North American Digital Signal Hierarchy
In the 1960’s The Bell System / AT&T came up with a transport system based off 64K (bits) “channels.” To come up with 64K, consider the voice frequency (VF) or voice band frequencies used in telephony, approximately 300 Hz to 3400 Hz. You can refer to this as baseband or narrowband. (Telecom likes to have several names for the same thing.)
A single voice transmission channel is about 4 kHz and is sampled at 8 kHz. Why? Because the Nyquist theorem says so.
(4,000 Hz is an adequate sweet-spot for human speech, it’s not exact but when reproduced at the other end, you can recognize it’s grandma when she calls.)
So what ya get is:
2 x 4K = 8K samples per second, each one of those sample is/used 8-bit pulse-code modulation which ends up as 8K x 8 = 64K bits per second – a DS0. (It will be called D-S-O or D-S-Zero interchangeably.)
Robbed bits for signaling
Here’s the catch – the low order bit is used for signaling purposes. For voice this created noise that you really can’t hear so Ma Bell didn’t care. For digital data you can’t fudge it like that, only 7 bits can be used. 8,000 7-bit samples gives you 56 kbps. Today you can get around this using different line codes and bit stuffing.
So the hierarchy using a DS0 of 64K or 64,000 bits per second:
|1st||1.544 Mbit/s||DS1||T-1||24||In ISDN PRI = 23B (user) + 1D (signaling) channels|
|IntermediateLevel||3.152 Mbit/s||DS1C||48||DS1C uses two DS1 signals combined and sent on a 3.152 megabit per second carrier which allows 64 kilobits per second for synchronisation and framing using pulse stuffing. Never common, you won’t see this in use.|
|2nd||6.312 Mbit/s||DS2||T-2||96||4 x DS1. Never common, you won’t see this in use.|
|3rd||44.736 Mbit/s||DS3||T-3||672||28 x DS1|
|Intermediate Level||139.264 Mbit/s||DS4NA||2016||3 x DS3 Highest designed in ANSI T1.107|
|4th Level||274.176 Mbit/s||DS4||T-4||4032||Replaced with Optical Carrier / OCx|
|5th Level||400.352 Mbit/s||DS5||T-5||5760||Replaced with Optical Carrier / OCx|
HOLD UP – 24 x 64,000 bits per second won’t get you 1.544 Mbit/s. What you have is 24 x 64,000 = 1.536 Mbit/s. Bits are lost between frames because a frame separator is needed for every 8 bit sample of the of the 24 channels . So yes, 24 x 8 = 192 but adding the separator, 193 bits per frame x 8K samples = 1.544 Mbit/s.
Anything above a T3 is now optical/fiber.
SONET (Synchronous Optical Network) in North America or SDH (Synchronous Digital Hierarchy) elsewhere is the modern day optical transmission systems. It’s nice because everything is a multiple of the OC-1 rate of 51.84 Mbps.
|Level Two||2488.32 Mbit/s||STS-48||STM-16||OC-48|
|Level Three||9953.28 Mbit/s||STS-192||STM-64||OC-192|
Optical Carrier Rates
|Optical Carrier||Data Rate||Payload-SONET (SPE)||User Data Rate||SONET||SDH|
|OC-1||51.84 Mbit/s||50.112 Mbit/s||49.536||STS-1||—|
|OC-3||155.52 Mbit/s||150.336 Mbit/s||148.608||STS-3||STM-1|
|OC-9||466.56 Mbit/s||451.044 Mbit/s||445.824||STS-9||STM-3|
|OC-12||622.08 Mbit/s||601.344 Mbit/s||594.824||STS-12||STM-4|
|OC-18||933.12 Mbit/s||902.088 Mbit/s||891.648||STS-18||STM-6|
|OC-24||1244.16 Mbit/s||1202.784 Mbit/s||1188.864||STS-24||STM-8|
|OC-36||1866.24 Mbit/s||1804.176 Mbit/s||1783.296||STS-36||STM-12|
|OC-48||2488.32 Mbit/s||2.4 Gbps||2377.728||STS-48||STM-16|
|OC-192||9953.28 Mbit/s||9.6 Gbps||9510.912||STS-192||STM-64|
To slice up the 51.84 Mbit/s, you can have a sub-STS-1 facilitie. VT1.5 is common because it can carry 1.728 Mbit/s (enough room for a DS1/T1 signal.) There’s a lot of overhead in SONET.