Transmission protocols relate to the physical transportation of raw data across the GSM
network and the various protocols associated with this function
At the transmission level, the physical layer produces user channels each comprising 13kbps
of user data. However, in order to adapt top the frailties of the radio interface additional
overheads are included for forward error detection and correction (FEC).
In addition the transmission layer control encryption as well as the air interface
FDMA/TDMA access procedures (see Section 4 of these course notes for more details).
At some stage the 13kbps channels must be converted into 64kbps for transportation across
the PSTN. This function is carried out by a network element known as the Transcoder and
Rate Adaption Unit (TRAU).
The primary function of the TRAU is to convert 16kps (inc signalling) GSM speech channels
to 64kbps PCM channels in the uplink direction and the reverse in the downlink direction.
The reason this process is necessary is because MSCs only switch at the 64kbps channel level.
Therefore, when making a MS-to-MS call, the originating channel has to be converted from
16kbps to 64kbps, switched by the MSC and then converted back to 16kbps for onward
transmission to the destination MS. The rate conversion is carried out using A-law coding.
Technically, the TRAU can be physically located in the BTS, BSC or MSC and hence leads to a
variety of installation configurations.
network and the various protocols associated with this function
GSM Transmission Protocol Stack
(TRAU at MSC)At the transmission level, the physical layer produces user channels each comprising 13kbps
of user data. However, in order to adapt top the frailties of the radio interface additional
overheads are included for forward error detection and correction (FEC).
In addition the transmission layer control encryption as well as the air interface
FDMA/TDMA access procedures (see Section 4 of these course notes for more details).
At some stage the 13kbps channels must be converted into 64kbps for transportation across
the PSTN. This function is carried out by a network element known as the Transcoder and
Rate Adaption Unit (TRAU).
The primary function of the TRAU is to convert 16kps (inc signalling) GSM speech channels
to 64kbps PCM channels in the uplink direction and the reverse in the downlink direction.
The reason this process is necessary is because MSCs only switch at the 64kbps channel level.
Therefore, when making a MS-to-MS call, the originating channel has to be converted from
16kbps to 64kbps, switched by the MSC and then converted back to 16kbps for onward
transmission to the destination MS. The rate conversion is carried out using A-law coding.
Technically, the TRAU can be physically located in the BTS, BSC or MSC and hence leads to a
variety of installation configurations.
TRAU Configurations
If the TRAU is installed at the BTS, each 16kbps GSM channel would need to be mapped to its
own 64kbps PCM channel. This results in 75% of the transmission bandwidth being wasted
across both the Abis (BTS-BSC) and A (BSC-MSC) interface.
However, if the TRAU is placed at the MSC, as is generally the case in current networks, a
multiplexer can be placed at the BTS which enables 4 x 16kbps GSM channels to be
multiplexed onto one 64kbps PCM channel, using 4 x 16kbps ISDN D-channels. In this
configuration, only at arrival at the MSC is the 16-64kbps channel conversion necessary,
thereby maximising the efficient usage of the transmission medium by increasing the GSM
channel throughput per PCM 2048 bearer from 30 to 120 channels.
Also, by centralising the TRAU function at the MSC, the number of TRAUs deployed across
the network is significantly reduced.
own 64kbps PCM channel. This results in 75% of the transmission bandwidth being wasted
across both the Abis (BTS-BSC) and A (BSC-MSC) interface.
However, if the TRAU is placed at the MSC, as is generally the case in current networks, a
multiplexer can be placed at the BTS which enables 4 x 16kbps GSM channels to be
multiplexed onto one 64kbps PCM channel, using 4 x 16kbps ISDN D-channels. In this
configuration, only at arrival at the MSC is the 16-64kbps channel conversion necessary,
thereby maximising the efficient usage of the transmission medium by increasing the GSM
channel throughput per PCM 2048 bearer from 30 to 120 channels.
Also, by centralising the TRAU function at the MSC, the number of TRAUs deployed across
the network is significantly reduced.
G.703
The TRAU converts the GSM channels into ISDN D-channels in accordance with the G.703
Recommendation. This Recommendation specifies physical and electrical characteristics of
the interfaces at hierarchical bit rates that are described in Recommendation G.702. The
interfaces are defined in terms of general characteristics, specifications at the output ports and
input ports and/or cross-connect points, earthing of outer conductor or screen and coding
rules etc.
The TRAU converts the GSM channels into ISDN D-channels in accordance with the G.703
Recommendation. This Recommendation specifies physical and electrical characteristics of
the interfaces at hierarchical bit rates that are described in Recommendation G.702. The
interfaces are defined in terms of general characteristics, specifications at the output ports and
input ports and/or cross-connect points, earthing of outer conductor or screen and coding
rules etc.
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