TDMA in telephone circuit switching - isn't there delay in transmitting a particular user's speech? [closed]

Question

Let's do a simple case on the left with X and A user telephones.

So the way an analog signal (user's speech X) is handled, is that it is sampled at 8000 bits / sec.

The high-speed C-channel has 128 kilo bits/sec of capacity.

X only transmits at 64kbps

A transmits at 64 kbps too.

However, if the switch 1 only "listens" to X for some time constant, and then switches to A, wouldn't it make the speech of X seem slury/ with pauses? Let's imagine user X is talking continuously for 5 minutes, if switch 1 is switching between X and A, then there will be short pauses in what user X is saying, isn't it?

So why is circuit switching even considered as a viable option in telephone communication? It's not.

Answer 1

The key thing to notice is that the wire can send information twice as fast as X or A or Y or B can actually speak or listen. So the switch on one side speeds up the speech and the switch on the other side slows it down again.

Okay, so X is reciting the alphabet, and A's ordering a pizza. And let's say the time slot is 4 words long, which means they actually only say 2 words in a time slot. And let's say A goes first.

The switch takes two slots' worth of speech and sends it in one time slot. Then the switch on the other end takes one slots' worth of sped up speech and plays it back in two time slots.

       | slot 1    | slot 2   | slot 3        | slot 4  | slot 5          | slot 6  | slot 7  |
 X:    | A B       | C D      | E F           | G H     | I J             | K L     |
 A:    | Hi my     | name  is | A  and        | I would | like to         | order a |

 TDMA: |     Hi my | A B C D  | name is A and | E F G H | I would like to | I J K L |

 Y:    | <silence> | A B      | C D           | E F     | G H             | I J     | K L     |
 B:    | <silence> | Hi my    | name is       | A and   | I would         | like to | order a |

So yes there's a delay, no there isn't any stuttering.

Answer 2

The high-speed channel C can transmit two bits in the same time that X and A each produce one bit, so it can simply transmit both bits, one after the other. No bits get lost. Actually, in the PSTN, samples (bytes), rather than bits, are interleaved. In this way, a 1.544 Mbps T1 line can handle 24 conversations simultaneously. 8000 times a second, a complete 193-bit frame of data is transmitted, which contains one 8-bit audio sample for each of the 24 circuits (plus one bit of "overhead").

In wireless TDMA, FIFO buffers are used to bunch the bits from X and A into larger groups called "packets", and the packets are interleaved in the high-speed radio channel. Each packet is transmitted in much less time than it took to create it. At the other end, another buffer captures the packet and then the data is transferred to the DAC at the original sample rate.

Answer 3

The key reason TDMA works without any apparent delay or pauses is the frequency at which the switching occurs. Audio sampling is best thought of as a stream of individual samples occurring at a certain rate. So for an example this might be 8000Hz, or one every 1/8000 of a second. Meaning we have 1/8000 of a second between transmitting samples. If a sample can be transmitted in 1/16000 of a second we can transmit 1 sample from each of 2 telephones in the same interval. And for each of those telephones, a sample will arrive every 1/8000th of a second. So the signal is transferred without any increased delay over a non-multiplexed line.

Answer 4

In the duration of time that sw1 only listens to A, A can't do anything meaningful because to do so would mean it would be faster than 8kbaud.

The two signals can be interleaved only because they are bandwidth-limited. On the receiving end, the "cut-up" signal can be filtered to recover the original bandwidth-limited signal.