-party conference call would require four TDM bus talk slots. A small PBX system based on a circuit switched network design consisting of a single TDM bus will require only two talk slots per two-party call, but intermediate and large PBX systems designed to support hundreds or thousands of station and trunk ports will have switching network designs based on many interconnected TDM bus segments, and more than two talk slots will be required for an internal two-party call. More than four talk slots will be required for a four-party conference call if the ports are housed in different port equipment cabinets. The answer to the question of how many talk slots are needed per call in an intermediate or large PBX system requires some knowledge of the PBX switch network design.