Eicon Networks ISDN NT1 Especificações descarregar pdf (Página 46)

Many Terminal Adapters support protocol conversion between synchronous PPP and

asynchronous PPP.

These protocols are not identical and conversion is required. The serial port of the PC is

asynchronous so this determines the type of PPP used between PC and Terminal

Adapter. The ISP, however, in natively supporting ISDN (rather than using Rate

Adaptation), requires synchronous PPP.

Summary

In this section you learned:

• Asynchronous traffic contains gaps

• Synchronous traffic is continuous

• Asynchronous and synchronous data streams therefore need a conversion protocol

between them

• V.110 or V.120 protocols are used to fill up the unused bandwidth

• This process is known as Rate Adaptation

• V.120 is also used to accommodate traffic flow between 56kbps and 64kbps ISDN

lines

• Many Terminal Adapters also perform protocol conversion between synchronous

PPP and asynchronous PPP

Q/A

1. Why is rate adaptation necessary?

Rate Adaptation is required when asynchronous and synchronous data streams need to be converted from

one to another. This is because the synchronous steam is constant while the asynchronous stream contains

gaps. Rate adaptation protocols fill the gaps and are also used to strip the gaps out again at the other end.

2. What are the two protocols which perform rate adaptation?

The two rate adaptation protocols are V.110 and V.120.

3. What determines which of these two protocols to use?

The selection of either V.110 or V.120 depends simply upon which of these protocols is being used at the

other end of the connection.

4. Where else is rate adaptation employed?

Rate adaptation is also employed at the boundary points of different ISDN networks where the data rate is

not the same.

For example, between the Euro-ISDN networks, which run at 64 kbps, and the US ISDN networks which run

at 56 kbps, there will need to be V.120 rate adaptation.

5. What other method is there for converting synchronous and asynchronous data?

Another method of rate adaptation is called Protocol Conversion.

A typical example of this is when the Asynchronous PPP used by the serial port on a PC is converted to the

Synchronous PPP used by ISPs.

6. Where does this conversion normally take place?

The protocol conversion takes place in a Terminal Adapter.

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Eicon Networks ISDN NT1 Especificações Página 46