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1. NT S208 Northern Telecom NIS S208 5 Basic rate NT A211 Northern Telecom NIS A211 Primary rate ATT 41449 AT amp T TR41449 Primary rate ATT 5E6 AT amp T 5D5 900 321 Basic rate NIU 301 USA North American ISDN Users Forum 301 Basic rate NI1 1953 USA Bellcore National ISDN 1 SR NWT 001953 Basic rate NI2 USA Bellcore National ISDN 2 Televerkets Scandinavia VN3 VN4 France CNET VN3 VN4 JT Q 931 Japan CCITT 1988 0 931 9 932 1988 ITU 1993 0 931 9 932 1993 Figure 12 13 ISDN codesets ISDN Connection Setup To initiate an ISDN connection the TE sends a SETUP command containing information such as the subscriber numbers the type of service voice or data communication the B channel designation and the capabilities of the terminal device The ISDN network responds depending on the message content with either a SETUP ACK or a CALL PROCEEDING command to acknowledge the SETUP command and forwards the call to the receiving station When the call is successfully delivered at the destination the network sends an ALERTING message to the calling station Once the call is accepted by the called station the network reports this fact to the calling station in a CONNECT message The calling station has the option of sending a CONNECT ACKNOWLEDGE message to confirm receipt of the CONNECT message This concludes the connection setup the call is now in the active state Clearing Down the Connection Once t
2. Re transmission of corrupted frames Data flow control 8 or 8 bits 16 bits 16 bits Up to 260 bytes 16 bits 8 bits Flag Address Control F Flag 01111110 field field nace Cheessum 01111110 012345670 1234567 E C E AIR SAPI A TEI Bits 1 1 6 1 T Control field formats Information frame 176543210 716543210 I Frame N R N R o RNR Receive Not Ready Supervisory frame 7654321076543210 S Frame N R PF00000001 RR Receive Ready 76543210 76543210 N R PF00000101 RNR Receive Not Ready 76543210 76543210 N R PF0 0020120014 REJ Reject Unnumbered frame U Frame 0 1 1 P 1111 SABME 00 01 F 1 1 1 1 DM Disconnected Mode 000 P 0011 UI Unnumbered Information 010 P 001 1 DISC Disconnect 01 11F 001 1 UA Unnumbered Acknowledgement 100 F 011 1 FRMR Frame Reject 1 0 LPFI 1 1 1 XID Exchange Identification Figure 12 8 LAP D frame format TROUBLESHOOTING WIDE AREA NETWORKS 1 2 446 ISDN Each LAP D frame consists of a beginning and end flag address field control field an optional information field and a frame check sequence FCS field see Figure 12 8 The address field contains the Service Access Point Identifier SAPI which defines the network service to be provided on Layer 3 for a given connection and the Terminal Endpoint Identifier TEI which identifies th
3. Troubleshooting by Othmar Kyas ISDN An Agilent Technologies Publication ve o Agilent Technologies Section Ill Troubleshooting Wide Area Networks ISDN Experience is the worst teacher It always gives you the exam first and the lesson afterwards UNKNOWN 12 1 ISDN Specification and Implementation Integrated Services Digital Network or ISDN was the first communications infrastructure designed for transmission of both voice and data The transmis sion technigue is based on 64 Kbit s bearer channels B channels and out of band signaling channels D channels There are two distinct types of network access in ISDN each with different bandwidth capacities basic rate access provides an access rate of up to 128 Kbit s over two B channels and unused bandwidth in the D channel of up to 16 Kbit s primary rate access provides access rates of up to 1 536 Kbit s in North America 28 B channels and one D channel or up to 1 984 Kbit s in Europe 30 B channels and one D channel D 16 Kbit s 2B D Basic rate interface BRI E B1 B2 64 Kbit s T AE D 64 Kbit s Primary rate interface PRI E BI B23 23 64 Kbit s or Figure 12 1 ISDN basic rate and primary rate access TROUBLESHOOTING WIDE AREA NETWORKS 1 2 436 ISDN From today s standpoint the major drawbacks of ISDN are the limitation of the d
4. 0 doos 260 0260 260 0760 Ansi L 6 D 0 60 LE6D 0 6 D jauueyo q yuowdin ba IN euua NGSI YMS 890 youms yun jev6iq rej BIG sjauueud g UDJIMS 220 1N juaudinba eunua NGSI Figure 12 3 Signaling in ISDN TROUBLESHOOTING WIDE AREA NETWORKS 1 2 439 ISDN dent data network is available for the signaling plane using the Signaling System No 7 SS7 the discussion of which is outside the scope of this book The Message Transfer Part MTP the transport mechanism is divided into three levels that correspond to the first three OSI layers The higher SS7 layers correspond only loosely to the OSI model and include the following application protocols or user parts UPs ISUP ISDN User Part defines ISDN B channel switching TUP Telephone User Part governs telephone channels DUP Data User Part governs data channels Separate SS7 application protocols are defined for every self switching service For example a Mobile Application Part MAP is defined for mobile telephony and Broadband ISDN User Part B ISUP for ATM The combination of a large number of protocols with the necessity for interoperability among the various services can make the operation of an SS7 network an exceptionally complex task 12 1 2 1 ISDN Layer 1 Bit Transmission Layer Basic Rate Access The S T reference point The total bit rate of the ISDN ba
5. MAMI 440 N NT 1 NT 2 437 P Primary rate access 435 442 Primary Rate Interface PRI 443 Protocol discriminator 449 o 0 931 437 TROUBLESHOOTING WIDE AREA NETWORKS 1 2 Index p3 ISDN R Receive ready frames 462 Response times over ISDN 464 s S bus 440 S T reference point 439 S y interface 437 SABME command Set Asynchronous Balanced Mode Exte 447 Service Access Point Identifier SAPI 446 Signaling inISDN 438 Signaling System No 7 SS7 439 SPID values 462 T TE1 TE2 437 Terminal Endpoint Identifier TEI 446 TUP Telephone User Part 439 U U reference point 436 441
6. network is also called a passive S bus A S bus must be terminated at each end by two 100 Ohm resistors one connected across the transmit wire pair and one across the receive pair The terminating resistors may be either integrated in the last ISDN TE on the bus where they can be activated by a switch or installed in the last wall jack on the bus The NT supplies power to the passive ISDN telephones Because the NT can only power a maximum of four telephones no more than four phones may be connected to an S bus unless a line vo tage adapter powers the eguipment To ensure that telephone service is still available in the event of a power failure the local switch can power one ISDN telephone The NT itself can be installed at any point on the S bus If only one ISDN terminal device is connected to the bus the maximum distance between the NT and the ISDN jack is 1 000 meters If up to four terminals are connected to the bus within 50 meters the distance from NT to the farthest terminal device may be up to 500 meters 12 2 2 Primary Rate Interface PRI The primary rate ISDN line sometimes called the S interface or primary rate access requires a 4 wire El or T1 line to the local switch In contrast to the BRI NT the NT at the PRI subscriber line can only be connected to a single ISDN terminal device this is called point to point configuration The TE in this case is usually either a PBX or a router port The TE is connected to the NT by
7. Connects terminals with X 25 interfaces as defined in ITU X 31 Case B 12 1 2 The ISDN Protocol In an ISDN network the circuit switched transmission plane is separate from the signaling plane End to end connections in the transmission plane must be set up and a user channel allocated before user data including digitized voice data can be transmitted over ISDN A signaling protocol on a separate D channel using out of band signaling provides connection setup for user data transmitted over B channels The communication protocol on OSI Layer 2 the data link layer uses the Link Access Procedure D LAP D which is an option of the high level data link control HDLC protocol defined in ITU Recommenda tions Q 920 and Q 921 Worldwide there are many different manifestations of the network layer protocols however all of them are based on the Digital Subscriber Signaling System No 1 DSS1 described in ITU Recommendations Q 930 and Q 931 OSI Layer 1 the bit transmission layer is identical on B and D channels and is defined in ITU Recommendations I 430 and I 431 A completely indepen TROUBLESHOOTING WiDE AREA NETWORKS S son 12 438 090 01d JSS 090 0 1d guueyo q e cdilN Le oer Leg 0 e dLN 1z6 0 0260 1260 0760 doos Le60 0 60 Le60 0 60 dnsi x jauueyo BuljeuBbis jauueyo q uoWWOD ea 109030Jd j uueyo q c dlN e dLN Lev 10 Lev
8. E to NT line terminal devices on the S bus must make sure the D channel is not in use by another TE before starting with data transmission of its own For this purpose the echo channel transmit ted in an E bit from the NT to the TE always carries a copy of the D channel bits received by the NT from the TEs The D channel is considered available for transmission when the TE detects a certain number of consecutive idle bits binary 1 on the echo channel E bits for low priority terminals this number is 9 while higher priority terminals can begin transmission once they TROUBLESHOOTING WIDE AREA NETWORKS 1 2 441 ISDN detect 8 such bits Subseguent to a successful transmission the low priority class TEs have to count 11 consecutive idle bits high priority TEs count to 10 before transmission of the next frame can commence After that the TEs count to 9 or 8 consecutive idle bits again The connector for the BRI S T interface is the modular registered phone jack RJ 45 with 8 pin positions defined in the ISO 8877 specification see Figure 12 5 Optional wire pair eae 1 251 1 oS F 1 ZQ 1 L l DOWN gt O O N gt lt vo Transmit Receive Voltage sink 1 5 4 Receive Transmit 7 COIN I I N I s t lt 9 NS s 5a 00 To a o 3 2 z oO To D z lt go BS lt 1 en 1 ss Pin assignment
9. ESHOOTING WIDE AREA NETWORKS 1 2 459 ISDN 12 4 Troubleshooting ISDN 12 4 1 Gathering Information on Symptoms and Recent Changes The first step in any troubleshooting process is to gather information The more information you have about the symptoms and characteristics of a problem including when it first occurred the better your chances of solving the problem quickly and efficiently Typical questions you might ask at this stage include Was there any change in any hardware or software network component Do the symptoms occur regularly or intermittently Is it possible to reproduce or recreate the symptoms When was the first occurrence of the symptom Are the symptoms related to certain applications or connections for example outgoing or incoming calls only or do they affect all network operations e Has anyone connected or disconnected equipment to from the network e Has anyone replaced or installed an interface card in a computer Has anyone stepped on a cable e Has any maintenance work been performed in the building recently by a telephone company or building maintenance personnel for example e Has anyone including cleaning personnel moved any equipment or furniture e Has there been severe weather thunderstorms tornados in the vicinity North America rural areas Is the network up and running Does a service provider Internet Service provider experience general problems in your
10. GUIRY Figure 12 11 0 931 DSS1 signaling messages TROUBLESHOOTING WIDE AREA NETWORKS 1 2 451 ISDN 11111110 identify other protocols such as X 25 the range from 01000000 to 01001111 is reserved for national use Call Reference The call reference identifies a call to which particular signaling messages apply All messages specific to a given connection contain the same call reference The call reference consists of three fields The first octet of the call reference is the length field which contains the number of octets to follow For basic rate access the length is 1 for primary rate 2 if the length field is 0 the call reference is called a Dummy call reference or Null call reference Dummy call references are used for mes sages not pertaining to a particular call such as service profile SPID exchanges in North American ISDN applications e The most significant bit in the second octet contains the call reference flag this flag is necessary to distinguish between call references of the same value for different calls because the NT and TE may allocate the call refer ence numbers independently of one another Whichever side allocates the call reference number sets the value of the call reference flag to 0 Bits 0 to 7 in the second octet and all bits in subsequent octets depending on the call reference length field contain the actual call reference value A value of 0 is called the global call refe
11. OTING WIDE AREA NETWORKS 1 2 Index p1 ISDN Index of chapter 12 Symbols 2B10 coding 441 2B10 lines 442 AB3T lines 442 A Alternate Mark Inversion AMI 442 Alternate Space Inversion ASI 440 B channel 440 Basic rate access 435 Bayonet Neil Concelman 443 Bipolar with Eight Zero Substitution B8ZS 442 British Naval Connector 443 C Call reference 451 Codeset extensions message sets 453 Crossed wires in BRI 464 D D channel 443 Design guidelines for ISDN 456 DUP Data User Part 439 Duplicate TEI assignment 464 E Error symptoms in ISDN 461 Exchange Termination ET 436 H H channel 443 H channel 443 High Density Bipolar 3 HDB3 443 High level Data Link Control HDLC 437 TROUBLESHOOTING WIDE AREA NETWORKS 1 2 Index p2 ISDN Incorrect input of Multiple Subscriber Number MSN 464 INFO 1 INFO 2 INFO 3 INFO 4 440 Information elements 0 931 452 Integrated Services Digital Network ISDN 43b ISDN Basic Rate U Interface 442 ISDN BRI 441 ISDN connection setup 454 ISDN information elements 453 ISDN Layer 1 439 ISDN Layer 2 LAP D 445 ISDN Layer 3 DSS1 448 ISDN line blocked 464 ISDN primary rate interface 444 ISDN protocol 437 ISDN S bus 457 ISDN S T interface 456 ISDN wall jacks 456 ISUP ISDN User Part 439 K Keep alive frames 462 L LAP D frame format 445 Line termination LT 436 Link Access Procedure D LAP D 437 M Modified Alternate Mark Inversion
12. SDN ISDN network interface card incorrectly configured wrong interrupt driver or timer configuration etc ISDN router port defective Incompatible ISDN services services not available not ordered from ISDN provider ISDN telephone defective e Line breaks in plug or cabling e Long response times due to automatic connection clear down settings in the router e Loose or defective connectors on network interface cards in wall jacks or patch panels e No grounding PRI e No terminating resistors on the S bus e NT defective e PBX defective e Power supply defective e Protocol configuration in the router incorrect address tables mapping tables subnet masks default gateways routing tables timers Q 931 implementation incorrect e Q 931 variant incompatible national ISDN vs Euro ISDN e Router does not activate the second or nth in PRI B channel configuration error Router settings incorrectly configured port not active wrong operating mode protocol not active Slow terminal devices RR or Status Enquiry responses too slow TEI assignment problems manual vs automatic mode e Terminal adapter defective Timers expire in B channel application protocols Unshielded cabling between NT and TE PRI e Window size too small in the application protocol such as IP on the B channel Wrong number called Figure 12 18b The most common causes of ISDN problems TROUBLESHO
13. SS1 The DSS1 protocol detailed in ITU Recommendation Q 931 is used for Layer 3 transmission of such information as the subscriber numbers service type voice or data transmission and desired channel B1 B2 etc DSS1 recognizes 25 signaling messages which are divided into the following categories Call Estab lishment Connection Information Phase Call Clearing and Miscellaneous The following types of messages are available for standard signaling procedures Call Establishment ALERTING e CALL PROCEEDING CONNECT CONNECT ACKNOWLEDGE PROGRESS SETUP SETUP ACKNOWLEDGE Call Information Phase RESUME e RESUME ACKNOWLEDGE RESUME REJECT SUSPEND e SUSPEND ACKNOWLEDGE SUSPEND REJECT USER INFORMATION Call Clearing DISCONNECT RELEASE RELEASE COMPLETE RESTART RESTART ACKNOWLEDGE Miscellaneous INFORMATION CONGESTION CONTROL TROUBLESHOOTING WIDE AREA NETWORKS 1 2 449 ISDN NOTIFY SEGMENT e STATUS e STATUS ENQUIRY Each signaling message is made up of a number of sections called information elements The first three information elements listed here and described later in detail are mandatory Mandatory Information Elements Protocol discriminator Call reference Message type The inclusion of additional elements depends on the message type Figure 12 10 shows the schematic structure of these user to network interface UNI
14. a 2 pair copper cable no more than 250 meters in length Regenerators are reguired if the ISDN connection is to span greater distances TROUBLESHOOTING WiDE AREA NETWORKS 457 son 12 NT atone end B Four wire cable SA g of the bus Sy e AW NT ae 1 TE TE TE Central office 1 2 8 NT in the middle k om Four wire cable a of the bus S S termination resistors at both ends AW IAE NT IAE AW J Two wire 10m s cable TE Central TE office 1 NT and one 1 000 m s terminal device Sy AW fH IAE NT Two wire N cable 10m Four wire cable 1 TE Central office NT with remote bus B poom 4 50 m So AW IAE IAE I NT Two wire B4 cable AW Terminating resistors 2 x 100 Q Fourawife cable TE Terminal equipment 1 NT Network termination TE TE Central office 1 4 Pi I d S bus In assignments an termination of S bus Ine ne NT 2x 1002 lt 1 prev O ST E ie Ol5 O al O 4 O 4 O b2 3 erososniosso O 3 O a2 Figur
15. ally should be configured for Point to Point operation because all connections go through the PBX not between other devices on the same S bus If the line is configured for the wrong operating mode TEI assign ment cannot be performed correctly and network operation is impaired 12 4 3 Symptoms and Causes ISDN Symptom No Connection Cause 1 Faulty cabling or connectors Cause 2 Power supply failure Cause 3 Crossed wires in BRI Cause 4 Wrong number Cause 5 Faulty network components ISDN router port terminal adapter TA PBX interface card telephone Cause 6 Incorrect configuration of the ISDN interface ISDN card router port or PBX Cause 7 Noise high BER Cause 8 Problems in TEI assignment manual vs automatic mode Cause 9 Duplicate TEIs Cause 10 Incompatible Q 931 variants national ISDN vs Euro ISDN Cause 11 Incompatible ISDN services Cause 12 Q 931 implementation errors Cause 13 Wiring faults on the S bus Cause 14 Lack of terminating resistors on the S bus Cause 15 Faulty grounding PRI Cause 16 Non shielded cabling between NT and TE PRD Cause 17 Signaling messages are sent with wrong TEI value in the case of multiple signaling links BRI North America 464 TROUBLESHOOTING WIDE AREA NETWORKS 1 2 ISDN Symptom Freguent Connection Loss Cause 1 High BER Cause 2 Slow terminal equipment Receive Ready RR or Status Enquiry responses to
16. are defined as either a command or a response type frame however some supervisory frames can be either a command or a response The type of frame is determined by the knowledge of the data source NT or TE and the content value of the C R bitin the first octet of the address field NTs and TEs have to set this bit differently for commands and responses The first step in setting up a LAP D connection is the transmission of a Set Asynchronous Balanced Mode Extended SABME com mand with the P bit set The receiving side transmits an unnumbered acknowl edge UA frame with the F bit set to 1 in order to confirm this command Once this procedure has been completed I frames can be transmitted The T202 timer 2s monitors the TEI assignment procedure and the T200 timer 1s monitors the response to a SABME command If no response to the SABME TE NT UI TEI t eqe M u aer TEI allocation SABME EE UA Frame ie aaa PO a st ie ieee Figure 12 9 Setting up and clearing down Layer 2 connections in ISDN LAPD protocol TROUBLESHOOTING WIDE AREA NETWORKS 1 2 448 ISDN is received within 1 second the SABME command is repeated If answers are received from multiple terminal devices while T202 is running this indicates that a given TEI has been assigned to more than one terminal Check the terminals to determine which devices have identical TEIs 12 1 2 3 ISDN Layer 3 D
17. area As with any network topology troubleshooting ISDN is greatly facilitated if records of the main operating statistics have been maintained prior to the occurrence of the error in question In an ISDN network it is also important to have detailed descriptions and user guides manuals of all ISDN network compo nents such as bridges ISDN routers computer systems with ISDN cards PBXs and ISDN telephones including configuration data and details about physical BRI PRI interfaces as well as the protocols and applications that are operated over ISDN lines Statistics such as capacity use of B channels peak and average values sorted by service or by distribution of packet size in the case of data transmission can be compared with the corresponding data collected in the error situation this often points directly to the source of the problem Trouble shooting tools include an ISDN terminal device known to be in working order TROUBLESHOOTING WIDE AREA NETWORKS 1 2 460 ISDN an NT known to be in working order and an ISDN analyzer ideally an analyzer that can be used to simulate NT and TE interfaces both BRI and PRI as well as to monitor ISDN lines In most cases a telephone connected to a different service for example a cell phone can be used for testing outgoing and incoming connections The first step in diagnosis is to obtain a clear picture of the error symptoms Is it possible to establish any connection at all on the ISDN l
18. ata speed to a maximum of 1 536 or 1 984 Kbit s which cannot be incremen tally expanded and the synchronous structure of the transmission channels which does not permit dynamic allocation of bandwidth within the network The latter characteristic was one of the reasons for the development of Frame Relay a dedicated data transmission service that dynamically adjusts the bandwidth in use for more efficient data transfer However ISDN has the advantage of being an established international standard providing a uniform digital interface for voice and data transmission worldwide 12 1 1 ISDN Interface Reference Model Common ISDN Interfaces Customer Premises Central office TE1 NT 1 nF nn ao LT ET TE2 H TA R S T U U S and T Interface Expanded Detail TE1 NT E NT 2 NT 1 TA S T Figure 12 2 ISDN interface reference model Exchange termination ET and line termination LT comprise the Central Office interface called the V Interface e In North America the U reference point is the standard interface and de marcation line between the network and the customer premises equipment CPE Outside North America the send S transmit T reference point represents the border line between the CPE and the network For the basic rate access
19. bit s H These channels are rarely used however Unlike the S interface the primary rate interface PRI supports only continu ous point to point connections between two terminals Thus some of the func tions available with basic rate access such as Layer 1 activation and deactiva tion of terminal eguipment or collision detection are unnecessary here The transmission frame and its control fields which include the alarm indication signal AIS cyclical redundancy check CRC and remote alarm indication RAD correspond exactly to the El or T1 Plesiosynchronous Digital Hierarchy PDH frame DS1 see also Chapter 14 The 1 544 Mbit s T1 interface uses 193 bit frames each consisting of one F bit followed by 24 time slots Time slot 24 is reserved for the D channel if used while any other slot can be assigned to a B channel H channels can be assigned to any group of 6 not necessarily consecutive time slots while an H channel takes up all 24 time slots The E1 interface uses a 256 bit frame divided into 32 time slots numbered 0 through 31 Time slot 16 is reserved for the D channel H channels can again be assigned to any group of six not necessarily consecutive time slots while an H channel takes up time slots 1 15 and 17 31 and an H channel occupies time slots 1 15 and 17 25 The type of connectors used for primary rate access depends on the line impedance of various national standards The two pairs of a T1 line are a
20. e 12 15 Distance limitations on the ISDN S bus 458 TROUBLESHOOTING WIDE AREA NETWORKS 1 2 ISDN 4 250 m _ Som U Central TE ou NTzu office Four wire cable 12 3 Figure 12 16 Distance limitations of the ISDN primary rate interface ISDN Standards All standards for ISDN were developed by the International Telecommunication Union ITU formerly CCITT The various national codesets have been defined on the basis of the TTU Recommendations by national and international organi zations The most important standards related to ISDN are 1 480 1 481 Q 920 Q 921 Q 930 Q 931 Basic User Network Interface Layer 1 Specification Primary Rate User Network Interface Layer 1 Specification ISDN User Network Interface Data Link Layer General Aspects ISDN User Network Interface Data Link Layer Specification ISDN User Network Interface Layer 3 General Aspects ISDN User Network Interface Layer 3 Specification for Basic Call Control See ATM Standards in Chapter 10 ATM Networks for a more complete list of I and G series specifications The ITU can be reached on the Internet at http www itu int Other important addresses related to ISDN include North American ISDN Users Forum http www niuf nist gov National ISDN Council http www nationalisdncouncil com index html European ISDN Users Forum http www2 echo lu eiuf en eiuf html TROUBL
21. e been correctly received and acknowledged by the peer entity The transmit ting station increments the transmit number by one every time a frame is transmitted Received I frames must be in sequence without transmission and format errors before they can be acknowledged by means of an appropriate supervisory frame or I frame transmitted to the peer entity The maximum number of frames that may be sent without acknowledgment of receipt is called the window size The maximum permissible window size depends on the modulo of the sequence number counters and is 7 for modulo 8 counters 3 bit for the Nis TROUBLESHOOTING WIDE AREA NETWORKS 1 2 447 ISDN and N and can be up to 127 for modulo 128 counters extended sequencing established with Set Asynchronous Balanced Mode Extended SABME as op posed to SABM Since 1988 equipment using the modulo 8 sequencing has been obsolete extended sequencing is now mandatory The default window size for transmission and reception is 7 Larger window sizes require more buffers All frames have a bit identified as either P poll in commands or F final in responses A pollbitindicates that an immediate response is expected in other words after a command frame with P 1 has been transmitted only received response frames with the final bit F 1 are considered a valid response Received response frames with the final bit F 0 under these circumstances are consid ered unsolicited responses Most of the frame types
22. e destination terminal s The following three services are identified by the SAPI SAPI63 Management services such as TEI administration SAPIO Call control signaling SAPI 16 Data transmission over the D channel X 25 Other SAPI values are reserved for national uses The TEI values that identify the terminals can be assigned either automatically by the TEI management entity or manually at the terminal itself TEI values 0 to 63 are reserved for manual assignment the ISDN switch automatically assigns values 64 through 126 TEI assignment information is transmitted in U frames see the following with the address values SAPI 63 and TEI 127 The TEI value reserved for broadcasts the automatic TEI assignment procedure and the TEI verification procedure also known at the link audit is 127 The control field of the LAP D frame indicates one of the following three frame types Information frames I frames Supervisory frames S frames Unnumbered frames U frames U frames are used to establish and release Layer 2 logical links to exchange parameters for negotiation and report irrecoverable error situations I frames transport sequenced Layer 3 data and S frames are used for Layer 3 information acknowledgements and flow control procedures Frames are numbered to en sure complete transmission Each I frame has its own transmit number Ney and receive number Ney The receive number represents the number of frames that hav
23. f 120 ternary symbols 108 scrambled data symbols 11 symbols for synchronization and 1 symbol for maintenance pur poses Different constructions are used for frames from LT to NT1 and NT1 to LT synchronization symbols are in different bit positions that is offset by 60 symbols The connector for the BRI U Interface is the RJ 45 modular phone jack with 8 pin positions defined in the ISO 8877 specification see Figure 12 6 al though sometimes different mechanical connector build outs are used Pin assignments 1 U3 2 U3 3 4 a Pin 1 5 b Pin 8 6 7 U2 8 U2 Figure 12 6 Connector and pin assignments in the ISDN basic rate U Interface RJ 45 ISO 8877 Primary Rate Access For ISDN primary rate access only the U reference point is defined as the demarcation between network equipment and customer premises equipment Four wire full duplex physical interfaces with Alternate Mark Inversion AMI or Bipolar with Eight Zero Substitution B8ZS line encoding are used for North TROUBLESHOOTING WIDE AREA NETWORKS 1 2 443 ISDN American T1 lines 1 544 Mbit s or High Density Bipolar 3 HDB3 line encod ing on El interfaces 2 048 Mbit s outside North America described in ITU Recommendations G 703 and G 704 Each B channel has 64 Kbit s of available bandwidth The ISDN specification also defines a 384 Kbit s H channel as well as two H channels with 1 536 Kbit s H and 1 920 K
24. he connection has been ended by one subscriber by a person hanging up a telephone receiver for example the clearing terminal device transmits a DISCONNECT message to the ISDN network The network forwards this mes TROUBLESHOOTING WIDE AREA NETWORKS 1 2 455 ISDN Switch Switch User User Picks up handset and dials SETUP Start T 303 CALL PROC a a Nj SET san SP T 303 Telephone a Picks up HE NE CONNECT handset Ring tone heard e 7 CONNEC a TAC CONNECT AAA CONNECT Ack Stop T 303 DISCONNECT Hangs up 1 REL ais CONNECT A EASE RELEASE TE TE RELEASE COMPLE RELEASE COMPL Figure 12 14 Setting up and clearing down an ISDN connection sage to the other subscriber and sends a RELEASE message to the subscriber that ended the connection The clear down procedure is finished once a RE LEASE COMPLETE message has been transmitted In North American ISDN networks with NI 1 capability terminals are uniguely identified by a SPID which usually consists of a directory number DN phone number plus a two digit terminal identifier TID At least one SPID for every Layer 2 link TEI value is required The SPID has to be entered and stored in the terminal by an operator After power up and establishment of the supported Layer 2 links the network requests the SPID from the terminal for every Layer 2 link If this procedure fails the network w
25. ill not recognize the terminal that is outgoing calls will not proceed because the network does not respond to Layer 3 call control messages After completion of the SPID reguest procedure call control messages for a particular DN must be transmitted over the Layer 2 link matched with a registered SPID or otherwise the messages will be ignored by the network TROUBLESHOOTING WIDE AREA NETWORKS 1 2 456 ISDN 12 2 Design Guidelines for ISDN 12 2 1 Basic Rate Interface An ISDN basic rate line is carried over a 2 wire telephone line from the public telecommunications network s local switch to the subscriber s premises where a NT provides the interface to the ISDN subscriber s communication equipment In North America the NT is considered customer premises eguipment and has to be provided by the subscriber Most ISDN eguipment is eguipped with a built in NT collapsed NT and can be connected directly to the 2 wire pair from the service provider U Interface Whereas if a NT is installed the 4 wire ISDN S T interface is used to connect the NT to a maximum of 12 ISDN wall jacks which allows up to eight ISDN terminal devices such as terminal adapters telephones faxes routers or PC cards to be connected at any given time The maximum length of the S bus is 200 meters The cable connecting the NT to the first ISDN jack must not exceed a length of 10 meters Because there are no active elements on the bus except the NT this part of the ISDN
26. ine Are existing connections cut off Have response times in the ISDN line grown longer has throughput diminished What happens when the malfunctioning station is called from a telephone in another service for example a pay phone or cell phone Because most problems in networks occur in the physical layer ISDN is no exception the troubleshooting procedure should start with basic checks This includes checking the physical interfaces and monitoring ISDN Layer 1 func tions If the symptoms seem to indicate a particular network component an ISDN card an ISDN telephone or an NT the next step is to replace the u Agilent Advisor ISDN sampisdn dat Decode File Run View GoTo Setup Window Help Al Bo oe Tele 5 ins 5 HEI pes 2a Summary IM Detaled T Hex ASCII EBCDIC Filter Search LAPD SAPI Call Control Comnand Response B Command TEI 112 Network assigned Frame Tupe 6x62 Information Poll Final 6 Not Poll N s 001 N r 002 FCS x0a F7 Good 0 931 T amp THI 1 Protocol Discriminator Call Reference Length Call Reference Flag Call Reference Value Hessage Type Information Element Information Element Length 8x88 0 931 Call Control 1 8 FROM side that originated call ref 6x61 SETUP 6x65 Bearer Capability 6x64 3 Info Trans Cap Speech Transfer Mode 64 kbps Circuit Layer 1 User Info u law Information Element Information E
27. lement Length Channel Identification 6x18 Interface Id Present Implicitly Interface Type Basic Channel Exclusive D channel No Chan Select B1 on Basic Access Information Element Calling Party Number 6x6c Ready Mon kbps Util Figure 12 17 Using a protocol analyzer to decode ISDN O 931 signals SETUP message TROUBLESHOOTING WIDE AREA NETWORKS 1 2 461 ISDN component in guestion with one that is known to be in working order If this does not resolve the problem the next step is to check log data gathered by components in the affected network such as routers and ISDN cards for any error indication and to check the configuration of these components If the source of the error still cannot be found the use of an ISDN tester to check ISDN Layer 1 as well as the signaling and bearer channel protocols is suggested On a BRI line Layer 1 should reach the Info 3 or 4 state automatically On a PRI line test the voltage level the Layer 1 alarm states AIS RAI CRC the power supply to the NT green LED on and the PRI and U red LEDs off If the collected data corroborates that the physical layer connection can be established then the LAP D protocol should be analyzed If no errors are de tected there the next step is to examine signaling procedures on the D channel Figure 12 17 shows an ISDN SETUP message decoded by a protocol analyzer If problems persist despite the fact that ISDN connections can be set up succe
28. locking shift procedure described in Q 931 may only be made to a codeset with a higher numerical value than one currently used Codesets 1 3 Reserved for future use by ITU Codeset 4 Reserved for future use by ISO IEC Codeset 5 Reserved for national use Codeset 6 Reserved for information elements specific to the local network Codeset 7 Reserved for user specific information elements A locking shift information element switches the codeset permanently For example if codeset 0 the default codeset is active a locking shift element that specifies codeset 5 will cause codeset 5 to become active all information ele ments from that point onward are interpreted according to codeset 5 Following a non locking shift element the higher numbered codeset is active only for one subsequent information element Additional information elements are consid ered to belong to the previously used codeset A variety of codesets were used in Europe up to the early 1990s Since then the European Telecommunications Standards Institute ETSI codeset has become the standard for European ISDN The table in Figure 12 13 gives an overview of the most common codesets in use today TROUBLESHOOTING WIDE AREA NETWORKS 1 2 454 ISDN ETSI 102 ETS 300 102 1 Euro ISDN 1TR6 FTZ 1TR6 German National ISDN TPH 1962 Telecom Australia TPH_1962 Basic rate TPH 1856 Telecom Australia TPH_1856 Primary rate
29. lways operated in balanced mode with 100 Q impedance whereas the two pairs of E1 lines could be operated in balanced mode at 120 Q imped ance as well as unbalanced mode with 75 Q impedance The most common connector for primary rate is the modular phone jack RJ 48 for T1 lines and E1 lines with balanced pairs On rare occasions a DB9 connector is used for E1 lines in balanced mode For E1 lines using unbalanced pairs a BNC Bayonet Neil Concelman or sometimes British Naval Connector connector is most often used see Figure 12 7 TROUBLESHOOTING WiDE AREA NETWORKS 444 son 12 Pin assignme 000 NO O GQ N nts E1 NT gt TE Ground Ground Ground TE gt NT NT gt TE TE gt NT Pin assignme 0 NO O V N Pin assignme 0 NO O V N nts E1 NT gt TE NT gt TE TE gt NT TE gt NT nts T1 NT gt TE N C TE gt NT TE gt NT Figure 12 7 Pin assignments ISDN primary rate interface RJ 48 Sub D9 ISO 10173 TROUBLESHOOTING WIDE AREA NETWORKS 1 2 445 ISDN 12 1 2 2 ISDN Layer 2 LAP D Link Access Procedure D or LAP D is the Layer Z data link layer protocol for the D channel LAP D is responsible for the secure transport of ISDN Layer 3 data including the following functions Multiplexing of several logical channels over one D channel e Detection of transmission and format errors at the interface
30. nd which ones are for automatic TEI assignment In North American networks with at least NI 1 capabilities the SPID values have to be entered into the terminal equipment s memory therefore the user must be familiar with the configuration procedure for a particular terminal equipment Problems on ISDN Layer 3 can have any of a number of causes Typical examples include incompatibility of Q 931 variants national ISDN versus Euro ISDN for example incompatibility or unavailability of ISDN services incorrect imple mentation of ISDN protocols and incorrect input of the subscriber number for the call destination When established connections are cut off this is most likely due to the absence of keep alive frames such as Receive Ready frames Layer 2 or Status Enquiry frames Layer 3 This type of error is usually caused by a high bit error rate or by terminals that are too slow in responding to polling frames In North Ameri can networks the failure to respond to the periodic line audit that is identity check request messages over the broadcast link SAPI 63 TEI 127 will result ina TEI removal procedure by the network Long response times in applications operated over ISDN links may also be part of normal operating behavior if B channel bandwidth is restricted for example or if a system that is accessed over ISDN necessitates a call setup for each transaction If the ISDN router is configured to clear down connections auto matically af
31. o slow Cause 3 Application does not respond Symptom Long Application Response Times over ISDN Cause 1 Additional call setup time due to automatic connection clear down during idle times by the ISDN router Cause 2 Router does not activate additional B channels at high traffic load Cause 3 Small window size of the application protocol such as IP used over the B channel Cause 4 Timers expire in B channel application protocols Cause 5 Application is busy Cause 6 Rate adaptation handshake fails due to wrong terminal applica tion settings Cause 7 Galling called station is a mobile station Cause 8 Call not end to end ISDN The following list summarizes the most frequent sources of problems with ISDN in alphabetical order Call forwarding is active no incoming calls Crossed wires in BRI Duplicate TEI assignment Electromagnetic interference Incorrect filter settings in the router High bit error rates Incorrect input of multiple subscriber number MSN Incorrect physical installation of router or switch loose cabling connector plug in module or card faulty wiring on the back plane ISDN interface card defective ISDN interface incorrectly configured ISDN interface card router port PBX ISDN telephone ISDN line blocked or not enabled by service provider Figure 12 18a The most common causes of ISDN problems TROUBLESHOOTING WIDE AREA NETWORKS 1 2 46b I
32. odified Alternate Mark Inversion MAMI line encoding see Figure 12 4 After power up of the NT and TE equipment the framing on the S T interface is established by exchanging predefined bit patterns between TE and NT This procedure is called activation and is achieved by means of a finite state machine with five states The bit patterns used for the transition from NO SIGNAL S bus deactivated which is called INFO 0 are as follows INFO 1 A continuous signal consisting of a positive pulse a negative pulse and a 6 byte interval with no pulse is transmitted repeatedly at 192 Kbit s INFO 2 Layer 1 frames with binary Os on channels D B E and A provide the activation request from the NT to the TE or the NT s response to an activation request from a TE INFO 3 Synchronized frames carry user data on the D and B channels TE gt NT INFO 4 Frames carry user data on the D B and E channels The A bit is set to 1 both sides continuously exchange frames and the S bus is considered activated In some jurisdictions the NT will activate the S bus only the NT is allowed to deactivate the S bus interface during idle times when no signaling or data message exchange is required The NT initiates deactivation by sending INFO 0 In North America the S T interface always remains activated because of the line audit procedure which is periodically checking the TEs connected to the network To prevent blocking or contention on the T
33. ommon source of errors A terminal device on a BRI line operated in point to point mode rather than in bus mode for example results in frame collisions that make it impossible to establish a connection on the physical layer TE equipment for TROUBLESHOOTING WIDE AREA NETWORKS 1 2 462 ISDN primary rate interfaces generally reguire the correct settings of the framing format line encoding and the selection of the correct time slot assigned for the signaling channel In the North American T carrier systems it is also essential to know the T1 line service type Due to some legacy services T1 lines with Zero Code Substitution ZC S or Digital Data System DDS services cannot transmit unrestricted digital data at 64 Kbit s Certain local exchange carriers still offer a maximum data rate of 56 Kbit s only The TE eguipment must be configured accordingly The most common errors in Layer 2 occur in the TEI assignment procedure If the ISDN components involved do not agree on the mode of TEI assignment manual or automatic then no communication is possible on the LAP D protocol level and connection setup fails Another error associated with TEI assignment is the duplicate assignment of a single TEI value this also makes it impossible to establish a connection For the evaluation of Layer 2 TEI assignment problems it is essential to know how many Layer 2 links are required and configured TEI values which links are configured for fixed TEI values a
34. rence which is used for generic messages pertaining to all active calls on a TE NT interface The global call reference is used only for RESTART RESTART ACKNOWLEDGE and STATUS messages Message Type The message type is a one or two octet field and identifies the function of the message see Figure 12 11 Two octet message types are used for network Bit 8 7 6 5 4 3 2 1 Byte Information Element Identifier 1 Information Element Length 2 Information Element Contents 3 Figure 12 12a Format and coding of information elements 0 931 452 TROUBLESHOOTING WIDE AREA NETWORKS sou 12 Bits 87654321 Information Element ldentifiers 1000 1001 10100000 10100001 1011 11017 00000000 00000100 00001000 00010000 00011000 00011110 00100000 00100111 00101000 00101000 00101001 00101100 00110100 01000000 01000010 01000011 01000100 01000101 01000110 01000111 01001010 01101100 01101101 011 011 011 011 011 1 1 1 1 01 01 01 11 Reserved Shift More data Sending complete Congestion level Repeat indicator Segmented message Bearer capability Cause Call identity Call state Channel identification Progress indicator Network specific facilities Notification indicator Display Date time Keypad facility Signal Information rate End to end transit delay Transit delay selection and indication Packet layer binary parameters Packet layer window si
35. s a U3 U3 TE gt NT NT gt TE NT gt TE TE gt NT U2 U2 Pin 1 Pin 8 1 2 3 4 5 6 7 8 Figure 12 5 Connector and pin assignments in the ISDN basic rate S T interface RJ 45 ISO 8877 The U Reference Point The total bit rate of the ISDN BRI at the U reference point is 160 Kbit s The D channel and the two B channels require 144 Kbit s the other 16 Kbit s of bandwidth are used for synchronization and framing The frame structure depends on the line encoding and modulation scheme Today the dominant line encoding scheme is 2B1Q coding where 2 bits are TROUBLESHOOTING WIDE AREA NETWORKS 1 2 442 ISDN combined to form a guaternary 4 line voltage state Outside North America a second coding scheme the 4B3T 4 bits form 3 ternary states has been used Echo cancellation transmission techniques make the use of a single copper pair possible In the past so called ping pong techniques have been used in Ger many also referred to as the U interface On 2B1Q lines one octet of each B channel and 2 bits from the C channel 18 bits are passed through a scrambler and then concatenated into a frame consisting of 12 times the 18 bit scrambled frames forming a data field of 216 bits Together with 18 synchronization bits and 6 maintenance bits a data frame of 240 bits is created Eight of these 240 bit frames form a superframe On 4B3T lines a frame is composed o
36. sic rate interface BRI at the S T reference point also referred to as the S bus is 192 Kbit s The D channel and the two B channels require 144 Kbit s the other 48 Kbit s of bandwidth is reserved for synchronization interface control and other overhead Data is transferred in To TE EEN 8bits la e N bis Jelly ets lelos 8 bis ell Wu B1 B2 B1 B2 1 2 bit A A TE e Echo ee Echo Echo NT pas z F 7 8 bits 8 bits 8 bits 8 bits LFL B1 LIDIL F L B2 LIDIL B1 LID L B2 LIDIL F Framing bit To NT L Parity bi D D channel bi E D channel echo bit M Multiframing bit A Activation bit 1 frame 48 bits every 250 us 192 Kbit s B1 B2 B channel bits S S channel Figure 12 4 The frame format in the ISDN S T interface BRI ITU 1 430 TROUBLESHOOTING WIDE AREA NETWORKS 1 2 440 ISDN 48 bit frames which carry the D channel and B channel data bits as well as the following control and status bits A device activation F frame start for synchronization L parity S1 and SZ padding bits E echo channel for contention resolution on the passive bus F additional framing bit M mul tiple frame and N this bit is always set to 1 The L parity bit ensures that there is no DC imbalance in the resulting bit stream The BRI uses Alternate Space Inversion ASI or M
37. signal ing messages Bit 8 7 6 5 4 3 2 1 Byte Protocol Discriminator 0 0 0 0 Call Reference Length Flag Call Reference Call Reference continued Message Type Information Elements oO X Q A U N Information Elements Figure 12 10 0 931 message format Protocol Discriminator The protocol discriminator is the first element in each Q 931 message It is usually set to 8 decimal or 00001000 binary which indicates the 9 931 protocol The values 00010000 through 00111111 and 01010000 through 450 TROUBLESHOOTING VVIDE AREA NETWORKS son 12 Bit 8 7 5 4 3 2 1 Byte Ext Message Type Bits 87654321 Q 931 Message 00000000 Escape sequence for national message types Connection setup 00000001 ALERTING 00000111 CONNECT 00001111 CONNECT ACKNOWLEDGE 00000010 CALL PROCEEDING 00000011 PROGRESS 00000101 SETUP 00001101 SETUP ACKNOWLEDGE Connection information 00100110 RESUME 00101110 RESUME ACKNOWLEDGE 00100010 RESUME REJECT 00100101 SUSPEND 00101101 SUSPEND ACKNOWLEDGE 00100001 SUSPEND REJECT 00100000 USER INFORMATION Connection clear down 01000101 DISCONNNECT 01001101 RELEASE 01011010 RELEASE COMPLETE 01000110 RESTART 01001110 RESTART ACKNOWLEDGE Other messages 01100000 SEGMENT 01111001 CONGESTION CONTROL 01111011 INFORMATION 01101110 NOTIFY 01111101 STATUS 01110101 STATUS EN
38. ss fully the user data on the B channels must be analyzed Factors to be checked include the bit error rate BER capacity use and the transport protocols used such as IP or X 25 12 4 2 Error Symptoms in ISDN The three most common error symptoms in ISDN networks are a problems establishing connections b interruptions of active connections and c signifi cant losses of network performance accompanied by long response times in ISDN applications By far the most freguent symptom is the failure to establish a connection when a call is placed from an ISDN telephone or from a computer system with an ISDN card This type of problem is usually the result of errors in the physical layer OSI Layer 1 the cause is seldom found in higher layers Typical causes include faulty power supplies line breaks miswired connectors lack of terminating resistors BRI faulty network components ISDN router ports ISDN switches ISDN PC cards ISDN telephones or noise on the line Additional causes in PRI lines can include faulty grounding or faulty shielding in the cables between NT and TE T1 E1 lines are usually terminated at digital cross connects or patch panels which may introduce additional reasons for faulty connections starting with using the wrong type of patch cords or termination resistors ambiguous labeling of the patch panels or simply patch cords plugged into the wrong connectors Incorrect configuration of ISDN interfaces is another c
39. ter a short period of line inactivity to minimize connection costs the necessity of re establishing the connection means a few seconds delay every time the application is accessed If connections can be established only through the dial back mode that is the calling party is called back by the remote station so that two call setup procedures take place for each call this can slow network TROUBLESHOOTING WIDE AREA NETWORKS 1 2 463 ISDN performance considerably Another reason for long response times however might be incorrect configuration of the B channel transport protocol If this is the case timers may expire too often or the window size may be too small Other causes include problems in activating B channels which is often the case when using multilink Point toPoint protocol PPP If the threshold configured in the router for activation of the second B channel of a BRI is too high then the second channel may be activated too late deactivated too early or not activated at all As a result the ISDN line appears to be overloaded even though the theoretical total bandwidth is not being used It is important to keep in mind the difference between the Point to Point and Point to Multipoint that is bus ISDN line configurations As described previ ously the PRI can be operated only inPoint to Point mode while BRI lines can be configured in either mode If a large PBX is operated over multiple S lines for example these gener
40. the U Interface is a 2 wire 2B1Q coded line from the Central Office for primary rate access the U Interface consists of a 4 wire T1 Digi TROUBLESHOOTING WIDE AREA NETWORKS 1 2 437 ISDN tal Speech Interpolation or DSI in North America or an El outside North America In Germany the U Interface for primary rate is often referred to as a S interface e NT 1 is a network termination unit that converts the 2 wire 2B1Q line code into a 4 wire code used on the S and T interfaces Most eguipment used in North America will not have an S or T interface connector e NT 2 is a network termination unit that is ISDN capable at both sides of the unit A good example is an ISDN PBX or ISDN concentrator S and T interface terms are largely interchangeable They are both 4 wire interfaces In Germany the terminology S is used for the S bus interface of basic access TEl is ISDN capable terminal equipment e TE2 is non ISDN capable terminal equipment for example a standard tele phone set etc e TA is a terminal adapter and is responsible for interfacing non ISDN equip ment to the ISDN line The following interfaces are most commonly found at the reference point R a b for connecting analog terminal equipment such as telephones telefax machines and modems V 24 Connects data terminals with V 24 interfaces X 21 Connects terminals with X 21 interfaces using bit rate adaptation as defined in ITU Recommendations X 30 and X 31 X 25
41. ze Packet size Closed user group Reverse charge indication Calling party number Calling party subaddress Called party number Called party subaddress Redirecting number Transit network selection Restart indication Low layer compatibility High layer compatibility User user Escape for extension Figure 12 12b Format and coding of information elements 0 931 TROUBLESHOOTING WIDE AREA NETWORKS 1 2 453 ISDN specific messages and the first octet has to be coded 00000000 binary with all bits set to 0 ISDN Information Elements Some messages include other information elements in addition to the manda tory elements described previously These elements may vary in length Each information element contains an information element identifier a length field and the actual contents of the information element A single message can contain zero one or multiple information elements Figure 12 12 shows the general structure of information elements and the different element types that are defined Codeset Extensions Message Sets The entire set of all information elements valid in a given operating mode is called a codeset or message set The information elements described in ITU Recommendation Q 931 comprise codeset 0 which is the default codeset Defin ing additional codesets numbered 1 through 7 expands the number of available information elements Transitions from one codeset to another performed using the locking or non

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