ISDNL2n3Tutorial03.docx

1、ISDNL2n3Tutorial03ISDN Layer 2 (Q.921) and Layer 3 (Q.931) TutorialISDN protocols described here include:LAPDInternational Variants of ISDNISDN Frame StructureISDN TerminologyISDN (Integrated Services Digital Network) is an all digital communications line that allows for the transmission of voice, d

2、ata, video and graphics, at very high speeds, over standard communication lines. ISDN provides a single, common interface with which to access digital communications services that are required by varying devices, while remaining transparent to the user. Due to the large amounts of information that I

3、SDN lines can carry, ISDN applications are revolutionizing the way businesses communicate. ISDN is not restricted to public telephone networks alone; it may be transmitted via packet switched networks, telex, CATV networks, etc.The ISDN is illustrated here in relation to the OSI model:ISDN applicati

4、onsLAPDThe LAPD (Link Access Protocol - Channel D) is a layer 2 protocol which is defined in CCITT Q.920/921. LAPD works in the Asynchronous Balanced Mode (ABM). This mode is totally balanced (i.e., no master/slave relationship). Each station may initialize, supervise, recover from errors, and send

5、frames at any time. The protocol treats the DTE and DCE as equals.The format of a standard LAPD frame is as follows:FlagAddress fieldControl fieldInformationFCSFlagLAPD frame structureFlagThe value of the flag is always (0x7E). In order to ensure that the bit pattern of the frame delimiter flag does

6、 not appear in the data field of the frame (and therefore cause frame misalignment), a technique known as Bit Stuffing is used by both the transmitter and the receiver.Address fieldThe first two bytes of the frame after the header flag is known as the address field. The format of the address field i

7、s as follows:87654321SAPIC/REA1TEIEA2LAPD address fieldEA1First Address Extension bit which is always set to 0.C/RCommand/Response bit. Frames from the user with this bit set to 0 are command frames, as are frames from the network with this bit set to 1. Other values indicate a response frame.EA2Sec

8、ond Address Extension bit which is always set to 1.TEITerminal Endpoint Identifier. Valid values are as follows:0-63Used by non-automatic TEI assignment user equipment.64-126Used by automatic TEI assignment equipment.127Used for a broadcast connection meant for all Terminal Endpoints.Control fieldTh

9、e field following the Address Field is called the Control Field and serves to identify the type of the frame. In addition, it includes sequence numbers, control features and error tracking according to the frame type.FCSThe Frame Check Sequence (FCS) enables a high level of physical error control by

10、 allowing the integrity of the transmitted frame data to be checked. The sequence is first calculated by the transmitter using an algorithm based on the values of all the bits in the frame. The receiver then performs the same calculation on the received frame and compares its value to the CRC.Window

11、 sizeLAPD supports an extended window size (modulo 128) where the number of possible outstanding frames for acknowledgement is raised from 8 to 128. This extension is generally used for satellite transmissions where the acknowledgement delay is significantly greater than the frame transmission times

12、. The type of the link initialization frame determines the modulo of the session and an E is added to the basic frame type name (e.g., SABM becomes SABME).Frame typesThe following are the Supervisory Frame Types in LAPD:RRInformation frame acknowledgement and indication to receive more.REJRequest fo

13、r retransmission of all frames after a given sequence number.RNRIndicates a state of temporary occupation of station (e.g., window full).The following are the Unnumbered Frame Types in LAPD:DISCRequest disconnectionUAAcknowledgement frame.DMResponse to DISC indicating disconnected mode.FRMRFrame rej

14、ect.SABMInitiator for asynchronous balanced mode. No master/slave relationship.SABMESABM in extended mode.UIUnnumbered Information.XIDExchange Information.There is one Information Frame Type in LAPD:InfoInformation transfer frame.ISDN decodeInterested in more details about testing this protocol?Inte

15、rnational Variants of ISDNThe organization primarily responsible for producing the ISDN standards is the CCITT (Now ITU). The CCITT study group responsible for ISDN first published a set of ISDN recommendations in 1984 (Red Books). Prior to this publication, various geographical areas had developed

16、different versions of ISDN. This resulted in the CCITT recommendation of a common ISDN standard for all countries, in addition to allocated variants definable for each country.The use of nation-specific information elements is enabled by using the Codeset mechanism which allows different areas to us

17、e their own information elements within the data frames.Following is a description of most ISDN variants:National ISDN1 (Bellcore)This variant is used in the USA by Bellcore. It has four network-specific message types. It does not have any single octet information elements. In addition to Codeset 0

18、elements it has four Codeset 5 and five Codeset 6 information elements.National ISDN-2 (Bellcore)The main difference between National ISDN-1 and ISDN-2 is parameter downloading via components (a component being a sub-element of the Extended Facility information element). These components are used to

19、 communicate parameter information between ISDN user equipment, such as an ISDN telephone, and the ISDN switch.Other changes are the addition of the SEGMENT, FACILITY and REGISTER message types and the Segmented Message and Extended Facility information elements. Also, some meanings of field values

20、have changed and some new accepted field values have been added.5ESS (AT&T)This variant is used in the USA by AT&T. It is the most widely used of the ISDN protocols and contains 19 network-specific message types. It has no Codeset 5, but does have 18 Codeset 6 elements and an extensive information m

21、anagement element.Euro ISDN (ETSI)This variant is to be adopted by all of the European countries. Presently, it contains single octet message types and has five single octet information elements. Within the framework of the protocol there are no Codeset 5 and Codeset 6 elements, however each country

22、 is permitted to define its own individual elements.VN3, VN4 (France)These variants are prevalent in France. The VN3 decoding and some of its error messages are translated into French. It is a sub-set of the CCITT document and only has single octet message types. The more recent VN4 is not fully bac

23、kward compatible but closely follows the CCITT recommendations. As with VN3, some translation has taken place. It has only single octet message types, five single octet information elements, and two Codeset 6 elements.1TR6 (Germany)This variant is prevalent in Germany. It is a sub-set of the CCITT v

24、ersion, with minor amendments. The protocol is part English and part German.ISDN 30 DASS-2 (England)This variant is used by British Telecom in addition to ETSI (see above). At layers 2 and 3 this standard does not conform to CCITT structure. Frames are headed by one octet and optionally followed by

25、information. However most of the information is IA5 coded, and therefore ASCII decoded.AustraliaIn 1989 Australian ISDN was introduced. This used Telecom Australia specified protocols TPH 1856 for PRI and TPH 1962 for BAI. These were adopted by the Regulator Austel as Australian Technical Standards

26、in 1990 - TS 014 and TS013 respectively. These protocols were developed from CCITT Red Book ISDN recommendations.In 1996, a new ISDN was established using EuroISDN protocols. The Regulator (Austel) issued new Standards, these being TS031 for BAI and TS 038 for PRI. These were replaced by new industr

27、y Standards in 2001, these being AS/ACIF S.031 and AS/ACIF S.038 for BAI and PRI respectively.There are currently no Australian ISDN BAI (TS 013) services in operation, while there are a small and declining number of Australian ISDN PRI (TS 014) in service.All Australian carrier networks are EuroISD

28、N capable, but there may be some differences in Supplementary Services offered. Some smaller carrier networks are also Australian ISDN (TS 014) capable.The major carrier only provides EuroISDN based services.NTT-JapanThe Japanese ISDN service provided by NTT is known as INS-Net and its main features

29、 are as follows: Provides a user-network interface that conforms to the CCITT Recommendation Blue Book. Provides both basic and primary rate interfaces. Provides a packet-mode using Case B. Supported by Signalling System No. 7 ISDN User Part with the network. Offered as a public network service.ARIN

30、C 746In passenger airplanes today there are phones in front of each passenger. These telephones are connected in a T1 network and the conversation is transferred via a satellite. The signalling protocol used is based on Q.931, but with a few modifications and is known as ARINC 746. The leading compa

31、nies in this area are GTE and AT&T. In order to analyze ARINC, the LAPD variant should also be specified as ARINC.ARINC 746 Attachment 11ARINC (Aeronautical Radio, INC.) Attachment 11 describes the Network Layer (layer 3) message transfer necessary for equipment control and circuit switched call con

32、trol procedures between the Cabin Telecommunications Unit (CTU) and SATCOM system, North American Telephone System (NATS), and Terrestrial Flight Telephone System (TFTS). The interface described in this attachment is derived from the CCITT recommendations Q.930, Q.931 and Q.932 for call control and the ISO/OSI standards DIS 9595 and DIS 95