Isdn, Adsl

ISDN
• The user interfaces to ISDN are defined as combinations of three types of channels: B, D, and H.
• B-channel (Bearer channel), D-channel (Data Channel), and H-channel (Hybrid Channel).
• The Β channel is a 64-Kbps channel that transports a circuit-switched connection, an X.25 service
(packet- switched, virtual circuit), or a permanent digital point-to-point connection.
• The D channel is a 16-Kbps or a 64-Kbps channel used for signaling information (call control) and
for low bit rate packet-switched services.
• An Η channel is a 384-Kbps, 1,536-Kbps, or 1,920-Kbps channel used like a Β channel but for
higher-rate services.
• The ISDN standards specify the basic access and the primary access for users.
• The basic access is 2B + D; it consists of two full- duplex Β channels and a full-duplex 16-Kbps D
channel.
• The primary access is 30B + D (64 Kbps) in Europe, and it is 23B + D (64 Kbps) in the United
States, Japan, and Canada.
ISDN
LAPB : Link Access Protocol
on B channel
LAPD : Link Access Protocol

on D channel
ISDN Standards
• ISDN standards for implementing the lower three OSI layers. For basic access, the physical
layer of ISDN specifies an 8-pin connector to attach to the network, a pseudo-ternary encoding
(1 is represented by 0 volt and 0 by alternatively +0.75 volt and —0.75 volt), a frame format
that includes synchronization and DC-level balancing bits, and a line rate of 192 Kbps
corresponding to the 144 Kbps of user data rate (2*64 + 16 Kbps) plus the overhead bits. In
addition, the physical layer specifies a contention-resolution protocol for access to the D
channel by up to eight terminals attached to a common (multidrop) line.
• Summary:
• ISDN is an attempt to diversify the bearer services offered by the telephone network. The different services
are provided by different networks (rather than in one single network) accessed through a common ISDN
switch. Diversity is limited because these services are built on top of the traditional 64-Kbps channels, which
constrains the bit stream rates that can be supported.
ADSL
• Asymmetric Digital Subscriber Line (ADSL) is a modem technology
that uses existing twisted pairs to create three channels:
1. a high-speed downstream channel, (1.5 to 6.1 Mbps)
2. a medium-speed upstream or duplex channel, depending on the
implementation of the ADSL architecture (16 to 640 Kbps)
3. POTS (Plain Old Telephone Service) or an ISDN channel.
ADSL
ADSL
The physical layer
• There is an ADSL modem at each end of the wire-pair, one at the subscriber end and the other at
the central office. The central office modem is called ATU-C (ADSL Terminal Unit-Central
Office); the subscriber modem is called ATU-R (Remote).
• The 1-MHz bandwidth of the wire-pair is divided into three regions.
• Frequencies below 4 kHz are split off into a separate channel for voice (if POTS is provided).
• Frequencies above 4 kHz are divided into an upstream or duplex channel (10-100 kHz) and
• a higher frequency downstream channel (100-1,000 kHz).
• Thus ADSL is a passband technique, bypassing the 0 to 4 kHz voice band.
• By contrast, ISDN uses baseband modulation, overlapping with the voice band.
ADSL
• A modulation scheme converts the serial bit stream into an electrical signal suitable for
transmission. The ANSI ADSL standard specifies Discrete Multitone (DMT) modulation
in which the 1-MHz bandwidth is partitioned into 256 4-kHz subchannels. A separate
carrier in each band is quadrature- amplitude-modulated.
• A competitive, less complex technique called carrierless amplitude-modulation-phase
modulation or CAP has recently emerged. In this technique there is a single downstream
and a single upstream channel
ADSL An ADSL service provider locates a
Digital Subscriber Loop Access Multi-
plexer (DSLAM) in the central office.
(If POTS is also provided, the voice
signal split off at the ATU-C is sent to
the telephone switch.)
The DSLAM concentrates a number of
ADSL subscriber lines to a single ATM
line connected via a router or layer 2
switch to the provider's ATM backbone
network through which a subscriber is
connected to a corporate gateway
(telecommuting applications) or an
Internet service provider(ISP).
INTELLIGENT NETWORKS
• Intelligent Network or IN is the name given to the network of programmable elements,
organized to facilitate introduction of new services.
• Modern switches in the telephone networks are programmable computers, which makes them
very flexible. By sending instructions to a switch, one can modify its configuration. This
contrasts with the earlier electromechanical switches, whose functions were built into their
hardware.
• In modern switches, the control is separated from the hardware that executes the elementary
switching operations. This separation of control and basic operations is also present in the other
network elements.
• As a result, these elements, too, are programmable. The separation enables telephone
companies to develop their own services and implement them on switches and other network
elements obtained from different equipment vendors.
INTELLIGENT NETWORKS
• A number of telephone companies have implemented their own version of IN.
Some wireless network operators are also implementing IN for mobile
subscribers. Finally, the capability to implement new services that IN offers to
a telephone company can in part be delegated to customers who can use the
capability to design their own services.
INTELLIGENT NETWORKS –Service
Examples
Plain old telephone service or POTS- POTS is the basic telephone call service.
In the old telephone network, with electromechanical switches, the call functions of the network elements are wired into
those elements. This arrangement of the hardware is inflexible. Eg: a customer cannot instruct the switches to block calls
dialed from specific numbers or to forward a call to another number at certain hours of the day .

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ISDN

LAPD : Link Access Protocol

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