Integrated Services
Digital Network
(ISDN)
Developed by ITU-T in 1976
Set of protocols that combines digital telephony and data
transport services. The whole idea is to digitize the
telephone network to permit the transmission of audio, video
and text over existing telephone lines.
The goal is to form a WAN that provides universal end-to-end
connectivity over digital media.
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Figure 16-1
ISDN Services
Bearer services – provide the means to transfer
information (voice, data, video) between users without the
network manipulating the content of that information.
Belongs to the first 3 layers of the OSI model.
Teleservices – the network my change or process the
contents of the data.
- correspond to layers 4-7 of the OSI model
- rely on the facilities of bearer services
Supplementary services – provide additional
functionality to the bearer services and teleservices.
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Figure 16-1
ISDN Services
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Figure 16-2
Voice Communication over an
Analog Telephone Network
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Figure 16-3
Voice and Data Communication
over an Analog Telephone Network
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Figure 16-4
Analog and Digital Services
over the Telephone Network
3 types of customers – the first one is the most prominent
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Figure 16-5
Integrated Digital Network (IDN)
A combination of networks available for different purposes.
Access to these networks is by digital pipes
digital pipes – time-multiplexed channels sharing very-high-speed
paths.
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Figure 16-6
Integrated Services Digital Network
(ISDN)
ISDN incorporates all communication connections
in a home or building into a single interface
With ISDN all customer services will become digital
rather than analog.
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Subscriber Access to the ISDN
• Subscriber loops – organized
into multiple channels of different
sizes
• B channel
– basic user channel
– Can carry any type of digital info
in Full-duplex mode
– Carries transmissions end-to-end
Channel
Data Rates
(Kbps)
Bearer (B)
64
Data (D)
16, 64
Hybrid (H)
384 (H0),
1536(H11),
1920(H12)
• D channel
– Primary function is to carry control
signaling for the B channel
– Acts like an operator between the
user and the network at the
network layer
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H Channels
- For high data rate applications
such as video, teleconferencing and
so on
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Two Types of Digital Subscriber Loops
• Basic rate interface (BRI)
• Primary rate interface (PRI)
– Each type is suited to a different level of
customer needs
– Both include one D channel and some
number of either B or H channels
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Figure 16-7
Basic Rate Interface (BRI)
Designed to meet the needs of residential and small-office customers
The same twisted pair local loop that delivers analog transmission
can be used to handle digital transmission
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Figure 16-8
Primary Rate Interface (PRI)
Other channel combinations:
– 3H0 + D
– 4H0 + D
– H12 + D
PRI was designed to be compatible with existing T-1 lines
In Europe, the PRI includes 30 B channels and 2 D channels
= capacity of 2.048 Mbps – the capacity of E-1 line
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Figure 16-9
Functional Grouping
Defines the function of
each type of equipment
NT – network termination
TE – terminal equipment
NT1
• Controls the physical & electrical termination of the ISDN at the user’s
premises
• Connects the user’s internal system to the digital subscriber loop
• Organizes the data stream s from a connected subscriber into frames that can
be sent over the digital pipe
• Translates the frames received from the network into a format usable by the
subscriber’s devices.
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NT2 ( ex. Digital PBX, LAN)
• Performs functions at the physical (multiplexing), data link (flow control),
network layers (packetizing) of the OSI model
• Provides intermediate signal processing between the data-generating devices
and an NT1
• Used primarily to interface between a multiuser system and an NT1 in a PRI.
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TE1 – same thing as DTE
• Any device that supports the ISDN standards
• Ex. Digital telephones,integrated voice/data terminals
TE2
• Any non-ISDN device
• Ex. Terminal workstation, regular telephone
• Can be used with the help of Terminal Adapter (TA)
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Figure 16-10
Reference Points
Label used to identify individual interfaces between two elements of an
ISDN installation
Defines the functions of the connections
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Figure 16-11
ISDN Layers
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Figure 16-12
Simplified Layers of ISDN
Mechanical and
electrical specs of
interfaces R, S, T, U
Encoding
Multiplexing
Power supply
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Figure 16-13
BRI Interfaces
R interface
– not defined by ISDN, it can be any EIA standards or V or X series
S interface
– ISO standard, ISO 8887, four-, six-, or eight-wire connections.
U interface
- Single-pair twistedpair cable in each
direction
- Two binary, one
quaternary (2B1Q)
encoding
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Figure 16-14
S Interface
Name
TE
NT
a
Power source Power sink 3
3
b
Power source Power sink 3
3
c
Transmit
Receive
d
Receive
Transmit
e
Receive
Transmit
f
Transmit
Receive
g
Power sink 2
Power
source 2
h
Power sink 2
Power
source 2
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Figure 16-15
2B/1Q Encoding
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00
01
10
11
-3
-1
+3
+1
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Figure 16-16
BRI Frame
overhead
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BRI
Topology
Point-to-point
Multipoint
-limited distance to
ensure frame
synchronization
As many as 8
devices can be
connected to NT1,
two can access the
B-channels at 1 time
D channels use a
mechanism like
CSMA
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Figure 16-18
PRI Interfaces
R and S – same
as those defined
for the BRI
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T – identical to S
standard
- B8ZS Encoding
U – same for
both, different
rates
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Figure 16-19
PRI Frame
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Figure 16-20
LAPD Address Field
SAPI – Service Access Point Identifier
- identifies the type of upperlayer service (network layer)
C/R – Command (0) / Response (1)
TEI – Terminal Equipment Identifier
000000 – call control for network
layer (signaling use of D-channel)
000001 – call control for upper
layer (ene-to-end signaling), not
yet in use
010000 – packet communication (
data use of D channel)
111111 - management
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Figure 16-21
Network Layer Packet Format
Protocol discriminator – identifies the protocol in use, for Q.931 = 0000 1000
Call Reference – sequence number of the call
Message Type – identifies the purpose of the message
- call establishment
Reading assignment:
- call information
Different messages under
- call clearing
these four message types
- call miscellaneous
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Figure 16-22
Call Reference Field
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Figure 16-23
Information Elements
Carries specific details about the connection that are required for call
establishment:
- addresses of the sender and receiver
- routing information
- type of network that is desired for the B-channel exchange
(X.25, ATM, Frame Relay)
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Figure 16-24
Information Element Types
ID – identify the information being sent
Length – length of the content in bytes
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Figure 16-25
Addressing in ISDN
Addressing – important information element type in ISDN
NC – national Code
Subaddress – identify the device connected to NT1
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Figure 16-26
Bit Rates for Different Applications
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Figure 16-27
B-ISDN Services
Broadband ISDN – based on a change from metal cable to fiber
optic cable at all levels of telecommunications
Interactive services – those that require two-way exchanges
between either 2 subscribers or between a subscriber & a service
provider
- conversational – real time exchanges such as telephone calls
- messaging – store & forward exchanges such as voice mail
- retrieval –retrieve info from a central office
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Figure 16-27
B-ISDN Services
Distributive services – unidirectional services sent from a provider to
subscribers, broadcast to the user
- without user control – user choice is limited to whether or not
to receive the service at all
- with user control – allow the user a choice of times during
which to receive them
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Figure 16-28
B-ISDN Accesses
155.520 Mbps – fill the needs of most residential and many
business subscribers
155.520/622.080 Mbps – fill the needs of businesses that require
the simultaneous receipt of multiple services and video
conferencing
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