What is an ACD (automatic call distributor)?

In a traditional on-premises call center, a call passes through other equipment before the ACD sees it. Noah Gans, Ger Koole and Avishai Mandelbaum trace that path in their 2003 review of call center operations. The phone network delivers the call to the company's private branch exchange, or PBX, which is the switch that connects a business to outside lines. The PBX usually connects the caller first to an interactive voice response system, the IVR, which is the recorded menu that asks what the caller needs. Callers who want a person are then handed to the ACD, which the authors describe as "a specialized switch" built to route calls to individual agents. A cloud platform runs the same roles as software inside one service.

That path clears up two mix-ups. The IVR talks to the caller and the ACD chooses who answers, so they are separate jobs even when one vendor sells both. Genesys makes the same point in its definition of an ACD, which says the ACD "focuses on routing, not self-service." The PBX is different again. Its own job is to give employees extensions and outside lines. Choosing the agent is the ACD function, even when it ships inside the same phone system.

The IVR and the ACD do work together, because whatever the caller tells the menu becomes an input to the ACD's choice. The paper's example is an airline whose callers press 3 to identify themselves as Spanish speakers. The ACD then looks for an agent whose login record shows Spanish.

The choice happens in two steps. First the ACD works out which agents are allowed to take the call. Then it picks one of them.

The first step depends on how agents are grouped. Xima, the platform behind Atlantech's contact center service, sorts them into skill groups such as billing or technical support. A supervisor gives each agent a skill level from 1 to 10 in every group that agent works, and 10 marks the most skilled. An agent with no level in a group never receives its calls.

The second step is a setting Xima calls the routing algorithm, and each skill group has its own. Four of the options deliver calls to agents.

Suppose three billing agents are free when a call arrives. One has a skill level of 9 and finished a call 40 seconds ago. The other two are both level 6. One of them has been idle for three minutes and the other for one minute. Most Idle sends the call to the level 6 agent who has waited three minutes. Linear sends it to the level 9 agent, and it will do the same every time that agent is free. Circular takes turns. As long as all three are available when their turn comes, each gets a call before any of them gets a second.

The algorithm you pick says what the queue is for. Most Idle and Circular spread the work evenly, which suits a team whose agents can all handle the same calls. Linear puts most calls on the strongest agents and uses the rest as backup. The fourth option is built for agents who belong to several skill groups. When two agents tie on skill, it gives the call to the one whose absence costs the other groups least, and that keeps the more versatile agent free for them.

Other vendors use different names for the same ideas. RingCentral's ACD page describes round robin, idle time and weighted distribution, among other methods. Genesys Cloud calls its versions evaluation methods. One of them, Best Available Skills, averages each agent's ratings on the skills a call asks for and picks the highest average. The labels won't line up between products, so compare what each setting does to a call.

The ACD holds the call in a queue and plays announcements or music. Callers are normally answered in the order they arrived, and Genesys Cloud tracks that order "down to the millisecond." A business can move chosen callers forward by giving them priority. How that works is part of routing in a contact center.

A queue needs limits, because a caller who waits too long may hang up. In Xima each skill group has a Queue Timeout that transfers a call elsewhere after a set wait. It has a Queue Overflow destination for calls that arrive when every agent is busy or none are logged in, and a cap on how many calls can wait at once. A setting called Agent Ring Time covers the agent who doesn't pick up. When the time runs out, the call goes back to the queue for the next available agent.

Queue callback replaces time on hold. Atlantech lists it among its contact center features, and Xima's version keeps the caller's place in line with a placeholder after the caller hangs up. When the placeholder reaches the front, the system dials the caller back. What happens next depends on the strategy the supervisor chose. Under Wait in Queue, the caller rejoins at position one and takes the next free agent. Under Reserve Agent, the system sets an agent aside before it dials and holds that agent for up to about two and a half minutes while the customer decides.

Each strategy has a cost. A reserved agent takes no other calls while the system dials. Without the reservation, a customer who accepts the callback has to wait again if every agent is busy. Xima suggests Reserve Agent for small teams with long waits, where that second wait is most likely.

The ACD's decisions feed the contact center's reports. Atlantech's service lists real-time wallboards that show agent status and call activity, along with historical reporting it describes as "cradle to grave." The most common figure built from that data is service level. Gans, Koole and Mandelbaum define it as the fraction of calls whose wait fell below a target, and they note the target is typically 20 or 30 seconds.

The target is your choice, so the report tells you how often you met the number you picked.

Cloud platforms use the ACD for more than phone calls. Genesys Cloud says its ACD matches "calls, chats, emails, messages, and callbacks" with available agents. How one set of routing rules shares agents across those channels is covered in what a unified queue is. The ACD is one piece of a larger set of features in a modern contact center.

The rules can differ by channel, though. In Xima, email routing always uses Linear distribution, whatever algorithm the skill group is set to. Each email goes to the highest-skilled agent who is available, with idle time as the tiebreak, even in a group where calls are shared evenly. We'd check for exceptions like that on any platform before assuming the setting chosen for calls applies to every channel.