Telecom & Transport Speed Schemes

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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:

Hierarchy Speed Digital
Carrier DS0’s Notes
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.
Speed DS0 Carrier
64 Kbps 1
1.544 Mbit/s 24 T-1
2.048 Mbit/s 32
6.312 Mbit/s 96 T-2
7.786 Mbit/s 120
8.448 Mbit/s 128
32.064 Mbit/s 480
34.368 Mbit/s 512
44.736 Mbit/s 672 T-3
97.728 Mbit/s 1440
139.264 Mbit/s 2016 DS4NA
139.264 Mbit/s 2048
274.176 Mbit/s 4032 T-4
400.352 Mbit/s 5760 T-5
565.148 Mbit/s 8192

Anything above a T3 is now optical/fiber.

Optical Carrier

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.

Hierarchy Data Rate SONET SDH OCx
Level Zero 155.52 STS-3 STM-1 OC-3
Level One 622.08 STS-12 STM-4 OC-12
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
OC-768 40Gbit/s STS-768 STM-256
OC-3072 160Gbit/s STS-3072 STM-1024

Virtual Tributary

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.

Name Speed
VT-1.5 1.728Mbit/s
VT-2 2.304Mbit/s
VT-3 3.456Mbit/s
VT-6 6.912Mbit/s
STS-1 50.112Mbit/s
STS-3 150.336Mbit/s

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