Capacity Planning
Understanding Capacity Planning
Traffic Engineering and Erlang Theory
Capacity planning is a critical discipline in telecommunications network engineering. While coverage planning ensures that users have a usable signal strength across a geographic area, capacity planning focuses on ensuring there are enough radio resources, switches, and backhaul bandwidth to support the volume of active calls and data sessions. If a cell site has excellent signal coverage but insufficient radio channels, users will experience blocked calls or slow data rates.
To calculate the required resources, engineers utilize Erlang traffic models. Offered traffic is measured in Erlangs, where one Erlang represents one continuous hour of channel usage. Using the Erlang B formula, which assumes that blocked calls are cleared and not retried immediately, planners can determine the minimum number of RF channels required to maintain a specific Grade of Service (GoS), typically defined as a blocking probability of 1% or 2% during the peak busy hour.
Transition from Coverage-Limited to Capacity-Limited Networks
When a wireless network is first deployed, it is typically coverage-limited. Sites are spaced widely, and the primary goal is to overcome path loss. As the subscriber base grows and data consumption increases, the network transitions into a capacity-limited regime. In this state, adding more transmit power does not help because the channels are congested and co-channel interference becomes the dominant issue. Capacity planners resolve this by splitting cells, deploying small cells, sectorizing existing towers, and implementing carrier aggregation to increase the density of available radio resources.
Key Mathematical Relations
Technical Specifications Comparison
| Planning Aspect | Coverage Planning | Capacity Planning |
|---|---|---|
| Primary Objective | Ensure signal availability and adequate SNR | Ensure channel availability for user traffic |
| Key Metrics | RSSI, RSRP, Path Loss, Cell Edge Signal | Blocking Probability (GoS), Throughput, Erlangs |
| Core Limitation | Geographical terrain, free-space path loss | Subscriber density, frequency reuse, backhaul bandwidth |
| Common Solutions | High-gain antennas, taller towers, lower frequency bands | Small cells, sectorization, carrier aggregation, Wi-Fi offload |
Frequently Asked Questions
What is an "Erlang" in telecommunication capacity planning?
An Erlang is a dimensionless unit of traffic intensity. It represents the continuous occupancy of a single voice channel or resource for one hour. For example, if a group of users generates a total of 60 minutes of call duration during a single hour, that represents 1 Erlang of traffic.
How does cell splitting help resolve capacity issues?
Cell splitting is the process of dividing a single congested cell area into smaller cells, each with its own low-power base station. This allows the same frequency bands to be reused over shorter distances, multiplying the number of active channels available within the same geographical footprint.
Why is backhaul capacity crucial in RF network planning?
Increasing the capacity of the radio link (such as upgrading a sector to 5G) is ineffective if the link connecting the base station to the core network (the backhaul) is congested. Planners must size the optical fiber or microwave backhaul link to handle the aggregate peak traffic from all sectors on a tower.