Telecom & Transport Speed Schemes

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 Signal	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.

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.

 

http://en.wikipedia.org/wiki/Digital_Signal_0

http://encyclopedia2.thefreedictionary.com/DS1