ISDN
ISDN
provides a raw data rate of 144 Kbps on a single telephone company (called
telco in the business) twisted pair. To better suit voice applications, this
144 Kbps channel is partitioned into subchannels: two 64 Kbps B (for bearer)
channels and one 16 Kbps D (for data) channel. Each B channel can carry a
separate telephone call and usually has its own telephone number, called a
Directory Number (DN). You can combine the two B channels together to form a
single 128 Kbps data channel through a process called bonding. A minimal
ISDN set-up connects two computers. The incoming twisted pair enters a
Telco-provided box called the network terminator (NT1), which breaks the 144
Kbps channel into the two B and single D sub channels.
The B channels carry customer
voice or data signals. The D channel carries signals between your ISDN
equipment and the phone company's central office. The two-bearer plus one
data channel is called the Basic Rate Interface (BRI) in telco lingo, or
sometimes-just 2B+D for short. You also can buy ISDN in bulk: 23 B channels
with a single 64 Kbps D channel. This service, called the Primary Rate
Interface (PRI), inherits most of the capabilities and limitations of BRI.
A typical TA for data-only
applications might simply emulate a pair of ordinary Hayes-compatible
modems, translating standard modem set-up and dialling commands into ISDN
call-set-up commands. You connect your computer to this kind of TA with a
normal RS-232 cable and use your usual modem or fax software set to 64 Kbps
(or as high as you can go). The TA provides automatic rate adaptation to
match whatever data rate your computer supports with ISDN's 64 Kbps channel,
so that if your computer can't communicate faster than, say, 38.4 Kbps, it
will still work fine under ISDN. An example of a more sophisticated TA is
the ISDN router, which connects to an ISDN line on one side and your office
or home LAN on the other.
An ISDN router can carry your
network traffic -- AppleTalk, IPX, TCP/IP -- either down the street to your
main office or around the world on the Internet. The advantage of an ISDN
router over the simpler modem-replacement TA is the ability to support many
different kinds of computers without special ISDN software; the router
contains all the intelligence necessary to move traffic over an ISDN link,
literally moving your local LAN to the far-away destination of your choice.
Because ISDN is purely digital,
the telco can more easily deliver data intact from end to end, largely
eliminating the effects of noise. In addition, because the 64 Kbps channel
is essentially a pure "bit pipe," with no rate negotiation or handshaking
involved, there are no modem speed or protocol differences to cause
conflicts. In fact, because the negotiation phase with ISDN is so simple,
ISDN takes only a second or two to dial and establish a connection (modems
may take as long as a minute to accomplish the same thing). These benefits
alone are worth the cost of two high-speed modems, which is about what a
bare-bones TA costs.
In an Internet-access
application, your computer treats the basic TA just as it would a modem,
using the PPP (Point-to-Point) serial line protocol to carry your Internet
traffic. From your point of view, then, the ISDN connection set-up is
identical to the setup a PPP modem Internet connection. Although you could
technically run the popular SLIP (Serial Line Internet Protocol) over ISDN,
PPP is the ISDN transport protocol of choice for several reasons. First, PPP
is built into a number of ISDN-capable routers on the market, and your ISP
will likely be using one of these routers to provide ISDN dialup service.
Second, a variant of PPP, called MPP (for Multichannel Point-to-Point
Protocol) lets you combine the two 64 Kbps D channels to create one 128 Kbps
bonded channel. This is also called inverse multiplexing, and is usually set
up to provide bandwidth on demand -- only adding the second channel when
network traffic warrants. Bandwidth-on-demand is a cost-saving feature. Each
D-channel ISDN connection is treated as a separate phone call, so having two
channels up costs twice as much as one if your ISDN connection has
per-minute usage fees associated with it. For flat-rate ISDN calls, you can
permanently bond the D channels. |
