Components of an ADSL System
Internet
Aggregation
Router
DSLAM
PSTN Switch
Central Office
Splitter
NIDS
Filter
DSL
Modem
PSTN
Voice
DSL Voice + DSL
Carrierless Amplitude and Phase Line Coding
Carrierless Amplitude and Phase (CAP) is a DSL line-coding method that
divides the bandwidth into three channels: one for voice, one for downstream
data, and one for upstream data. Each type of traffic is carried within one
frequency band, and so CAP is termed a single-carrier modulation
technique. The bands are fairly wide. Voice uses 0 to 4 kHz, upstream traffic
uses 25 to 160 kHz, and downstream uses 240 kHz to 1.5 MHz. CAP is
simple to understand and implement but does not scale as well as Discrete
Multi-Tone (DMT) modulation.
Discrete Multi-Tone Line Coding
Discrete Multi-Tone (DMT) is the most widely used method of ADSL line
coding. It divides the DSL frequency band into 256 channels of 4 kHz each.
Some channels are duplex and used for both downstream and upstream
traffic. Others are used only for downstream. Channel quality is constantly
monitored, and the channels used can be changed when conditions warrant.
DMT is more complex than CAP but is also more flexible and scalable and
can achieve higher speeds.
A version of DMT, G.Lite ADSL, uses half the number of channels as DMT.
Layer 2 over DSL
Recall that DSL is a Layer 1 (physical layer) technology. There are three
methods of carrying data at Layer 2 over DSL:
�¡ Bridging.Based on RFCs 1483 and 2684. Ethernet traffic is just
bridged from the subscriber PCs, through the DSL modem and the
DSLAM, to a provider router. Bridging is not as secure or scalable as
other methods.
�¡ PPP over Ethernet (PPPoE).The most common Layer 2 method of
carrying data over DSL. PPP traffic is encapsulated in Ethernet
frames.
�¡ PPP over ATM (PPPoA).PPP packets are routed over ATM
between the subscriber equipment and the provider.