Understanding the Mobile Disconnect Problem for Important Conference Calls

Conference calls are very different from a typical mobile phone call. In our previous post, Why Your Mobile is Killing Your Client Calls (and What to do About it!), we mentioned that one study found that the huge majority of mobile calls last less than 30 minutes. This is in sharp contrast to conference calls which range from 30 minutes to over two hours long.

Of course, we live and breathe conference calls literally every day. Experience shows us that key participants in a conference call, those who are actively engaged in the conversation, should not connect to calls with their mobile phones.

The reason for that is what we’ve come to think of as “the mobile disconnect problem.”

A simple call between two people

Suppose for the sake of argument that a mobile connection has a 50% chance of being dropped over the course of a 30 minute call (and that’s probably conservative).

Landlines are almost flawless with respect to disconnects, so let’s just assume they don’t contribute to the problem.

One person is on a land-line and you are on a mobile phone. You intend to talk for 30 minutes. There is a 50/50 chance that you’ll suffer a disconnection before you’re done.

Now suppose that the call is going to last 90 minutes. The probability that your call will be dropped increases dramatically.

Time (Over the course of 90 minute call)	Probability of Call Being Dropped
First 30 Minutes	0.5
Middle of the Call	0.5
Last 30 Minutes	0.5

In reality, the chance that your mobile call does not get dropped over the course of 90 minutes is:

0.5 x 0.5 x 0.5 = 0.125

That means there’s a 0.875, or seven out of eight chance, that you won’t go the full 90 minutes without getting interrupted.

While this is admittedly a simplified analysis, it’s accurate in illustrating the nature of the issue. It correctly highlights the magnitude of the potential for disruption.

More Than One Mobile Phone

Our initial example was a simple call between two people.

Conference calls further alter the probabilities since there are often more than two people involved. That makes it possible that two or more people will join the conference using their mobile phones, which further reduces the probability of a a trouble-free call.

For example, a 60 minute call with two mobiles has a 93% chance of having at least one party suffer a disconnect, using our same 50% chance of disconnect during 30 minutes (per connection).

A Further Disconnect Example

Let me bore you with one more statistical exercise. Suppose in the morning I am on 20 successive mobile phone calls, each lasting six minutes. So I’m on the phone for 120 minutes, and I get dropped once.

That’s one in 20 calls that have been dropped or just 5%.

But in the afternoon, I am on two conference calls, each lasting an hour. I still suffer one drop, but that means that 50% of my conference calls have been interrupted – 10 times worse than in the morning.

Conditions are no worse in the afternoon than in the morning, but the impact appears far greater.

You can see from this analysis that the biggest reason that mobile drops occur during conference calls is that they last a long time compared to most mobile calls.

What Happens When a Call is in Motion

The wireless environment over which a mobile call must travel is dynamic. When a mobile handset user is in motion several things are always happening:

Radio Link Re-Negotiation

The radio link, from the handset to the tower, is routinely getting re-negotiated. As the user moves away from one tower, the signal weakens. As long as another tower is in-range and has capacity, a “hand-off” will occur.

But if there is no tower in range, or no capacity at that tower, the call will drop. Sometimes, in the fraction of a second it takes for a hand-off to occur, another user initiates a call and takes the slot that your phone thought it would use.

Blocked Phone Signal

As the user moves about, the path from the phone to the tower may become blocked by a building or a hill or an overpass or trucks on the road, weakening or entirely obliterating the signal.

If this only lasts a few seconds, the user may just suffer some silence; if longer, the call will drop.

Radio-Frequency Noise

Also as the user moves, they become subject to more (or less) interference. As the user rises in elevation (going up a hill or to a higher floor in a building), they will be subject to more radio-frequency noise from other active mobile phones.

While this noise is not usually audible, it interferes with the phone’s reception and transmission capabilities and can cause a call to drop.

What Happens When Mobile Users Are Not In Motion?

Even when a mobile user is not in motion the radio environment is changing, causing phone calls to get dropped.

How “In Range” Phones Drop Calls

As other users in the same “cell” get on their mobile phones, the total radio noise goes up; this causes the mobile system to instruct transmitters to reduce power to limit interference.

That results in the effective coverage of the cell decreasing; a phone that was “in range” moments ago may no longer be, resulting in a dropped call.

Moving Vehicles Cause Interference

Vehicles (trucks, trains, airplanes) moving about nearby can block the radio path, causing a call to drop.

When You’re On a Coverage Border

A stationary user may unknowingly be “toggling” between two adjacent cell towers if they are on the coverage border; this means they are repeatedly being handed-off from one tower to another and then back again, with a finite possibility of being dropped each time.

The Advantages of Land-Lines

All of the above is in sharp contrast to the legacy landline environment, which traditionally was engineered to be deterministic and is much less prone to “dropping” mid-call:

• Dedicated Wires: From the user’s location, the connection usually travels to a nearby (few blocks to few miles) Central Office (CO) over a wire that is dedicated to that user and explicitly shielded from interference.
• Dedicated Channel: On a higher-capacity business connection, the user’s call will still be assigned to a “channel” between the office and the CO; the channel will be dedicated to the user for the duration of the call. A new call will not be permitted to start if no channel is available.
• Protected Wires (Fiber): The wire (or fiber) link is either strung high on pole-tops or buried underground, but is generally isolated from physical upset.
• Failover Protection: Once at the CO, the signal will be assigned to an onward channel that again will be dedicated for the duration of the call. Most higher-capacity network connections are protected by duplicate or redundant facilities with automatic failure detection and near-instantaneous cutover.

Dropped Calls on ZipDX

Here’s one question that we’re often asked about when dropped calls occur using the ZipDX platform.

Q: Why doesn’t ZipDX call the participant back automatically if they are dropped?

A: The biggest reason is because we can’t tell the difference between the participant being dropped or them disconnecting intentionally on their own.

Further, if they’ve been dropped because they’ve just stepped into an elevator or driven into a tunnel, we might need to wait an indeterminate amount of time for them to become available again.

Fortunately, our Identity Conferencing scheme makes it very simple for someone to rejoin a call if they’ve been disconnected. When the disconnected participant redials ZipDX the system will recognize their caller-ID, quickly returning them to the call. There’s no need to enter a conference code.

Ultimately, to avoid the hassle and wasted time of calls getting dropped and callers not understanding each other, avoid using your mobile phone for important business conference calls.