Telecom and Network Speeds

We see network speed terms (e.g. T1, DS0, OC-192) all over the place and we always get confused (with our modest brain sizes) over what they all mean. So we set out to try and pull all the pieces into one place and this is what we came up with:

Some Background

First some basic stuff. You will see references to 64K (bits) 'channels' all over the place. This is the basic digital voice signal (called Digital Signal 0 or the infamous DS0 for short). The digital voice signal is encoded using PCM (Pulse Code Modulation) and TDM (Time Division Multiplexing). All other classic copper signal hierarchies (e.g. T3) are defined as multiples of DS0. Why 64K. Well... to digitize 'narrowband' speech (voice) you take a 4KHz spectrum (well actually 3.1K see notes below). Normal sampling techniques only give reasonable resolution if sampled at twice the frequency (which gives 2 x 4K(ish) = 8K samples per second). Each sample is 8 bits which gives 8K x 8 = 64K bits per second.

Notes:

1. K here is 1,000 not 1,024.
2. 'Narrowband' speech is from 0.3 to 3.4KHz. The newer 'wideband' speech (a.k.a. 'hi-fi speech') is proposed from 0.15 to 6.8KHz.

Contents

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 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.

Notes:
BITDROPPING
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 Mbps (that would be 24 * 64,000 = 1.536 Mbps. 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 Mbps. Easy really.

European Digital Signal Hierarchy

The fiendish Europeans left the US to blaze the digital voice trail, so when they came standardize things they 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.

Notes:
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...

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

Optical Carriers

Optical transmission systems are known as SONET (Synchronous Optical NETwork) in North America and SDH (Synchronous Digital Hierarchy) in the Rest of the World. Optical Carriers are typically known by their OC-x number where x is a multiple of the OC-1 rate of 51.84 Mpbs (shades of DS0). While there is a common world-wide approach to optical systems there are differences but they are accomodated within the standard. North America uses an STS-x format for frames (packets) and Europe an STM-x format. One day if we ever understand the differences we will add some more information.

Optical Signal Hierarchy

Optical Carrier Rates