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Megabit Modem 400F User Manual 61
9
TECHNICAL REFERENCE
This chapter provides technical information about how your modem transmits data between
users on your LAN and a service provider over the WAN.
ADSL
Asymmetric Digital Subscriber Line (ADSL) is the technology used to transmit data between
the modem and service provider at the physical layer. It provides data at asymmetric rates so
that downstream traffic from a service provider to you is faster than upstream traffic from you
to the service provider. The downstream transmission rate is up to 7.552 Mbps, while the
upstream rate is up to 928 kbps. Included in the ADSL bandwidth is analog POTS.
Discrete Multitone (DMT) is the line coding used for ADSL. Basically, it divides the
bandwidth into subchannels. Some of the subchannels are reserved for analog POTS. The
other subchannels are allocated to upstream and downstream traffic. Within the upstream
and downstream subchannels, some subchannels are used for management and
performance functions.
DMT ADSL provides rate-adaptive transmission that allows the service provider to deliver you
the best transmission rate determined by distance and line conditions.
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 Service provides UBR.
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