basic isdn
DESCRIPTION
An introduction to ISDNTRANSCRIPT
BASIC ISDN
Bean Cans !
Analogue Speech - SILENCE
N S
Analogue Speech - Positive Pressure
Analogue Speech - Negative Pressure
The Traditional ‘Oscilloscope View’
MAXMAX+VE Volts+VE Volts
MAXMAX-VE Volts-VE Volts
0 Volts0 Volts
The Traditional ‘Oscilloscope View’
MAXMAX+VE Volts+VE Volts
MAXMAX-VE Volts-VE Volts
0 Volts0 Volts
The Traditional ‘Oscilloscope View’
MAXMAX+VE Volts+VE Volts
MAXMAX-VE Volts-VE Volts
0 Volts0 Volts
The Traditional ‘Oscilloscope View’
MAXMAX+VE Volts+VE Volts
MAXMAX-VE Volts-VE Volts
0 Volts0 Volts
The Analogue Telephone
The Analogue Telephone
This signal is analogue ‘cos it is analogous to my vocal
chords vibrating
This signal is analogue ‘cos it is analogous to my vocal
chords vibrating
Digital Telephony
What is Digital?
1’s & 0’s
on/off
Analogue Waveform
Volt
s
Time
Take Samples
Volt
s
Time
How many samples ?
The active band of frequency on the telephone network is from 300 to 3400 cycles per second
Nyquist Theorem...........You must sample at, at least twice the highest
frequency of the analogue signal
3400 x 2 = 8000 Samples Per Second = 1 sample every 0.000125 seconds
Pulse Amplitude Modulation
t1
t2
t3
t4
t5
t6
TimeVolt
s
125s
Pulse Amplitude ModulationTimed Samples (every 125s)
Pulse Amplitude Modulation
01111111
01111100
01101011
01010101
01011010
01001001
11011010
11010101
11101010
11101101
11110011
11111111
00000000
11001001
Am
plit
ud
e S
am
ple
s
Timed Samples (every 125s)
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
XAm
plit
ud
e S
am
ple
s
Timed Samples (every 125s)
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
XAm
plit
ud
e S
am
ple
s
Timed Samples (every 125s)
01111111
01111100
01101011
01010101
01011010
01001001
X
X
X X
X
X
01111111
01111100
01101011
01010101
01011010
01001001
X
X
X X
X
X
01011010 01010101
0111110001111100
s1 s2 s3 s4
8000 samples of 8 bits = 64,000 bits per second
01111111
01111100
01101011
01010101
01011010
01001001
X
X
X X
X
X
01011010
01010101
01111100
01111100
s1 s2 s3 s4
? ? ?
125s
01111111
01111100
01101011
01010101
01011010
01001001
X
X
X X
X
X
01011010
01010101
01111100
01111100
s1 s2 s3 s4011111110111110001101011010101010101101001001001
X
X
X
X XX
01011010
01010101
01111100
01111100
s1
s2
s3
s4
125s
s2
01011010s1
01101011
s1
125s
125s
0100011010101100
s2s1
125s
0100011010101100
32 samples
s2s1
125s32 samples
s1 s2
8000 times per second
8bits x 32 x 8000 = 2,048,000 bits per second
1 frame
125s32 samples
s1 s2
8000 times per second
0 16
125s32 samples
s2
0 16
Signalling
DASSDPNSSQ931
Synchronisation
30 samples
30 samples
OF WHAT ?
Audio
SpeechSpeech
TonesTones
MusicMusic
Video
SecuritySecurity
NewsNews
Video-conferenceVideo-conference
Fax
Group 2/3 sent as audioGroup 2/3 sent as audio
Group 4 after handshake 1.5 seconds/pageGroup 4 after handshake 1.5 seconds/page
Data
FilesFiles
ProgrammesProgrammes
ControlControl
Two Types Of ISDN Channel
Function Groups & Reference PointsLT
NT 1
NT 2
TE 1
TE 2
TA
Function Groups & Reference PointsLT
NT 1
NT 2
TE 1
TE 2
TA
U
T
S
R
ITU Reference Model for ISDN
PTT
Equipment at
Phone Company
Switch
U InterfaceTermination Point
T InterfaceTermination Point
S InterfaceTermination Point
Standard PSTN Equipmenthas an R InterfaceTermination Point
U T S
R
TE2TA
TE1 TE1
NT1 NT2
ISDN Equipment thatcan connect directlyto an ISDN Line
Terminal Adapter usedto connect TE2 devices
to an ISDN line
Equipment thatcannot connect to an ISDN line
Network Termination
used to convert U
into T interface
Network Termination
used to convert T
into S interface
Function Groups & Reference Points
NT1 Network Termination 1, Handles physical layer interface functions such as line termination (eg NTE 8).
NT2 Network Termination 2, Handles physical layer plus layer 2 and 3 functions such as multiplexing, switching and concentration (e.g. an ISPBX).
LT Line Termination, Handles termination of 2 wire pair at the exchange, operating 2B1Q or 4B3T line coding.
TA Terminal Adaptor, Equipment that supports ISDN call set up and provides an interface for connecting to non ISDN equipment.
Function Groups & Reference Points
TE1 Terminal Equipment 1, End user equipment such as ISDN telephones or data terminals compliant with ISDN call set up procedures and capable of interfacing directly to the S-bus.
TE2 Terminal Equipment 2, End user equipment for non ISDN environments (typically uses an RS232 interface)
Basic Rate Interface
The BRI is defined as two 64Kb/s Bearer (B) channels and one 16Kb/s Data (D) channel
Basic Rate ISDN
Two separate ‘B’ channels over a single line. Combined voice, data, video. ‘T’ interface allows both channels to be used
independently. ‘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, fast call set-up.
What IS ISDN 2e?
ISDN2e is the standard basic 2 channel ISDN service
ISDN2e fully comply with European Telecom Standards
ISDN2e provides a network platform that is capable of supporting supplementary services
NTE 8
The NTE8 (opposite) is the The NTE8 (opposite) is the normal NTE within the normal NTE within the customers premises for an customers premises for an ISDN2e connection.ISDN2e connection.The NTE8 has TWO RJ45 The NTE8 has TWO RJ45 sockets - ONLY ONE IS TO sockets - ONLY ONE IS TO BE USED - the other is for BE USED - the other is for testing purposestesting purposes
NTE 8
The NTE8 has a green LED which indicates the presence of the ISDN service.
NTE8 is only available on local exchanges which use ‘Line Cards’
I-MUX exchanges will use NTE6
ISDN RJ 45 Connection
EIA 568A Commercial Building
Cabling Specification Draft 9.0
Preferred termination of UTP data cabling
International ISDN standard
Point To MultipointReferred to by BT as;
Standard Access
Or
S/T Reference
Additional telephone numbers are normally provided by MSN
Basic Rate For The Home Or Office
Multiple Subscriber Numbering
ISDN
NTE
TA
0208 988 6643
0208 988 9102
0208 988 5106
• Allows the programming of separate telephone numbers into each device connected to an ISDN2e line.
• Currently 4 Options• - 2 Numbers• - 3 Numbers• - 8 Numbers• - 10 Numbers
Sub Addressing
For ISDN2e to ISDN2e calls.
Allows up to 20 Alphanumeric characters to be sent (not #).
For ISDN2e to ISDN2 or ISDN 30
Allows up to 6 Alphanumeric characters to be sent (not #).
Multiple Devices And Multiple Numbers
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
Point To PointReferred to by BT as;
System Access
Or
T Reference
Additional telephone numbers are normally provided by DDI
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
ISDN2e Supplementary Services
Calling Line Identity Presentation
Multiple Subscriber Numbering Direct Dialling InCall Forwarding Sub Addressing
Terminal Portability Call Barring Options Maintenance Options
Terminal Portability
Allows a terminal to be disconnected from an ISDN2e socket and to be reconnected to another socket (on the same line) during a call without losing the call.
The terminal equipment must be capable of supporting this facility.
This facility is not available with the DDI service.
AODI
BT’s ISDN Connect
The data connection is initiated using X.25 on the D channel where it maintains an open link.
When extra bandwidth is required the bandwidth Allocation Control Protocol automatically switches in the B Channels.
When the additional channels are no longer require they will be automatically ‘un-nailed.
PRIMARY RATEPRIMARY RATE
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
Primary Rate Interface
The PRI is supplied through a standard 2.048Mb/s E1 channel.
This comprises of 30 64Kb/s B channels and one 64Kb/s D channel
DASS IIDASS II
BT’s 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
DASS II Is presented Like This
ISDN 30ISDN 30
E1 Is Presented Like this
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 +
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
Signalling
For the I.421 service, DASS 2 signalling is replaced with Q.931 signalling to the ETSI standard
Numbering
The numbering options for the I.421 service are different than for 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)
Numbering
In most cases it will be possible for a user to keep their existing analogue directory number when they migrate to ISDN - but this can not be 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 not possible.
A New NTE
For the I.421 service the interface connector for the ISDN 30 (DASS 2) BNC 75 Ohm Unbalanced (G703) is replaced with an I.421 socket, EN28877 (RJ45) 120 Ohm Balanced connector to the CCITT I.421 standard.
ISDN 30 Resilience Options
Alternate Routing This option delivers ISDN 30 over 2 separate cables to
guard against cable failure. Diverse Routing This option delivers ISDN 30 from 2 separate exchanges
to guard against exchange failure. DDI Dual Parenting This option delivers ISDN 30 from 2 separate local
exchange processors to guard against processor failure.
Supplementary ServicesSupplementary Services
Calling Line Identity Presentation
This is a service which must be subscribed to
Allows the reception and display of the incoming callers telephone number.
Can be restricted by the incoming caller
Not provided for International speech calls.
Call Forwarding
Only speech or 3.1Khz calls can be forwarded to the analogue network (PSTN).
Call Forwarding UnconditionalAll incoming calls are immediately forwarded to a
prearranged, nominated directory number. Call Forwarding on No Reply
Automatically forwards all calls to a prearranged, nominated number if the call is unanswered for 20 seconds.
Call Forwarding on BusyAutomatically forwards all calls to a prearranged,
nominated number if the line is engaged.
Call Barring Options
Incoming Calls BarredAll incoming calls are permanently barredOutgoing calls only allowed
Outgoing Calls BarredNo outgoing calls can be madeIncoming calls are not effected
Selective Outgoing Calls BarredVarious options are available including;
International Barred, National and international Barred, All calls except 999, 112, 150, 151, 152, 154, 0800 and 0500
Digital Circuits Circuit Description Bandwidth
K2 Kilostream (2.4Kbps)
K4 Kilostream (4.8Kbps)K9 Kilostream (9.6Kbps)K19 Kilostream (19.2Kbps)K48 Kilostream (48Kbps)K64 Kilostream (64Kbps)K Kilostream N (64Kbps)K Kilostream N (128Kbps)K Kilostream N (256Kbps)K Kilostream N (512Kbps)K Kilostream N (1024Kbps)M2 Megastream (2Mbps)M8 Megastream (8Mbps)M34 Megastream (34Mbps)M45 Megastream (45Mbps)M140 Megastream (140Mbps)M155 Megastream (155Mbps)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)
Useful Numbers
Engineers Co-Op
0800 282 212
ISDN PRI Desk
0800 679 079
ISDN Helpdesk
0800 181 514
THE END
PDH
Plesiochronous Digital Hierarchy
The basic 2.048 Mbit/s frame
The set of 30 time slots for telephone channels, one slot for synchronization/transmission of alarms and another for signalling make up what is known as the basic 2.048 Mbit/s frame or primary digital group. For the sake of simplicity it is more usual to talk of the 2 Mbit/s frame and from here on we will refer to this digital group. The main characteristics of the 2 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
In an E1 channel communication consists of sending successive frames from the transmitter to the receiver. The receiver must receive an indication showing when the first interval of each frame begins so that, since it knows 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 the correct channel. A synchronization process is then established that is known as frame alignment.
FAS
In order to implement the frame alignment system, that is, so that the receiver of the frame can tell where it begins, there is a Frame Alignment Signal (FAS). In the 2 Mbit/s frames, the FAS is a combination of seven fixed bits ("0011011") transmitted in the first time slot in the frame (slot zero or TS0). For the alignment mechanism to be maintained, the FAS does not need to be transmitted in every frame. Instead, this signal can be sent in alternate frames (in the first, in the third, in the fifth and so on). In this case, TS0 is used as the synchronization slot. The TS0 of the rest of the frames is therefore available for other functions, such as transmitting alarms.
FAS
CRC-4 multiframe
• In the TS0 of frames with FAS, this word only takes up bits 2 to 8 of the interval. The first bit is dedicated to carrying the bits of certain code words. This code, known as the Cyclic Redundancy Checksum, tells us whether there are one or more bit errors in a specific group of bits received (called a block).
CRC-4 procedure
• The aim of this system is to avoid a loss of synchronization due to the coincidental appearance of the sequence "0011011" in a time slot other than the TS0 of a frame with FAS. To implement the CRC code in the transmission of 2 Mbit/s frames a CRC-4 multiframe is built, made up of 16 frames. These are then grouped in two blocks of eight frames called submultiframes, over which a CRC checksum or word of 4 bits (CRC-4) is put in the positions Ci (bits nº1, frames with FAS) of the next submultiframe.
CRC-4 procedure
At the receiving end the CRC of each submultiframe is calculated locally and compared 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.
CRC-4 multiframe alignment The 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.
Advantages of the CRC-4 method
A CRC-n method is mainly used to protect the communication against a wrong frame alignment word and also to provide a certain degree of monitoring 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 around 10-3 (each block contains at least one errored bit).
Another advantage in using the CRC is that all the bits transmitted are checked, 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 not completely infallible, since there exists a probability of around 1/16 that an error may occur and not be detected, that is, 6.25% the blocks may contain errors that are not detected by the code.
Monitoring errors The aim of monitoring errors is to continously check transmission quality without disturbing the traffic of information and, when this quality is not of the required standard, 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 for transmission must be considered.
The CRC-4 multiframe alignment word only takes up six of the first eight bits of the TS0 without FAS. There are two bits in every second block or submultiframe whose task is to indicate block errors in the far end of the communication. The mechanism 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 the frame being sent along the return path to the transmitter. This way, the near end of the communication is informed that an errored block has been detected and both ends have the same information: one from the CRC-4 procedure and the other 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 far end and the E bit of frame 15 refers to the submultiframe II (block II).
Monitoring errors
Supervision bits
The bits that are in position two of the TS0 in the frame that do not contain the FAS are called supervision bits and are set to "1" in order to avoid simulations of the FAS signal.
NFAS - Spare bits
• The bits of the TS0 that does not contain the FAS in positions 3 to 8 make up what is known as the Non-Frame Alignment Signal or NFAS. This signal is sent in alternate frames (Frame 1, Frame 3, Frame 5, etc.). The first bit of the NFAS (bit nº 3 of the TS0) is used to indicate that an alarm has occured at the far end of the communication. When operating normally it is set to "0", while a value of "1" indicates an alarm.
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 the Telecommunications Carrier. In accordance with the ITU-T recommendation G.704, these bits can be used in specific point-to-point applications, or to establish a data link based on messages for operations management, maintenance or monitoring of the transmission quality, etc. If these spare bits in the NFAS are not used, they must be set to "1" in international links.
NFAS - Alarm bit
The method used to transmit the alarm makes use of the fact that in telephone systems transmission is always two-way. Multiplexing/demultiplexing devices (known generically as multiplex devices) are installed at both ends of the communication for the transmission and reception of frames. When a device detects either of the following in its multiplexer or demultiplexer
•a power failure •a 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". The transmitter then considers how serious the alarm is and goes on to generate a series of operations depending on the type of alarm condition detected.
NFAS - Alarm bit
Signalling channel
As well as transmitting information generated by the users of the telephone network, it is also necessary to transmit signalling information. Signalling refers to the protocols that must be established between exchanges so that the users who are communicating with each other can exchange information. There are signals that indicate when a subscriber has picked up the telephone, when they can start to dial a number, when another subscriber calls, signals that let the communication link be maintained, etc.
In the E1 PCM system signalling information can be transmitted by two different methods: the Common 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 the signalling 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 the channels in the frame. Each message contains the information that determines the channel that is signalling. The signalling circuits access the 64 kbit/s channel of the TS16, and are also common to all the channels signalled. There are different CCS systems that constitute complex protocols. In the following section and by way of example, Channel Associated Signalling will be looked at in detail. Channel Associated Signalling is defined in the ITU-T recommendation G.704, which defines the structure of the E1 frame
Signalling channel
In CAS signalling, a signalling channel is associated with each information channel (there is no common signalling channel), meaning that the signalling circuits are personalized for each channel
CAS signalling multiframe
In the case of channel associated signalling (CAS), each 64 kbit/s telephone channel is assigned 2 kbit/s for signalling. This signalling is 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 TS16 therefore carries the signalling for two telephone channels.
Given that there are only 4 signalling bits available for each channel, to transmit all the signalling words from the 30 PCM channels 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, which consists of a set of the TS16 of 16 consecutive E1 frames
CAS signalling multiframe
CAS multiframe alignment signal
In order to synchronize the CAS multiframe, that is, to identify where it begins, a multiframe 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 CAS multiframe, called frame 0.
CAS non-multiframe alignment signal
The remaining four bits of the interval are divided between one alarm bit and three spare bits, making up the non-multiframe alignment signal (NMFAS). In short, the signalling information for the 30 channels is transmitted in 2 ms, which is fast enough if we consider that the shortest signalling pulse (the one which corresponds to dialling the number) lasts 100 ms.
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 that correspond to signalling (TS16). When in its multiplexer or demultiplexer a CAS multiplex signalling device detects:
•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 return frame 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 consecutive MFAS words with error, that is, with a value other than "0000". In this case bit 6 of the TI16 of the frame 0 that this multiplexer transmits to the near end multiplexer is set to "1". When it receives this indication of loss of multiframe 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).
THE END