The figure shows an analog phone and a DSL modem connected to a
single wall plate with two receptacles. (Most people's home phones plug into a
wall plate with just one receptacle; to use DSL, you can buy a little plastic
device that will create two receptacles for about $5 retail at most office
supply stores.) Physically, you connect your DSL modem to a wall socket just
like any of the phones in your house. You can pick up your same old phone and
make a call, just like always. Simultaneously, the DSL
modem can send data over the same phone line to the router in the ISP
network, as shown in the figure.
DSL uses the same local loop wiring that's already run between
the CO and your house, but now the CO connects the local loop wiring to a device
called a DSL access multiplexer (DSLAM).
The DSLAM splits out the digital signal and
analog signal from the local loop. The DSLAM gives the analog voice signalthe
frequency range between 04000 Hzto a telephone switch. The voice switch treats
that signal just like any other analog voice line.
Conversely, the DSLAM does not pass the digital electrical
signal to a telephone switch. The DSLAM forwards the data traffic to a router
owned by the ISP that provides the service in this figure. Notice that the ISP's
router is actually depicted as being resident in the local telco's COthat is
actually true in many cases. To support DSL, the ISP works with the local telco
to install some of the ISP's gear in the COa process called co-location, or co-lo for short. The local telco DSLAM forwards
and receives the IP packets to and from the ISP router, while the telco
maintains control over the local voice traffic.
If you can get some binary digits from a PC, through a DSL
modem, to the DSLAM, and finally to a router at an ISP, you can get IP packets
to and from that router. Incidentally, DSL uses PPP as the Layer 2 protocol.
Likewise, because DSL can get an analog electrical signal from the phone, over
the local loop, to the DSLAM, and then to the telco switch, you can make voice
calls.
Interestingly, with DSL, the ISP is often not the same company
that provides local telephone service. Typically, the consumer requests DSL
high-speed Internet access from an ISP, the ISP charges the customer for the
service, and then the ISP pays the local telco some cut of the fee. You still
need to pay for the local phone services as well. You end up with two services,
basicallyphone service and Internetand you pay two bills, assuming that you get
the Internet service from someone besides the local telco.
Sections
Comments (0 posted)
Syndication
