Quantize your voice

Say what? First conceived in 1937 by Alex Reeves, a voice digitization technique known as Pulse Code Modulation started to be deployed in the United States Public Switched Telephone Network in 1962.

Basically, you start with a 4 KHz analog voice channel. Then you take a "snapshot" of the voice signal's amplitude every 1/8000th of a second (you have to sample at twice the maximum frequency to avoid a problem known as "aliasing"). Then you convert the measured amplitude to a number (the "quantization" process) that is represented by 8 bits. Thus, PCM requires 64 KBPS of digitial bandwidth (8 KHz * 8 bits). This basic channel represents the first level of a digital heirachy, known as a DS0.

A special type of Time-Division Multiplexer (TDM) called a "Channel Bank" takes 24 of these 64K DS0 channels and combines (multiplexes) them into a single aggregate rate of 1.544 MBPS. This rate is the combination of the channel data payload of 1.536 MBPS (64 KBPS * 24 Channels) + 8 KBPS of framing and synchronization bits. The 1.544 MBPS rate is known as the DS1 level in the digital hierarchy. Facilities that support this rate are usually referred to as "T-Spans" or "T1" circuits.

International standards were developed later. Although the basic heirarchial DS0 rate of 64 KBPS was preserved, the algorithm for converting the voice signal to a digital signal is different. Also, the International standard calls for 30 voice channels + a 64 KBPS synchronization channel + a 64 KBPS signaling channel. Therefore, these systems operate at a rate of 2.048 MPBS (1.920 MBPS + 64 KBPS + 64 KBPS). Facilities that support this rate are usually referred to as "E1" circuits.

The 1960s

In addition to the development of 8-bit communication codes and Pulse Code Modulation systems, the sixties brought forth a number of other significant contributions.

The deployment of digitial transmission facilities resulted in the development of standard digital hierarchies, as noted in the previous Pulse Code Modulation section.

Integrated circuit (IC) development created Large Scale Integration (LSI) IC technology.

CRT terminals, developed in the 1950s, saw increased use as the preferred I/O device for computer systems. Computer architectures changed to accomodate interactive I/O.

The first communications satellites are launched.

The Carterphone decision in 1968/1969 allowed devices which were beneficial and not harmful to the network to be connected to the PSTN. This spawned the development of many modem and data communications companies!

The 1970s

Dataphone Digital Service (DDS) started deployment in 1974, bringing digital transmission facilities to the customer's premise. DDS circuit deployment also accelerated the conversion to digital networking within the Bell System.

X.25 began widescale deployments at the end of the 70s, introducing packet switched networking. Large X.25 public networks evolved; such as Telenet (now "Sprintnet") and Tymnet.

The continued development of Integrated Circuits results in widespread availability of LSI and VLSI (Very Large Scale Integration) devices, increased reliability, and decreased costs.

The 1980s

During the 1980s, the development of Dial Modem technology accelerated at a frantic rate.

On January 1, 1984, AT&T divested itself of its 22 Bell System operating companies based upon a 7 year antitrust suit filed against AT&T by the U.S. Department of Justice, and an agreed upon settlement. Ultimately, the Bell Operating Companies ("BOC"s) were grouped together into seven Regional Bell Operating Companies ("RBOC"s):

 - Ameritech Corporation
- Bell Atlantic Corporation
- Bell South Corporation
- Nynex Corporation
- Pacific Telesis Group
- Southwestern Bell Corporation
- US West Incorporated

AT&T itself, now divested, consists of two basic organizations:

 - AT&T Communications:

  Provides long distance services.
  Provides inter-LATA and network services.

- AT&T Technologies:

  AT&T Bell Labs
  AT&T International
  AT&T Information Systems
  AT&T Network Systems

Divestiture caused the carriers to compete in the only unregulated area; business communications services. This resulted in an explosion in business communications, starting with the availability of T1 (1.544 MBPS) services in 1984.

Multiplexing vendors launched new, network-savvy, Time Division Multiplexers. Company networks consolidated voice and data circuits into single high-speed aggregate bit streams; saving money and manpower, while improving network survivability. These new "microprocessor muxes" offered features such as redundancy and automatic circuit rerouting while supporting a wide variety of data and voice I/O types.

Local Area Network deployment accelerated, offering users a new view of data communications networking; the ability to access anything from anywhere, bandwidth-on-demand for data transfers, standardized connectivity, etc. The migration of computer networks has shifted from "Centralized host" to "Client-Server" architectures.

Signaling System #7 (SS7), a digital switch protocol used in the PSTN, began widescale deployment in the US PSTN. Sweden was among the first countries to implement SS7 networking while Bell Atlantic was among the first Local Exchange Carriers (LECs) to complete SS7 network implementation. This offered additional CLASS (Customer Local Area Signaling Services) services: Automatic Callback, Automatic Recall, Computer Access Restriction, Distinctive Alert, Caller ID, Selective Call Acceptance/Blocking, etc.

The 1990s

After the completion of SS7 within the PSTN backbone, additional telephone networking services were offered to business customers. Particularly, enhanced PBX network services such as Virtual Private Networks (VPNs) evolved. These services allowed flexible dialing for business users, and allowed the carrier to integrate Public and Business communications throughout the carrier's SS7 network.

The attractive Virtual Network options for voice services, combined with continued cost reductions in T1 services, have resulted in the segregation of voice and data in the Wide Area Network (WAN). As such, a "new" standard, known as Frame Relay, began deployment. Frame Relay is particularly adept at transporting LAN and X.25 traffic, and Public Frame Relay transport services are available from many carriers.

Wireless communications system use has exploded, with dramatic growth in Cellular voice and data technologies. Of particular interest are the merger of AT&T and McCaw Cellular (Cellular 1) and the development of the Cellular Digital Packet Data (CDPD) standards. Additional frequency allocations have recently occured for the development of wireless Personal Communications Systems (PCS), in an unprecedented spectrum auction by the Federal Communications Commission (FCC).

Slow, but steady increases are seen in the use of Integrated Services Digital Networks (ISDN); providing higher speed digital access capabilities to the residence and businesses.

New methods of integrating voice and data, as well as Local Area and Wide Area networks, are under development. These new "cell-based" transmission technologies are known as Switched Multimegabit Data Service (SMDS), Asynchronous Transfer Mode (ATM), and Broadband ISDN.