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Data Line Speeds

North American Digital Signal Hierarchy

The North American signal hierarchy was created by the old US 'Bell system' (AT&T) in the early 1960's and was the world's first digital voice system. It is based on multiples of the DS0 signal with a little bit of overhead to show its age. The fiendish Europeans who waited longer to define a digital hierarchy were able to live without the small overhead largely due to improved electronics.

The hierarchy defines the levels of multiplexing i.e. the first level of the hierarchy multiplexes (combines) a number of DS0s into a single digital signal (with a DSx designator) which is then placed on a carrier (with a T-x designator). The DSx defines an abstract signal or speed and the T-x defines a physical 'pipe' or format. The DSx and T-x series specs and most other telecom related specifications are standardized by the ANSI accredited Committee T1 (T1E1) (now part of Alliance for Telecommunications Industry solutions - ATIS) which then represents (via the US State Department) the US at ITU standard sessions.

Remember: a DS0 is 64K or 64,000 bits per second.

Hierarchy Speed Digital
Carrier DS0's Notes
First Level 1.544 Mbit/s DS1 T-1 24 In ISDN PRI = 23B (user) + 1D (signaling) channels
3.152 Mbit/s DS1C - 48 -
Second Level 6.312 Mbit/s DS2 T-2 96 4 x DS1
Third Level 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
Fourth Level 274.176 Mbit/s DS4 T-4 4032 Replaced with OCx
Fifth Level 400.352 Mbit/s DS5 T-5 5760 Replaced with OCx

Now if you have not been sleeping you will have figured out that for a T1 if you multiply 24 x DS0 (64,000) you do NOT get 1.544 Mbit/s (that would be 24 * 64,000 = 1.536 Mbit/s). The extra bits are lost between 'frames' where a frame consists of one 8 bit sample for each of the 24 channels (remember the DS0 basics). So every 192 bits (24 x 8 = 192) we add a 'frame separator' bit to give 193 bits per frame. The final arithmetic is 193 bits x 8K samples = 1.544 Mbit/s. Easy really.

If you do the same arithmetic for DS1C, T2 etc. the above will not give the right answer. In short above T1 things get even messier with M-Frames and M-subframes. Its mind numbing stuff and if you really want to know you need ANSI T1.107-2002 and lots of coffee.

European Digital Signal Hierarchy

Us fiendish Europeans left the US to blaze the digital voice trail, so when we came standardize things we could forget all this 'frame separator' stuff. Euro Telecom standards are defined by CEPT (a Euro Telecom 'club'). Here in all its glory is the super simple European hierarchy. Again all based on our good friend the ever popular 64,000 DS0.

Hierarchy Speed Carrier DS0's Notes
First Level 2.048 Mbit/s E-1 32 In ISDN PRI = 30B (user) + 2D (signaling) channels
Second Level 8.448 Mbit/s E-2 128 -
Third Level 34.368 Mbit/s E-3 512 -
Fourth Level 139.264 Mbit/s E-4 2048 -
Fifth Level 565.148 Mbit/s E-5 8192 -

While the table above shows the European carriers as E-1, E-3 etc. in similar format to the American T-1 etc. this is of relatively recent vintage. The original carrier names were CEPT-1, CEPT-3 etc.

Summary Table

The following table summarises a number of digital signal hierarchies currently in operation. We have used the terms J-1 etc. for convenience without actually knowing if they are used in practice. Maybe you know...

Speed DS0's North
Europe Japan
64 Kbps 1 - - -
1.544 Mbit/s 24 T-1 - J-1
2.048 Mbit/s 32 - E-1 -
6.312 Mbit/s 96 T-2 - J-2
7.786 Mbit/s 120 - - J-2 (alt)
8.448 Mbit/s 128 - E-2 -
32.064 Mbit/s 480 - - J-3
34.368 Mbit/s 512 - E-3 -
44.736 Mbit/s 672 T-3 - -
97.728 Mbit/s 1440 - - J-4
139.264 Mbit/s 2016 DS4NA - -
139.268 Mbit/s 2048 - E4 -
274.176 Mbit/s 4032 T-4 - -
400.352 Mbit/s 5760 T-5 - -
565.148 Mbit/s 8192 - E-5 J-5

The rates above T-3, E-3 etc are normally now optical (see below)

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

This page was last updated: 29th September 2004 at 10:20pm BST
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