ISDN is an old technology but always up to date ! Today it is used still so much in any enterprises. Vendors know it and always keep their technology with ISDN.
The ISDN-PRA (Integrated Services Digital Network – Primary Rate Access) is a facility to carry a number of synchronous digital communication channels to the user over a 2048 kbit/s structured signal; the ISDN- PRA structure is defined in recommendation ETS 300 233.
Organization of ISDN – PRA frames : For each direction of transmission the following functions are provided by the synchronous ISDN-PRA 2048 Kbit/s frames (refer to recommendation ETS 300 233): 30 time slots at 64 kbit/s (numbered 1 to 15 and 17 to 31) for the transport of any appropriate allocation of the B, H0 and H1 channels. 1 time slot at 64 kbit/s (numbered 16) for the transport of a D channel. 1 time slot at 64 kbit/s (numbered 0) for frame alignment, multiframe alignment, CRC-4 procedure and maintenance (rec. G.704)
ISDN – PRA communication channels : The communication channels transported by the ISDN-PRA (Integrated Services Digital Network – Primary Rate Access) signal are the following:
B-channel it is rated 64 kbit/s, as defined in ITU-T Rec. I.412; it permits the allocation of up to 30 independent B-channels over the 2Mbit/s synchronous frame.
H0-channel it is rated 384 kbit/s, as defined in ITU-T Rec. I.412; it permits the allocation of up to 5 independent H0-channels over the 2Mbit/s synchronous frame.
H1-channel it is rated 1920 kbit/s, as defined in ITU-T Rec. I.412; it permits the allocation of one H1-channel over the 2Mbit/s synchronous frame.
D-channel it is rated 64 kbit/s, as defined in ITU-T Rec. I.412.
Here is a topology example for ISDN network :
