Eicon Networks ISDN NT1 Specifikace stáhnout pdf (Strana 44)

Rate Adaption

In this section you will:

• Learn the difference between asynchronous and synchronous data streams

• Understand how they can be connected

• Examine the technologies available to achieve this

• Extend your understanding of the functions of an ISDN TA (Terminal Adapter)

Note: Rate adaptation is sometimes called 'rate adaption' in North America and some

other countries.

Synchronous & Asynchronous data

The data stream on an analogue modem is intermittent because the traffic is

asynchronous - that is to say, it starts and stops.

If you watch the lights on a modem when connected to an ISP, for example, you will notice

that the transmit and receive data lights are off for much of the time.

On the other hand, B channel activity on an ISDN line never stops. This is because it is

synchronous. This means that there is always data flowing.

There are two approaches that are used to handle the transition between these different

types of data:

• encapsulation - the asynchronous data is carried across the digital network, and is

then converted back to asynchronous data on the other side of the network.

• conversion - the asynchronous data is irretrievably converted to synchronous data.

This is appropriate where the destination device can handle synchronous data; this

method depends on an appropriate higher level protocol such as PPP. (Strictly

speaking, this is not Rate Adaptation. However, Rate Adaptation is normally

performed by terminal adapters, that are often also capable of performing protocol

conversion.)

Encapsulation

The diagram below shows asynchronous traffic being transmitted across the ISDN network

to a device that understands asynchronous data (e.g. an asynchronous port on a router

providing Internet access at an ISP).

In order to allow the asynchronous traffic to pass across the ISDN, the terminal adapter

(TA) between the PC's serial port and the network must fill all the empty gaps in the

asynchronous data stream with something.

In addition, it must fill in the difference between the speed of the serial port and the speed

of the B channel. This process is sometimes called bit stuffing because extra bits are

inserted into the data stream to fill it out to 64kbps.

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Eicon Networks ISDN NT1 Specifikace Strana 44