Making WAN Connections
Different flavors of PRI configurations are available in different parts of the world. The
configuration named 24B+D is common, and you might also see variations such as 22 B-channels with
a 64 Kbps D-channel, 24 56 Kbps B-channels, or even 30 standard B-channels (totaling 1.92 Mbps).
ISDN connections are usually formed as needed--they are switched. For a WAN
link, you use on-demand ISDN routers at each end, which can "dial up" the other
router when data is pending. Because ISDN has extremely fast call setup times, ISDN
connections are formed much more quickly than POTS connections--usually in less
than a second.
Although many systems can also use the Internet for videoconferencing, most firms
rely on ISDN as the mainstay connection type for these types of calls. If you are setting up a
videoconferencing system, you should plan on installing at least two BRI connections (three is
better) and purchase a videoconferencing system that supports at least 256 Kbps of bandwidth.
Videoconferencing calls over a single BRI (128 Kbps) are fairly poor quality, two BRIs (256 Kbps)
are much better, and three BRI (384 Kbps) connections are very good. Note also that both ends of a
call need to support the same speed and number of BRIs.
ISDN pricing changes occur regularly. ISDN prices also vary considerably in
different parts of the country. Getting full pricing information from your own regional
Bell operating company (RBOC) before choosing ISDN is important. Then, using your
projected usage data, you should be able to calculate the cost to use ISDN. Generally,
the installation of an ISDN-BRI line, assuming no wiring changes are necessary, costs
about $150. Some RBOCs might waive the installation charge if you sign an agreement
to keep the ISDN line for one to two years.
Monthly ISDN usage charges and long-distance ISDN call charges are similar to
POTS charges. But remember that connecting with two B-channels is equivalent to
making two separate calls, and whatever charge exists for a single call will double
when you use both B-channels.
Digital Subscriber Line (DSL)
The digital subscriber line (DSL) connection type has become widely available. A
number of different flavors of DSL exist. Each of these types begins with a different
initial or combination of initials, which is why DSL is often called xDSL. The available
flavors include the following:
Asymmetric DSL (ADSL) allows for up to 8 Mbps of data to be
received and up to 1 Mbps of data to be sent. However, many RBOCs offer
only up to 1.5 Mbps to be received (which is called the downstream direction)
and 256 Kbps to be sent (called the upstream direction), and distance from the
RBOC's local CO (the place where the RBOC equipment is located) might
affect the speeds available at any particular location. At further distances,
connections might be available only at much slower speeds (although in all
cases, ADSL is still faster than POTS connections using a modem).