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53

ETSI ES 201 980 V4.1.2 (2017-04)

6.3.6FAC repetition

The FAC channel parameters shall be sent in each FAC block. The FAC service parameters for one or two services shall be sent in each FAC block. When more than one FAC block is needed to signal all the services in the multiplex, the repetition pattern is significant to the receiver scan time. When all services are of the same type (e.g. all audio or all data) then the services shall be signalled sequentially. When a mixture of audio and data services is present then the patterns shown in table 20 shall be signalled. In the case when there is only one service and the FAC block signals two sets of service parameters, both sets shall contain identical content.

Table 20: Service parameter repetition patterns for mixtures of audio and data services

Number of

Number of

Repetition pattern when FAC block

Repetition pattern when FAC block

audio

data

contains one set of service parameters

contains two sets of service parameters

services

services

 

 

1

1

A1A1A1A1D1

A1D1

1

2

A1A1A1A1D1A1A1A1A1D2

A1D1 A1D2

1

3

A1A1A1A1D1A1A1A1A1D2A1A1A1A1D3

A1D1 A1D2 A1D3

2

1

A1A2A1A2D1

A1A2 D1A1 A2D1

2

2

A1A2A1A2D1A1A2A1A2D2

A1A2 A1D1 A2D2

3

1

A1A2A3A1A2A3D1

A1A2 A3D1

Where An designates an audio service and Dn designates a data service.

6.4Service Description Channel (SDC)

6.4.1Introduction

This clause describes the format and content of the SDC. The SDC gives information on how to decode the MSC, how to find alternative sources of the same data, and gives attributes to the services within the multiplex.

The data capacity of the SDC varies with the spectrum occupancy of the multiplex and other parameters. The SDC capacity can also be increased by making use of the AFS index.

Alternative frequency checking may be achieved, without loss of service, by keeping the data carried in the SDC quasi-static. Therefore, the data in the SDC frames has to be carefully managed.

6.4.2Structure

An SDC block is the SDC data contained in one transmission super frame.

The SDC is treated as a single data channel. The total amount of data to be sent may require more than a single SDC block to send. An AFS index is therefore provided to permit a receiver to know when the next occurrence of the current SDC block will be transmitted, and so allow for alternative frequency checking and switching (AFS). A validity function is provided in the FAC to indicate whether the AFS index is valid or not, indicating to a receiver when the AFS function can operate.

The SDC block is made up as follows:

AFS index

4 bits.

data field

n bytes.

CRC

16 bits.

padding

k bits.

The AFS index is an unsigned binary number in the range 0 to 15 that indicates the number of transmission super frames which separate this SDC block from the next with identical content when the identity field in the FAC is set to 00. The AFS index shall be identical for all SDC blocks. The AFS index may be changed at reconfiguration.

ETSI

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