As with BRI, you have several switch types to select from. Check with your provider to configure
the correct one. Otherwise, you might have to reboot your router for the switch type change
to take effect.
Table 26.3 shows the typical available switch types used with PRI.
TABLE 2 6 . 3
PRI Switch Types
Switch Type Typically Used
PRIMARY-5ESS AT&T 5ESS switch type for the U.S.
PRIMARY-4ESS AT&T 4ESS switch type for the U.S.
PRIMARY-DMS100 Northern DMS-100 switch type for the U.S.
PRIMARY-NET5 NET5 switch type for the U.K. and most of Europe
VN3 VN3 and VN4 switch types for France
PRIMARY-NTT Japanese ISDN PRI switches
PRIMARY-NI AT&T National ISDN switch type for the U.S.
T-1– and E-1–based PRIs use different line-coding and framing schemes. A T-1–based PRI
uses binary eight-zero substitution (B8ZS) for encoding and Extended Super Frame (ESF) for
framing. An E-1–based PRI uses high-density bipolar Order 3 (HDB3) for encoding and cyclic
redundancy check, level 4 (CRC-4) for framing.
863
IT Certification CCIE,CCNP,CCIP,CCNA,CCSP,Cisco Network Optimization and Security Tips
Primary Rate Interface (PRI)
Most Internet service providers use
Primary Rate Interface (PRI)
ISDN to connect to the public
switched telephone network (PTSN). A PRI enables service to analog modem users, digital modem
users, and ISDN customers. The calls are routed to the appropriate modems after the access server
receives the calling number’s bearer capability. ISDN also provides a means to deliver calling line ID
(CLID), as well as called number or automatic number identification (ANI). These features can be
used to determine the correct authentication server for this customer.
PRIs have the following capacities:
A T-1–based PRI has 23 64Kbps B channels and one 64Kbps D channel, which equals a
bandwidth of 1.536Kbps. An 8Kbps channel for framing and synchronization is also used,
resulting in a total bandwidth of 1.544Mbps for a U.S. T-1/PRI. The last T-1 channel is
used as the D channel.
An E-1–based PRI has 30 B channels and one 64Kbps D channel. An E-1 uses channel 15
for signaling (D channel). An E-1 has 2.048Mbps of total bandwidth.
Primary Rate Interface (PRI)
ISDN to connect to the public
switched telephone network (PTSN). A PRI enables service to analog modem users, digital modem
users, and ISDN customers. The calls are routed to the appropriate modems after the access server
receives the calling number’s bearer capability. ISDN also provides a means to deliver calling line ID
(CLID), as well as called number or automatic number identification (ANI). These features can be
used to determine the correct authentication server for this customer.
PRIs have the following capacities:
A T-1–based PRI has 23 64Kbps B channels and one 64Kbps D channel, which equals a
bandwidth of 1.536Kbps. An 8Kbps channel for framing and synchronization is also used,
resulting in a total bandwidth of 1.544Mbps for a U.S. T-1/PRI. The last T-1 channel is
used as the D channel.
An E-1–based PRI has 30 B channels and one 64Kbps D channel. An E-1 uses channel 15
for signaling (D channel). An E-1 has 2.048Mbps of total bandwidth.
BRI Switch Options
Several BRI switch options are available for configuring your router. These switch types vary
according to geographic location. The available switch types are listed in Table 26.2.
TABLE 2 6 . 2
ISDN BRI Switch Types
Switch Type Typically Used
BASIC-1TR6 1TR6 switch type for Germany
BASIC-5ESS AT&T 5ESS switch type for the U.S.
BASIC-DMS100 Northern DMS-100 switch type for the U.S.
BASIC-NET3 NET3 switch type for the U.K. and most of Europe
BASIC-NI National ISDN switch type for the U.S.
BASIC-TS013 TS013 switch type for Australia
NTT NTT switch type for Japan
VN3 VN3 and VN4 switch types for France
EZ-ISDN North American ISDN standard service package
A benefit to using a BRI is being able to make a voice call while maintaining your Internet
connection. This is a great solution for SOHO deployments.
The D channel can also be used to transport packet-switched data communications
such as X.25. In fact, Cisco has enabled this feature in version 12 of its IOS
software. The feature is called Always On/Dynamic ISDN (AO/DI). Basically, it
enables the low-bandwidth traffic to use the D channel and initiates a call by
using one or two B channels if the traffic warrants. This feature will be most useful
for point-of-sale applications but is not supported by all service providers.
according to geographic location. The available switch types are listed in Table 26.2.
TABLE 2 6 . 2
ISDN BRI Switch Types
Switch Type Typically Used
BASIC-1TR6 1TR6 switch type for Germany
BASIC-5ESS AT&T 5ESS switch type for the U.S.
BASIC-DMS100 Northern DMS-100 switch type for the U.S.
BASIC-NET3 NET3 switch type for the U.K. and most of Europe
BASIC-NI National ISDN switch type for the U.S.
BASIC-TS013 TS013 switch type for Australia
NTT NTT switch type for Japan
VN3 VN3 and VN4 switch types for France
EZ-ISDN North American ISDN standard service package
A benefit to using a BRI is being able to make a voice call while maintaining your Internet
connection. This is a great solution for SOHO deployments.
The D channel can also be used to transport packet-switched data communications
such as X.25. In fact, Cisco has enabled this feature in version 12 of its IOS
software. The feature is called Always On/Dynamic ISDN (AO/DI). Basically, it
enables the low-bandwidth traffic to use the D channel and initiates a call by
using one or two B channels if the traffic warrants. This feature will be most useful
for point-of-sale applications but is not supported by all service providers.
Basic Rate Interface (BRI)
A
Basic Rate Interface (BRI)
uses a single pair of copper wires to provide up to 192Kbps of
bandwidth for both voice and data calls A BRI uses two 64Kbps B channels and one 16Kbps
D channel. An additional 48Kbps are used for framing and synchronization.
To review the math, each B channel is 64Kbps, so that totals 128Kbps. Add the 16Kbps
D channel, and the usable bandwidth for ISDN BRI is now at 144Kbps. Finally, add the 48Kbps
for framing and synchronization to get a total circuit speed of 192Kbps. Figure 26.1 shows the
ISDN protocol layers.
Both the B and D channels share layer 1. Layers 2 and 3 operate over the D channel, but the B
channel operates in either an HDLC or PPP encapsulation mode. This architecture is used to encapsulate
the upper layer protocols instead of using layer 2 and layer 3 directly. LAPD is the framing
protocol used for the D channel data. DSS1 (digital subscriber signaling system number 1) is the
layer 3 protocol for the D channel where Q.931 is used. B channels are used by the IP or IPX protocols
for data transfer, and the D channel is used by dial-on-demand routing (DDR), which builds
the connection over ISDN.
Basic Rate Interface (BRI)
uses a single pair of copper wires to provide up to 192Kbps of
bandwidth for both voice and data calls A BRI uses two 64Kbps B channels and one 16Kbps
D channel. An additional 48Kbps are used for framing and synchronization.
To review the math, each B channel is 64Kbps, so that totals 128Kbps. Add the 16Kbps
D channel, and the usable bandwidth for ISDN BRI is now at 144Kbps. Finally, add the 48Kbps
for framing and synchronization to get a total circuit speed of 192Kbps. Figure 26.1 shows the
ISDN protocol layers.
Both the B and D channels share layer 1. Layers 2 and 3 operate over the D channel, but the B
channel operates in either an HDLC or PPP encapsulation mode. This architecture is used to encapsulate
the upper layer protocols instead of using layer 2 and layer 3 directly. LAPD is the framing
protocol used for the D channel data. DSS1 (digital subscriber signaling system number 1) is the
layer 3 protocol for the D channel where Q.931 is used. B channels are used by the IP or IPX protocols
for data transfer, and the D channel is used by dial-on-demand routing (DDR), which builds
the connection over ISDN.
ISDN Line Options
ISDN is available in many configurations, or line options. In this section, you will learn about
two of the most common: Basic Rate Interface (BRI) and Primary Rate Interface (PRI). These
flavors of ISDN vary according to the type and number of channels that carry data. Each option
has two or more DS0s, or
B (bearer) channels,
and a
D (data) channel
. ISDN is characterized
by the presence of a D channel, which carries control and signaling information, freeing up the
B channels exclusively for voice and data transport.
Each DS0 is capable of carrying 64,000 bits per second of either voice or data. Telephone
companies (telcos) can provide ISDN on their current infrastructure with little additional work.
Table 26.1 shows the relationship between the DS level, speed, designations, and number of DS0s
per circuit. Only the DS1 level is associated with ISDN, which is the transport that a PRI circuit uses.
TABLE 2 6 . 1
North American Digital Hierarchy
Digital Signal Level Speed Designation Channel(s)
DS0 64K None 1
DS1 1.544Mbps T-1 24
DS2 6.312Mbps T-2 96
DS3 44.736Mbps T-3 672
DS4 274.176Mbps T-4 4,032
Different standards, called Synchronous Optical Network (SONET) and Synchronous
Digital Hierarchy (SDH), were developed for Fiber Optics Transmission
Systems (FOTS). These standards are not covered in this book.
Another ISDN element is the
service profile identifier (SPID)
. A SPID identifies the characteristics
of your ISDN line. SPIDs might or might not be needed, depending on the type of switch
your service provider uses. ISDN National-1 and DMS-100 switches require a SPID for each
B channel, whereas a SPID is optional with an AT&T 5ESS switch type. Please consult your
ISDN provider if you are not sure whether you need a SPID. The format of a SPID is usually the
10-digit phone number, plus a prefix and possibly a suffix. For example, say that your telephone
number is 949-555-1234. Now add a prefix of 01 and a suffix of 0100. This gives you a SPID
of 0194955512340100.
SPIDs are only used in the U.S.
To place an ISDN call, you will also need a
directory number
, or DN. A DN is the actual
number you would call to reach that B channel. In the example from the previous paragraph,
the DN would be 9495551234 or 5551231. Knowing the SPID, switch type, and DN will speed
up the configuration of your router. Your service provider should provide you with this information.
Other than the directory number, the rest might be automatically detected.
two of the most common: Basic Rate Interface (BRI) and Primary Rate Interface (PRI). These
flavors of ISDN vary according to the type and number of channels that carry data. Each option
has two or more DS0s, or
B (bearer) channels,
and a
D (data) channel
. ISDN is characterized
by the presence of a D channel, which carries control and signaling information, freeing up the
B channels exclusively for voice and data transport.
Each DS0 is capable of carrying 64,000 bits per second of either voice or data. Telephone
companies (telcos) can provide ISDN on their current infrastructure with little additional work.
Table 26.1 shows the relationship between the DS level, speed, designations, and number of DS0s
per circuit. Only the DS1 level is associated with ISDN, which is the transport that a PRI circuit uses.
TABLE 2 6 . 1
North American Digital Hierarchy
Digital Signal Level Speed Designation Channel(s)
DS0 64K None 1
DS1 1.544Mbps T-1 24
DS2 6.312Mbps T-2 96
DS3 44.736Mbps T-3 672
DS4 274.176Mbps T-4 4,032
Different standards, called Synchronous Optical Network (SONET) and Synchronous
Digital Hierarchy (SDH), were developed for Fiber Optics Transmission
Systems (FOTS). These standards are not covered in this book.
Another ISDN element is the
service profile identifier (SPID)
. A SPID identifies the characteristics
of your ISDN line. SPIDs might or might not be needed, depending on the type of switch
your service provider uses. ISDN National-1 and DMS-100 switches require a SPID for each
B channel, whereas a SPID is optional with an AT&T 5ESS switch type. Please consult your
ISDN provider if you are not sure whether you need a SPID. The format of a SPID is usually the
10-digit phone number, plus a prefix and possibly a suffix. For example, say that your telephone
number is 949-555-1234. Now add a prefix of 01 and a suffix of 0100. This gives you a SPID
of 0194955512340100.
SPIDs are only used in the U.S.
To place an ISDN call, you will also need a
directory number
, or DN. A DN is the actual
number you would call to reach that B channel. In the example from the previous paragraph,
the DN would be 9495551234 or 5551231. Knowing the SPID, switch type, and DN will speed
up the configuration of your router. Your service provider should provide you with this information.
Other than the directory number, the rest might be automatically detected.
What Is Integrated Services Digital Network (ISDN)?
What Is Integrated Services
Digital Network (ISDN)?
Integrated Services Digital Network (ISDN) has been under development for a couple of decades
but has been hampered by the lack of applications that can use its speed. It wasn’t until recently
that telecommuting, video conferencing, and
small offices/home offices (SOHOs)
have needed the
capabilities that ISDN offered. Another factor slowing the development of ISDN was that it was
somewhat proprietary in nature. However, this ended when National ISDN-1 became available
in 1992. National ISDN-1 is a standard switch type used by ISDN providers. This standard
enabled vendors to interoperate among devices.
Different service providers adopted different standards, but on a national basis,
so several different ISDN switch types are now “standard.”
Before getting into what ISDN is and does, you first need to understand how our traditional,
or
plain old telephone service (POTS)
, operates. Typically, you pick up the telephone receiver,
you dial the number, and the party answers at the other end. Your voice—which is an analog
wave—is converted into a digital signal through a process called
pulse code modulation (PCM)
.
PCM samples your voice 8,000 times per second and converts the audio level into an 8-bit value.
This 64Kbps channel, or
DS0
, is multiplexed with 23 other channels to form a T-1.
If you do the math, you’ll notice that a T-1 is 1.544Mbps; however, 24 64Kbps is only
1.536Mbps. Where are the other 8Kbps? Before we answer that question, think of the purpose
of a T-1. Each telephone call in the past required two copper wires to carry the voice traffic. A
T-1 was originally designed to carry 24 individual voice calls on the same wire. Each voice call
received its own channel. The underlying technique to carry all 24 channels on the same wire
is called time division multiplexing (TDM). TDM breaks up the circuit into 24 separate channels
and provides a distinct time slot for each.
Now back to the math. Each of the 24 channels is composed of 8 bits, for a total of 192 bits
(8
×
24). According to the Nyquist theorem, we know that we need to sample at 8,000 times per
second to replicate the human voice. Therefore, to produce all 24 channels, the entire 192 bits
must be transmitted 8,000 times each second, for a subtotal of 1,536,000 bits per second, or
1.536Mbps (8,000
×
192).
Specifically, Nyquist states that we should sample at twice the highest data rate
of the sampled signal, and rounding the voice spectrum up to 4Kbps gives us
the 8,000. The 8 bits for each channel comes from the 256 sampling
levels
used
at each sample time.
Now for the missing 8Kbps. A single framing bit is added between each 24-channel frame. Therefore,
an additional 8,000 framing bits are sent each second (remember the sampling rate for human
voice), raising our total to 1,544,000 bits per second, or 1.544Mbps (1,536,000 + 8,000). This
number is the bit rate of the line itself, and the one you commonly see with reference to a T-1 circuit.
Because 8,000 of the bits sent each second are used for framing and not data, however, the maximum
data you could theoretically put on the wire is the smaller number: 1.536Mbps.
ISDN differs from POTS in a couple of ways. First, ISDN data starts off as digital signaling,
so there is no analog-to-digital conversion. Second, call setup and teardown is accomplished
through a dedicated 16Kbps channel also known as a D (data) channel. By using “out of band”
signaling, you have the entire 64Kbps for data. This leaves one or two B (bearer) channels for
your data or voice traffic that does not have an intrusion on the line for clocking or error control.
ISDN then provides unadulterated bandwidth to end users.
ISDN benefits include improved speed over an analog modem, fast call setup (one second or
less, typically), and lower cost than a dedicated point-to-point circuit. DSLs and cable modems
are replacing ISDN in some areas and will continue to do so as they fit the need for high-speed
Internet access to the home. However, ISDN has some advantages over these newer, faster technologies.
Here is a list of the advantages that ISDN can provide:
Ability to dial into many locations simultaneously
High-speed dial-up services for traveling telecommuters
A fault-tolerant link for dedicated lines
Remote SOHO connectivity
Video conferencing
Digital Network (ISDN)?
Integrated Services Digital Network (ISDN) has been under development for a couple of decades
but has been hampered by the lack of applications that can use its speed. It wasn’t until recently
that telecommuting, video conferencing, and
small offices/home offices (SOHOs)
have needed the
capabilities that ISDN offered. Another factor slowing the development of ISDN was that it was
somewhat proprietary in nature. However, this ended when National ISDN-1 became available
in 1992. National ISDN-1 is a standard switch type used by ISDN providers. This standard
enabled vendors to interoperate among devices.
Different service providers adopted different standards, but on a national basis,
so several different ISDN switch types are now “standard.”
Before getting into what ISDN is and does, you first need to understand how our traditional,
or
plain old telephone service (POTS)
, operates. Typically, you pick up the telephone receiver,
you dial the number, and the party answers at the other end. Your voice—which is an analog
wave—is converted into a digital signal through a process called
pulse code modulation (PCM)
.
PCM samples your voice 8,000 times per second and converts the audio level into an 8-bit value.
This 64Kbps channel, or
DS0
, is multiplexed with 23 other channels to form a T-1.
If you do the math, you’ll notice that a T-1 is 1.544Mbps; however, 24 64Kbps is only
1.536Mbps. Where are the other 8Kbps? Before we answer that question, think of the purpose
of a T-1. Each telephone call in the past required two copper wires to carry the voice traffic. A
T-1 was originally designed to carry 24 individual voice calls on the same wire. Each voice call
received its own channel. The underlying technique to carry all 24 channels on the same wire
is called time division multiplexing (TDM). TDM breaks up the circuit into 24 separate channels
and provides a distinct time slot for each.
Now back to the math. Each of the 24 channels is composed of 8 bits, for a total of 192 bits
(8
×
24). According to the Nyquist theorem, we know that we need to sample at 8,000 times per
second to replicate the human voice. Therefore, to produce all 24 channels, the entire 192 bits
must be transmitted 8,000 times each second, for a subtotal of 1,536,000 bits per second, or
1.536Mbps (8,000
×
192).
Specifically, Nyquist states that we should sample at twice the highest data rate
of the sampled signal, and rounding the voice spectrum up to 4Kbps gives us
the 8,000. The 8 bits for each channel comes from the 256 sampling
levels
used
at each sample time.
Now for the missing 8Kbps. A single framing bit is added between each 24-channel frame. Therefore,
an additional 8,000 framing bits are sent each second (remember the sampling rate for human
voice), raising our total to 1,544,000 bits per second, or 1.544Mbps (1,536,000 + 8,000). This
number is the bit rate of the line itself, and the one you commonly see with reference to a T-1 circuit.
Because 8,000 of the bits sent each second are used for framing and not data, however, the maximum
data you could theoretically put on the wire is the smaller number: 1.536Mbps.
ISDN differs from POTS in a couple of ways. First, ISDN data starts off as digital signaling,
so there is no analog-to-digital conversion. Second, call setup and teardown is accomplished
through a dedicated 16Kbps channel also known as a D (data) channel. By using “out of band”
signaling, you have the entire 64Kbps for data. This leaves one or two B (bearer) channels for
your data or voice traffic that does not have an intrusion on the line for clocking or error control.
ISDN then provides unadulterated bandwidth to end users.
ISDN benefits include improved speed over an analog modem, fast call setup (one second or
less, typically), and lower cost than a dedicated point-to-point circuit. DSLs and cable modems
are replacing ISDN in some areas and will continue to do so as they fit the need for high-speed
Internet access to the home. However, ISDN has some advantages over these newer, faster technologies.
Here is a list of the advantages that ISDN can provide:
Ability to dial into many locations simultaneously
High-speed dial-up services for traveling telecommuters
A fault-tolerant link for dedicated lines
Remote SOHO connectivity
Video conferencing
Integrated Services Digital Network (ISDN)
Describe how different WAN technologies can be used to
provide remote access to a network, including asynchronous
dial-in, Frame Relay, ISDN, cable modem, and DSL.
Explain the operation of remote network access control
methods.
Identify PPP components, and explain the use of PPP as an
access and encapsulation method.
Configure an ISDN solution for remote access.
Plan a Cisco ISDN solution for remote access or primary
link backup.
Troubleshoot nonfunctional remote access systems.
Integrated Services Digital Network (ISDN)
has gained quite
a following over the past few years. It offers a switched highspeed
data connection that you can also use to support voice,
video, or fax calls, making it an excellent choice for small office/home office (SOHO) users.
However, digital subscriber line (DSL) will probably replace ISDN completely within the
next few years because DSL is cheaper and faster—which means it must be better, right?
Maybe. Just like ISDN, DSL can also provide data, voice, and fax services to end users.
Cable modems have also been around for a few years. They provide a large amount of bandwidth
for a neighborhood to access the Internet, but cable modems are really just composed
of a large Thinnet network in which all your neighbors share the same bandwidth. Thinnet
is the type of wiring used for 10Base2 Ethernet networks, which was popular before the
10BaseT standard. It runs over a thin coaxial cable similar to RG-6 wiring used by cable
providers, hence the term
Thinnet network
.
Now, you might be thinking, “Hey, I thought this was an ISDN chapter; what’s with DSL
taking over the discussion?” It is an ISDN chapter, and you do need to know about the topic.
ISDN won’t be replaced overnight, and although DSL will probably replace it, it is possible that
it won’t. Remember about six or seven years ago when everyone was saying that ATM was
going to take over the world? Pretty glad we didn’t buy stock in that rumor. ATM is a contender,
but the expense and difficult technical administration make it unpopular compared to
Gigabit Ethernet for the LAN and to DSL for the WAN. In defense of ISDN, it does have a few
benefits over DSL and cable modems that we will describe in this chapter.
ISDN is still a good choice for WAN services because of its high speed (Cisco calls ISDN high
speed). It can run anywhere from 56K to T-1 speeds (1.544Mbps). 128Kbps is the most common,
though. Although 128Kbps is not high speed to most people, compared to a 33Kbps dialup
analog modem, it is.
Outside of the U.S., the maximum speed of ISDN is 2.048 Mbps (E-1 standard).
Unlike a modem (which is analog), ISDN is digital from end to end. Analog modems translate
from digital on the computer, to analog between modems, and then back to digital on the
remote end. ISDN is more efficient and faster, and it also has a faster setup connection speed
than an analog modem.
In this chapter, you will learn about ISDN, beginning with the Physical layer and working
up. Topics covered in this chapter include ISDN device types, layer 2 (Q.921) and layer 3
(Q.931) specifications, ISDN reference points (R, S, T, U, and V), configuring dial backup and
bandwidth on Demand configurations, and commonly used ISDN commands.
provide remote access to a network, including asynchronous
dial-in, Frame Relay, ISDN, cable modem, and DSL.
Explain the operation of remote network access control
methods.
Identify PPP components, and explain the use of PPP as an
access and encapsulation method.
Configure an ISDN solution for remote access.
Plan a Cisco ISDN solution for remote access or primary
link backup.
Troubleshoot nonfunctional remote access systems.
Integrated Services Digital Network (ISDN)
has gained quite
a following over the past few years. It offers a switched highspeed
data connection that you can also use to support voice,
video, or fax calls, making it an excellent choice for small office/home office (SOHO) users.
However, digital subscriber line (DSL) will probably replace ISDN completely within the
next few years because DSL is cheaper and faster—which means it must be better, right?
Maybe. Just like ISDN, DSL can also provide data, voice, and fax services to end users.
Cable modems have also been around for a few years. They provide a large amount of bandwidth
for a neighborhood to access the Internet, but cable modems are really just composed
of a large Thinnet network in which all your neighbors share the same bandwidth. Thinnet
is the type of wiring used for 10Base2 Ethernet networks, which was popular before the
10BaseT standard. It runs over a thin coaxial cable similar to RG-6 wiring used by cable
providers, hence the term
Thinnet network
.
Now, you might be thinking, “Hey, I thought this was an ISDN chapter; what’s with DSL
taking over the discussion?” It is an ISDN chapter, and you do need to know about the topic.
ISDN won’t be replaced overnight, and although DSL will probably replace it, it is possible that
it won’t. Remember about six or seven years ago when everyone was saying that ATM was
going to take over the world? Pretty glad we didn’t buy stock in that rumor. ATM is a contender,
but the expense and difficult technical administration make it unpopular compared to
Gigabit Ethernet for the LAN and to DSL for the WAN. In defense of ISDN, it does have a few
benefits over DSL and cable modems that we will describe in this chapter.
ISDN is still a good choice for WAN services because of its high speed (Cisco calls ISDN high
speed). It can run anywhere from 56K to T-1 speeds (1.544Mbps). 128Kbps is the most common,
though. Although 128Kbps is not high speed to most people, compared to a 33Kbps dialup
analog modem, it is.
Outside of the U.S., the maximum speed of ISDN is 2.048 Mbps (E-1 standard).
Unlike a modem (which is analog), ISDN is digital from end to end. Analog modems translate
from digital on the computer, to analog between modems, and then back to digital on the
remote end. ISDN is more efficient and faster, and it also has a faster setup connection speed
than an analog modem.
In this chapter, you will learn about ISDN, beginning with the Physical layer and working
up. Topics covered in this chapter include ISDN device types, layer 2 (Q.921) and layer 3
(Q.931) specifications, ISDN reference points (R, S, T, U, and V), configuring dial backup and
bandwidth on Demand configurations, and commonly used ISDN commands.
Verifying a Dial-Up Connection
Dial-up connections work without a significant amount of troubleshooting under most
circumstances. When they don’t, Windows generally provides an indication of the error and
a recommended course of action, as shown in Figure 25.14. This screen shows error 680,
which means that there was no dial tone.
On the access server, the administrator can choose to use the show line command to view
the status of the connection. Unfortunately, this requires that much of the connection is already
established—a presumption that does not always coincide with troubleshooting.
Remote access solutions provide connectivity beyond the local area network. In prior chapters, you
read about solutions that use Cisco routers to communicate to other Cisco routers. This chapter
differs in that it is completely focused on a non-Cisco technology—Microsoft Windows—the leading
desktop operating system in use today. Although the current versions of Windows (XP and Windows
2000) are not covered in this chapter, the Cisco position to focus only on Windows 95/98 is
not completely without merit. While outdated and no longer supported, Windows 95/98 shares
many comparable traits with its offspring, and learning the old operating system can provide a solid
foundation for newer implementations. Having said that, Cisco should update their exam materials
to reflect shipping versions of software, and readers will need to augment this chapter’s material,
which focuses on the exam, with study and practice on newer versions in order to transition to realworld
practical usage.
Windows dial-up networking interoperates with Cisco remote access solutions via each of
the three layer 2 and layer 3 protocols offered by Microsoft. These are TCP/IP, which is actually
IP; IPX, the Novell networking protocol; and NetBEUI. NetBEUI is a bridged protocol and technically
operates at layer 2. The most common of these in production networks is IP.
At the Data Link layer, Microsoft installations are typically configured with PPP. This is the
most common implementation with Cisco solutions and is the most important to understand.
For some reason, Cisco stresses knowing the method used in configuring the dial-up networking
options within the Windows operating system. Microsoft places these options (unlike most
other network settings) under the Accessories option, and not the Control Panel or Network
icons. This is very important to know for successful implementation of the remote access solution;
however, it would be fair to note that many users have already learned the quirks of Windows
configuration and would therefore question Cisco’s judgment in stressing a process that is more
than seven years old. Suffice it to say that familiarity is important, and it would be prudent to
focus on this if you are approaching the exam or practical usage without Microsoft experience.
There are other minor elements in Windows remote access that are valuable to know. Microsoft
supports bonding and Multilink Protocol. Troubleshooting tools and terminal options are also
available. Terminal windows are often used with third-party authentication solutions.
The use of Windows devices directly attaching to Cisco routers or AS5000 series aggregation
routers can be an efficient way to provide remote connectivity. As a final point, readers are cautioned
on using this model to provide new remote access solutions. Although outside the scope
of this chapter due to Cisco’s focus and objectives, modern solutions would likely take advantage
of VPN, DSL, cable modem, and other more economical, secure, and scalable solutions.
Know which Cisco remote access protocols Windows 95 supports. Windows 95 supports IP,
IPX, and NetBEUI protocols, which are also supported by Cisco remote access. The most common
of these is IP.
Know the configuration settings location. The dial-up networking options are located under
Start Programs Accessories Communications Dial-Up Networking.
Understand that the Windows Control Panel is not used to configure a dial-up networking
session. These options are controlled under Programs.
Realize that each dial-up networking session is started by a specific icon. The dial-up networking
icons are located in Start Programs Accessories Communications Dial-Up
Networking, followed by the specific icon created for that connection.
Know how to use the terminal window option. Remember that the terminal window can be
used to add parameters to a dial-up session or to integrate with enhanced authentication products.
circumstances. When they don’t, Windows generally provides an indication of the error and
a recommended course of action, as shown in Figure 25.14. This screen shows error 680,
which means that there was no dial tone.
On the access server, the administrator can choose to use the show line command to view
the status of the connection. Unfortunately, this requires that much of the connection is already
established—a presumption that does not always coincide with troubleshooting.
Remote access solutions provide connectivity beyond the local area network. In prior chapters, you
read about solutions that use Cisco routers to communicate to other Cisco routers. This chapter
differs in that it is completely focused on a non-Cisco technology—Microsoft Windows—the leading
desktop operating system in use today. Although the current versions of Windows (XP and Windows
2000) are not covered in this chapter, the Cisco position to focus only on Windows 95/98 is
not completely without merit. While outdated and no longer supported, Windows 95/98 shares
many comparable traits with its offspring, and learning the old operating system can provide a solid
foundation for newer implementations. Having said that, Cisco should update their exam materials
to reflect shipping versions of software, and readers will need to augment this chapter’s material,
which focuses on the exam, with study and practice on newer versions in order to transition to realworld
practical usage.
Windows dial-up networking interoperates with Cisco remote access solutions via each of
the three layer 2 and layer 3 protocols offered by Microsoft. These are TCP/IP, which is actually
IP; IPX, the Novell networking protocol; and NetBEUI. NetBEUI is a bridged protocol and technically
operates at layer 2. The most common of these in production networks is IP.
At the Data Link layer, Microsoft installations are typically configured with PPP. This is the
most common implementation with Cisco solutions and is the most important to understand.
For some reason, Cisco stresses knowing the method used in configuring the dial-up networking
options within the Windows operating system. Microsoft places these options (unlike most
other network settings) under the Accessories option, and not the Control Panel or Network
icons. This is very important to know for successful implementation of the remote access solution;
however, it would be fair to note that many users have already learned the quirks of Windows
configuration and would therefore question Cisco’s judgment in stressing a process that is more
than seven years old. Suffice it to say that familiarity is important, and it would be prudent to
focus on this if you are approaching the exam or practical usage without Microsoft experience.
There are other minor elements in Windows remote access that are valuable to know. Microsoft
supports bonding and Multilink Protocol. Troubleshooting tools and terminal options are also
available. Terminal windows are often used with third-party authentication solutions.
The use of Windows devices directly attaching to Cisco routers or AS5000 series aggregation
routers can be an efficient way to provide remote connectivity. As a final point, readers are cautioned
on using this model to provide new remote access solutions. Although outside the scope
of this chapter due to Cisco’s focus and objectives, modern solutions would likely take advantage
of VPN, DSL, cable modem, and other more economical, secure, and scalable solutions.
Know which Cisco remote access protocols Windows 95 supports. Windows 95 supports IP,
IPX, and NetBEUI protocols, which are also supported by Cisco remote access. The most common
of these is IP.
Know the configuration settings location. The dial-up networking options are located under
Start Programs Accessories Communications Dial-Up Networking.
Understand that the Windows Control Panel is not used to configure a dial-up networking
session. These options are controlled under Programs.
Realize that each dial-up networking session is started by a specific icon. The dial-up networking
icons are located in Start Programs Accessories Communications Dial-Up
Networking, followed by the specific icon created for that connection.
Know how to use the terminal window option. Remember that the terminal window can be
used to add parameters to a dial-up session or to integrate with enhanced authentication products.
Launching Terminal Windows
On the Modem Properties Options tab, the user is offered the option of launching a terminal
window either before or after the connection is made. The option of opening a terminal window
after the connection is made is frequently necessary for hard authentication options such as
SecureID. This tab is shown in Figure 25.13.
Typically, the terminal window is launched with a challenge sent from the SecureID or a similar
third-party product. The challenge is a dynamically created value that is entered into a physical
calculator programmed to generate the proper response. This response is valid only for the
duration of the challenge—typically a minute—and it is a single-use password. These security
solutions require physical possession of the token, or password generator, and the PIN that
allows access. This security model is sometimes referred to as “something you have and something
you know.” Bank ATM cards use a similar principle.
window either before or after the connection is made. The option of opening a terminal window
after the connection is made is frequently necessary for hard authentication options such as
SecureID. This tab is shown in Figure 25.13.
Typically, the terminal window is launched with a challenge sent from the SecureID or a similar
third-party product. The challenge is a dynamically created value that is entered into a physical
calculator programmed to generate the proper response. This response is valid only for the
duration of the challenge—typically a minute—and it is a single-use password. These security
solutions require physical possession of the token, or password generator, and the PIN that
allows access. This security model is sometimes referred to as “something you have and something
you know.” Bank ATM cards use a similar principle.
Locking DTE Speed
At times the user might want to lock the DTE speed to complete a connection. Locking the DTE
speed can provide better performance on degraded lines if the speed is locked to a value lower
than would otherwise be possible—a result of fewer retransmissions to cope with the errors. For
most connections, this step is unnecessary.
To lock the DTE speed, select the Only Connect at This Speed box in the Modem Properties
dialog box, as shown in Figure 25.12. Recall that this is DTE-to-DCE speed, and as such, it
should relate to the capacity of the DCE device, as defined in Chapter 23.
speed can provide better performance on degraded lines if the speed is locked to a value lower
than would otherwise be possible—a result of fewer retransmissions to cope with the errors. For
most connections, this step is unnecessary.
To lock the DTE speed, select the Only Connect at This Speed box in the Modem Properties
dialog box, as shown in Figure 25.12. Recall that this is DTE-to-DCE speed, and as such, it
should relate to the capacity of the DCE device, as defined in Chapter 23.
Setting Additional Configuration Options
This section addresses two of the most common optional configurations that administrators
and users select in dial-up networking:
Lock DTE speed
Launch terminal windows
The first option, locking DTE (data terminal equipment) speed, is predominantly used for
troubleshooting or for improving performance on degraded circuits—circuits that are impaired
due to line conditions. This option is becoming less significant as phone line quality and termination
equipment improve.
The second option, launching terminal windows, is usually used for third-party authentication;
however, it can also be used for manual control of the session.
Unlike the previous options, both of these selections are grouped with the modem controls as
opposed to the networking configuration options. This is due to their relationship with the Physical
and Data Link layers—both DTE speed and a terminal window are independent of the Network
Layer protocol in use.
and users select in dial-up networking:
Lock DTE speed
Launch terminal windows
The first option, locking DTE (data terminal equipment) speed, is predominantly used for
troubleshooting or for improving performance on degraded circuits—circuits that are impaired
due to line conditions. This option is becoming less significant as phone line quality and termination
equipment improve.
The second option, launching terminal windows, is usually used for third-party authentication;
however, it can also be used for manual control of the session.
Unlike the previous options, both of these selections are grouped with the modem controls as
opposed to the networking configuration options. This is due to their relationship with the Physical
and Data Link layers—both DTE speed and a terminal window are independent of the Network
Layer protocol in use.
Multilink Tab
You learned about multilink services and the Multilink Protocol (MP) in Chapter 24. Multilink
provides the ability to create a single logical connection through two or more physical modems,
which can provide greater aggregate bandwidth for a remote user. Note that Microsoft’s multilink
feature does not support the Cisco proprietary Multilink Multipoint Protocol (MPP), only
the standards-based MP. Users or administrators need to provide only the phone number to
configure the service, as shown in Figure 25.11. The Edit Extra Device dialog box shown in Figure
25.11 opens when the user selects Use Additional Devices and clicks the Add button.
provides the ability to create a single logical connection through two or more physical modems,
which can provide greater aggregate bandwidth for a remote user. Note that Microsoft’s multilink
feature does not support the Cisco proprietary Multilink Multipoint Protocol (MPP), only
the standards-based MP. Users or administrators need to provide only the phone number to
configure the service, as shown in Figure 25.11. The Edit Extra Device dialog box shown in Figure
25.11 opens when the user selects Use Additional Devices and clicks the Add button.
Scripting Tab
Scripts enable the administrator or user to automate functions, including login or program
execution. A parallel of a script is a to-do list for getting ready in the morning—get up, brush
teeth, get dressed, and so forth. Scripts should be approached with care because they are not
stored in a secure manner and therefore can present a security risk.
To select a script, type the script name in the File Name text box (see Figure 25.10). The Step
Through Script option (grayed out in this figure because a script file was not defined) can be useful
for timing a script or for general debugging, and the Start Terminal Screen Minimized option
can be used to hide the script’s execution from being displayed to the user.
execution. A parallel of a script is a to-do list for getting ready in the morning—get up, brush
teeth, get dressed, and so forth. Scripts should be approached with care because they are not
stored in a secure manner and therefore can present a security risk.
To select a script, type the script name in the File Name text box (see Figure 25.10). The Step
Through Script option (grayed out in this figure because a script file was not defined) can be useful
for timing a script or for general debugging, and the Start Terminal Screen Minimized option
can be used to hide the script’s execution from being displayed to the user.
Allowed Network Protocols
The Allowed Network Protocols section of the Server Types tab enables eligible protocols to be
included or omitted from the dial-up networking connection. All three—NetBEUI, IPX, and
IP—are allowed in Figure 25.8 because PPP was selected. The TCP/IP Settings button enables
the user or administrator to choose DHCP-assigned IP address information (the default), or to
enter static entries.
included or omitted from the dial-up networking connection. All three—NetBEUI, IPX, and
IP—are allowed in Figure 25.8 because PPP was selected. The TCP/IP Settings button enables
the user or administrator to choose DHCP-assigned IP address information (the default), or to
enter static entries.
Viewing a Log File
The output shown next provides an example of the log output. Note that the software automatically
recovered from an error condition found when hanging up the modem via hardware
command by lowering DTR (data terminal ready).
The log is a standard text file that can be viewed by choosing Connection
Advanced from
the modem’s properties dialog box and then clicking the View Log button in the Advanced Connection
Settings dialog box that opens (see Figure 25.9).
Following is a sample log file that shows the preliminary handshake with the modem. This
identifies the information file (INF) that is used, in addition to the status of connections, error
control, compression, and hang-up characteristics. Note that in this case, the modem did not
respond to the lowering of DTR for the hang-up and was disconnected with software. This
might indicate a configuration problem with the modem; however, it is benign in this case.
02-15-2000 22:36:33.15 - Lucent Win Modem in use.
02-15-2000 22:36:33.16 - Modem type: Lucent Win Modem
02-15-2000 22:36:33.16 - Modem inf path: LTMODEM.INF
02-15-2000 22:36:33.16 - Modem inf section: Modem_PNP_DSVD
02-15-2000 22:36:34.80 - 115200,N,8,1
02-15-2000 22:36:34.80 - 115200,N,8,1
02-15-2000 22:36:34.80 - Initializing modem.
02-15-2000 22:36:34.80 - Send: AT
02-15-2000 22:36:34.81 - Recv: AT
02-15-2000 22:36:34.81 - Recv:OK
02-15-2000 22:36:34.81 - Interpreted response: Ok
02-15-2000 22:36:34.81 - Send: AT &F E0 &C1 &D2 V1 S0=0\V1
02-15-2000 22:36:34.85 - Recv: AT &F E0 &C1 &D2 V1 S0=0\V1
02-15-2000 22:36:34.85 - Recv:OK
02-15-2000 22:36:34.85 - Interpreted response: Ok
02-15-2000 22:36:34.85 - Send: ATS7=60S30=0L0M1\N3%C1&K3B0B15B2N1\J1X4
02-15-2000 22:36:34.86 - Recv:OK
02-15-2000 22:36:34.86 - Interpreted response: Ok
02-15-2000 22:36:34.86 - Dialing.
02-15-2000 22:36:34.86 - Send: ATDT;
02-15-2000 22:36:37.38 - Recv:OK
02-15-2000 22:36:37.38 - Interpreted response: Ok
02-15-2000 22:36:37.38 - Dialing.
02-15-2000 22:36:37.38 - Send: ATDT#######
02-15-2000 22:37:10.81 - Recv:CONNECT 26400 V42bis
02-15-2000 22:37:10.81 - Interpreted response: Connect
02-15-2000 22:37:10.81 - Connection established at 26400bps.
02-15-2000 22:37:10.81 - Error-control on.
02-15-2000 22:37:10.81 - Data compression on.
02-15-2000 22:37:44.27 - Hanging up the modem.
02-15-2000 22:37:44.27 - Hardware hangup by lowering DTR.
02-15-2000 22:37:45.47 - WARNING: The modem did not respond to lowering
➥
DTR. Trying software hangup...
02-15-2000 22:37:45.47 - Send: +++
02-15-2000 22:37:45.55 - Recv:OK
02-15-2000 22:37:45.55 - Interpreted response: Ok
02-15-2000 22:37:45.55 - Send: ATH E1
02-15-2000 22:37:45.63 - Recv:OK
02-15-2000 22:37:45.63 - Interpreted response: Ok
02-15-2000 22:37:45.63 - 115200,N,8,1
02-15-2000 22:37:46.69 - Session Statistics:
02-15-2000 22:37:46.69 - Reads : 811 bytes
02-15-2000 22:37:46.69 - Writes: 2991 bytes
02-15-2000 22:37:46.69 - Lucent Win Modem closed.
recovered from an error condition found when hanging up the modem via hardware
command by lowering DTR (data terminal ready).
The log is a standard text file that can be viewed by choosing Connection
Advanced from
the modem’s properties dialog box and then clicking the View Log button in the Advanced Connection
Settings dialog box that opens (see Figure 25.9).
Following is a sample log file that shows the preliminary handshake with the modem. This
identifies the information file (INF) that is used, in addition to the status of connections, error
control, compression, and hang-up characteristics. Note that in this case, the modem did not
respond to the lowering of DTR for the hang-up and was disconnected with software. This
might indicate a configuration problem with the modem; however, it is benign in this case.
02-15-2000 22:36:33.15 - Lucent Win Modem in use.
02-15-2000 22:36:33.16 - Modem type: Lucent Win Modem
02-15-2000 22:36:33.16 - Modem inf path: LTMODEM.INF
02-15-2000 22:36:33.16 - Modem inf section: Modem_PNP_DSVD
02-15-2000 22:36:34.80 - 115200,N,8,1
02-15-2000 22:36:34.80 - 115200,N,8,1
02-15-2000 22:36:34.80 - Initializing modem.
02-15-2000 22:36:34.80 - Send: AT
02-15-2000 22:36:34.81 - Recv: AT
02-15-2000 22:36:34.81 - Recv:
02-15-2000 22:36:34.81 - Interpreted response: Ok
02-15-2000 22:36:34.81 - Send: AT &F E0 &C1 &D2 V1 S0=0\V1
02-15-2000 22:36:34.85 - Recv: AT &F E0 &C1 &D2 V1 S0=0\V1
02-15-2000 22:36:34.85 - Recv:
02-15-2000 22:36:34.85 - Interpreted response: Ok
02-15-2000 22:36:34.85 - Send: ATS7=60S30=0L0M1\N3%C1&K3B0B15B2N1\J1X4
02-15-2000 22:36:34.86 - Recv:
02-15-2000 22:36:34.86 - Interpreted response: Ok
02-15-2000 22:36:34.86 - Dialing.
02-15-2000 22:36:34.86 - Send: ATDT;
02-15-2000 22:36:37.38 - Recv:
02-15-2000 22:36:37.38 - Interpreted response: Ok
02-15-2000 22:36:37.38 - Dialing.
02-15-2000 22:36:37.38 - Send: ATDT#######
02-15-2000 22:37:10.81 - Recv:
02-15-2000 22:37:10.81 - Interpreted response: Connect
02-15-2000 22:37:10.81 - Connection established at 26400bps.
02-15-2000 22:37:10.81 - Error-control on.
02-15-2000 22:37:10.81 - Data compression on.
02-15-2000 22:37:44.27 - Hanging up the modem.
02-15-2000 22:37:44.27 - Hardware hangup by lowering DTR.
02-15-2000 22:37:45.47 - WARNING: The modem did not respond to lowering
➥
DTR. Trying software hangup...
02-15-2000 22:37:45.47 - Send: +++
02-15-2000 22:37:45.55 - Recv:
02-15-2000 22:37:45.55 - Interpreted response: Ok
02-15-2000 22:37:45.55 - Send: ATH E1
02-15-2000 22:37:45.63 - Recv:
02-15-2000 22:37:45.63 - Interpreted response: Ok
02-15-2000 22:37:45.63 - 115200,N,8,1
02-15-2000 22:37:46.69 - Session Statistics:
02-15-2000 22:37:46.69 - Reads : 811 bytes
02-15-2000 22:37:46.69 - Writes: 2991 bytes
02-15-2000 22:37:46.69 - Lucent Win Modem closed.
Record a Log File for This Connection
When you select this check box, a log file will be
recorded. You might find that log files are useful for troubleshooting purposes, but most administrators
find the lack of information provided by this output frustrating. The log file might help
to augment the diagnostic process, however. When used with caution, the Cisco
debug
commands
provide substantially better troubleshooting output.
recorded. You might find that log files are useful for troubleshooting purposes, but most administrators
find the lack of information provided by this output frustrating. The log file might help
to augment the diagnostic process, however. When used with caution, the Cisco
debug
commands
provide substantially better troubleshooting output.
Require Data Encryption
By selecting this check box, you are making sure that information
passing through your connection will be encrypted. Unlike data compression, encryption protects
the contents of the data during transmission. Even though this option provides relatively weak
encryption, you might want to use it when you are transmitting critical data. Note that your performance
will suffer slightly with this option because the encryption is processed in software.
passing through your connection will be encrypted. Unlike data compression, encryption protects
the contents of the data during transmission. Even though this option provides relatively weak
encryption, you might want to use it when you are transmitting critical data. Note that your performance
will suffer slightly with this option because the encryption is processed in software.
Require Encrypted Password
By selecting the Require Encrypted Password check box, you are
precluding the use of cleartext authentication. Microsoft supports several encrypted password
options, including Shiva Password Authentication Protocol (SPAP), Data Encryption Standard
(DES), Challenge Handshake Authentication Protocol (CHAP), and MS-CHAP. MS-CHAP is
based on Rivest-Shamir-Adleman (RSA) MD4 (Message Digest type 4). On Windows NT, this is
enhanced to MD5 with Service Pack 3 or greater, and is standard in newer versions of Windows.
Remember when choosing your password that passwords are generally
case sensitive.
precluding the use of cleartext authentication. Microsoft supports several encrypted password
options, including Shiva Password Authentication Protocol (SPAP), Data Encryption Standard
(DES), Challenge Handshake Authentication Protocol (CHAP), and MS-CHAP. MS-CHAP is
based on Rivest-Shamir-Adleman (RSA) MD4 (Message Digest type 4). On Windows NT, this is
enhanced to MD5 with Service Pack 3 or greater, and is standard in newer versions of Windows.
Remember when choosing your password that passwords are generally
case sensitive.
Enable Software Compression
Software-based compression is different from the modem-based
compression features that were presented in Chapter 23, “Asynchronous Connections.” By selecting
this option, you can improve throughput by enabling compression, but this depends on the
type of data and equipment you use. By compressing with software, you are substituting a repetitious
series of characters to reduce the amount of bandwidth required. When decompressing, the
compressed data stream is translated back into an uncompressed form.
compression features that were presented in Chapter 23, “Asynchronous Connections.” By selecting
this option, you can improve throughput by enabling compression, but this depends on the
type of data and equipment you use. By compressing with software, you are substituting a repetitious
series of characters to reduce the amount of bandwidth required. When decompressing, the
compressed data stream is translated back into an uncompressed form.
Log On to Network
If you are connecting to an NT domain, you use this option to establish
a network connection and to attempt to log into the domain. Leave this option unselected to
improve performance on networks where this service is not required.
a network connection and to attempt to log into the domain. Leave this option unselected to
improve performance on networks where this service is not required.
Subscribe to:
Posts (Atom)