very, very basically...
To supply you with an ADSL connection Telkom needs to install equipment in the exchange to split your line between voice and data.
The data connection is via a DSLAM, IMAX or MSAN.
In the beginning Telkom used older Marconi DSLAMs and the maximum download speed was 4Mbps.
Then they started using newer Marconi DSLAMs and you could get a maximum download speed was 10Mbps.
In many cases Telkom did NOT remove the older DSLAMs so you would have a mix of 4 and 10 Mbps DSLAMs in the same exchange.
Then... along came the IMAX, capable of providing a maximum download speed was 24Mbps.
Telkom installed them in some exchanges, not all.
And again, in many cases Telkom did NOT remove the older DSLAMs so you would have a mix of 4 and 10 Mbps DSLAMs and a 24Mbps IMAX in the same exchange.
And then... along came the MSAN, capable of variable speeds up to 100Mbps and on copper or fibre.
And again, in many cases Telkom did NOT remove the older DSLAMs and IMAXs so you would have a mix of 4 and 10 Mbps DSLAMs, a 24Mbps IMAX and MSANs in the same exchange.
The end goal is to only have MSANs an to recover all the DSLAMs and IMAXs but that will take a few years...
PS. By MSAN I refer to the Huawei MSAN and the Alcatel Lucent ISAM.