3 ms·

I've recently been reading about T1 and E1 cables, which were used to transmit most calls inside and between telecom companies back in the day, and I was astoni

I've recently been reading about T1 and E1 cables, which were used to transmit most calls inside and between telecom companies back in the day, and I was astonished that they transmitted data one sample at a time. Unlike IP, those were synchronous, circuit-switched systems. You'd first use a signaling protocol (mostly SS7) to establish a call, reserving a particular timeslot on a particular link for it, and you'd then have an opportunity to transmit 8 bits of data on that timeslot 8000 times a second. There was no need for packet headers, as the timeslot you were transmitting on was enough to identify which call the byte belonged to. Because all data from a single call always took the same path, and everything was very tightly synchronized, there was also no variability in latency. This basically eliminated any need for buffers, which are the main cause of latency in digital systems.

rasz 2y ago
Didnt GSM(2G) work same way with dedicated regular timeslots per call? I dont know about 3G, but 4G finally introduced and 5G cemented packetized voice data with Volte.
- joha4270 2y ago
  The interesting point wasn't the timeslots, but their size. Yes, 2G has fixed time slots, but a slot is used for a lot longer than a single (half?) sample.
  - miki123211 2y ago
    2g (and all other standards after it) use 20-millisecond frames. It needs to send 8KHz audio at much lower bitrates (~14Kbps instead of 64Kbps), and you can't do that with raw PCM if you want decent quality. This means you need lossy compression and a codec, and those need far more than a single sample to work well. CDMA was similar, not sure what their frame size was exactly, but it was somewhere in the vicinity.
toast0 2y ago
> This basically eliminated any need for buffers, which are the main cause of latency in digital systems. You still need a buffer at each switching point, because the timeslots on each cable aren't likely to line up. But the buffer for each channel only needs to be 2 samples wide in the worst case where the timeslots overlap and you need to send from the buffer while receiving into the buffer. Given the timeframe when T1/E1 were developed, a more accurate perspective is not that buffers were eliminated, it's that they were never created.

rasz 2y ago

Didnt GSM(2G) work same way with dedicated regular timeslots per call? I dont know about 3G, but 4G finally introduced and 5G cemented packetized voice data with Volte.

joha4270 2y ago
The interesting point wasn't the timeslots, but their size. Yes, 2G has fixed time slots, but a slot is used for a lot longer than a single (half?) sample.
- miki123211 2y ago
  2g (and all other standards after it) use 20-millisecond frames. It needs to send 8KHz audio at much lower bitrates (~14Kbps instead of 64Kbps), and you can't do that with raw PCM if you want decent quality. This means you need lossy compression and a codec, and those need far more than a single sample to work well. CDMA was similar, not sure what their frame size was exactly, but it was somewhere in the vicinity.

toast0 2y ago

> This basically eliminated any need for buffers, which are the main cause of latency in digital systems. You still need a buffer at each switching point, because the timeslots on each cable aren't likely to line up. But the buffer for each channel only needs to be 2 samples wide in the worst case where the timeslots overlap and you need to send from the buffer while receiving into the buffer. Given the timeframe when T1/E1 were developed, a more accurate perspective is not that buffers were eliminated, it's that they were never created.