Eicon Networks ISDN NT1 Especificaciones descargar pdf (Pagina 55)

The bearer capabilities are communicated to the destination, so that he may act upon them. For example, an

analogue call could be presented to a modem, but a transparent data call would be presented to an ISDN

adapter.

The ISDN might also use the bearer capabilities for its own purpose. For example, in North America, some

network operators make different charges for voice and for data calls. Some networks are able to optimise

their internal performance by using the bearer capabilities.

PSTN or POTS

PSTN stands for Public Switched Telephone Network. POTS stands for Plain Old Telephone Service.

Both of these terms refer to the conventional analogue telephone system. Often these expressions are used

to make contrast with ISDN.

DSP (Digital Signal Processor)

A digital signal processor a high-performance microprocessor that is optimised to provide real-time of

analogue data. They typically have a small instruction set, but can perform many millions of floating point

operations per second.

For example, the are capable of performing the Fast Fourier Transforms necessary for sampling analogue

signals input and creating a bit stream representing the bit stream that represents this, and vice versa, in real

time.

Start-Stop Protocol

In the 1960s and 1970s, IBM used to call asynchronous traffic by the name Start-Stop Protocol. This is

because there are not always bits present in the communications link.

In such a protocol, the data is divided into bytes. These bytes may be of any length, normally between four

and eight bits, but all will be the same size.

Each byte is followed by one, one-and-a-half or two stop bits. These are used as an inter-byte filler and allow

the receiving modem to re-synchronise its bit timing. This is especially important when one byte immediately

follows another. Following a gap in transmission, the receiving modem is able to re-establish bit

synchronisation at the moment it receives it receives the first bit of next byte.

This means that an asynchronous channel has three states:

Transmitting a zero

Transmitting a one

Not transmitting

By contrast a synchronous channel has only two states:

Transmitting a zero

Transmitting a one

Bit Rate and Information Rate

There is a distinction between these two ideas. The bit rate is the advertised bit rate of a communications

channel. In the case of an ISDN B channel in Europe it is 64 000 bits per second.

Information rate is the rate at which information is passed over the channel. The information rate can never

be higher than the bit rate, but it might be lower.

An ISDN telephone transmits and receives 64 000 bits per second, even when there is total silence.

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Eicon Networks ISDN NT1 Especificaciones Pagina 55