# Time Division Multiplexing and Transmission Rate

According to my textbook, with TDM, each circuit gets all of the bandwidth periodically during brief intervals of time (that is, during slots).

If we have a circuit switched network where each link uses TDM with 24 slots and has a bit rate of 1.536 Mbps. Also suppose we have to send a file of 640,000 bits.

How come we divide the bit rate (bandwidth) by 24? I thought each slot gets the full 1.536 Mbps?

• Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer. Aug 7, 2017 at 19:10

The transmission rate 1.536 MBps is the full transmission rate of the link. In TDM, each link is divided up into time slots for each channel. In this example, there are 24 slots so each channel's transmission rate is 1/24 * the full transmission rate.

I just came across this example in Kurose and Ross so here's some explicit working out in case anyone still has this question and isn't familiar with all the terminology in the accepted answer:

``````Time taken, T = time to establish connection + time to transmit the file
T = 500 msec + 640,000 bits / (1.536 Mbps / 24)
= 0.5 s + 640,000 bits / (1.536 (Mb/s) / 24)
= 0.5 s + (640,000 * 24 / 1.536) b/(Mb/s)
= 0.5 s + (640,000 * 24 / 1.536) b/(Mb) s
= 0.5 s + (640,000 * 24 / 1.536)/1,000,000 s
= 0.5 s + (640 * 24 / 1.536)/1,000 s
= 0.5 s + (640 * 24 / 1536) s
= 0.5 s + 10 s = 10.5s
``````

According to my textbook, with TDM, each circuit gets all of the bandwidth periodically during brief intervals of time (that is, during slots).

Yes, that's correct.

If we have a circuit switched network where each link uses TDM with 24 slots and has a bit rate of 1.536 Mbps.

That's typical for T1 line, in which line is made from 24 DS0 64kbit/s channels.

How come we divide the bit rate (bandwidth) by 24? I thought each slot gets the full 1.536 Mbps?

No, the line would be then 24x1.5Mbit/s, not 1.5Mbit/s. The whole idea of TDM is that you take a line, clocked appropriately (for T1 it's usually Stratum 3 class clock), to fit a defined number of DS0 channels. Your specific service can get one DS0 (64kbit/s), two (128kbit/s), three (192kbit/s) - and up to full bandwidth of the link (1.5Mbit/s).

Think about it this way: each second, 1.5Mbit is sent over the link according to clocking of the line. Depending on how many channels (DS0's) you were allocated in your service specification, you'll be able to send that much of bits over the link.

BTW, in Europe we use E1's, that have 30 channels, each at 64kbit/s for 2Mbit/s, but again, the service can get anything from one channel up to full set of channels - to the level of 2Mbit/s.