ISDN Basics

5/21/2018 ISDN Basics

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BASIC ISDN


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Bean Cans !


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Analogue Speech - SILENCE

N S


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Analogue Speech - Positive Pressure


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Analogue Speech - Negative Pressure


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The Traditional Oscilloscope View

MAX

+VE VoltsMAX

-VE Volts

0 Volts


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MAX

+VE VoltsMAX

-VE Volts

0 Volts


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MAX

+VE VoltsMAX

-VE Volts

0 Volts


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MAX

+VE VoltsMAX

-VE Volts

0 Volts


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The Analogue Telephone


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The Analogue Telephone

This signal is analogue cosit is analogous to my vocal

chords vibrating


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Digital Telephony


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What is Digital?

1s & 0s

on/off


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Analogue Waveform

Volts

Time


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Take Samples

Volts

Time


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How many samples ?

The active band of frequency on the telephonenetwork is from 300 to 3400 cycles per second

Nyquist Theorem...........You must sample at, at least tw ice the highest frequency of

the analogue signal

3400 x 2=8000 Samples Per Second

= 1 sample every 0.000125 seconds


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Pulse Amplitude Modulation

t1

t2

t3

t4

t5

t6

TimeVolts

125s


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Timed Samples (every 125ms)


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01111111

01111100

01101011

01010101

01011010

01001001

11011010

11010101

11101010

11101101

11110011

11111111

00000000

11001001

Amp

litudeSamples



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Pulse Code Modulation

01111111

01111100

01101011

01010101

01011010

01001001

11011010

11010101

11101010

11101101

11110011

11111111

00000000

11001001

X

X

X X

X

X

X

X

X X

X

X

X

X

X

X

X

X X

X

X X

X

XAmp

litudeSamp

les



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Pulse Code Modulation

01111111

01111100

0110101101010101

01011010

01001001

11011010

11010101

11101010

11101101

11110011

11111111

00000000

11001001

X

X

X X

XX

X

X

X X

X

X

X

X

X

X

X

X X

X

X X

X

XAmp

litudeSamp

les



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0111111101111100

01101011

01010101

01011010

01001001

X

X

X X

X

X


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0111111101111100

01101011

01010101

01011010

01001001

X

X

X X

X

X

01011010 01010101 01111100 01111100

s1 s2 s3 s4

8000 samples of 8 bits = 64,000 bits per second


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0111111101111100

01101011

01010101

01011010

01001001

X

X

X X

X

X

01011010 01010101 01111100 01111100

s1 s2 s3 s4? ? ?

125ms


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0111111101111100

01101011

01010101

01011010

01001001

X

X

X X

X

X

01011010 01010101 01111100 01111100

s1 s2 s3 s4011111110111110001101011010101010101101001001001

X

X

X

X X

X

01011010 01010101 01111100 01111100

s1 s2 s3 s4

125ms


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s2

01011010

s1

01101011

s1

125ms


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125ms

0100011010101100

s2s1


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125ms

0100011010101100

32 samples

s2s1


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125ms32 samples

s1 s2

8000 times per second

8bits x 32 x 8000 =2,048,000bits per second

1 frame


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125ms

32 samples

s1 s28000 times per second

0 16


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125ms32 samples

s2

0 16

Signalling

DASSDPNSSQ931

Synchronisation


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30 samples


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30 samples

OF WHAT ?


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Audio

Speech

Tones

Music


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Video

Security

News

Video-conference


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Fax

Group 2/3 sent as audio

Group 4 after handshake 1.5 seconds/page


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Data

Files

Programmes

Control


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Two Types Of ISDN Channel


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Function Groups & Reference Points

LT

NT

1

NT

2

TE 1

TE 2

TA


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LT

NT

1

NT

2

TE 1

TE 2

TA

U

T

S

R


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ITU Reference Model for ISDN

PTT

Equipmentat PhoneCompany

Switch

U InterfaceTermination Point T InterfaceTermination Point S InterfaceTermination Point Standard PSTN Equipmenthas an R Interface

Termination Point

U T S

RTE2TA

TE1 TE1

NT1 NT2

ISDN Equipment thatcan connect directlyto an ISDN Line

Terminal Adapter usedto connect TE2 devices

to an ISDN line

Equipment thatcannot connectto an ISDN line

NetworkTermination

used toconvert U into

T interface

NetworkTermination

used toconvert T into

S interface


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NT1 Network Termination 1, Handles physical layerinterface functions such as line termination (eg NTE 8).

NT2Network Termination 2, Handles physical layer pluslayer 2 and 3 functions such as multiplexing, switching andconcentration (e.g. an ISPBX).

LT Line Termination, Handles termination of 2 wire pair atthe exchange, operating 2B1Q or 4B3T line coding.

TA Terminal Adaptor, Equipment that supports ISDN callset up and provides an interface for connecting to non ISDNequipment.


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TE1 Terminal Equipment 1, End user equipmentsuch as ISDN telephones or data terminals compliantwith ISDN call set up procedures and capable ofinterfacing directly to the S-bus.

TE2 Terminal Equipment 2, End user equipmentfor non ISDN environments (typically uses an RS232interface)


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Basic Rate Interface

The BRI isdefined as two64Kb/s Bearer(B) channelsand one

16Kb/s Data(D) channel


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Basic Rate ISDN

Two separate B channels over a single line.

Combined voice, data, video.

T interface allows both channels to be usedindependently.

S interface may use one or both B channels.

Bandwidth 2 x 64k or 1 x 128k per S port

Flexible, high speed, high quality, low error rate, fastcall set-up.


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What IS ISDN 2e?

ISDN2e is the standard basic 2 channel ISDNservice

ISDN2e fully comply with European TelecomStandards

ISDN2e provides a network platform that iscapable of supporting supplementaryservices


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NTE 8

The NTE8 (opposite) is the

normal NTE within thecustomers premises for an

ISDN2e connection.

The NTE8 has TWO RJ45

sockets - ONLY ONE IS TO

BE USED - the other is for

testing purposes


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NTE 8

The NTE8 has a green LED which

indicates the presence of the ISDNservice.

NTE8 is only available on local

exchanges which use Line CardsI-MUX exchanges will use NTE6


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ISDN RJ 45 Connection

EIA 568A

Commercial Building

Cabling Specification

Draft 9.0

Preferred termination of

UTP data cabling International ISDN

standard


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Point To MultipointReferred to by BT as;

Standard Access

Or

S/T Reference

Additional telephone numbers are normally provided by MSN


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Basic Rate For The Home Or Office


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Multiple Subscriber Numbering

ISDN

NTE

TA

0208 988 6643

0208 988 9102

0208 988 5106

Allows theprogramming ofseparate telephone

numbers into eachdevice connected to anISDN2e line.

Currently 4 Options

- 2 Numbers

- 3 Numbers - 8 Numbers

- 10 Numbers


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Sub Addressing

For ISDN2e to ISDN2e calls.

Allows up to 20 Alphanumeric characters to besent (not #).

For ISDN2e to ISDN2 or ISDN 30

Allows up to 6 Alphanumeric characters to besent (not #).


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Multiple Devices And Multiple Numbers


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S0 Bus

RJ 45

Outlet

Terminating

RJ 45

Outlet

RJ 45

Outlet

RJ 45

Outlet

RJ 45

Outlet

RJ 45

Outlet

RJ 45

Outlet

RJ 45

Outlet


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Point To Point

Referred to by BT as;

System Access

Or

T Reference

Additional telephone numbers are normally provided by DDI


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Direct Dialling In

ISDN

ISPBX

7100

7101

7103

7104

DDI provides 10 or

more directory

numbers to an ISDN

line or group of lines.

Requires ISDN2e to

be configured for

System Access

Must be connected

to a PBX


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ISDN2e Supplementary Services

Calling Line Identity Presentation

Multiple Subscriber Numbering

Direct Dialling In

Call Forwarding

Sub Addressing

Terminal Portability

Call Barring Options

Maintenance Options


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Terminal Portability

Allows a terminal to be disconnected from anISDN2e socket and to be reconnected to another

socket (on the same line) during a call without losingthe call.

The terminal equipment must be capable of

supporting this facility.

This facility is not available with the DDI service.


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AODI

BTs ISDN Connect

The data connection is initiated using X.25 on the D channelwhere it maintains an open link.

When extra bandwidth is required the bandwidth AllocationControl Protocol automatically switches in the B Channels.

When the additional channels are no longer require they will beautomatically un-nailed.


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PRIMARY RATE


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NTE Status Lights

Light Status Indication

Power Green ON NTE on mains power

Power Green OFF No power to NTE

Power Green Flashing NTE on Standby Battery

Customer Amber OFF Inputs OK

Customer Amber ON One input disconnected

Customer Amber Flashing Two or more inputs disconnected

BT Red OFF Network OK

BT Red ON Network Faulty

Test Amber OFF OK

Test Amber ON or Flashing BT Testing


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Primary Rate Interface

The PRI is supplied

through a standard2.048Mb/s E1 channel.

This comprises of 3064Kb/s B channels andone 64Kb/s D channel


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DASS II

BTs own standard.

Equivalent of up to 30 exchange lines.Available from 8 channels upward.

Normally provided over fibre cable.

Can be provided over Microwave or Copper.

Each system is 2Mbit/s

Presented as a G703 BNC Connector


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DASS II Is presented Like This


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ISDN 30


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E1 Is Presented Like this


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The Connection

Pin Signal Polarity

1 Power Source/ Sink 3 +

2 Power Source/ Sink 3 -

3 Transmit/ Receive +

4 Receive/ Transmit +

5 Receive Transmit -

6 Transmit/ Receive -

7 Power Source/ Sink 2 -

8 Power Source/ Sink 2 +


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ISDN 30 I.421

European standard for ISDN.

Uses Q.931 signalling protocol.Available from 8 channels upward.

Normally provided over fibre cable.

Can be provided over Microwave or Copper.

Each system is 2Mbit/s.

Presented as an RJ45 connector


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Signalling

For the I.421 service, DASS 2 signalling is

replaced with Q.931 signalling to theETSI standard


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Numbering

The numbering options for the I.421 service are different thanfor DASS;

Numbers can not be allocated to dedicated channels.

The options per 2 Meg Bearer are;

Single Directory Number (Hunt Group)

DDI Range (up to 5)


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Numbering

In most cases it will be possible for a user tokeep their existing analogue directory number

when they migrate to ISDN - but this can notbe guaranteed.

It is dependant on whether or not the number

can be transferred to a local digital exchange- which in a small number of cases is notpossible.


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A New NTE

For the I.421 service theinterface connector for the

ISDN 30 (DASS 2) BNC 75Ohm Unbalanced (G703) isreplaced with an I.421socket, EN28877 (RJ45)120 Ohm Balancedconnector to the CCITTI.421 standard.


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ISDN 30 Resilience Options

Alternate Routing

This option delivers ISDN 30 over 2 separate cables to guardagainst cable failure.

Diverse Routing

This option delivers ISDN 30 from 2 separate exchanges toguard against exchange failure.

DDI Dual ParentingThis option delivers ISDN 30 from 2 separate local exchangeprocessors to guard against processor failure.


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Supplementary Services


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Calling Line Identity Presentation

This is a service which must be subscribed to

Allows the reception and display of theincoming callers telephone number.

Can be restricted by the incoming caller

Not provided for International speech calls.


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Call Forwarding

Only speech or 3.1Khz calls can be forwarded to theanalogue network (PSTN).

Call Forwarding UnconditionalAll incoming calls are immediately forwarded to a prearranged,nominated directory number.

Call Forwarding on No ReplyAutomatically forwards all calls to a prearranged, nominatednumber if the call is unanswered for 20 seconds.

Call Forwarding on BusyAutomatically forwards all calls to a prearranged, nominatednumber if the line is engaged.


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Call Barring Options

Incoming Calls BarredAll incoming calls are permanently barred

Outgoing calls only allowed

Outgoing Calls BarredNo outgoing calls can be made

Incoming calls are not effected

Selective Outgoing Calls BarredVarious options are available including; InternationalBarred, National and international Barred, All calls except

999, 112, 150, 151, 152, 154, 0800 and 0500


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Digital CircuitsCircuit Description Bandwidth

K2 Kilostream (2.4Kbps)




K48 Kilostream (48Kbps)K64 Kilostream (64Kbps)

K Kilostream N (64Kbps)





M2 Megastream (2Mbps)



M45 Megastream (45Mbps)M140 Megastream (140Mbps)


B Basic Rate ISDN (2 X 64Kbps)

E1 Primary Rate ISDN (2.048Mbps)

E2 Carries four multiplexed E1's (8.448Mbps)

E3 Carries sixteen E1's (34.368Mbps)

E4 Carries four E3's (139.246Mbps)

E5 Carries four E4's (565.148Mbps)


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Useful Numbers


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Engineers Co-Op

0800 282 212


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ISDN PRI Desk

0800 679 079


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ISDN Helpdesk

0800 181 514


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THE END


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PDH


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Plesiochronous Digital Hierarchy


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The basic 2.048 Mbit/s frame

The set of 30 time slots fortelephone channels, one slot forsynchronization/transmission of

alarms and another forsignalling make up what isknown as the basic 2.048 Mbit/sframe or primary digital group.For the sake of simplicity it ismore usual to talk of the 2

Mbit/s frame and from here onwe will refer to this digital group.The main characteristics of the2 Mbit/s frame are:

Nominal bit rate 2048 kbit/s

Tolerance 50 ppm

Line code HDB3

Frame length 256 bits

Frame rate 8000 frames/s

Bits per time interval 8 bits

Multiplexing method Byte-by-byte
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In an E1 channel communication consists of sendingsuccessive frames from the transmitter to the receiver.The receiver must receive an indication showing whenthe first interval of each frame begins so that, since itknows which channel the information corresponds to in

each time slot, it can demultiplex correctly. This way,the bytes received in each slot are assigned to thecorrect channel. A synchronization process is thenestablished that is known as frame alignment.


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FAS

In order to implement the frame alignment system, thatis, so that the receiver of the frame can tell where itbegins, there is a Frame Alignment Signal (FAS). In the

2 Mbit/s frames, the FAS is a combination of sevenfixed bits ("0011011") transmitted in the first time slot inthe frame (slot zero or TS0). For the alignmentmechanism to be maintained, the FAS does not need tobe transmitted in every frame. Instead, this signal can

be sent in alternate frames (in the first, in the third, inthe fifth and so on). In this case, TS0 is used as thesynchronization slot. The TS0 of the rest of the framesis therefore available for other functions, such astransmitting alarms.


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FAS


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CRC-4 multiframe

In the TS0 of frames with FAS, this word only takes up bits 2 to8 of the interval. The first bit is dedicated to carrying the bits ofcertain code words. This code, known as the Cyclic

Redundancy Checksum, tells us whether there are one or morebit errors in a specific group of bits received (called a block).


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CRC-4 procedure

The aim of this system is to avoid a loss ofsynchronization due to the coincidental appearance

of the sequence "0011011" in a time slot other thanthe TS0 of a frame with FAS. To implement the CRCcode in the transmission of 2 Mbit/s frames a CRC-4multiframe is built, made up of 16 frames. These arethen grouped in two blocks of eight frames called

submultiframes, over which a CRC checksum orword of 4 bits (CRC-4) is put in the positions Ci (bitsn1, frames with FAS) of the next submultiframe.


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CRC-4 procedure

At the receiving end the CRC of each submultiframe is calculated locally andcompared to the CRC value received in the next submultiframe. If these do not

coincide, one or most bit errors are determined to have been found in in the

block, and an alarm is sent back to the transmitter, indicating that the block

received at the far end contains errors.


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CRC-4 multiframe alignmentThe receiving end has to know which is the first bit of the CRC-4 word (C1). For

this reason, a CRC-4 multiframe alignment word is needed, that is, the receiver

has to be told where the multiframe begins (synchronization).

The CRC-4 multiframe alignment word is the set combination "001011", which is

introduced in the first bits of the frames that do not contain the FAS signal.


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Advantages of the CRC-4 method

A CRC-n method is mainly used to protect the communication against awrong frame alignment word and also to provide a certain degree ofmonitoring of the bit error rate when this has low values (around 10-6).

This method is not suitable for cases in which the bit error rate is around10-3 (each block contains at least one errored bit).

Another advantage in using the CRC is that all the bits transmitted arechecked, unlike those systems that only check 7 bits (those of the FAS,which are the only ones known in advance) out of every 512 bits (those

between one FAS and the next). However, the CRC-4 code is notcompletely infallible, since there exists a probability of around 1/16 that anerror may occur and not be detected, that is, 6.25% the blocks may containerrors that are not detected by the code.


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Monitoring errorsThe aim of monitoring errors is to continously check transmission quality withoutdisturbing the traffic of information and, when this quality is not of the requiredstandard, taking the necessary steps to improve it. Telephone traffic is two-way,that is, information is transmitted in both directions between the ends of the

communication. This means that two 2 Mbit/s channels and two directions fortransmission must be considered.

The CRC-4 multiframe alignment word only takes up six of the first eight bits ofthe TS0 without FAS. There are two bits in every second block or submultiframewhose task is to indicate block errors in the far end of the communication. Themechanism is as follows: Both bits (called E bits) have "1" as their default value.When the far end of the communication receives a 2 Mbit/s frame and detects

an errored block, it puts a "0" in the E bit that corresponds to that block in theframe being sent along the return path to the transmitter. This way, the near endof the communication is informed that an errored block has been detected andboth ends have the same information: one from the CRC-4 procedure and theother from the E bits. If we number the frames in the multiframe from 0 to 15,the E bit of frame 13 refers to the submultiframe I (block I) received at the farend and the E bit of frame 15 refers to the submultiframe II (block II).


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Monitoring errors


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Supervision bits

The bits that are in position two of the TS0 in the frame that do notcontain the FAS are called supervision bits and are set to "1" inorder to avoid simulations of the FAS signal.


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NFAS - Spare bits

The bits of the TS0 that does not contain the FAS in positions 3 to 8 make up what is knownas the Non-Frame Alignment Signal or NFAS. This signal is sent in alternate frames (Frame1, Frame 3, Frame 5, etc.). The first bit of the NFAS (bit n 3 of the TS0) is used to indicatethat an alarm has occured at the far end of the communication. When operating normally itis set to "0", while a value of "1" indicates an alarm.


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NFAS - Spare bits

The bits in positions four to eight are spare bits, that is,they do not have one single application, but can be

used in a number of different ways as decided by theTelecommunications Carrier. In accordance with theITU-T recommendation G.704, these bits can be usedin specific point-to-point applications, or to establish adata link based on messages for operationsmanagement, maintenance or monitoring of the

transmission quality, etc. If these spare bits in the NFASare not used, they must be set to "1" in internationallinks.


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NFAS - Alarm bit

The method used to transmit the alarm makes use of the fact that in telephone systems transmission is alwaystwo-way. Multiplexing/demultiplexing devices (known generically as multiplex devices) are installed at both endsof the communication for the transmission and reception of frames. When a device detects either of the followingin its multiplexer or demultiplexer

a power failurea failure of the coder/decoder

or any of the following in its demultiplexer:

loss of the 2 Mbit/s signal received

loss of frame alignment (synchronization)

bit error rate (BER) greater than or equal to 10-3

an alarm must be sent to the transmitter.

This Remote Alarm Indication (RAI) is sent in the NFAS of the return frames, with bit 3 being set to "1". Thetransmitter then considers how serious the alarm is and goes on to generate a series of operations depending onthe type of alarm condition detected.


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NFAS - Alarm bit


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Signalling channel

As well as transmitting information generated by the users of the telephone network, it is alsonecessary to transmit signalling information. Signalling refers to the protocols that must beestablished between exchanges so that the users who are communicating with each other canexchange information. There are signals that indicate when a subscriber has picked up thetelephone, when they can start to dial a number, when another subscriber calls, signals that letthe communication link be maintained, etc.

In the E1 PCM system signalling information can be transmitted by two different methods: theCommon Channel Signalling (CCS) method and the Channel Associated Signalling (CAS)method. In both cases the time slot TS16 of the basic 2 Mbit/s frame is used to transmit thesignalling information.

For CCS signalling, messages of several bytes are transmitted through the 64 kbit/s channel

provided by the TS16 of the frame, with these messages providing the signalling for all thechannels in the frame. Each message contains the information that determines the channel thatis signalling. The signalling circuits access the 64 kbit/s channel of the TS16, and are alsocommon to all the channels signalled. There are different CCS systems that constitute complexprotocols. In the following section and by way of example, Channel Associated Signalling will belooked at in detail. Channel Associated Signalling is defined in the ITU-T recommendationG.704, which defines the structure of the E1 frame


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Signalling channel

In CAS signalling, a signalling channel is associated with each information channel (there is nocommon signalling channel), meaning that the signalling circuits are personalized for eachchannel


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CAS signalling multiframe

In the case of channel associated signalling (CAS), each 64 kbit/stelephone channel is assigned 2 kbit/s for signalling. This signallingis formed by a word of 4 bits (generically known as a, b, c and d)

that is situated in the TS16 of all the frames sent. Each TS16therefore carries the signalling for two telephone channels.

Given that there are only 4 signalling bits available for eachchannel, to transmit all the signalling words from the 30 PCMchannels that make up a 2 Mbit/s frame (120 bits) it is necessary to

wait until the TS16 of 15 consecutive frames have been received.The grouping of frames defines a CAS signalling multiframe, whichconsists of a set of the TS16 of 16 consecutive E1 frames


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CAS signalling multiframe


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CAS multiframe alignment signal

In order to synchronize the CAS multiframe, that is, to identify where it begins, amultiframe alignment signal (MFAS) is defined, made up of the sequence of bits"0000" located in the first four bits of the TI16 of the first frame of the CASmultiframe, called frame 0.


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CAS non-multiframe alignment signalThe remaining four bits of the interval are divided between one alarm bit and threespare bits, making up the non-multiframe alignment signal (NMFAS). In short, thesignalling information for the 30 channels is transmitted in 2 ms, which is fastenough if we consider that the shortest signalling pulse (the one which correspondsto dialling the number) lasts 100 ms.


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CAS non-multiframe alignment signal

The alarm bit in the NMFAS is dealt with in a similar way to the non-frame alignment signal (NFAS). In this case,the alarms are transmitted between multiplex signalling devices connected to the 64 kbit/s circuits thatcorrespond to signalling (TS16). When in its multiplexer or demultiplexer a CAS multiplex signalling devicedetects:

a power failure

or detects the following in its demultiplexer:

loss of incoming signalling

loss of CAS multiframe alignment

an indication must be sent to the multiplex signalling device at the far end, setting bit 6 of the TS16 in the returnframe 0 to "1". Additionally, the value "1" is applied to all the signalling channels.

Example: a remote multiplexer is considered to have lost multiframe alignment when it receives two consecutiveMFAS words with error, that is, with a value other than "0000". In this case bit 6 of the TI16 of the frame 0 thatthis multiplexer transmits to the near end multiplexer is set to "1". When it receives this indication of loss ofmultiframe alignment at the far end, the near end multiplexer sends a signal made up entirely of bits at "1",known as AIS64 (Alarm Indication Signal-64 kbit/s) in the TS16 (64 kbit/s channel).


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THE END

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