Wednesday, 24 April 2013
TELECOMMUNICATION(ISDN)
ISDN
(INTEGRATED SERVICES DIGITAL NETWORK)
ISDN
is a set of standards which define an end to end Digital Network
Features of ISDN
• Uses
Digital Signal
• Uses
Existing telephone wiring
• Charges are
generally based on the duration of
call (How long the WAN link was used)
• Alternate
to using leased lines
• Can
transport many types of Network traffic
(Voice, Data, Video, Text, Graphics etc)
• Faster Data
transfer rate than modems
• Faster Call
setup than Modems
ISDN Components
•
Terminal Equipment type 1 (TE1)
*
ISDN compatible device (Router with ISDN Interface)
*
TE1s connect to the ISDN network through a four-wire,twisted-pair digital link
•
Terminal Equipment type 2 (TE2)
*
ISDN Non-compatible devices.
*
Will require a terminal adapter.
•
Terminal Adapter (TA)
*
Converts standard electrical signals into the form used by ISDN
*
Needed for connection with TE2 devices
*
The ISDN TA can be either a standalone device or a board inside
the
TE2
•
Network termination type 1 (NT1)
*
Network-termination devices that connect the four-wire
Subscriber
wiring to the conventional two-wire local loop
*
Is a customer premises equipment (CPE) device (North America)
•
Network termination type 2 (NT2)
*
Intelligent device that performs switching & concentrating.
*
Provides multiple ISDN interfaces on an ISDN line. The NT2
may
be as simple as a bridging device connected to an NT1 unit
or
it may be as complicated as a PBX (Private
Branch
exchanges)
ISDN Reference points
•
R---The reference point between non-ISDN equipment and a TA.
•
S---The reference point between user terminals and the NT2.
•
T---The reference point between NT1 and NT2 devices.
•
U---The reference point between NT1 devices and line-termination
equipment
in the carrier network. The U reference point is
relevant
only in North America, where the NT1 function is not
provided
by the carrier network
INTERFACES OF ISDN
BRI (Basic Rate Interface)
•
Connection from the ISDN office to the user location provides for
access
to three channels. The channels are two 64Kb
B-channels and
one 16Kb D-channel
•
The B-channels and the D-channel provide the user with access to the
circuit
switched network
PRI (Primary Rate Interface)
•
ISDN Primary Rate Interface service provides digital access via a T1
line.
A T1 line provides a 1.544 bandwidth. This bandwidth is divided
into
24 64Kb channels. The ISDN PRI service uses 23
B channel
access
and uses the 24th (D) channel for signaling
purposes
ISDN Protocols
• E -
Protocols recommend telephone network standards for ISDN
(International
Telephone Numbering Plans)
• I -
Protocols for I.400=User-Network
Interface & I.100=Concepts,
Structures
& Terminology
• Q -
Protocols, how switching and signaling should operate, call
setup etc.
(Q.921=LAPD & Q.931=ISDN Network layer)
LAPD
is used to deliver signaling messages to the switch (call setup)
Link
Access Procedure
on the D channel
(not between
routers, only between router and ISDN switches)
TELECOMMUNICATION(DTMF)
Dual-tone
multi-frequency signaling
(DTMF)
(DTMF)
Introduction:
Dual-tone multi-frequency signaling (DTMF) is used for telecommunication signaling over analog
telephone lines in the voice-frequency band between telephone handsets and
other communications devices and the switching center. The version of DTMF that is used in push-button telephones for tone dialing is known as Touch-Tone. It was developed by Western Electric
and first used by the Bell System in commerce, using that name as a registered trademark.
DTMF is standardized by ITU-T Recommendation Q.23. It is also known in the UK as MF4.Other multi-frequency
systems are used for internal signaling within the telephone network.Introduced by AT&T in 1963, the Touch-Tone system using the telephone keypad gradually replaced the use of rotary dial and has become the industry standard for landline service.
Prior to the development of DTMF, numbers were dialed on automated telephone systems by means of pulse dialing (Dial Pulse or DP in the U.S.) or loop disconnect (LD) signaling, which functions by rapidly disconnecting and re-connecting the calling party's telephone line, imilar to flicking a light switch on and off. The repeated interruptions of the line, as the dial spins, sounds like a series of clicks. The exchange equipment interprets these dial pulses to determine the dialed number. Loop disconnect range was restricted by telegraphic distortion and other technical problems, and placing calls over longer distances required either operator assistance (operators used an earlier kind of multi-frequency dial) or the provision of subscriber trunk dialing equipment.
Multi-frequency signaling is a group of signaling methods that use a mixture of two pure tone (pure sine wave) sounds. Various MF signaling protocols were devised by the Bell System and CCITT. The earliest of these were for in-band signaling between switching centers, where long-distance telephone operators used a 16-digitkeypad to input the next portion of the destination telephone number in order to contact the next downstream long-distance telephone operator. This semi-automated signaling and switching proved successful in both speed and cost effectiveness. Based on this prior success with using MF by specialists to establish long-distance telephone calls, Dual-tone multi-frequency (DTMF) signaling was developed for the consumer to signal their own telephone-call's destination telephone number instead of talking to a telephone operator.
Touch tone dial telephones : DTMF technology
The touch tone
dialing arrangement is as shown below:
The rotary dial is
replaced by a push button keyboard. ‘Touching’ a button generates a ‘tone’
which is a combination of two frequencies, one from the lower band and the
other from the upper band. The Dtmf application is associated with digital
telephony and provides two selected output frequencies (one high band, one low
band) for a duration of 100ms. The keypad is arranged as a matrix for selecting
high and low frequencies associated with each key. Each key is uniquely
referenced by selecting one of the four low band frequencies associated with
the matrix rows, coupled with selecting one of the three high band frequencies
associated with the matrix column.
Design Considerations
1.
Choice of bands
2.
Band separation
3.
Choice of frequencies
4.
Choice of power
levels
5.
Signaling duration
The choice of code
for touch tone signaling should be such that imitation of code signals by
speech and music should be difficult. So, some form of multi frequency code is
required.
Separating the band
of the two frequencies is needed for the
following reasons:
·
Each frequency
component can be amplitude regulated separately
·
Before attempting to
determine two specific frequencies at the receiving end, band filtering can be
used to separate the frequency groups. This renders determination of specific
frequencies simpler.
The following fig. shows a simplified block diagram of
a touch tone receiver. The limiters attenuate differences in levels between
components of an incoming multifrequency signal.
If two frequencies
reach the limiter with one of the being relatively stronger , the o/p of the
limiter peaks with the stronger signal and weaker signal is further attenuated.
If both reach with same strength, limiter o/p is much below full o/p and
neither signal dominates. The selective circuitry is designed to recognize
signal as bonafide when it falls within
the specified narrow passband and has an amplitude within about 2.5 db of full
o/p of the limiter. The limiter and the selective circuitry together reduce the
probability mistaking touch signal to audio or vice versa. Band elimination
filters are sometimes used in place of band separation filters as they permit a
wider spectrum of speech to pass through the filters.
CCITT Q.23 standards
Consideration of human factors and mechanical design
factors include aspects like button size and spacing, stroke lengths, strike
force, numbering scheme and button arrangement. User preference and performance
studies coupled with design considerations have resulted in following
specifications.
·
3/8 inch square
buttons,separated by ¼ inch ,1/8 inch stroke length
·
100g force at the
bottom of the stroke
·
4*3 array with digits
1,2,3 in top row. 0 in middle of last row. # in third row usually to redial
·
The push button is
reserved for special features.
MAJOR ADVANTAGES OF THE TOUCH TONE DIALING :
1. Potential
for data transmission and remote control.
2. Data in
voice answer system – a power application.
3. Allows a
higher dialing rate of more than 10 signals per second.
4. Can
accurately determine the signaling frequencies by testing for very small
durations also.
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