7-8
ADSL WAN Connections
ADSL Overview
Figure 7-4. ADSL Connection to the Internet
Moving high-speed WAN connections onto a separate network infrastructure
alleviates a serious problem for most public carriers: congestion in the tradi-
tional public carrier network. With the increasing popularity of the Internet,
more and more businesses and residential users are connecting to the Internet
through the public carrier network, which is not built to handle the high-
volume caused by many Internet connections.
ADSL Annex A and Annex B: Sharing the Line with Analog or
ISDN Voice Traffic
ADSL is designed to share the local loop with analog voice or Integrated
Services Digital Network (ISDN) traffic used for either voice or fax transmis-
sions. (ADSL cannot share the local loop with an ISDN WAN connection, which
is used to transmit data.) To share the local loop, ADSL devices reserve the
bottom frequencies for analog voice and ISDN traffic. (See Figure 7-5.)
In the ADSL standards, support for analog voice is called ADSL over Plain Old
Telephone Service (POTS), or ADSL Annex A. The customer’s existing tele-
phone equipment can continue to send voice traffic over the same pair of wires
that carry ADSL traffic.
Central OfficeCustomer’s Premises
WAN router
DSLAM
Local
loop
Regional
broadband
network
LAN
Internet
Internet core
router
Broadband
switch (ATM)
Broadband
access server
Other
DSLAMs
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