SIB or System Information Block 1

Last updated on Mar 14, 2023

SIB or System Information Block 1 is a crucial element of the radio interface protocol used in GSM (Global System for Mobile Communications), which is a widely used cellular network standard. SIB 1 is transmitted by the base station to the mobile device, and it contains essential information about the network, such as the PLMN (Public Land Mobile Network) ID, cell ID, supported frequencies, and other network parameters. This information is used by the mobile device to establish a connection with the network and to choose the most suitable cell and frequency to communicate with.

In this article, we will discuss the technical aspects of SIB 1 in detail, including its structure, contents, and transmission procedures. We will also explain the role of SIB 1 in the overall GSM system and how it affects the performance of the network.

Structure of SIB 1

SIB 1 is a type of control channel message that is transmitted on the BCCH (Broadcast Control Channel) of the GSM network. The BCCH is a dedicated frequency channel that is continuously broadcast by the base station to all mobile devices in its coverage area. The BCCH carries various types of control information, including SIBs, paging messages, and synchronization information.

SIB 1 consists of the following fields:

Message type (MT): A 6-bit field that identifies the message type as SIB 1.
Message length (ML): An 8-bit field that indicates the length of the SIB 1 message in bytes.
PLMN ID: A 24-bit field that identifies the public land mobile network (PLMN) to which the base station belongs. The PLMN ID is assigned by the national regulatory authority and consists of a 3-digit Mobile Country Code (MCC) and a 2- or 3-digit Mobile Network Code (MNC).
Cell ID: A 16-bit field that identifies the cell within the PLMN to which the base station belongs. The cell ID is assigned by the network operator and is unique within the PLMN.
Access parameters: A 14-bit field that contains various access parameters, such as the maximum transmit power level, the minimum received signal level, and the frequency hopping sequence number.
Frequency list: A variable-length field that contains the list of frequencies supported by the cell. Each frequency is represented by a 12-bit value that encodes the ARFCN (Absolute Radio Frequency Channel Number), which is a unique identifier for the frequency channel.
Location area ID (LAI): A 24-bit field that identifies the location area within the PLMN to which the cell belongs. The LAI is assigned by the network operator and is used for location management and handover purposes.
Cell reselection parameters: A 14-bit field that contains the cell reselection parameters, such as the minimum received signal level and the cell reselection hysteresis.
Power control parameters: A 9-bit field that contains the power control parameters, such as the target received signal level and the power control hysteresis.
Other network parameters: A variable-length field that contains other network parameters, such as the paging group identity, the cell identity type, and the broadcast control channel description.

Contents of SIB 1

The contents of SIB 1 can be divided into two categories: mandatory and optional. The mandatory contents are required by the GSM specifications and must be included in every SIB 1 message, while the optional contents are defined by the network operator and may or may not be included in the message.

The mandatory contents of SIB 1 include the PLMN ID, cell ID, access parameters, and frequency list. These parameters are essential for the mobile device to identify and select the most suitable cell and frequency to communicate with.

The PLMN ID is a globally unique identifier that identifies the network operator to which the base station belongs. The mobile device uses the PLMN ID to determine whether it is currently in the home network or in a visited network, and to select the appropriate network to register with.

The cell ID is a locally unique identifier that identifies the cell within the PLMN to which the base station belongs. The mobile device uses the cell ID to determine which cell it is currently located in and to choose the most suitable cell to communicate with.

The access parameters define the maximum transmit power level, the minimum received signal level, and the frequency hopping sequence number. These parameters are used by the mobile device to adjust its transmit power and to synchronize with the base station.

The frequency list contains the list of frequencies supported by the cell. Each frequency is represented by an ARFCN, which is a unique identifier for the frequency channel. The mobile device uses the frequency list to determine which frequencies are available for communication and to select the most suitable frequency based on the received signal quality.

The optional contents of SIB 1 include the LAI, cell reselection parameters, power control parameters, and other network parameters. These parameters are defined by the network operator and may vary depending on the specific implementation of the GSM network.

The LAI is a geographic area within the PLMN to which the cell belongs. The mobile device uses the LAI to determine its current location and to assist in the handover process when moving between cells.

The cell reselection parameters define the minimum received signal level and the cell reselection hysteresis. These parameters are used by the mobile device to determine when to reselect to another cell with a stronger signal.

The power control parameters define the target received signal level and the power control hysteresis. These parameters are used by the base station to adjust the transmit power of the mobile device to maintain a consistent signal quality.

The other network parameters include various network configuration parameters, such as the paging group identity, the cell identity type, and the broadcast control channel description. These parameters are used by the mobile device to configure its communication with the network.

Transmission of SIB 1

SIB 1 is transmitted by the base station on the BCCH frequency channel using a specific transmission scheme defined by the GSM specifications. The BCCH frequency channel is a dedicated channel that is continuously broadcast by the base station to all mobile devices in its coverage area.

The transmission of SIB 1 is divided into two parts: the broadcast of the SIB 1 message and the scheduling of the next broadcast. The broadcast of the SIB 1 message occurs at a regular interval, typically once every 5 seconds, and it contains the current values of the mandatory and optional parameters.

The scheduling of the next broadcast is determined by the network operator based on various factors, such as the network load and the mobility of the mobile devices. The network operator may also send a special command to the base station to modify the scheduling parameters, such as the broadcast interval and the broadcast time offset.

The mobile device listens for the BCCH frequency channel and decodes the SIB 1 message to obtain the necessary network information. The mobile device may also store the received SIB 1 message in its memory for future reference and comparison with subsequent SIB 1 messages.

Role of SIB 1 in the GSM system

SIB1 plays a critical role in the operation of the GSM system. It provides the necessary network information to mobile devices, enabling them to identify and select the most suitable cell and frequency to communicate with. Without SIB 1, the mobile device would be unable to establish a connection with the network and would be unable to make or receive calls or data.

SIB 1 is also essential for the proper operation of various network functions, such as handover and cell reselection. Handover is the process of transferring a call from one cell to another without interrupting the call. Handover requires accurate measurement of the signal quality of the current and target cells, which is facilitated by the information provided in SIB 1.

Cell reselection is the process of selecting the most suitable cell to communicate with based on the received signal quality. Cell reselection is essential for ensuring a consistent and reliable connection between the mobile device and the network, especially when the mobile device is moving between cells.

SIB 1 is also used by the network operator to configure and optimize the network parameters. The network operator may modify the contents of SIB 1 to adjust the network parameters, such as the cell reselection parameters and power control parameters, to optimize the network performance and improve the user experience.

Conclusion

SIB 1 is a critical component of the GSM system that provides the necessary network information to mobile devices. It contains essential parameters, such as the PLMN ID, cell ID, access parameters, and frequency list, that enable the mobile device to establish a connection with the network and select the most suitable cell and frequency to communicate with. SIB 1 also contains optional parameters, such as the LAI, cell reselection parameters, power control parameters, and other network parameters, that enable the network operator to configure and optimize the network parameters. The proper operation of SIB 1 is essential for the reliable and efficient operation of the GSM system and the delivery of high-quality voice and data services to the users.