4 Transcoding and rate adaption

11.243GPPGSM Transcoding and Rate Adaptation: Base Station System Equipment SpecificationTS

The Transcoding and Rate Adaption Unit (TRAU) is a GSM entity which converts between the 64 kbit/s per channel bit rate at the MSC and a net rate of 13 kbit/s for the radio interface in the case of Full Rate (FR) speech. For terrestrial links the 13 kbit/s data is transmitted over 16 kbit/s after the addition of synchronisation and control information. On the radio interface the 13 kbit/s is padded to 22.8 kbit/s after channel coding. In the case of data the net bit rates used as the bearer can be either 12 kbit/s, 6 kbit/s or 3,6 kbit/s. Again channel coding results in a radio interface data rate of 22.8 kbit/s. While the transcoder is considered functionally part of the BSS it can be located at either the MSC location, BSC location or BTS location. This aspect is illustrated in figure 11.24‑1. The 16 kbit/s terrestrial link to the remote transcoder may be (4 to 1) multiplexed onto a single 64 bit/s link as shown in BSS types 2,4 and 6.

In situations where the TRAU is located remote from the radio interface (i.e. BSS type 2, 4‑7) speech or data is transferred between the TRAU and channel codec unit (CCU) in frames of fixed length of 320 bits (20ms). These "TRAU frames" also carry control signals that signal frame type and timing information. For example in‑call modifications from speech to data will be signalled from the CCU to the TRAU within the frame structure. In the case of timing control the CCU checks the synchronisation of the downlink radio link and signals advance/retard information to the TRAU over the uplink TRAU frames. Corrections are then made to the downlink phasing of the TRAUs. For more information see GSM 08.60 [17].

Testing for the transcoding function will be divided into three categories:

‑ speech transcoding/DTX which is independent of BSS type;

‑ data rate adaption;

and

‑ In-band control which relates to types 2,4‑7.

4.1 Transcoder test points

To facilitate the standardization of the testing the following interfaces are required:

4.1.1 External interfaces

The mandatory external interfaces of the BSS (according to figure 11.24‑1) are listed below:

1. The antenna connector.

2. The A‑interface to the MSC.

An additional optional external interface may be required:

  1. The separate OMC‑interface.

4.1.1.1 Test signals applied to the antenna connector

The purpose of the tests in this specification are to test the transcoding function of the BSS. Where tests are performed using the antenna connector as an external interface, the characteristics of the RF test signal should be chosen so as to minimise the contribution of the BTS (especially its RF parts) the measurement results.

NOTE: The BTS can contribute to the measurement result is due to the introduction of bit errors. This is most likely to occur if the level of the input signal is not large enough compared to the noise floor of the receiver, but can also occur if the input signal overloads the receiver.

The manufacturer may specify the RF characteristics of the test signal for these tests, which meet these requirements.

If the manufacturer does not specify the characteristics of the test signal, the signal and general test method defined in GSM 11.21 [19] for static reference sensitivity may be used, except that the level of the signal should be increased by at least 20dB.

Perfect radio conditions: Using a test signal as defined above, applied continuously to the antenna connector.

Error-free radio conditions: Using a test signal as defined above, applied for certain parts of the GSM radio interface frame structure, as defined in each test.

Alternatively, an internal interface point within the BTS may be used, with a test signal comprising the bit sequence which would result at the internal interface point for an error free test signal applied to the antenna connector.

4.1.2 Internal test points and interfaces

In addition to the external interfaces in clause 4.1.1 the following internal logical interfaces shall as a minimum be accessible in the equipment in order to carry out the measurements in this specification:

1. Input and output bit access to 104 kbit/s level, 13 bit linear PCM for full rate speech channels (TCH/FS) in both directions of transmission and reset control of the speech encoder and decoder.

2. Output bit access after channel decoding including frame erasure information (FEI or BFI) for all channel types.

3. Input bit access before channel encoding for all channel types.

4. The optional internal TRAU interface (referred to as Abis in GSM 08.60 [17]), if used.

Where an internal access is required, the implementation of this is up to the manufacturer. However, in order to physically interface with the BSSTE, the implementation is restricted to the following options:

a. Direct physical access to the logical interface.

b. Physical access via a dedicated external interface adapter.

c. Physical access to bits (insertion and monitoring) before channel encoding/after channel decoding via a loop‑back over the radio path as defined in GSM 11.10‑1 [18] for the Mobile Station.

d. Physical access to bits (insertion and monitoring) before channel encoding/after channel decoding from the A‑interface using the A‑interface rate adaptation functions.

e. Physical access to bits (insertion and monitoring) before channel encoding/after channel decoding from the Abis‑interface using the 16 kbit/s Abis‑interface rate adaptation and speech handling functions, if any.

f. Physical access to 13 bit/8 kHz PCM samples via a special combination of 2 x A‑ or Abis‑interface 64 kbit/s channels.

A dedicated external adapter shall, if used, be supplied by the BSS manufacturer.

The internal test points or interfaces are illustrated in figure 11.24‑2 and figure 11.24‑3 for the various Base Station System types described in figure 11.24‑1.

The detailed specifications of the interface points are:

‑ Interface point x:

TCH/FS: Input and output 104 kbit/s = 13 bit linear PCM at 8 kHz sampling rate

Reset control of the full rate speech encoder

TCH/HS: Input and output 104 kbit/s = 13 bit linear PCM at 8 kHz sampling rate

Reset control of the half rate speech encoder

No other channels are applicable.

‑ Interface point x’:

TCH/FS: Input and output 104 kbit/s = 13 bit linear PCM at 8 kHz sampling rate

Reset control of the full rate speech decoder (or at interface point y’)

TCH/HS: Input and output 104 kbit/s = 13 bit linear PCM at 8 kHz sampling rate

Reset control of the half rate speech decoder

No other channels are applicable.

‑ Interface point y:

Speech:

TCH/FS: Input/output 13 kbit/s = 260 bits every 20 ms

TCH/HS: Input/output 5.6 kbit/s = 112 bits every 20 ms

NOTE 1: For BSS types 4‑7 input/output control can be handled via the Abis‑interface.

Data:

TCH/F9,6: Input 12 kbit/s = 60 bits every 5 ms

TCH/F4,8: Input 6 kbit/s = 60 bits every 10 ms

TCH/F2,4: Input 3,6 kbit/s = 72 bits every 10 ms

TCH/H4,8: Input 6 kbit/s = 60 bits every 20 ms

TCH/H2,4: Input 3,6 kbit/s = 72 bits every 20 ms

Signalling:

FACCH/F: Input 184 bits (23 octets) every 20 ms

FACCH/H: Input 184 bits (23 octets) every 40 ms

SACCH/TF: Input 184 bits (23 octets) every 480 ms

SACCH/TH: Input 184 bits (23 octets) every 480 ms

SACCH/C8: Input 184 bits (23 octets) every 470.77 ms

SACCH/C4: Input 184 bits (23 octets) every 470.77 ms

SDCCH/8: Input 184 bits (23 octets) every 235.38 ms

SDCCH/4: Input 184 bits (23 octets) every 235.38 ms

BCCH: Input 184 bits (23 octets) every 235.38 ms

PCH: Input 184 bits (23 octets) every 235.38 ms

AGCH: Input 184 bits (23 octets) every 235.38 ms

RACH: Not applicable

SCH: Input 5 x 25 = 125 bits every 235.38 ms

‑ Interface point y’:

Speech:

TCH/FS: Input/Output 13 kbit/s = 260 bits every 20 ms

Reset control of the full‑rate speech decoder (or at interface point x’)

TCH/HS: Input/Output 5.6 kbit/s = 112 bits every 20 ms

NOTE 2: For BSS types 4‑7 input/output and reset control can be handled via the Abis‑interface.

All outputs of speech channels shall include the Bad Frame Indication (BFI). All bits shall be available, i.e. also bits in erased frames. No extrapolation shall be done.

Data:

TCH/F9,6: Output 12 kbit/s = 60 bits every 5 ms

TCH/F4,8: Output 6 kbit/s = 60 bits every 10 ms

TCH/F2,4: Output 3,6 kbit/s = 72 bits every 10 ms

TCH/H4,8: Output 6 kbit/s = 60 bits every 20 ms

TCH/H2,4: Output 3,6 kbit/s = 72 bits every 20 ms

Signalling:

FACCH/F: Output 184 bits (23 octets) every 20 ms

FACCH/H: Output 184 bits (23 octets) every 40 ms

SACCH/TF: Output 184 bits (23 octets) every 480 ms

SACCH/TH: Output 184 bits (23 octets) every 480 ms

SACCH/C8: Output 184 bits (23 octets) every 470.77 ms

SACCH/C4: Output 184 bits (23 octets) every 470.77 ms

SDCCH/8: Output 184 bits (23 octets) every 235.38 ms

SDCCH/4: Output 184 bits (23 octets) every 235.38 ms

BCCH: Not applicable

PCH: Not applicable

AGCH: Not applicable

RACH: Output 8 bits every 4.62 ms

SCH: Not applicable

All outputs of control channels shall include the Frame Erasure Indication (FEI). All bits shall be available, i.e. also bits in erased frames.

Position Z: The 64 kbit/s level at the MSC interface ie. the A interface.

Position T: The TRAU or Abis‑interface (Usually at 16 kbit/s submultiplexed into a 64 kbit/s channel). This is the link between the TRAU and the CCU in the BSS or BTS.

Figure 11.24‑1: BSS types

Figure 11.24‑2:Test points or interfaces (BSS type 1 & 3)

Figure 11.24‑3: Test points or interfaces (BSS types 2, 4‑7)