Data Communications | Connectors
Modem | Acoustic coupler | Multiplexer | Concentrator

Common data comm equipment

This page deals with the hardware components required to set up a data communications network.

And includes;
Modem, Acoustic coupler, concentrator, multiplexor

[Cartoon: modem]

Modem

Modem
- MODulator, and DEModulator

A piece of equipment that performs;

Modulation

Converts signals from a DTE in digital form, to an analog form.

Demodulation

Converts Analogue signals to Digital form

[modem speeds]

Allows the transmission and reception of digital data over an analog data circuit.

With the widespread usage of dial-up internet connections, transmission speeds are continually improving.

Two types ;

Synchronous modem

Packets of bytes are sent

Asynchronous modem

Single bytes are sent

Modem - how it works

A MODEST MODEM OVERVIEW

When two modems connect and begin to squeal at each other, they are negotiating the sending and/or delivery of an analog reference signal. This reference signal has a particular combination of loudness and pitch which is identified during the "handshaking" process. Without an acceptable reference signal, the connection between the two modems will be broken off.
It is these detectable changes in the signal that carry both the transfer speed in kilobits per second (Kbps) and the actual data itself.
The cleaner analog -and especially digital- lines installed by the long distance carriers and local phone companies mean that modems can detect smaller changes in the signal, and thereby communicate more information in less time.
Modems that generate 28.8 Kbps have reached the limit of the standard POTS (Plain Old Telephone Service). It's not a question of the modem chipset, or even the software - both of which can run faster. Modems running 33.6K and 56K are really depending on your local network to handle the higher data rates.
The new modems depend on a clean phone signal so that
1) the improved sampling technology can extract the data from the reference signal in a shorter amount of time,
2) it can more quickly analyse finer changes in the signal amplitude (loudness) and pitch (phase).
If the modem doesn't receive the high-speed reference signal (because the line hasn't been upgraded or if there is a lot of noise), the modem drops down to a slower speed until a connection is made.
The degree to which your local network has been upgraded varies depending on what part of the country you live, what part of the state, or even what part of the city.
Slow delivery of 56K modems (1997, Feb 28)

Modem - New industry standard:( Aug '95)

V.34

Designed to allow modems to establish consistent, reliable connections at 28.8 kbps. Its strength is that it can compensate for the wide variance in quality of a typical phone connection by continuously monitoring and compensating for this variance.

V.42 error control

Ensures the integrity of your data so no mistakes are received.

V.42bis data compression

a protocol that can compress data on the fly up to a quarter of its original size allowing for data transfer rates beyond 115 kbps.

Modems in this group also support 14.4kbps fax transmissions.

Ref [133]NZPC World Aug '95

V.90 56Kbps

Allows for downloading transmission speeds of up to 56Kbps and uploading speeds of 33.6Kbps..
Takes advantage of the typical network configuration found when an analogue subscriber connects to a digitally connected server.
Based on "encoding" rather than "modulation" it bypasses the analogue-to-digital conversion in the downstream path enabling it to use nearly all of the 64K network bandwidth.
With standard V.42 bis compression, can download at speeds up to 230.4Kbps.
Initially two competing standards
- x2 - US Robotics & 3Com
- k56flex - Lucent & Rockwell Semiconductor.
- Combined by International Telecommunications Union (ITU) to V.90 standard
(1998, Apr) Peripheral options extend PC power, (1997, Feb) Introducing x2 technology from US Robotics

Asynchronous digital subscriber line (ADSL) & cable modems

ADSL

Work over ordinary telephone lines at up to 9Mbps (initially 1.5Mbps) and uploads can be 10 times slower than downoads

Cable modems

Work over wiring that carries cable TV services

Possible downloads of 30Mbps (uploads significantly slower)

Affected by other users on the same line.

Refer to Data Comm(Networks)
Malcolm, A. (1997, Aug 4)

Acoustic coupler

Konexx acoustic coupler

Features
* Same function as Modems
* Are strapped to a telephone handset instead of plugging into a wall BTU telephone socket.
* Has microphones, and speakers to transmit, and receive data through the handset.
* Easy to install.
* Compatible with virtually all telephones throughout the world.
* Battery saver feature can conserve battery life when not operating. No switches to forget.
* May have adjustable rubber cups and Velcro strap assure noise and error free coupling to virtually any telephone.

Applications
* Where connection via a telephone adaptor is not possible due to hard wiring or a digital PBX
* Connecting to pay phones

Examples;
(Aug 12 1997) Konexx, Teleadapt

Multiplexor

Multiplexors are devices that allow the combination of several data channels independently into one physical circuit.

Data Streams that have been combined in this manner can be separated, and recovered at the other end of the circuit.

There are two main types ;

Frequency division Multiplexors (FDM)

Time division Multiplexors (TDM)

There are 2 types of TDM's

Statistical Multiplexor(SDM)

A multiplexor which uses the idle time of connected devices to carry data traffic from other active devices.
The statistical mux is the most efficient when there is a light to moderate data load.

Classical Multiplexor (TDM)

A device that permits the transmission of two or more independent data channels on a single high speed circuit by interleaving the data from each circuit by time
More efficient when there is a heavy data load.

Example : Cronyx-Sigma multiplexor

[multiplexor card]

The Cronyx-Sigma is a high speed multichannel multiplexor intended for use with IBM AT 286/386/486 compatible computers. Various data communication and data terminal equipment (DCE such as modems or DTE such as video terminals, serial printers, or other computers) that comply with RS-232C, V.35, RS-449 or EIA-530 standards can be connected to your computer via the multiplexor.

The Cronyx-Sigma supports a set of synchronous protocols (BISYNC, HDLC, SDLC, X.21.), as well as asynchronous data transmission.

Possible applications of the multiplexor include:

Global data transmission networks,
PC based terminal servers,
Routers for LAN interconnection,
High-speed serial interface graphical devices (X-terminals, laser printers, etc.).
(1997, Feb 28)Cronyx-Sigma multiplexor

Concentrator

A concentrator is a device that combines the functions of a controller and a multiplexer.

Most concentrators work by using a store-and-forward messaging approach - storing messages from low-speed devices until the quantity of data is sufficient enough to make it worthwhile to send over a high speed line.
Airline passenger reservation systems commonly use this approach.

Note that the volume of data is such that most terminal users would not be aware that a concentrator is being used.

[Ref 31:]