LTE And The Number Of Simultaneously Connected Users

Recently I read this very interesting post that analyzed how many users can be connected to an LTE eNodeB simultaneously. As the author says in the article there is no single answer to this as many factors are playing into the game. From a pure uplink signaling point of view there can be well over 1000 users simultaneously in RRC-Connected state. In practice I wouldn’t be surprised if networks set that number way lower as that number is far too high to give anyone a halfway decent speed experience. So how many might there be connected simultaneously to a base station today? Let’s play with some numbers.

It’s All About State

Before I start let’s define what we are looking for here. Basically, an LTE device can be in three states:

Off

It can be switched off or in flight mode in which case it is not connected to the network at all.

RRC-Idle

When it is switched on but not transferring data the network puts the device in the RRC-Idle state after some time. In this state it still has an IP address but there is no context for the device in the basestation (eNodeB). The only thing the device does in this state is listening to system information broadcasts, takes signal strength measurements, performs cell changes and listens to the paging channel so it can react to incoming circuit switched voice calls, SMS messages and incoming IP data packets. This is a listen only mode so no resources are required at the base station to keep the device happy.

RRC-Connected

When data needs to be transferred the UE establishes an active connection to an eNodeB. To keep the over-the-air connectivity, the mobile and the network have to send additional signaling data periodically every few tens of milliseconds, especially in the uplink direction even if no IP packets are transmitted. This is required so the network knows that a device is still reachable and to get information about the signal quality experienced in uplink and downlink so future IP packets can be coded and modulated for the current reception conditions. Once the transmit buffer is empty on both sides the connection remains in RRC-Connected state as the mobile device and eNodeB can’t know if there will be follow on traffic or not. Only if there is no further IP traffic for some time does the network set the physical connection from RRC-Connected to RRC-Idle. This state change delay is configurable and a typical value I’ve observed in practice is 60 seconds. That means that even if only few IP packets are exchanged for a second or two, e.g. for an instant messenger status update, the device stays in RRC-Connected state for over a minute and periodically transmits status information in the uplink direction during that time. During that time the network can set the device to Connected-DRX (Discontinuous Reception) state that reduces the amount of status signaling in the uplink direction at the expense of additional delay before the device can transmit data again once IP packets from an app end up in the transmission buffer.

Let’s Do Some Math

So taking the above into account let’s say that the majority of devices in an LTE network today are smartphones that generate a lot of background traffic from instant messengers and chatty other apps. Let’s assume that happens once every 4 minutes, i.e. 15 times an hour. In other words, each device is connected for 15 minutes every hour to the network. It sounds like a lot but let’s err on this side of the equation rather being too conservative. During that time a device transfers almost no data but it is still connected and depending on how Connected-DRX is configured sends more or less signaling data.

Two other numbers we require for a ballpark estimation of the number of simultaneous users in Connected state at an LTE base station is the number of base stations in the network and the number of LTE devices. While Vodafone doesn’t give specific details I take it from their press releases that they must have around 20.000 basestation sites in Germany from which they serve around 40 million customers. Let’s say out of those 40 million customers there are 15 million LTE smartphones. The number might be too high but let’s again err on the high end here.

In other words, 15 million devices are distributed among 20.000 base stations, which would be 15.000.000 / 20.000 = 750 devices per basestation. Each basestation has 3 sectors which results in 250 devices per sector. As each device is ‘only’ active one fourth of the time that would be around 63 devices that are in RRC-Connected state at the same time. As some basestations serve more devices than others let’s be generous and bump that number up to 100 devices in RRC-Connected state. When compared to the more than 1000 devices that could transfer signaling information in the uplink direction there is still a reassuringly big delta between the two numbers.

Agreed, it’s a crude calculation but it should give good ballpark estimation if how many simultaneously connected users we are talking about in practice today.