Download - Physical laer of ISDN
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ISDN Physical LayerAbhijith B N
Assistant Professor, Christ University
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Framing and Multiplexing
Basic Rate Interface 2B and 1D 144kbps
These channels are multiplexed over a 192 kbps
interface The extra 48 kbps in used for framing and synch
Data is divided into frames of 48 bits
4096 frames in a second
One frame per every 250 us.
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Frame Format
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TE to NT
F L B1 L D L FA L B2 L D L B1 L D L B2 L D L
NT to TE
F L B1 E D A FA
N B2 E D M B1 E D S B2 E D L
F- Framing bit
L- Load balancing bit
E- D-channel echo bit
FA- Auxiliary framing bit
N- Set to opposite of framing bitM- multiframing bit
B1- B channel bits (16 )
B12 B channel bits (16 )
D- D channel bits (16 )
S- Spare bits
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Synchronization
Pseudoternary encoding
Framing bit is always a positive pulse
L is a DC balancing bit, negative
F-L pattern indicates the beginning of a frame
The first occurrence of a zero bit will always be
negative pulse
Then follows pseudoternary coding
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Fame Alignment
Deliberate violations in psedoternary
codes to synchronize frames
First F bit is a positive zero
First zero bit after the first L(which is
negative) bit is also a negative pulse.
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Next 8 bits B channel
Balanced by L bit
1 D bit balanced by L
Auxiliary framing FA, followed by L
Set to 1 for multiframe structure
Next 8 bit B , D, B, D channel balanced by L D channel balanced by L
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TE to NT
F L B1 L D L FA L B2 L D L B1 L D L B2 L D L
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E- Echo signal used for LAPD and
contention resolution
Activate or deactivate TE, low power
consumption mode
M and N are for multi framing M
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Multiframe Structure
An additional channel for traffic in TE to NT direction
called Q channel
Every 20th frame M bit is set to 1
FA bit in every 5th
frame is used as Q bit Normally FA bit will be set to binary zero and N bit to
binary one in NT to TE frame
For multiframe communication FA=1 and N=0
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What does it look like?
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TE to NT
F L B1 L D L FA L B2 L D L B1 L D L B2 L D L
NT to TE
F L B1 L D L FA L B2 L D L B1 L D L B2 L D L
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Multi Drop Configurations
Normally Point to Point configuration is used 1km More than one TE on a passive bus called multi drop
Signal strength has to be optimum
Strong enough to sustain attenuation
Not too strong to overload the receiver circuitry
NT should adjust for different round trip delay the
transmitter has.
Short passive bus Maximum distance is 100 to 200
If all devices are clustered at the end, we can achieve
greater length
If we have to use the longer TE NT separation for multi
drop a star configuration is required7 August 2014 10
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P2P and passive bus
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Extended passive bus
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STAR Configuration
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Contention Resolution
For multi drop configuration the D channel
will be shared among the terminal
equipment.
Thus a control over the channel is
necessary so that all the TEs will be able
to send LAPD signals.
The E bit is used to control the traffic over
D channel
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Priority
The signalling information is given priority
compared to the other types of information
Two priority lever for two priority classes
1. Signalling information Normal X1=8
Lower X1=9
2. Non Signalling information Normal X2=10
Lower X2=9
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When subscriber has no LAPD bits it transmits a series
of 1s. Voltage level zero as per pseudoternary
NT on receiving D channel, reflects it back as Echo bit
When a terminal wants to transmit an LAPD frame, itlistens to stream of echo signals. If it detects a string of
1-bits of length equal to the threshold value Xi, it may
transmit
If multiple terminals transmit together, it causes collision.
Depending upon the priority some terminals backs off.
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To summarize any equipment transmits a
zero overrides any equipment that
transfers a one at the same instant.
The arrangement ensures successful
completion of transmission by at least one
terminal
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Primary Rate User-Network Interface
1.544 Mbps
193 bits frame
24 8 bit time slots and a framing bit
Frames repeat in 125muS. 8000 Frames per
second
23 B channel and 1 D channel
Framing bits are for synchronization
FAS, O&M, CRC
linecoding AMI using B8Zs
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2.048Mbps
256 bit frames
32 8 bit time slots
30 B and 1 D channel
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Primary Rate User-Network Interface
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U Interface
2B1Q Line coding
Multiplexed over 160kbps channel Framing and synchronization is less
5.5 km7 August 2014 21
Dibit Signal level
10 +450 mV
11 +150 mV01 150 mV
00 450 mV
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Frame Format for U-Interface
250 bit ling
One frame every 1.5ms
3 parts Synchronization word (18 bits)
User data: 12 18 bits B and D channel
M channel: 6 bits for maintanance
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Experiment
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