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PPP PRELIMINARY
56 Megabit Modem 500L Installation Manual
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ATM
Asynchronous Transfer Mode (ATM) is a technology that can simultaneously transmit voice,
data, and video over ADSL. ATM uses fixed-size cells that transmit over a preestablished
connection called a Permanent Virtual Circuit (PVC). Quality of Services can also be specified.
ATM cells are 53 bytes that comprise a 5-byte header and 48-byte payload. The header includes
the Virtual Path Identifier (VPI) and Virtual Channel Identifier (VCI) that you entered when you
configured each session in Chapter 4. The VPI and VCI provide the virtual connection between
the modem and the service provider. The VPI identifies the Virtual Path (VP) that transports
ATM cells in a Virtual Channel (VC). The Megabit Modem 500L has three VCs in the VP,
which provide the three sessions for the Internet or for other WAN applications. You can,
however, activate only one session at a time.
Mapping an ATM Session
Your service provider will give you a VCI and VPI address for each session. The VCI address,
for each session (each session is a VC), can be a number up to a maximum of 255 (32 to 255),
with the first 32 numbers (0 through 31) reserved. The VCI value for each VC must be unique
for each of the three sessions. The VPI value must always be 0.
PPP
PPP runs over ATM virtual circuits. PPP exists between the hardware layer and the
network-layer interface protocols. It is a widely used protocol for establishing connections on
the Internet. PPP provides the set up and release of connections for each session. PAP/CHAP
provide the authentication for the PPP sessions.
You can configure three sessions , but only one session can be established between the service
provider and the modem at a time. The user is mapped to the session that is active.