Signaling Types in EPABX/PBX Systems

In the realm of telephony, particularly with EPABX (Electronic Private Automatic Branch Exchange) and PBX (Private Branch Exchange) systems, signaling plays a vital role in call establishment and management. When we talk about the trunk side – the connections between telephone exchanges – there are primarily three signaling types used in analog telephony for supervisory signaling: Start Dial signaling, Ground Start signaling, and E & M signaling. Let’s dive into each of these.

Start Dial Signaling

Start Dial signaling encompasses a few different methods, each with its nuances.

• Wink Start: In this scenario, the originating trunk initiates the process by going off-hook (taking the line). The remote switch then responds by transmitting an off-hook pulse, signaling that it’s ready to receive digits. Finally, the originating switch sends the dial digits (the phone number being called).

• Immediate Start: As the name suggests, this method is more direct. The originating switch immediately places the trunk off-hook and starts transmitting the dial digits without waiting for a signal from the remote switch.

• Tone Start: Here, the originating switch goes off-hook. The receiving switch, upon detecting this, generates a dial tone, indicating it’s ready to receive the phone number. The dial digits are then transmitted.

Ground Start Signaling

Ground Start signaling is often used in PBX systems.

• It involves switching trunk lines and momentarily grounding the ring lead.
• PBXs commonly use Ground Start, detecting off-hook status via a grounded wire. This is actually a modified version of local loop signaling.
• A key advantage of Ground Start is that it prevents both switches from simultaneously gaining control of the trunk, which could lead to conflicts.
• It also provides a mechanism for current detection, enhancing reliability.

E & M Signaling

E & M signaling is arguably the most prevalent type.

• A significant feature of E & M signaling is that it uses separate paths for voice and signaling data.
• The M lead is dedicated to transmit signaling (outbound – from the originating switch).
• The E lead is used for receive signaling (inbound – to the originating switch).
• The process involves the originating switch requesting a trunk by raising the M lead. The other side of the switch responds by raising the E Lead.

Here’s a table summarizing the states of the E and M leads: