Content Overview Integrated Services
Digital Network (ISDN) is a network that provides end-to-end
digital connectivity to support a wide range of services
including voice and data services.ISDN allows multiple digital
channels to operate simultaneously through the same regular
phone wiring used for analog lines, but ISDN transmits a
digital signal rather than analog. Latency is much lower on an
ISDN line than on an analog line. Dial-on-demand routing (DDR)
is a technique developed by Cisco that allows the use of
existing telephone lines to form a wide-area network (WAN),
instead of using separate, dedicated lines. Public switched
telephone networks (PSTNs) are involved in this process. DDR is
used when a constant connection is not needed, thus reducing
costs. DDR defines the process of a router connecting using a
dialup network when there is traffic to send, and then
disconnecting when the transfer is complete. Students
completing this module should be able to: - Define the
ISDN standards used for addressing, concepts, and
signaling
- Describe how ISDN uses the physical and data
link layers
- List the interfaces and reference points
for ISDN
- Configure the router ISDN interface
- Determine what traffic is allowed when configuring
DDR
- Configure static routes for DDR
- Choose
the correct encapsulation type for DDR
- Be able to
determine and apply an access list affecting DDR traffic
- Configure dialer interfaces
Content
4.1 ISDN Concepts 4.1.1 Introducing ISDN There
are several WAN technologies used to provide network access
from remote locations. One of these technologies is ISDN. ISDN
can be used as a solution to the low bandwidth problems that
small offices or dial-in users have with traditional telephone
dial-in services. The traditional PSTN was based on an analog
connection between the customer premises and the local
exchange, also called the local loop. The analog circuits
introduce limitations on the bandwidth that can be obtained on
the local loop. Circuit restrictions do not permit analog
bandwidths greater than approximately 3000 Hz. ISDN technology
permits the use of digital data on the local loop, providing
better access speeds for the remote users. Telephone companies
developed ISDN with the intention of creating a totally digital
network. ISDN allows digital signals to be transmitted over
existing telephone wiring. This became possible when the
telephone company switches were upgraded to handle digital
signals. ISDN is generally used for telecommuting and
networking small and remote offices into the corporate LAN.
Telephone companies developed ISDN as part of an effort to
standardize subscriber services. This included the User-Network
Interface (UNI), better known as the local loop. The ISDN
standards define the hardware and call setup schemes for
end-to-end digital connectivity. These standards help achieve
the goal of worldwide connectivity by ensuring that ISDN
networks easily communicate with one another. In an ISDN
network, the digitizing function is done at the user site
rather than the telephone company. ISDN brings digital
connectivity to local sites. The following list provides some
of the benefits of ISDN: - Carries a variety of user
traffic signals, including data, voice, and video
-
Offers much faster call setup than modem connections
- B channels provide a faster data transfer rate than modems
- B channels are suitable for negotiated
Point-to-Point Protocol (PPP) links
ISDN is a
versatile service able to carry voice, video, and data traffic.
It is possible to use multiple channels to carry different
types of traffic over a single connection. ISDN uses
out-of-band signaling, the delta (D channel), for call setup
and signaling. To make a normal telephone call, the user dials
the number one digit at a time. Once all the numbers are
received, the call can be placed to the remote user. ISDN
delivers the numbers to the switch at D-channel rates, thus
reducing the time it takes to set up the call. ISDN also
provides more bandwidth than a traditional 56 kbps dialup
connection. ISDN uses bearer channels, also called B channels,
as clear data paths. Each B channel provides 64 kbps of
bandwidth. With multiple B channels, ISDN offers more bandwidth
for WAN connections than some leased services. An ISDN
connection with two B channels would provide a total usable
bandwidth of 128 kbps. Each ISDN B channel can make a separate
serial connection to any other site in the ISDN network. Since
PPP operates over both synchronous and asynchronous serial
links, ISDN lines can be used in conjunction with PPP
encapsulation. Web Links Integrated Services Digital
Network (ISDN) http://www.cisco.com/univercd/cc/
td/doc/cisintwk/ ito_doc/ isdn.htm
Content
4.1 ISDN Concepts 4.1.2 ISDN standards
and access methods Work on standards for ISDN began in the late
1960s. A comprehensive set of ISDN recommendations was
published in 1984 and is continuously updated by the
International Telecommunication Union Telecommunication
Standardization Sector (ITU-T), formerly known as the
Consultative Committee for International Telegraph and
Telephone (CCITT). The ISDN standards are a set of protocols
that encompass digital telephony and data communications. The
ITU-T groups and organizes the ISDN protocols according to the
following general topic areas: - E Protocols –
Recommend telephone network standards for ISDN. For example,
the E.164 protocol describes international addressing for
ISDN.
- I Protocols – Deal with concepts,
terminology, and general methods. The I.100 series includes
general ISDN concepts and the structure of other I-series
recommendations. I.200 deals with service aspects of ISDN.
I.300 describes network aspects. I.400 describes how the UNI is
provided.
- Q Protocols – Cover how switching and
signaling should operate. The term signaling in this context
means the process of establishing an ISDN call.
ISDN
standards define two main channel types, each with a different
transmission rate. The bearer channel, or B channel, is defined
as a clear digital path of 64 kbps. It is said to be clear
because it can be used to transmit any type of digitized data
in full-duplex mode. For example, a digitized voice call can be
transmitted on a single B channel. The second channel type is
called a delta channel, or D channel. There can either be 16
kbps for the Basic Rate Interface (BRI) or 64 kbps for the
Primary Rate Interface (PRI). The D channel is used to carry
control information for the B channel. When a TCP connection is
established, there is an exchange of information called the
connection setup. This information is exchanged over the path
on which the data will eventually be transmitted. Both the
control information and the data share the same pathway. This
is called in-band signaling. ISDN however, uses a separate
channel for control information, the D channel. This is called
out-of-band signaling. ISDN specifies two standard access
methods, BRI and PRI. A single BRI or PRI interface provides a
multiplexed bundle of B and D channels. BRI uses two 64 kbps B
channels plus one 16kbps D channel. BRI operates with many
Cisco routers. Because it uses two B channels and one D
channel, BRI is sometimes referred to as 2B+D. The B channels
can be used for digitized speech transmission. In this case,
specialized methods are used for the voice encoding. Also, the
B channels can be used for relatively high-speed data
transport. In this mode, the information is carried in frame
format, using either high-level data link control (HDLC) or PPP
as the Layer 2 protocol. PPP is more robust than HDLC because
it provides a mechanism for authentication and negotiation of
compatible link and protocol configuration. ISDN is considered
a circuit-switched connection. The B channel is the elemental
circuit-switching unit. The D channel carries signaling
messages, such as call setup and teardown, to control calls on
B channels. Traffic over the D channel employs the Link Access