Data Network Rate

Background Information

The most basic digital voice signal is Digital Signal 0 or DS0. DS0 is a 64 Kbit/s (or 64,000 bit/s) "channel".

This DS0 digital voice signal is encoded using PCM (Pulse Code Modulation) and TDM (Time Division Multiplexing). All other classic copper signal hierarchies (PDH - Plesiochronous Digital Hierarchy 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:

• K here is 1,000 not 1,024.
• '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.

North American Digital Signal Hierarchy (T-Carrier)

The North American signal hierarchy was created by Bell Labs(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.

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

The hierarchy defines the levels of multiplexing, i.e. the first level of the hierarchy multiplexes 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).

Technically, DSx is the transmission protocol used over a physical T-x line; however, the terms "DSx" and "T-x" are often used interchangeably.

The DSx and T-x series specs 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.

A DS1 circuit is made up of twenty-four 8-bit channels (also known as timeslots and DS0's), each channel being a 64 kbit/s DS0 multiplexed pseudo-circuit. A DS1 is also a full-duplex circuit, meaning, in theory, the circuit can send 1.544 Mbit/s and receive 1.544 Mbit/s concurrently.

A total of 1.536 Mbit/s of bandwidth is achieved by sampling each of the twenty-four 8-bit DS0's 8000 times per second. This sampling is referred to as 8-kHz sampling (see pulse-code modulation). An additional 8 kbit/s is obtained from the placement of a framing bit, for a total of 1.544 Mbit/s, calculated as follows:

Note: The calculation of DS2 and above is different from the DS1 calculation, you will need a copy of ANSI T1.107-2002 to see the details.

ITU/European Digital Signal Hierarchy (PDH)

European telecom standards are defined by CEPT. Here is the European hierarchy. They are also based on 64,000 bit/s DS0.

Summary Table

The following table summarizes a number of digital signal hierarchies currently in operation.

Notes: The data rates above T-3, E-3 etc are now obsolete and replaced by Optical Carriers (see reference 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 Mbps. While there is a common world-wide standard for optical systems there are differences but they are accommodated within the standard. North America uses an STS-x format for frames (packets) and Europe an STM-x (Synchronous Transport Module) format.

Optical Signal Hierarchy

Optical Carrier Rates

Notes:

1. SPE (Synchronous Payload Envelope) = AU (Administrative Unit)

Tributaries and Virtual Containers

SDH/SONET defines a way or packaging capacity into Virtual Containers (VCs) which may be Higher Order Virtual Container (HVC) or Lower Order Virtual Containers (LVC). The term Tributary Unit (TU - used in RoW) or Virtual Tributary (VT - North America) describes a method of mapping PDH (e.g. T1) carriers onto SDH/SONET.