Eicon Networks ISDN NT1 Specifikace stáhnout pdf (Strana 20)

Analogue fax and modem over ISDN

Another important idea is introduced here; this is the Terminal Adapter.

A Terminal Adapter (TA) is always necessary to connect non-ISDN devices (such as a

serial port of a PC) to the ISDN. However, a TA can also contain a CODEC if it is intended

to support analogue phones, fax machines and modems.

In diagram below, the modem at the top left can plug in to the POTS ports on the TA. The

TA will then convert sounds generated by the modem on its POTS port into a bit stream

(and vice-versa). This bit stream is identical to that created by an ISDN telephone; that's to

say it represents sounds.

Starting in the bottom right-hand corner, data leaves the PC as bits that are converted into

sounds by the modem. We now have data encapsulated in sounds.

These sounds cross the PSTN network until they are encapsulated inside a bit stream by

the CODEC at the boundary between the ISDN and the PSTN. This bit stream is then

passed from the ISDN network to the Terminal Adapter, which contains a CODEC that

converts the bit stream back into sounds. These sounds are sent to the modem at the top

left, which converts this back into the original data that entered the modem at the bottom

left.

Starting in the bottom right-hand corner, the data leaving the PC is converted into sounds

by the modem. We now have data encapsulated in sounds. These sounds cross the PSTN

network until they are encapsulated inside a bit stream by the CODEC at the boundary

between the ISDN and the PSTN. This bit stream then passes through the ISDN network

to the Terminal Adapter, which contains a CODEC that converts the bit stream back into

sounds. These sounds are sent to the modem at the top left, that converts this back into

the original data.

The process runs in the opposite direction to send data from the PC in the top left-hand

corner to the PC in the bottom right-hand corner. This appears to be a lot of work: the data

sent across the ISDN has been encapsulated twice.

Double Encapsulation of Analogue Modem Data Across an ISDN:

Between the PSTN and the ISDN there is a CODEC that samples the sounds the modem

is making and creates a bit stream representing these sounds. This means that the data

that originally entered the modem is now encapsulated as bits representing sounds that

represent the data.

Why couldn't the CODEC, merely interpret the sounds and recover the original data - like

this?

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Eicon Networks ISDN NT1 Specifikace Strana 20