physical laer of isdn

8/12/2019 Physical laer of ISDN

1/23

ISDN Physical LayerAbhijith B N

Assistant Professor, Christ University


2/23

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.

7 August 20142


3/23

Frame Format

7 August 20143

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


4/23

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

7 August 20144


5/23

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.

7 August 2014 5


6/23

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

7 August 2014 6

TE to NT



7/23

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

7 August 2014 7


8/23

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

7 August 2014 8


9/23

What does it look like?

7 August 20149

TE to NT


NT to TE



10/23

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


11/23

P2P and passive bus

7 August 2014 11


12/23

Extended passive bus

7 August 2014 12


13/23

STAR Configuration

7 August 2014 13


14/23

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

7 August 2014 14


15/23

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

7 August 2014 15


16/23

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.

7 August 2014 16


17/23

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

7 August 2014 17


18/23

7 August 2014 18


19/23

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

7 August 2014 19


20/23

2.048Mbps

256 bit frames

32 8 bit time slots

30 B and 1 D channel

7 August 2014 20

Primary Rate User-Network Interface


21/23

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


22/23

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

7 August 2014 22


23/23

Experiment

7 August 2014 23

youtube.com

GO

physical laer of isdn

Documents