GSM Bearer Services
• Bearer Services represent layers 1-3 of the OSI Model
• Transparently transports application data between TEs
• Bearer Services are uniquely numbered (BSxx)
• Data transfer bearer services currently defined for GSM include:
• Asynchronous circuit-switched data (BS 21-26)
• Synchronous circuit-switched data (BS 31-34)
• PAD access (BS 41-46)
• Packet data (BS 51-53)
• Alternate speech and data (BS 61)
• Speech followed by Data (BS 81)
GSM bearer services refer to those services provided to transport binary data across the GSM
network. They represent the functionality of layers 1 to 3 of the OSI 7-layer model.
Both circuit switched (BS21-34) and packet switched (BS 41-53) data bearer services are
supported. The packet data services are in the form of asynchronous access to PADs (BS41-
44) or as direct synchronous packet access (BS 51-53).
Two modes of bearer services are offered; Transparent (T) or Non-Transparent (NT). The
transparent mode services provide a circuit-switched connection between the TE and the IWF
module. They are generally constant bit rate and are only forward error protected (FEC).
Non-transparent mode services are protected by level 2 error protection over the air interface
using a Radio Link Protocol (RLP). This protocol terminated at the MSC and uses backward
(ARQ) error protection. Non-transparent mode operation generally offers a more reliable
transmission path but at lower potential data rates than transparent mode services.
• Each Bearer Service is uniquely identified by its characteristics which
include:
• Service Type (data, PAD, packet etc)
• Structure (asynch, synch)
• Bit Rate (300-14400 bps)
• Mode (transparent, non-transparent)
• Transmission (Unrestricted Data Information (UDI) or 3.1kHz)
Bearer services 21-53 are further categorised into Unrestricted Digital Information (UDI) or
3.1kHz. The distinction is only important when operating outside the PLMN i.e. what type of
Interworking functions needs to be implemented. The distinction is whether the data should
be handled as the equivalent of 3.1kHz bandwidth audio signals over a modem or raw data
over a digital link.
The last two categories of bearer services shown in the diagram above refer to services which
enable switching between voice and data during a session. BS 61 refers to the ability change
between voice and data at will during a session (‘alternate voice and data’). BS 81 refers to the
ability to initiate a call in voice and then switch to data (‘speech followed by data’)
• Bearer Services represent layers 1-3 of the OSI Model
• Transparently transports application data between TEs
• Bearer Services are uniquely numbered (BSxx)
• Data transfer bearer services currently defined for GSM include:
• Asynchronous circuit-switched data (BS 21-26)
• Synchronous circuit-switched data (BS 31-34)
• PAD access (BS 41-46)
• Packet data (BS 51-53)
• Alternate speech and data (BS 61)
• Speech followed by Data (BS 81)
GSM bearer services refer to those services provided to transport binary data across the GSM
network. They represent the functionality of layers 1 to 3 of the OSI 7-layer model.
Both circuit switched (BS21-34) and packet switched (BS 41-53) data bearer services are
supported. The packet data services are in the form of asynchronous access to PADs (BS41-
44) or as direct synchronous packet access (BS 51-53).
Two modes of bearer services are offered; Transparent (T) or Non-Transparent (NT). The
transparent mode services provide a circuit-switched connection between the TE and the IWF
module. They are generally constant bit rate and are only forward error protected (FEC).
Non-transparent mode services are protected by level 2 error protection over the air interface
using a Radio Link Protocol (RLP). This protocol terminated at the MSC and uses backward
(ARQ) error protection. Non-transparent mode operation generally offers a more reliable
transmission path but at lower potential data rates than transparent mode services.
GSM Bearer Service Characteristics
• Each Bearer Service is uniquely identified by its characteristics which
include:
• Service Type (data, PAD, packet etc)
• Structure (asynch, synch)
• Bit Rate (300-14400 bps)
• Mode (transparent, non-transparent)
• Transmission (Unrestricted Data Information (UDI) or 3.1kHz)
Bearer services 21-53 are further categorised into Unrestricted Digital Information (UDI) or
3.1kHz. The distinction is only important when operating outside the PLMN i.e. what type of
Interworking functions needs to be implemented. The distinction is whether the data should
be handled as the equivalent of 3.1kHz bandwidth audio signals over a modem or raw data
over a digital link.
The last two categories of bearer services shown in the diagram above refer to services which
enable switching between voice and data during a session. BS 61 refers to the ability change
between voice and data at will during a session (‘alternate voice and data’). BS 81 refers to the
ability to initiate a call in voice and then switch to data (‘speech followed by data’)