ISDN
Integrated Services Digital Network, a circuit-switching network
used for voice, data and video transfer over existing copper
telephone lines. ISDN is a bit similar to the normal telephone
system but it is faster and needs less time to setup a call.
ISDN runs on the bottom three layers of the OSI reference model.
There are several types of ISDN channels, the two main being
the 64 Kilobits per second B-channel for data, and the D-channel
for control information. Two B-channels + one D-channel make
up ISDN BRI (Basic-Rate Interface), some Remote Access servers
support a feature called multilink allowing both B-channels
to be combined in a single virtual link of 128 Kbps. In SOHO
networks often 1 B-channel is used for data (an internet connection
for example) and 1 B-channel is used for voice (connected to
a digital telephone for example). The US and Japanese version
of ISDN PRI (Primary-Rate Interface) is made up of 23 B-channels
(total rate of 1.472 Mbps) and 1 D-channel. The European and
Australian version supports 30 B-channels (total rate of 1.984
Mbps) and 1 D-channel.
A common implementation of these two types of ISDN is a remote
access solution with ISDN PRI at the corporate network supporting
23 dial-in connections for employees with ISDN BRI at home.
Also an ISDN BRI connection is often implemented as a backup
line between routers in WANs such as in a Frame Relay network
as shown in the following image:
Besides this dial-up ISDN configuration for backup and other
Dial on Demand Routing (DDR) configurations another service
offered are ISDN BRI leased-line connections, the difference
is they always use both data channels for the connection to
the ISDN service provider and ISDN BRI leased-lines are always
active.
ISDN Function groups
The ISDN function groups represent the devices in an ISDN environment
such as terminals, terminal adapters, network-termination devices
and line-termination equipment. The following table lists these
devices:
TE1
(Terminal Equipment 1) |
Specialized ISDN terminals that understand
the ISDN standards, for example an ISDN telephone. |
TE2
(Terminal Equipment 2) |
Non-ISDN Terminals that need a Terminal
Adapter (TA) to connect to an ISDN network, for example
a regular telephone. |
TA
(Terminal Adapter) |
Converts some other form of signaling
to ISDN to allow non-ISDN devices (TE2) to work the 2-wire
ISDN network. |
NT1
(Network Termination 1) |
Connects TE1 or TA devices to the ISDN
network. In the US, the NT1 is located at the customer's
premises and owned by the customer. In other parts of
the world the NT1 is usually provided by the carrier (typically
a telephone company). |
NT2
(Network Termination 2) |
The NT2 is a physical device that interfaces
the NT1 to different types of devices (TE1 or TA). In
most cases it is a PBX at the customer's premises.
Take for example an apartment building or campus, if have
a demand for ISDN lines from your renters (customers)
you can order an ISDN PRI and connect it to your local
PBX. You can then extend the ISDN service to any place
in the building(s). |
The following image shows the various function groups and reference
points.
The following image illustrate some real-life situations. As
you can see the NT2 is left out, most NT1 adapters today have
a U interface on one side and an s/t on the other so you simply
plug your TE1 or TA into the NT1 and you're good to go.
The following image shows two type of routers, the upper is
usually used in North America where the demarcation point between
the customer premises and the carrier's network is the U reference
point, this router is actually a TE1 with a built-in NT1 and
is also known as a 'U router'. The other router is used in most
other parts of the world where the NT1 is provided by the telco,
this router is actually a TE2 with a built-in TA and is also
known as a 'S/T router'.
ISDN Reference points
ISDN specifies four reference points that define the logical
interfaces/connections between function groups (also represented
in the mage below):
R defines the reference
point between non-ISDN equipment (TE2) and a TA.
S
defines the reference point between and an NT2.
T
defines the reference point between NT1 and NT2 devices.
U defines the reference point between NT1 devices
and line-termination equipment in a carrier network. Relevant
in North America where the NT1 function isn’t provided
by the carrier network.
ISDN protocols
ISDN protocols are defined in ITU protocols that operates on
the Physical, Data Link and Network layer of the OSI model.
There are several series of protocols dealing with different
issues:
E series defines the use of ISDN
on the existing telephone network.
I series
deals with concepts, aspects, and services.
Q
series covers switching and signaling. The LAPD protocol is
formally specified in ITU-T Q.920 and ITU-T Q.921. LAPD is the
signaling protocol used on the D-channel in ISDN BRI and PRI.
Configure ISDN BRI and Legacy
DDR
Configuring ISDN may seem to be complex but is rather simple
in basic situations. The diagram below shows a typical setup
connecting two remote offices using an ISDN dial-up configuration.
First the ISDN switch type must be configured and should match
the carrier's equipment. You can use the isdn
switch-type command in both global config mode (required)
and interface configuration mode (optional if different per
interface). For example:
Router(config)#isdn switch-type basic-dms100
The correct switch type should be supplied by the carrier. Click
here
for a table at Cisco.com listing the ISDN BRI service provider
switch types. If you change the switch-type, you must reload
the router for the new switch type to take effect.
Although ISDN supports several upper-layer protocols such as
IP, IPX and Appletalk, typically IP is used and this is also
the one relevant to the CCNA exam. Configuring an IP address
on an ISDN BRI interface is done in the same way as configuring
an IP address for any other interface such as Ethernet or Serial:
Router(config)#interface bri 0 (to
enter interface config mode)
Router(config-if)#ip
address 172.16.22.115 255.255.255.0
Some service providers require the use of SPIDs for your ISDN
device to be able to place or receive calls. A SPID is usually
the telephone number of the channel with some optional numbers
which can be used to identity the service(s) the customer is
subscribed to. The SPID numbering scheme depends on the service
provider and the switch-type. For example, the DMS-100 switch
type requires a SPID for each B channel.
Router(config-if)#isdn
spid1 5055551234 0111 (B1 channel)
Router(config-if)#isdn
spid2 5055551235 0111 (B2 channel)
The default encapsulation type for each B-channel is HDLC, however
PPP encapsulation is recommended over HDLC in order to allow
the use of CHAP authentication. The encapsulation type can be
configured using the following command in interface configuration
mode:
Router(config-if)#encapsulation
ppp
Now to configure the actual part that maps the link to the network
layer using the dialer map command,
it defines the remote host where the calls are going, specifies
whether broadcast messages will be sent and the dialing string
to use to set up the call. Here's the syntax of the command:
Router(config-if)#dialer map protocol
next-hop-address name remote-name speed 56|64
dial-string
We'll break down the command using example options:
Router(config-if)#dialer map ip 172.16.22.114
name RouterB speed 64 broadcast 55588613213
- The IP address of the remote router's BRI interface used in
this command is the next hop. In the global configuration
you will have to define a static route to the remote network
pointing to the next hop address used in the dialer
map command. The use of static routes is very important,
since you don't want to use dynamic routing protocols for this
type of connection because the routing updates will keep the
link up.
- The remote name in name remote-name
is the hostname of the other router.
- speed defaults to 64 (in kilobits)
but you may need to set it to 56 in some situations.
- The broadcast option specifies whether
broadcast packets such as routing updates are sent.
- The dial-string is the
telephone number that should be dialed when making an outgoing
connection. You can leave out this number to configure the interface
to only accept incoming connections.
The following commands will define "interesting" traffic
that will cause the router to place a call make the connection.
For example if you want the router to dial-in for all IP traffic
you need to configure a dialer-list and bind it to the BRI interface:
Router(config)#dialer-list 1 protocol ip
permit
Router(config)#int bri0
Router(config-if)#dialer-group 1
You can also use regular or extended access lists to permit
all traffic except HTTP/HTTPs for example. Instead of using
the options in the dialer-list command
above you would specify the access list:
Router(config)#dialer-list
1 protocol ip list 101
The following command makes the router disconnect calls that
haven't had any interesting traffic for the configured time:
Router(config-if)#dialer idle-timeout seconds
To add some level of security and to identify the router when
it dials out, you should use the Challenge Handshake Authentication
Protocol (CHAP). The hostname of the router is used to identify
the router to another router when sending messages.
Router(config-if)#ppp
authentication chap
The global configuration username
command is required when CHAP is used to specify the CHAP secret
message to use when challenged by another router. Important
to know is that the two routers that need to talk must share
the same password.
Router(config)#username
routerB password password
PPP Multilink
Multilink is a feature that enables the use of both B-channels
combined for one call. To turn on multilink use the following
command:
Router(config-if)#ppp multilink
Use the following command to specify when the second B-channel
should kick-in (bandwidth on demand). When the total
load for this connection reaches this threshold, it brings up
the other B channel. This value represents a utilization percentage;
it is a number between 1 and 255, where 255 is 100 percent.
Router(config-if)#dialer
load-threshold 60
TROUBLESHOOTING
AND MONITORING ISDN
Here are some commonly used show commands used to monitor and
troubleshoot ISDN:
Router(config)#show
interfaces bri number
Displays information about the physical attributes of the ISDN
BRI B and D channels.
Router(config)#show
controllers bri number
Displays protocol information about the ISDN B and D channels.
Checks Layer 1 (physical layer) of the BRI.
Router(config)#show
isdn {active | history | memory | status | timers}
Displays information about calls, history, memory, status, and
Layer 2 and Layer 3 timers.
Router(config)#show
dialer interface bri number
Obtains general diagnostic information about the specified interface.
Checks Layer 3 (network layer).
Router(config)#show
isdn status
Use to verify that ISDN BRI Layer 1 is ACTIVE, LAYER 2 State
is MULTIPLE_FRAME_ESTABLISHED, and the service profile identifiers
(SPIDs) are valid.
Router(config)#debug
q921
Checks Layer 2 (data link layer).
The following three commands offer more advanced methods to
check Layer 3 (network layer) operation:
Router(config)#debug
isdn events
Router(config)#debug
q931
Router(config)#debug
dialer
REFERENCES TO CISCO
ISDN DOCUMENTATION
- Integrated
Services Digital Network (ISDN)
- ISDN
Glossary
- DDR
Dialup Technology Overviews and Explanations
- Basic
ISDN Sample Configuration
- More
ISDN Configuration Examples
- Designing
ISDN Internetworks
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