05-21-2004 11:25 AM - edited 03-02-2019 03:52 PM
"BRI also provides for framing control and other overhead, bringing its total bit rate to 192 kbps. "
-- Cisco Internetworking Technology Handbook
http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/isdn.htm
I'm confused by the statement above. Isn't BRI have a total capability of 2B+D = 2*64+16 = 144kbps ? Can framing control increase bandwidth? Anybody can clarify this? Thanks in advance.
05-21-2004 11:48 AM
You have to look at in timeslots as it running over a time division network.information in each time slot is 8 bits and sampled 8000 times per second to give the 64kbps.
The b channel will fill the whole timeslot, the d timeslot will have 16kbs of info and the rest is filler.
When the data is sent over layer 1 the frames consists of bits from the 2 d channels, 1 b channel framing bit dc balancing bit d-echo channel bit and and seven other types of bit.
A good book is isdn explained by john m griffiths.
05-21-2004 12:21 PM
a slight correction is required.
On isdn 2, the layer 1 frames are 48 bits long and are samples 4000 times a second to give the 192kbps.
each frame consists bits from 2b and d channels plus a few others.
05-22-2004 01:50 PM
You can think of the total bandwidth of ISDN BRI as 3 X 64 = 192 Kbps = 2 X B (2 X 64K) + 1 X D (16) + 48 Kbps (framin, synchronization etc. overhead.). The last part of 48 K is out of use.
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