HFN (Hyper-Frame Number)

Introduction

Hyper-Frame Number (HFN) is a term that is often used in the field of wireless communication, specifically in Long-Term Evolution (LTE) networks. HFN is a critical component of the security algorithm used in LTE networks, and it plays a crucial role in preventing unauthorized access and interception of data. In this article, we will discuss the concept of HFN in detail and how it is used in LTE networks.

What is HFN?

In simple terms, HFN is a counter that keeps track of the number of Hyper-frames that have been transmitted between a mobile device and a base station in an LTE network. A Hyper-frame is a unit of time that consists of 1024 frames, and each frame is 10 milliseconds long. Therefore, a Hyper-frame is approximately 10.24 seconds long. The HFN value is used in the security algorithm used in LTE networks to ensure that only authorized devices can access the network and that data transmitted over the network is secure.

Why is HFN important?

HFN is an essential component of the security algorithm used in LTE networks. It is used to prevent unauthorized access and interception of data transmitted over the network. In an LTE network, the base station and the mobile device authenticate each other using a security key called the KASME (Key for Access to the Security Functions of the MME). The KASME is derived from the SIM card inserted in the mobile device and is used to generate security keys that are used to encrypt and decrypt data transmitted over the network.

The security algorithm used in LTE networks is based on the Advanced Encryption Standard (AES) and is designed to provide a high level of security. The algorithm uses a 128-bit key, which is generated using the KASME and the HFN. The HFN value is used to ensure that the same key is not used for too long, as using the same key for too long can make it easier for an attacker to break the encryption and intercept the data transmitted over the network.

How is HFN used in the security algorithm?

In an LTE network, the security algorithm is used to encrypt and decrypt data transmitted over the air interface between the mobile device and the base station. The security algorithm uses a 128-bit key, which is generated using the KASME and the HFN. The key is used to encrypt the data before it is transmitted over the air interface and is decrypted by the recipient using the same key.

The HFN value is used to ensure that the same key is not used for too long. The HFN value is incremented each time a Hyper-frame is transmitted between the mobile device and the base station. When the HFN value reaches a certain threshold, the security key is updated, and a new key is generated using the KASME and the new HFN value.

The threshold at which the HFN value is updated is determined by the network operator and can vary depending on the network configuration. In most LTE networks, the HFN value is updated after a certain number of Hyper-frames have been transmitted. For example, the HFN value may be updated after every 1000 Hyper-frames.

The use of HFN in the security algorithm ensures that the same key is not used for too long, which makes it more difficult for an attacker to break the encryption and intercept the data transmitted over the network.

How is HFN implemented in an LTE network?

In an LTE network, the HFN value is implemented at the Radio Link Control (RLC) layer of the protocol stack. The RLC layer is responsible for ensuring that data is transmitted reliably over the air interface between the mobile device and the base station.

The RLC layer uses a technique called Segmentation and Reassembly (SAR) to divide the data into smaller segments before transmitting it over the air interface. Each segment is assigned a Sequence Number (SN), which is used to ensure that the segments are received in the correct order and that no segments are lost during transmission.

The HFN value is implemented as a 10-bit counter at the RLC layer, and it is incremented each time a Hyper-frame is transmitted. The HFN value is used to derive the key used in the security algorithm, as discussed earlier.

The HFN value is synchronized between the mobile device and the base station during the initial synchronization procedure. The base station sends a random value to the mobile device during the synchronization procedure, which is used to initialize the HFN value. The mobile device then uses the same value to initialize its own HFN counter.

The HFN value is also used in the RLC layer to ensure that no duplicate segments are transmitted over the air interface. Each segment is assigned a unique SN, and the HFN value is used to determine whether a segment with a particular SN has already been transmitted. If a segment with a particular SN has already been transmitted, it is not transmitted again, which helps to conserve bandwidth and reduce network congestion.

Conclusion

Hyper-Frame Number (HFN) is a critical component of the security algorithm used in LTE networks. It is a counter that keeps track of the number of Hyper-frames that have been transmitted between a mobile device and a base station. The HFN value is used to ensure that the same key is not used for too long, which makes it more difficult for an attacker to break the encryption and intercept the data transmitted over the network.

The HFN value is implemented at the RLC layer of the protocol stack in an LTE network and is incremented each time a Hyper-frame is transmitted. The HFN value is synchronized between the mobile device and the base station during the initial synchronization procedure.

In summary, the use of HFN in the security algorithm used in LTE networks helps to ensure that data transmitted over the network is secure and that only authorized devices can access the network.