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PPP Concepts

308639-14.00 Rev 00

2-23

Synchronous Versus Asynchronous Connections

When you configure a dial (switched) circuit, you can specify whether you want

transmission on that circuit to be synchronous or asynchronous. Switched services

include dial-on-demand, bandwidth-on-demand, and dial backup. These function

independently of the physical modem communication method.

In synchronous transmissions, the timing of each data transfer has a specific time

relationship to the previous and next data transfer. Synchronous data transfers are

fast because of the low network overhead relative to the amount of data transferred

in each block, but they require more expensive, clock-driven data transmission

equipment. In general, you might use synchronous data transmission for large data

transfers.

Asynchronous communication, on the other hand, does not rely on a clock to

define the beginning and end of a transmission. It uses lower-cost, asynchronous

modems and appends a start and a stop bit to each transmission. This adds

overhead to each data transfer, but it is often acceptable in lower-speed (less than

56KB, for example) and lower-volume data transfers. You can, for example, use

an asynchronous modem connected to either a router or a PC to dial into a Nortel

Networks AN, ASN, BN, or BCN router or terminal server (such as a 5390) with

asynchronous modem banks.

You configure most of the synchronous/asynchronous parameters when you set up

dial services on a circuit and configure your modem pools. A modem pool can

contain synchronous lines, asynchronous, or ISDN lines.

When you create or add a line to a dial services modem pool for a line configured

to use PPP, the parameters that you must supply differ, depending on whether it is

a synchronous or an asynchronous line. Refer to Configuring Dial Services for

details on setting up and configuring both synchronous and asynchronous lines.

Recognizing Asynchronous Modem Control Characters

For a link that uses asynchronous modems, you can configure PPP to recognize

and “escape” specified control characters that may occur in data packets. An

example of such control characters is the XON/XOFF flow control mechanism

that asynchronous modems use. The escape mechanism removes spurious control

characters that external hardware may have introduced on the link.

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Avaya Configuring PPP Services Manual do Utilizador Página 51