Basic ISDN

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An introduction to ISDN

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  • T1 = Time Slot 1 T2 = Time Slot 2 etc…. Every time slot is 0.000125 seconds apart (which is the sampling rate of 8KHz)
  • We now have our samples of an entire conversation taken every 0.000125 seconds
  • We now have a number of amplitude measurements taken at 0.000125 second intervals Each one of these samples is represented by an 8 bit binary number 8000 samples are taken every second Each sample is 8 bits long Therefore every second we need to transmit 8000 samples of 8 bits or 64,000 bits of information every second At this point the light should be starting to glow regarding the 64K channels available on Primary Rate ISDN
  • However – now that we have our samples of our conversation we now also have a big empty hole between each sample!
  • As nature abhors a vacuum – we need to fill this hole with something (MORE SAMPLES!!!)
  • So, in our 0.000125 second space we are now starting to build up a nice collection of data samples
  • BUT – we still have a hole I wonder how many samples we can cram into our 0.000125 second hole?
  • WRONG!!! According to Nyquist there are; Lets look at a DS-1 signal which passes over a T-1 circuit. For DS-1 transmissions, each frame contains 8 bits per channel and there are 24 channels. Also, 1 "framing bit" is required for each of the 24 channel frames. 24 channels * 8 bits per channel + 1 framing bit = 193 bits per frame. 193 bits per frame + 8,000 "Nyquist" samples = 1,544,000 bits per second. And it just so happens that the T-1 circuit is 1.544 Mbps.--not a coincidence. Each of the 24 channels in a T-1 circuit carries 64Kbps. HOWEVER – as an American Nyquist was obviously WRONG! In the UK we have BT and a thing called an E1 This gives you a total available bandwidth of 2.048 Mb 2.048 Mb / 64K = 32 Available Channels SORRY!!!! In 1965 the standard appeared that permitted the TDM multiplexing of 24 digital telephone channels of 64 kbit/s into a 1.544 Mbit/s signal whose format was called T1 and which was also adopted by Japan. For the T1 signal, a synchronization bit is added to the 24 TDM time slots that correspond to the telephone channels, in such a way that the aggregate transmission rate is the result of the following calculation: (24channels x 8bit/channel + 1bit) / 125 ms = 1.544 Mbit/s where 125 ms is the PCM sampling period or interval between one sample and the next in the same channel . Europe developed its own TDM multiplexing standard a little later (1968), although its capacity was greater: 30 digital telephone channels of 64 kbit/s plus one for synchronization/transmission of alarms and another for signalling. The resulting signal was transmitted at 2.048 Mbit/s and its format was called E1. This European standard was adopted by the rest of the world except for the USA and Japan. For an E1 signal, the aggregate transmission rate can be obtained from the following calculation: (32channels x 8bit/channel) / 125 ms = 2.048 Mbit/s
  • HOWEVER – now that we have our 32 samples we had probably better send some other information as well (synchronisation, maybe the callers telephone number etc). So, where’s a good place for this – at the beginning and in the middle! So, we will use channels 0 and 16 for control information and channels 1 to 15 and 17 to 31 for call information A BIT MESSY – Lets use channel 0 for synchronisation and channel 16 for everything else MUCH NEATER!
  • Primary Rate NTE 4
  • YES I KNOW we said 32 channels earlier BUT if you remember we gave channel 16 a specific purpose (DPNSS, QSIG etc) SO for many customers channel 16 does not exist (YEAH I KNOW channel 0 does not add up to much for them but for the sake of customer relations we will admit to one and keep the other up our sleeves for later!!
  • Basic ISDN

    1. 1. BASIC ISDN
    2. 2. Bean Cans !
    3. 3. Analogue Speech - SILENCE N S
    4. 4. Analogue Speech - Positive Pressure
    5. 5. Analogue Speech - Negative Pressure
    6. 6. The Traditional ‘Oscilloscope View’ MAX +VE Volts MAX -VE Volts 0 Volts
    7. 7. The Traditional ‘Oscilloscope View’ MAX +VE Volts MAX -VE Volts 0 Volts
    8. 8. The Traditional ‘Oscilloscope View’ MAX +VE Volts MAX -VE Volts 0 Volts
    9. 9. The Traditional ‘Oscilloscope View’ MAX +VE Volts MAX -VE Volts 0 Volts
    10. 10. The Analogue Telephone
    11. 11. The Analogue Telephone This signal is analogue ‘cos it is analogous to my vocal chords vibrating
    12. 12. Digital Telephony
    13. 13. What is Digital? <ul><li>1’s & 0’s </li></ul><ul><li>on/off </li></ul>
    14. 14. Analogue Waveform Volts Time
    15. 15. Take Samples Volts Time
    16. 16. How many samples ? <ul><li>The active band of frequency on the telephone network is from 300 to 3400 cycles per second </li></ul><ul><li>Nyquist Theorem........... </li></ul><ul><ul><li>You must sample at, at least twice the highest frequency of the analogue signal </li></ul></ul><ul><li>3400 x 2 = 8000 Samples Per Second </li></ul><ul><li>= 1 sample every 0.000125 seconds </li></ul>
    17. 17. Pulse Amplitude Modulation t1 t2 t3 t4 t5 t6 Time Volts 125  s
    18. 18. Pulse Amplitude Modulation Timed Samples (every 125  s)
    19. 19. Pulse Amplitude Modulation 01111111 01111100 01101011 01010101 01011010 01001001 11011010 11010101 11101010 11101101 11110011 11111111 00000000 11001001 Amplitude Samples Timed Samples (every 125  s)
    20. 20. 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 X Amplitude Samples Timed Samples (every 125  s)
    21. 21. 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 X Amplitude Samples Timed Samples (every 125  s)
    22. 22. 01111111 01111100 01101011 01010101 01011010 01001001 X X X X X X
    23. 23. <ul><li>8000 samples of 8 bits = 64,000 bits per second </li></ul>01111111 01111100 01101011 01010101 01011010 01001001 X X X X X X 01011010 01010101 01111100 01111100 s1 s2 s3 s4
    24. 24. 01111111 01111100 01101011 01010101 01011010 01001001 X X X X X X 01011010 01010101 01111100 01111100 s1 s2 s3 s4 ? ? ? 125  s
    25. 25. 01111111 01111100 01101011 01010101 01011010 01001001 X X X X X X 01011010 01010101 01111100 01111100 s1 s2 s3 s4 01111111 01111100 01101011 01010101 01011010 01001001 X X X X X X 01011010 01010101 01111100 01111100 s1 s2 s3 s4 125  s
    26. 26. s2 01011010 s1 01101011 s1 125  s
    27. 27. 125  s 01000110 10101100 s2 s1
    28. 28. 125  s 01000110 10101100 32 samples s2 s1
    29. 29. 125  s 32 samples 8000 times per second 8bits x 32 x 8000 = 2,048,000 bits per second 1 frame s1 s2
    30. 30. 125  s 32 samples s1 s2 8000 times per second 0 16
    31. 31. 125  s 32 samples s2 0 16 Signalling DASS DPNSS Q931 Synchronisation
    32. 32. 30 samples
    33. 33. 30 samples OF WHAT ?
    34. 34. Audio Speech Tones Music
    35. 35. Video Security News Video-conference
    36. 36. Fax Group 2/3 sent as audio Group 4 after handshake 1.5 seconds/page
    37. 37. Data Files Programmes Control
    38. 38. Two Types Of ISDN Channel
    39. 39. Function Groups & Reference Points LT NT 1 NT 2 TE 1 TE 2 TA
    40. 40. Function Groups & Reference Points LT NT 1 NT 2 TE 1 TE 2 TA U T S R
    41. 41. ITU Reference Model for ISDN PTT Equipment at Phone Company Switch U Interface Termination Point T Interface Termination Point S Interface Termination Point Standard PSTN Equipment has an R Interface Termination Point U T S R TE2 TA TE1 TE1 NT1 NT2 ISDN Equipment that can connect directly to an ISDN Line Terminal Adapter used to connect TE2 devices to an ISDN line Equipment that cannot connect to an ISDN line Network Termination used to convert U into T interface Network Termination used to convert T into S interface
    42. 42. Function Groups & Reference Points <ul><li>NT1 Network Termination 1, Handles physical layer interface functions such as line termination (eg NTE 8). </li></ul><ul><li>NT2 Network Termination 2, Handles physical layer plus layer 2 and 3 functions such as multiplexing, switching and concentration (e.g. an ISPBX). </li></ul><ul><li>LT Line Termination, Handles termination of 2 wire pair at the exchange, operating 2B1Q or 4B3T line coding. </li></ul><ul><li>TA Terminal Adaptor, Equipment that supports ISDN call set up and provides an interface for connecting to non ISDN equipment. </li></ul>
    43. 43. Function Groups & Reference Points <ul><li>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. </li></ul><ul><li>TE2 Terminal Equipment 2, End user equipment for non ISDN environments (typically uses an RS232 interface) </li></ul>
    44. 44. Basic Rate Interface The BRI is defined as two 64Kb/s Bearer (B) channels and one 16Kb/s Data (D) channel
    45. 45. Basic Rate ISDN <ul><li>Two separate ‘B’ channels over a single line. </li></ul><ul><li>Combined voice, data, video. </li></ul><ul><li>‘ T’ interface allows both channels to be used independently. </li></ul><ul><li>‘ S’ interface may use one or both ‘B’ channels. </li></ul><ul><li>Bandwidth 2 x 64k or 1 x 128k per ‘S’ port </li></ul><ul><li>Flexible, high speed, high quality, low error rate, fast call set-up. </li></ul>
    46. 46. What IS ISDN 2e? <ul><li>ISDN2e is the standard basic 2 channel ISDN service </li></ul><ul><li>ISDN2e fully comply with European Telecom Standards </li></ul><ul><li>ISDN2e provides a network platform that is capable of supporting supplementary services </li></ul>
    47. 47. NTE 8 The NTE8 (opposite) is the normal NTE within the customers premises for an ISDN2e connection. The NTE8 has TWO RJ45 sockets - ONLY ONE IS TO BE USED - the other is for testing purposes
    48. 48. NTE 8 <ul><li>The NTE8 has a green LED which indicates the presence of the ISDN service. </li></ul><ul><li>NTE8 is only available on local exchanges which use ‘Line Cards’ </li></ul><ul><li>I-MUX exchanges will use NTE6 </li></ul>
    49. 50. ISDN RJ 45 Connection <ul><li>EIA 568A </li></ul><ul><li>Commercial Building Cabling Specification Draft 9.0 </li></ul><ul><li>Preferred termination of UTP data cabling </li></ul><ul><li>International ISDN standard </li></ul>
    50. 51. Point To Multipoint <ul><li>Referred to by BT as; </li></ul><ul><li>Standard Access </li></ul><ul><li>Or </li></ul><ul><li>S/T Reference </li></ul>Additional telephone numbers are normally provided by MSN
    51. 52. Basic Rate For The Home Or Office
    52. 53. Multiple Subscriber Numbering <ul><li>Allows the programming of separate telephone numbers into each device connected to an ISDN2e line. </li></ul><ul><li>Currently 4 Options </li></ul><ul><li> - 2 Numbers </li></ul><ul><li> - 3 Numbers </li></ul><ul><li> - 8 Numbers </li></ul><ul><li> - 10 Numbers </li></ul>ISDN NTE TA 0208 988 6643 0208 988 9102 0208 988 5106
    53. 54. Sub Addressing <ul><li>For ISDN2e to ISDN2e calls. </li></ul><ul><li>Allows up to 20 Alphanumeric characters to be sent (not #). </li></ul><ul><li>For ISDN2e to ISDN2 or ISDN 30 </li></ul><ul><li>Allows up to 6 Alphanumeric characters to be sent (not #). </li></ul>
    54. 55. Multiple Devices And Multiple Numbers
    55. 56. 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
    56. 57. Point To Point <ul><li>Referred to by BT as; </li></ul><ul><li>System Access </li></ul><ul><li>Or </li></ul><ul><li>T Reference </li></ul>Additional telephone numbers are normally provided by DDI
    57. 58. Direct Dialling In <ul><li>DDI provides 10 or more directory numbers to an ISDN line or group of lines. </li></ul><ul><li>Requires ISDN2e to be configured for ‘System Access’ </li></ul><ul><li>Must be connected to a PBX </li></ul>ISDN ISPBX 7100 7101 7103 7104
    58. 59. ISDN2e Supplementary Services <ul><li>Calling Line Identity Presentation </li></ul><ul><li>Multiple Subscriber Numbering </li></ul><ul><li>Direct Dialling In </li></ul><ul><li>Call Forwarding </li></ul><ul><li>Sub Addressing </li></ul><ul><li>Terminal Portability </li></ul><ul><li>Call Barring Options </li></ul><ul><li>Maintenance Options </li></ul>
    59. 60. Terminal Portability <ul><li>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. </li></ul><ul><li>The terminal equipment must be capable of supporting this facility. </li></ul><ul><li>This facility is not available with the DDI service. </li></ul>
    60. 61. AODI <ul><li>BT’s ISDN Connect </li></ul><ul><li>The data connection is initiated using X.25 on the D channel where it maintains an open link. </li></ul><ul><li>When extra bandwidth is required the bandwidth Allocation Control Protocol automatically switches in the B Channels. </li></ul><ul><li>When the additional channels are no longer require they will be automatically ‘un-nailed. </li></ul>
    61. 62. PRIMARY RATE
    62. 64. NTE Status Lights BT Testing ON or Flashing Test Amber OK OFF Test Amber Network Faulty ON BT Red Network OK OFF BT Red Two or more inputs disconnected Flashing Customer Amber One input disconnected ON Customer Amber Inputs OK OFF Customer Amber NTE on Standby Battery Flashing Power Green No power to NTE OFF Power Green NTE on mains power ON Power Green Indication Status Light
    63. 65. 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
    64. 66. DASS II <ul><li>BT’s own standard. </li></ul><ul><li>Equivalent of up to 30 exchange lines. </li></ul><ul><li>Available from 8 channels upward. </li></ul><ul><li>Normally provided over fibre cable. </li></ul><ul><li>Can be provided over Microwave or Copper. </li></ul><ul><li>Each system is 2Mbit/s </li></ul><ul><li>Presented as a G703 BNC Connector </li></ul>
    65. 67. DASS II Is presented Like This
    66. 68. ISDN 30
    67. 69. E1 Is Presented Like this
    68. 70. The Connection + Power Source/ Sink 2 8 - Power Source/ Sink 2 7 - Transmit/ Receive 6 - Receive Transmit 5 + Receive/ Transmit 4 + Transmit/ Receive 3 - Power Source/ Sink 3 2 + Power Source/ Sink 3 1 Polarity Signal Pin
    69. 71. ISDN 30 I.421 <ul><li>European standard for ISDN. </li></ul><ul><li>Uses Q.931 signalling protocol. </li></ul><ul><li>Available from 8 channels upward. </li></ul><ul><li>Normally provided over fibre cable. </li></ul><ul><li>Can be provided over Microwave or Copper. </li></ul><ul><li>Each system is 2Mbit/s. </li></ul><ul><li>Presented as an RJ45 connector </li></ul>
    70. 72. Signalling <ul><li>For the I.421 service, DASS 2 signalling is replaced with Q.931 signalling to the ETSI standard </li></ul>
    71. 73. Numbering <ul><li>The numbering options for the I.421 service are different than for DASS; </li></ul><ul><li>Numbers can not be allocated to dedicated channels. </li></ul><ul><li>The options per 2 Meg Bearer are; </li></ul><ul><li>Single Directory Number (Hunt Group) </li></ul><ul><li>DDI Range (up to 5) </li></ul>
    72. 74. Numbering <ul><li>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. </li></ul><ul><li>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. </li></ul>
    73. 75. A New NTE <ul><li>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. </li></ul>
    74. 76. ISDN 30 Resilience Options <ul><li>Alternate Routing </li></ul><ul><li>This option delivers ISDN 30 over 2 separate cables to guard against cable failure. </li></ul><ul><li>Diverse Routing </li></ul><ul><li>This option delivers ISDN 30 from 2 separate exchanges to guard against exchange failure. </li></ul><ul><li>DDI Dual Parenting </li></ul><ul><li>This option delivers ISDN 30 from 2 separate local exchange processors to guard against processor failure. </li></ul>
    75. 77. Supplementary Services
    76. 78. Calling Line Identity Presentation <ul><li>This is a service which must be subscribed to </li></ul><ul><li>Allows the reception and display of the incoming callers telephone number. </li></ul><ul><li>Can be restricted by the incoming caller </li></ul><ul><li>Not provided for International speech calls. </li></ul>
    77. 79. Call Forwarding <ul><li>Only speech or 3.1Khz calls can be forwarded to the analogue network (PSTN). </li></ul><ul><li>Call Forwarding Unconditional </li></ul><ul><li>All incoming calls are immediately forwarded to a prearranged, nominated directory number. </li></ul><ul><li>Call Forwarding on No Reply </li></ul><ul><li>Automatically forwards all calls to a prearranged, nominated number if the call is unanswered for 20 seconds. </li></ul><ul><li>Call Forwarding on Busy </li></ul><ul><li>Automatically forwards all calls to a prearranged, nominated number if the line is engaged. </li></ul>
    78. 80. Call Barring Options <ul><li>Incoming Calls Barred </li></ul><ul><li>All incoming calls are permanently barred </li></ul><ul><li>Outgoing calls only allowed </li></ul><ul><li>Outgoing Calls Barred </li></ul><ul><li>No outgoing calls can be made </li></ul><ul><li>Incoming calls are not effected </li></ul><ul><li>Selective Outgoing Calls Barred </li></ul><ul><li>Various options are available including; International Barred, National and international Barred, All calls except 999, 112, 150, 151, 152, 154, 0800 and 0500 </li></ul>
    79. 81. Digital Circuits <ul><li>K2 Kilostream (2.4Kbps) </li></ul><ul><li>K4 Kilostream (4.8Kbps) </li></ul><ul><li>K9 Kilostream (9.6Kbps) </li></ul><ul><li>K19 Kilostream (19.2Kbps) </li></ul><ul><li>K48 Kilostream (48Kbps) </li></ul><ul><li>K64 Kilostream (64Kbps) </li></ul><ul><li>K Kilostream N (64Kbps) </li></ul><ul><li>K Kilostream N (128Kbps) </li></ul><ul><li>K Kilostream N (256Kbps) </li></ul><ul><li>K Kilostream N (512Kbps) </li></ul><ul><li>K Kilostream N (1024Kbps) </li></ul><ul><li>M2 Megastream (2Mbps) </li></ul><ul><li>M8 Megastream (8Mbps) </li></ul><ul><li>M34 Megastream (34Mbps) </li></ul><ul><li>M45 Megastream (45Mbps) </li></ul><ul><li>M140 Megastream (140Mbps) </li></ul><ul><li>M155 Megastream (155Mbps) </li></ul><ul><li>B Basic Rate ISDN (2 X 64Kbps) </li></ul><ul><li>E1 Primary Rate ISDN (2.048Mbps) </li></ul><ul><li>E2 Carries four multiplexed E1's (8.448Mbps) </li></ul><ul><li>E3 Carries sixteen E1's (34.368Mbps) </li></ul><ul><li>E4 Carries four E3's (139.246Mbps) </li></ul><ul><li>E5 Carries four E4's (565.148Mbps) </li></ul>Circuit Description Bandwidth
    80. 82. Useful Numbers
    81. 83. Engineers Co-Op 0800 282 212
    82. 84. ISDN PRI Desk 0800 679 079
    83. 85. ISDN Helpdesk 0800 181 514
    84. 86. THE END
    85. 87. PDH
    86. 88. Plesiochronous Digital Hierarchy
    87. 89. The basic 2.048 Mbit/s frame <ul><li>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: </li></ul><ul><li>Nominal bit rate 2048 kbit/s </li></ul><ul><li>Tolerance 50 ppm </li></ul><ul><li>Line code HDB3 </li></ul><ul><li>Frame length 256 bits </li></ul><ul><li>Frame rate 8000 frames/s </li></ul><ul><li>Bits per time interval 8 bits </li></ul><ul><li>Multiplexing method Byte-by-byte </li></ul>
    88. 90. <ul><li>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. </li></ul>
    89. 91. FAS <ul><li>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 (&quot;0011011&quot;) 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. </li></ul>
    90. 92. FAS
    91. 93. CRC-4 multiframe <ul><li>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). </li></ul>
    92. 94. CRC-4 procedure <ul><li>The aim of this system is to avoid a loss of synchronization due to the coincidental appearance of the sequence &quot;0011011&quot; 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. </li></ul>
    93. 95. 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.
    94. 96. CRC-4 multiframe alignment <ul><li>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). </li></ul>The CRC-4 multiframe alignment word is the set combination &quot;001011&quot;, which is introduced in the first bits of the frames that do not contain the FAS signal.
    95. 97. Advantages of the CRC-4 method <ul><li>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). </li></ul><ul><li>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. </li></ul>
    96. 98. Monitoring errors <ul><li>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. </li></ul><ul><li>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 &quot;1&quot; 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 &quot;0&quot; 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). </li></ul>
    97. 99. Monitoring errors
    98. 100. Supervision bits <ul><li>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 &quot;1&quot; in order to avoid simulations of the FAS signal. </li></ul>
    99. 101. NFAS - Spare bits <ul><li>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 &quot;0&quot;, while a value of &quot;1&quot; indicates an alarm. </li></ul>
    100. 102. NFAS - Spare bits <ul><li>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 &quot;1&quot; in international links. </li></ul>
    101. 103. NFAS - Alarm bit <ul><li>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 </li></ul><ul><li>a power failure </li></ul><ul><li>a failure of the coder/decoder </li></ul><ul><li>or any of the following in its demultiplexer: </li></ul><ul><li>loss of the 2 Mbit/s signal received </li></ul><ul><li>loss of frame alignment (synchronization) </li></ul><ul><li>bit error rate (BER) greater than or equal to 10-3 </li></ul><ul><li>an alarm must be sent to the transmitter. </li></ul><ul><li>This Remote Alarm Indication (RAI) is sent in the NFAS of the return frames, with bit 3 being set to &quot;1&quot;. 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. </li></ul>
    102. 104. NFAS - Alarm bit
    103. 105. Signalling channel <ul><li>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. </li></ul><ul><li>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. </li></ul><ul><li>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 </li></ul>
    104. 106. Signalling channel <ul><li>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 </li></ul>
    105. 107. CAS signalling multiframe <ul><li>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. </li></ul><ul><li>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 </li></ul>
    106. 108. CAS signalling multiframe
    107. 109. CAS multiframe alignment signal <ul><li>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 &quot;0000&quot; located in the first four bits of the TI16 of the first frame of the CAS multiframe, called frame 0. </li></ul>
    108. 110. CAS non-multiframe alignment signal <ul><li>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. </li></ul>
    109. 111. CAS non-multiframe alignment signal <ul><li>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: </li></ul><ul><li>a power failure </li></ul><ul><li>or detects the following in its demultiplexer: </li></ul><ul><li>loss of incoming signalling </li></ul><ul><li>loss of CAS multiframe alignment </li></ul><ul><li>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 &quot;1&quot;. Additionally, the value &quot;1&quot; is applied to all the signalling channels. </li></ul><ul><li>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 &quot;0000&quot;. In this case bit 6 of the TI16 of the frame 0 that this multiplexer transmits to the near end multiplexer is set to &quot;1&quot;. 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 &quot;1&quot;, known as AIS64 (Alarm Indication Signal-64 kbit/s) in the TS16 (64 kbit/s channel). </li></ul>
    110. 112. THE END

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