You can credit Scott Morris for this explanation: Let's go way back to voice sampling days (where our DS0s and T-1 originate) and look at the bandwidth of each voice line. The human voice occupies 3,200 Hz of sound waves. We cover some area on the outside of this and round it up to a 4,000 Hz range.
The Nyquist Theorem tells us that to adequately sample and represent any signal, we should sample at twice the frequency rate. That means if the sound waves cycle through 4,000 cycles per second (Hz), then we should sample at twice that, or 8,000 times per second. In addition, each sample is represented on an 8-bit scale. So we have eight bits of information in each of 8,000 times per second. That's where we come up with 64,000 bits per second for a voice channel (64k per DS0). Now, looking at a T-1, we have 24 individual channels (or DS0s). Doing more math, 64,000 times 24 is 1,536,000 or 1,536k as your Cisco 7000 allows. So where does your other router come up with that extra 8,000 bits? Magic? No, that boils down to framing and some interpretation. On a T-1, a frame consists of the information from each of the 24 channels. So each frame (8,000 per second) represents 24 times 8 bits, or 192 bits of information. In addition, there's one synchronization bit in each frame. This gives us 193 bits per frame. While the synchronization bit moves across the line, it doesn't contain usable data. But, when looking at the raw numbers here... 193 bits (each frame) times 8,000 frames per second yields 1,544,000 bits per second. Usable information, however is 24 channels times 8 bits times 8,000 frames per second... Or 1,536,000 bits per second. -----Original Message----- From: Kelly Cobean [mailto:[EMAIL PROTECTED]] Sent: Thursday, April 18, 2002 11:33 AM To: [EMAIL PROTECTED] Subject: How do you get that? [7:41832] All, I was wondering if any of you have a good link to a site that explains how the bandwidth is derived for T-1/T-3 circuits. What I'm looking for specifically is how we come to 1.544 Mb/s for a T-1 that is 24x64K channels which = 1536K, and what happens to the other 8K, and the same calculation for a T-3. Descriptions of Robbed-bit signaling, etc., too. Thanks much for your help. Kelly Cobean, CCNP, CCSA, ACSA, MCSE, MCP+I Network Engineer GRC International, Inc., an AT&T company Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=41841&t=41832 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
