The principle operation of ISDN?

Question

The principle operation of ISDN?

Answer 1

integrated services digital network, an international communications standard for sending voice, video, and data over digital telephone lines or normal telephone wires. ISDN supports data transfer rates of 64 Kbps (64,000 bits per second).

There are two types of ISDN:



Basic Rate Interface (BRI) -- consists of two 64-Kbps B-channels and one D-channel for transmitting control information.

Primary Rate Interface (PRI) -- consists of 23 B-channels and one D-channel (U.S.) or 30 B-channels and one D-channel (Europe).

The original version of ISDN employs baseband transmission. Another version, called B-ISDN, uses broadband transmission and is able to support transmission rates of 1.5 Mbps. B-ISDN requires fiber optic cables and is not widely available

Answer 2

Abbreviation of integrated services digital network, an international communications standard for sending voice, video, and data over digital telephone lines or normal telephone wires. ISDN supports data transfer rates of 64 Kbps (64,000 bits per second).



There are two types of ISDN:

* Basic Rate Interface (BRI) -- consists of two 64-Kbps B-channels and one D-channel for transmitting control information.

* Primary Rate Interface (PRI) -- consists of 23 B-channels and one D-channel (U.S.) or 30 B-channels and one D-channel (Europe).



The original version of ISDN employs baseband transmission. Another version, called B-ISDN, uses broadband transmission and is able to support transmission rates of 1.5 Mbps. B-ISDN requires fiber optic cables and is not widely available.

Answer 3

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Answer 4

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Answer 8

Integrated Services Digital Network (ISDN) is a type of circuit switched telephone network system, designed to allow digital transmission of voice and data over ordinary telephone copper wires, resulting in better quality and higher speeds than available with analog systems. More broadly, ISDN is a set of protocols for establishing and breaking circuit switched connections, and for advanced call features for the user. The English term is a "backronym", thought better for English-language advertisements than the original, "Integriertes Sprach- und Datennetz" (German for "Integrated Speech and Data Net").



In a videoconference, ISDN provides simultaneous voice, video, and text transmission between individual desktop videoconferencing systems and group (room) videoconferencing systems.



In the phrase "Integrated Services Digital Network",



Integrated Services refers to ISDN's ability to deliver two simultaneous connections, in any combination of data, voice, video, and fax, over a single line. Multiple devices can be attached to the line, and used as needed. That means an ISDN line can take care of most people's complete communications needs, without forcing the purchase of multiple analog phone lines at a much higher transmission rate.

Digital refers to its purely digital transmission, as opposed to the analog transmission of plain old telephone service. If you're using a modem for Internet access at this moment, your Internet service provider's modem has converted this site's digital content to analog signals before sending it to you, and your modem converts those signals back to digital when receiving (the same thing happens with every keystroke and mouse click you transmit). When you connect with ISDN, there is no analog conversion. ISDN transmits data digitally, resulting in a very clear transmission quality. There is none of the static and noise of analog transmissions that can slow transmission speed.

Network refers to the fact that ISDN is not simply a point-to-point solution like a leased line. ISDN networks extend from the local telephone exchange to the remote user and include all of the telecommunications and switching equipment in between



Configurations

In ISDN, there are two types of channels, B (for "Bearer") and D (for "Delta"). B channels are used for data (which may include voice), and D channels are intended for signalling and control (but can also be used for data).



There are two kinds of access to ISDN. Basic rate interface (BRI) — also Basic rate access (BRA) — consists of two B channels, each with bandwidth of 64 kbit/s, and one D channel with a bandwidth of 16 kbit/s. Together these three channels can be designated as 2B+D. Primary rate interface (PRI) — also Primary rate access (PRA) — contains a greater number of B channels and a D channel with a bandwidth of 64 kbit/s. The number of B channels for PRI varies according to the nation: in North America and Japan it is 23B+1D, with an aggregate bit rate of 1.544 Mbit/s (T1); in Europe and Australia it is 30B+1D, with an aggregate bit rate of 2.048 Mbit/s (E1).



Also ISDN Can be used in a configuration in which the B channels of an ISDN basic rate interface are bundled to provide a total duplex bandwidth of 128 kbit/s. This precludes use of the line for voice calls while the internet connection is in use.



Using bipolar with eight-zero substitution encoding technique, call data is transmitted over the data (B) channels, with the signalling (D) channels used for call setup and management. Once a call is set up, there is a simple 64 kbit/s synchronous bidirectional data channel between the end parties, lasting until the call is terminated. There can be as many calls as there are data channels, to the same or different end-points. Bearer channels may also be multiplexed into what may be considered single, higher-bandwidth channels via a process called B channel bonding.



The D channel can also be used for sending and receiving X.25 data packets, and connection to X.25 packet network, this is specified in X.31. In practice, X.31 was only commercially implemented in France and Japan.



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Reference points

A set of reference points are defined in the ISDN standard to refer to certain points between the telco and the end user ISDN equipment.



R - defines the point between a non-ISDN device and a terminal adapter (TA) which provides translation to and from such a device

S - defines the point between the ISDN equipment (or TA) and a Network Termination Type 2 (NT-2) device

T - defines the point between the NT-2 and NT-1 devices1

U - defines the point between the NT-1 and the telco switch2

1 Most NT-1 devices can perform the functions of the NT-2 as well, and so the S and T reference points are generally collapsed into the S/T reference point.

2 Inside North America, the NT-1 device is considered customer premises equipment and must be maintained by the customer, thus, the U interface is provided to the customer. In other locations, the NT-1 device is maintained by the telco, and the S/T interface is provided to the customer.





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Types of communications handled

Among the kinds of data that can be moved over the 64 kbit/s channels are pulse-code modulated voice calls, providing access to the traditional voice PSTN. This information can be passed between the network and the user end-point at call set-up time. In North America, ISDN is nowadays mostly used as an alternative to analog connection, most commonly for Internet access. Some of the services envisaged as being delivered over ISDN are now delivered over the Internet instead. In Europe, and in Germany in particular, ISDN has been successfully marketed as a phone with features, as opposed to a POTS phone (Plain Old Telephone Service) with few or no features. However meanwhile features that were first available with ISDN (such as Three-Way Call, Call Forwarding, Caller ID, etc.) are now commonly available for ordinary analog phones as well, eliminating this advantage of ISDN. Another advantage of ISDN was the possibility of multiple simultaneous calls (one call per B channel), e.g. for big families, but with the increased popularity and reduced prices of mobile telephony this has become less interesting as well, making ISDN rather unappealing to the private customer, although carriers don't all too often deliver the advanced call features over POTS. However, ISDN is typically more reliable than POTS, and has a significantly faster call setup time compared with POTS, and IP connections over ISDN typically have some 30-35ms round trip time, as opposed to 120-180ms (both measured with otherwise unused lines) over 56k or V.34 modems, making ISDN more pleasant for telecommuters.



Where an analog connection requires a modem, an ISDN connection requires a terminal adapter (TA). The function of an ISDN terminal adapter is often delivered in the form of a PC card with an S/T interface, and single-chip solutions seem to exist, looking at the plethora of combined ISDN- and ADSL-routers.



[edit]

A sample ISDN call

The following is an example of a Primary Rate (PRI) ISDN call showing the Q.921/LAPD and the Q.931/Network message intermixed (i.e. exactly what was exchanged on the D-channel). The call is originating from the switch where the trace was taken and goes out to some other switch, possibly an end-office LEC, who terminates the call.



The first line format is . If the message is an ISDN level message, then a decoding of the message is attempted showing the various Information Elements that make up the message. All ISDN messages are tagged with an ID number relative to the switch that started the call (local/remote). Following this optional decoding is a dump of the bytes of the message in ... ... format.



The RR messages at the beginning prior to the call are the keep alive messages. Then you will see a SETUP message that starts the call. Each message is acknowledged by the other side with a RR.



10:49:47.33 21/1/24 R RR

0000 02 01 01 a5 ....



10:49:47.34 21/1/24 T RR

0000 02 01 01 b9 ....



10:50:17.57 21/1/24 R RR

0000 02 01 01 a5 ....



10:50:17.58 21/1/24 T RR

0000 02 01 01 b9 ....



10:50:24.37 21/1/24 T SETUP

Call Reference : 000062-local

Bearer Capability : CCITT, Speech, Circuit mode, 64 kbit/s

Channel ID : Implicit Interface ID implies current span, 21/1/5, Exclusive

Calling Party Number : 8018023000 National number User-provided, not screened Presentation allowed

Called Party Number : 3739120 Type: SUBSCRB

0000 00 01 a4 b8 08 02 00 3e 05 04 03 80 90 a2 18 03 .......>........

0010 a9 83 85 6c 0c 21 80 38 30 31 38 30 32 33 30 30 ...l.!.801802300

0020 30 70 08 c1 33 37 33 39 31 32 30 0p..3739120



10:50:24.37 21/1/24 R RR

0000 00 01 01 a6 ....



10:50:24.77 21/1/24 R CALL PROCEEDING

Call Reference : 000062-local

Channel ID : Implicit Interface ID implies current span, 21/1/5, Exclusive

0000 02 01 b8 a6 08 02 80 3e 02 18 03 a9 83 85 .......>......



10:50:24.77 21/1/24 T RR

0000 02 01 01 ba ....



10:50:25.02 21/1/24 R ALERTING

Call Reference : 000062-local

Progress Indicator : CCITT, Public network serving local user,

In-band information or an appropriate pattern is now available

0000 02 01 ba a6 08 02 80 3e 01 1e 02 82 88 .......>.....



10:50:25.02 21/1/24 T RR

0000 02 01 01 bc ....



10:50:28.43 21/1/24 R CONNECT

Call Reference : 000062-local

0000 02 01 bc a6 08 02 80 3e 07 .......>.



10:50:28.43 21/1/24 T RR

0000 02 01 01 be ....



10:50:28.43 21/1/24 T CONNECT_ACK

Call Reference : 000062-local

0000 00 01 a6 be 08 02 00 3e 0f .......>.



10:50:28.44 21/1/24 R RR

0000 00 01 01 a8 ....



10:50:35.69 21/1/24 T DISCONNECT

Call Reference : 000062-local

Cause : 16, Normal call clearing.

0000 00 01 a8 be 08 02 00 3e 45 08 02 8a 90 .......>E....



10:50:35.70 21/1/24 R RR

0000 00 01 01 aa ....



10:50:36.98 21/1/24 R RELEASE

Call Reference : 000062-local

0000 02 01 be aa 08 02 80 3e 4d .......>M



10:50:36.98 21/1/24 T RR

0000 02 01 01 c0 ....



10:50:36.99 21/1/24 T RELEASE COMPLETE

Call Reference : 000062-local

0000 00 01 aa c0 08 02 00 3e 5a .......>Z



10:50:36.00 21/1/24 R RR

0000 00 01 01 ac ....



10:51:06.10 21/1/24 R RR

0000 02 01 01 ad ....



10:51:06.10 21/1/24 T RR

0000 02 01 01 c1 ....



10:51:36.37 21/1/24 R RR

0000 02 01 01 ad ....



10:51:36.37 21/1/24 T RR

0000 02 01 01 c1 ....



Basic rate interface (BRI, 2B+D, 2B1D) is an Integrated Services Digital Network configuration defined in the physical layer standard I.430 produced by the ITU. This configuration consists of two 64 kbit/s "bearer" channels (B channels) and one 16 kbit/s "data" channel (D channel). The B channels are used for voice or user data, and the D channel is used for any combination of: data, control/signalling and X.25 packet networking. The two B channels can be bonded together giving a total data rate of 128 kbit/s. BRI is the kind of ISDN interface most likely to be found in residential service.



The I.430 protocol defines 48-bit packets comprising 16 bits from the B1 channel, 16 bits from B2 channel, 4 bits from the D channel, and 12 bits used for synchronization purposes. These packets are sent at a rate of 4 kHz, giving the data rates listed above for a maximum possible throughput of 144 kbit/s.



ISDN (Integrated Services Digital Network) is an all digital communications line that allows for the transmission of voice, data, 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 ISDN 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.



LAPD



The 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 frames at any time. The protocol treats the DTE and DCE as equals.



The format of a standard LAPD frame is as follows:



Flag

Address field

Control field

Information

FCS

Flag



LAPD frame structure



Flag

The value of the flag is always (0x7E). In order to ensure that the bit pattern of the frame delimiter flag does 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 field

The first two bytes of the frame after the header flag is known as the address field. The format of the address field is as follows:



8 7 6 5 4 3 2 1

SAPI C/R EA1

TEI EA2

LAPD address field

EA1 First Address Extension bit which is always set to 0.

C/R Command/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.

EA2 Second Address Extension bit which is always set to 1.

TEI Terminal Endpoint Identifier. Valid values are as follows:

0-63 Used by non-automatic TEI assignment user equipment.

64-126 Used by automatic TEI assignment equipment.

127 Used for a broadcast connection meant for all Terminal Endpoints.



Control field

The 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.



FCS

The Frame Check Sequence (FCS) enables a high level of physical error control by 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 size

LAPD 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. 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 types

The following are the Supervisory Frame Types in LAPD:



RR Information frame acknowledgement and indication to receive more.

REJ Request for retransmission of all frames after a given sequence number.

RNR Indicates a state of temporary occupation of station (e.g., window full).



The following are the Unnumbered Frame Types in LAPD:



DISC Request disconnection

UA Acknowledgement frame.

DM Response to DISC indicating disconnected mode.

FRMR Frame reject.

SABM Initiator for asynchronous balanced mode. No master/slave relationship.

SABME SABM in extended mode.

UI Unnumbered Information.

XID Exchange Information.



There is one Information Frame Type in LAPD:



International Variants of ISDN



The organization primarily responsible for producing the ISDN standards is the CCITT. 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 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 use 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 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 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 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 management 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 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 backward 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 version, 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 information. However most of the information is IA5 coded, and therefore ASCII decoded.



Australia

In 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 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 industry 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 EuroISDN 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-Japan

The Japanese ISDN service provided by NTT is known as INS-Net and its main features 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.

ARINC 746

In 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 companies 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 11

ARINC (Aeronautical Radio, INC.) Attachment 11 describes the Network Layer (layer 3) message transfer necessary for equipment control and circuit switched call control 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 9596 for equipment control. These Network Layer messages should be transported in the information field of the Data Link Layer frame.



ARINC 746 Attachment 17

ARINC (Aeronautical Radio, INC.) Attachment 17 represents a system which provides passenger and cabin crew access to services provided by the CTU and intelligent cabin equipment. The distribution portion of the CDS transports the signalling and voice channels from headend units to the individual seat units. Each zone within the aircraft has a zone unit that controls and services seat units within that zone.



Northern Telecom - DMS 100

This variant represents Northern Telecom’s implementation of National ISDN-1. It provides ISDN BRI user-network interfaces between the Northern Telecom ISDN DMS-100 switch and terminals designed for the BRI DSL. It is based on CCITT ISDN-1 and Q Series Recommendations and the ISDN Basic Interface Call Control Switching and Signalling Requirements and supplementary service Technical References published by Bellcore.



DPNSS1

DPNSS1 (Digital Private Network Signalling System No. 1) is a common-channel signalling system used in Great Britain. It extends facilities normally only available between extensions on a single PBX to all extensions on PBXs that are connected together in a private network. It is primarily intended for use between PBXs in private networks via time-slot 16 of a 2048 kbit/s digital transmission system. Similarly it may be used in time-slot 24 of a 1.544 kbit/s digital transmission system. Note that the LAPD variant should also be selected to be DPNSS1.



Swiss Telecom

The ISDN variant operated by the Swiss Telecom PTT is called SwissNet. The DSS1 protocol for SwissNet is fully based on ETS. Amendments to this standard for SwissNet fall into the category of definitions of various options in the standard and of missing requirements. They also address SwissNet-specific conditions, e.g., assuring compatibility between user equipment and SwissNet exchanges of different evolution steps.



QSIG

QSIG is a modern, powerful and intelligent inter-private PABX signalling system. QSIG standards specify a signalling system at the Q reference point which is primarily intended for use on a common channel; e.g. a G.703 interface. However, QSIG will work on any suitable method of connecting the PINX equipment. The QSIG protocol stack is identical in structure to the DSSI protocol stack. Both follow the ISO reference model. Both can have an identical layer 1 and layer 2 (LAPD), however, at layer 3 QSIG and DSS1 differ.



ISDN Frame Structure



Shown below is the general structure of the ISDN frame.



8

7

6

5

4

3

2

1



Protocol discriminator



0

0

0

0

Length of reference call value



Flag

Call reference value



0

Message type



Other information elements as required



ISDN frame structure



Protocol discriminator

The protocol used to encode the remainder of the Layer.



Length of call reference value

Defines the length of the next field. The Call reference may be one or two octets long depending on the size of the value being encoded.



Flag

Set to zero for messages sent by the party that allocated the call reference value; otherwise set to one.



Call reference value

An arbitrary value that is allocated for the duration of the specific session, which identifies the call between the device maintaining the call and the ISDN switch.



Message type

Defines the primary purpose of the frame. The message type may be one octet or two octets (for network specific messages). When there is more than one octet, the first octet is coded as eight zeros. A complete list of message types is given in ISDN Message Types below.



ISDN Information Elements



There are two types of information elements: single octet and variable length.



Single octet information elements

The single octet information element appears as follows:



8 7 6 5 4 3 2 1

1

Information element identifier

Information element



Single octet information element



Following are the available single octet information elements:



1 000 ---- Reserved

1 001 ---- Shift

1 010 0000 More data

1 010 0001 Sending Complete

1 011 ---- Congestion Level

1 101 ---- Repeat indicator



Variable length information elements

The following is the format of the variable length information element:



8

7

6

5

4

3

2

1



0

Information element identifier



Length of information elements



Information elements (multiple bytes)



Variable length information element





The information element identifier identifies the chosen element and is unique only within the given Codeset. The length of the information element informs the receiver as to the amount of the following octets belonging to each information element. Following are possible variable length information elements:



0 0000000 Segmented Message

0 0000100 Bearer Capability

0 0001000 Cause

0 0010100 Call identify

0 0010100 Call state

0 0011000 Channel identification

0 0011100 Facility

0 0011110 Progress indicator

0 0100000 Network-specific facilities

0 0100111 Notification indicator

0 0101000 Display

0 0101001 Date/time

0 0101100 Keypad facility

0 0110100 Signal

0 0110110 Switchhook

0 0111000 Feature activation

0 0111001 Feature indication

0 1000000 Information rate

0 1000010 End-to-end transit delay

0 1000011 Transit delay selection and indication

0 1000100 Packet layer binary parameters

0 1000101 Packet layer window size

0 1000110 Packet size

0 1101100 Calling party number

0 1101101 Calling party subaddress

0 1110000 Called party number

0 1110001 Called Party subaddress

0 1110100 Redirecting number

0 1111000 Transit network selection

0 1111001 Restart indicator

0 1111100 Low layer compatibility

0 1111101 High layer compatibility

0 1111110 User-user

0 1111111 Escape for ex

Other values Reserved



ISDN Message Types



Following are possible ISDN message types:



Call Establishment



000 00001 Alerting

000 00010 Call Proceeding

000 00011 Progress

000 00101 Setup

000 00111 Connect

000 01101 Setup Acknowledge

000 01111 Connect Acknowledge



Call Information Phase



001 00000 User Information

001 00001 Suspend Reject

001 00010 Resume Reject

001 00100 Hold

001 00101 Suspend

001 00110 Resume

001 01000 Hold Acknowledge

001 01101 Suspend Acknowledge

001 01110 Resume Acknowledge

001 10000 Hold Reject

001 10001 Retrieve

001 10011 Retrieve Acknowledge

001 10111 Retrieve Reject



Call Clearing



010 00101 Disconnect

010 00110 Restart

010 01101 Release

010 01110 Restart Acknowledge

010 11010 Release Complete



Miscellaneous



011 00000 Segment

011 00010 Facility

011 00100 Register

011 01110 Notify

011 10101 Status inquiry

011 11001 Congestion Control

011 11011 Information

011 11101 Status



Interested in more details about testing this protocol?





ISDN Terminology



BRI

The Basic Rate Interface is one of the two services provided by ISDN. BRI is comprised of two B-channels and one D-channel (2B+D). The B-channels each operate at 64 Kbps and the D-channel operates at 16 Kbps. It is used by single line business customers for typical desk-top type applications.



C/R

C/R refers to Command or Response. The C/R bit in the address field defines the frame as either a command frame or a response frame to the previous command.



Codeset

Three main Codesets are defined. In each Codeset, a section of the information elements are defined by the associated variant of the protocol:



Codeset 0 The default code, referring to the CCITT set of information elements.

Codeset 5 The national specific Codeset.

Codeset 6 The network specific Codeset.



The same value may have different meanings in various Codesets. Most elements usually appear only once in each frame.



In order to change Codesets two methods are defined:



Shift This method enables a temporary change to another Codeset. Also termed as non-locking shift, the shift only applies to the next information element.

Shift Lock This method implements a permanent change until indicated otherwise. Shift-Lock may only change to a higher Codeset.



CPE

Customer Premises Equipment - refers to all ISDN compatible equipment connected at the user sight. Examples of devices are telephone, PC, Telex, Facsimile, etc. The exception is the FCC definition of NT1. The FCC views the NT1 as a CPE because it is on the customer sight, but the CCITT views NT1 as part of the network. Consequently the network reference point of the network boundary is dependent on the variant in use.



ISDN Channels B, D and H

The three logical digital communication channels of ISDN perform the following functions:



B-Channel Carries user service information including: digital data, video, and voice.

D-Channel Carries signals and data packets between the user and the network

H-Channel Performs the same function as B-Channels, but operates at rates exceeding DS-0 (64 Kbps).



ISDN Devices

Devices connecting a CPE and a network. In addition to facsimile, telex, PC, telephone, ISDN devices may include the following:



TA Terminal Adapters - devices that are used to portray non-ISDN equipment as ISDN compatible.

LE Local Exchange - ISDN central office (CO). The LE implements the ISDN protocol and is part of the network.

LT Local Termination - used to express the LE responsible for the functions associated with the end of the Local Loop.

ET Exchange Termination - used to express the LE responsible for the switching functions.

NT Network Termination equipment exists in two forms and is referred to accordingly. The two forms are each responsible for different operations and functions.

NT1 - Is the termination of the connection between the user sight and the LE. NT1 is responsible for performance, monitoring, power transfer, and multiplexing of the channels.

NT2 - May be any device that is responsible for providing user sight switching, multiplexing, and concentration: LANs, mainframe computers, terminal controllers, etc. In ISDN residential environments there is no NT2



TE

Terminal Equipment - any user device e.g.: telephone or facsimile. There are two forms of terminal equipment:

TE1 - Equipment is ISDN compatible.

TE2 - Equipment is not ISDN compatible





ISDN Reference Points

Reference points define the communication points between different devices and suggest that different protocols may be used at each side of the point. The main points are as follows:



R A communication reference point between a non-ISDN compatible TE and a TA.

S A communication reference link between the TE or TA and the NT equipment.

T A communication reference point between user switching equipment and a Local Loop Terminator.

U A communication reference point between the NT equipment and the LE. This reference point may be referred to as the network boundary when the FCC definition of the Network terminal is used.



The following diagram illustrates the ISDN Functional Devices and Reference Points:







LAPD

The Link Access Protocol on the D-channel. LAPD is a bit orientated protocol on the data link layer of the OSI reference model. Its prime function is ensuring the error free transmission of bits on the physical layer (layer 1).



PRI

The Primary Rate Interface is one of the two services provided by ISDN. PRI is standard dependent and thus varies according to country. In North America, PRI has twenty-three B-channels and one D-channel (23B+D). In Europe, PRI has thirty B-channels and one D-channel (30B+D).



The American B- and D-channels operate at an equal rate of 64 Kbps. Consequently, the D-channel is sometimes not activated on certain interfaces, thus allowing the time slot to be used as another B-channel. The 23B+D PRI operates at the CCITT designated rate of 1544 Kbps.



The European PRI is comprised of thirty B-channels and one D-channel (30B+D). As in the American PRI all the channels operate at 64 Kbps. However, the 30B+D PRI operates at the CCITT designated rate of 2048 Kbps.



SAPI

Service Access Point Identifier -the first part of the address of each frame.



TEI

Terminal End Point Identifier - the second part of the address of each frame.