DAB was specifically developed to use E1 connections between MUX and the Transmitter (ETI data connection).
E1 is a synchronous frame (continuously repeated with binary data). It has a raw capacity of 2048 kbps, enough for the ETI connection between MUX and Transmitter.
The E1 Frame
The frame has 32 time slots of 8-bit each. This is a legacy of the first digital telephone trunks between telephone exchanges.
The 8 bits were used as one telephone channel, and the frame is repeated every 8kHz. It matched the Nyquist frequency for the 3,6kHz bandwidth used for telephony.
The result was 8 bits * 8000 = 64kbps (bandwidth of a telephone channel).
The 32 slots of 64kbps result in a raw data capacity of 2048 kbps.
The frame could transport 30 telephone channels. In addition, the first slot was used to supervise the connection, and the 16th slot was used for telephony signalling.
In the days DAB was developed, the E1 was still a very often used service. This was the reason DAB adopted de E1 as the connection between MUX and the transmitter. We can tell that the BBC used this type of connection when they started to roll out their DAB network in the UK.
Nowadays, telephony is more of an IP-based service, and today, the E1 is rarely used for telephone purposes. It is a service that may be phased out in some countries or, if needed, simulated over an asynchronous IP network or an underlying SDH (Synchronous Digital Hierarchy) fiber network. But even today, in an overall IP world, there is no reason not to use E1 for DAB when available.
The E1 variant G.703
G.703 is the raw frame of 32 slots of 8 bits each with an 8kHz repetition frequency (being 256 bits (8 bits * 32 slots) that are sent with 8 kHz timing (per 125 µs).
256 bits * 8000 Hz = 2048 kbps
DAB recognizes this format as ETI-NI
The E1 variant G.704
With G.704, two of the 32 slots are used for signalling, so only 30 slots remain or 240 bits.
(slot 1 and slot 16 are still from digital telephony, where slot 16 is used for signalling for the 30 telephone channels) and slot 1 for error indication. This results in 16 bits being lost per frame.
But even G.704 with 30 slots (instead of 32) has sufficient throughput for DAB ETI transport.
240 bits * 8000 Hz = 1920 kbps
DAB recognizes this signal as ETI-NA