3.3.1 Function of the Transcoding Rate and Adaptation Unit
One of the most interesting functions in GSM involves the TRAU, which typi- cally is located between the BSC and the MSC. The task of the TRAU is to compress or decompress speech between the MS and the TRAU. The used method is called regular pulse excitation–long term prediction (RPE-LTP). It is able to compress speech from 64 Kbps to 16 Kbps, in the case of a fullrate channel (net bit rate with fullrate is 13 Kbps) and to 8 Kbps in the case of a hal- frate channel (net bit rate with halfrate is 6.5 Kbps).
Note that the TRAU is not used for data connections.
3.3.2 Site Selection for Transcoding Rate and Adaptation Unit
Although speech compression is intended mainly to save resources over the Air-interface, it also is suitable to save line costs when applied on terrestrial links, as illustrated schematically in Figure 3.6. When the TRAU is installed at the MSC site (see top portion of Figure 3.6), a fullrate speech channel uses only 16 Kbps over the link from the BSC to the MSC.
The specifications allow for the installation of the TRAU between the BTS and the BSC. That requires, however, the use of 64-Kbps channels between the BSC and the MSC and hence the use of more links (see bottom portion of Figure 3.6).
This variant is, therefore, used only infrequently. In fact, most of the time, the TRAU is installed at the site of the MSC to get the most benefit from the compression.
MSC
BSC
TRAU frame
16 Kbit/s 16 Kbit/s
16 Kbit/s 16 Kbit/s 16 Kbit/s
64 Kbit/s 64 Kbit/s 64 Kbit/s
TRAU
TRAU TRAU frame
BTS TRX
BTS TRX
BSC MSC
Figure 3.6 Possible sites for the TRAU in the signal chain.
3.3.3 Relationship Between the Transcoding Rate and Adaptation Unit, and Base Station Subsystem
The TRAU is functionally assigned to the BSS, independently of where it actu- ally is located. The reason for that is the following.
Both the BTS and the TRAU have an interface for payload that is trans- parent for the BSC. The payload is formatted in TRAU frames, then transpar- ently sent over PCM links between the TRAU and the BTS in cycles of 20 ms.
That applies to both directions. The data contained in the TRAU frames form the input and output values for channel coding.
For data connections, the compression functionality has to be switched off. The type of connection (data/speech) is communicated to the TRAU during the assignment of the traffic channel. As illustrated in Figure 3.7, the BTS starts to transmit TRAU frames in the uplink, immediately after receiving the CHAN_ACT message. Those TRAU frames carry inband signaling, which is exchanged between the BTS-TRX (or more precisely the coding unit) and the TRAU, to consolidate the characteristics of a connection. Part of the con- trol information is, in particular, synchronization data, discontinuous transmis- sion (DTX) on/off, and the connection type (halfrate/fullrate).
Air-interface Abis-interface A-interface
DT1/BSSM ASS_REQ [A-Itf.-chann.]
I/DCM/CHAN_ACT [Fullrate/Halfrate/DTX]
I/DCM/
CHAN_ACT_ACK [FN]
TRAU 20 ms
TRAU 20 ms
TRAU I/RLM/DATA_REQ
ASS_CMD [TCH-Data]
SDCCH/I/RR ASS_CMD [TCH-Data]
BTS TRX
MSC VLR
BSC
TRAU-Frame (activation of the TRAU)
TRAU-Frame (activation of the TRAU)
TRAU-Frame (activation of the TRAU)
Figure 3.7 Activation of the TRAU during assignment of a traffic channel.
Note that TRAU frames are sent over traffic channels and not over the associated control channels and hence are transparent to protocol analysis.
The TRAU frames are, nevertheless, very important for error analysis on data connections.
4
The Network Switching Subsystem
The NSS plays the central part in every mobile network. While the BSS pro- vides the radio access for the MS, the various network elements within the NSS assume responsibility for the complete set of control and database functions required to set up call connections using one or more of these features: encryp- tion, authentication, and roaming. To satisfy those tasks, the NSS consists of the following:
• MSC (mobile switching center);
• HLR (home location register)/authentication center (AuC);
• VLR (visitor location register);
• EIR (equipment identity register).
The subsystems are interconnected directly or indirectly via the worldwide SS7 network. The network topology of the NSS is more flexible than the hierarchi- cal structure of the BSS. Several MSCs may, for example, use one common VLR; the use of an EIR is optional, and the required number of subscribers determines the required number of HLRs.
Figure 4.1 provides an overview of the interfaces between the different network elements in the NSS. Note that most interfaces are virtual, that is, they are defined as reference points for signaling between the network elements.
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