Information Technology Reference
In-Depth Information
17
CHAPTER
Troubleshooting ISDN Connections
Introduction
Dialup is simply the application of the Public Switched Telephone Network (PSTN) to carry data on
behalf of the end user. It involves customer premises equipment (CPE) sending the telephone switch a
phone number to direct a connection to. The Cisco 3600, AS5200, AS5300, and AS5800 are all examples
of routers that have the capability to run a PRI along with banks of digital modems. The AS2511, on the
other hand, is an example of a router that communicates with external modems.
Since the time of the last
Internetworking Troubleshooting Handbook
, the carrier market has grown
significantly, and there has been a demand for higher modem densities. The answer to this need was a
higher degree of interoperation with the telco equipment and the development of the digital modem. This
type of modem is capable of direct digital access to the PSTN. This has allowed the development of
faster CPE modems that can take advantage of the clarity of signal that the digital modems enjoy. The
fact that digital modems connecting into the PSTN through a PRI or BRI can transmit data at greater
than 53K using the V.90 communication standard attests to the success of the idea.
The first Cisco access servers were the Cisco 2509 and Cisco 2511. The 2509 could support 8 incoming
connections using external modems, and the 2511 could support 16. The AS5200 was introduced with 2
PRIs and could support 48 users using digital modems, which represented a major leap forward in
technology. Modem densities have increased steadily, with the AS5300 supporting four and then eight
PRIs. Recently, the AS5800 was introduced to fill the needs of carrier class installations that needed to
handle dozens of incoming T1s and hundreds of user connections.
A few outdated technologies bear mentioning in a historical discussion of dialer technology. 56K flex is
an older (pre-V.90) 56K modem standard that was proposed by Rockwell. Cisco supports version 1.1 of
the 56K flex standard on its internal modems, but the company recommends migrating the CPE modems
to V.90 as soon as possible. Another outdated technology is the AS5100, which was a joint venture
between Cisco and a modem manufacturer. Created as a way to increase modem density through the use
of quad modem cards, the AS5100 involved a group of 2511s built as cards that inserted into a backplane
shared by quad modem cards and a dual T1 card.
Troubleshooting Incoming Calls
Troubleshooting an incoming call starts at the bottom—the physical layer—and works up the protocol
stack. The general flow of reasoning looks for the following (a “yes” answer advances to the next
question):
Do we see the call arrive?
•
Does the receiving end answer the call?
•
