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There are two basic types of ISDN service: Basic Rate Interface (BRI) and Primary Rate Interface (PRI). BRI consists of two 64 kb/s B channels and one 16 kb/s D channel for a total of 144 kb/s. This basic service is intended to meet the needs of most individual users.

PRI is intended for users with greater capacity requirements. Typically the channel structure is 23 B channels plus one 64 kb/s D channel for a total of 1536 kb/s. In Europe, PRI consists of 30 B channels plus one 64 kb/s D channel for a total of 1984 kb/s. It is also possible to support multiple PRI lines with one 64 kb/s D channel using Non-Facility Associated Signaling (NFAS).

• H0=384 kb/s (6 B channels)
• H10=1472 kb/s (23 B channels)
• H11=1536 kb/s (24 B channels)
• H12=1920 kb/s (30 B channels) - International (E1) only

To access BRI service, it is necessary to subscribe to an ISDN phone line. Customer must be within 18000 feet (about 3.4 miles or 5.5 km) of the telephone company central office for BRI service; beyond that, expensive repeater devices are required, or ISDN service may not be available at all. Customers will also need special equipment to communicate with the phone company switch and with other ISDN devices. These devices include ISDN Terminal Adapters (sometimes called, incorrectly, "ISDN Modems") and ISDN Routers.

In the early 1990s, an industry-wide effort began to establish a specific implementation for ISDN in the U.S. Members of the industry agreed to create the National ISDN 1 (NI-1) standard so that end users would not have to know the brand of switch they are connected to in order to buy equipment and software compatible with it. However, there were problems agreeing on this standard. In fact, many western states would not implement NI-1. Both Southwestern Bell and U.S. West (now Qwest) said that they did not plan to deploy NI-1 software in their central office switches due to incompatibilities with their existing ISDN networks.

Ultimately, all the Regional Bell Operating Companies (RBOCs) did support NI-1. A more comprehensive standardization initiative, National ISDN 2 (NI-2), was later adopted. Some manufacturers of ISDN communications equipment, such as Motorola and U S Robotics (now owned by 3Com), worked with the RBOCs to develop configuration standards for their equipment. These kinds of actions, along with more competitive pricing, inexpensive ISDN connection equipment, and the desire for people to have relatively low-cost high-bandwidth Internet access have made ISDN more popular in recent years.

Most recently, ISDN service has largely been displaced by broadband internet service, such as xDSL and Cable Modem service. These services are faster, less expensive, and easier to set up and maintain than ISDN. Still, ISDN has its place, as backup to dedicated lines, and in locations where broadband service is not yet available.

The modem was a big breakthrough in computer communications. It allowed computers to communicate by converting their digital information into an analog signal to travel through the public phone network. There is an upper limit to the amount of information that an analog telephone line can hold. Currently, it is about 56 kb/s bidirectionally. Commonly available modems have a maximum speed of 56 kb/s, but are limited by the quality of the analog connection and routinely go about 45-50 kb/s. Some phone lines do not support 56 kb/s connections at all. There were currently 2 competing, incompatible 56 kb/s standards (X2 from U S Robotics (recently bought by 3Com), and K56flex from Rockwell/Lucent). This standards problem was resolved when the ITU released the V.90, and later V.92, standard for 56 kb/s modem communications.

ISDN allows multiple digital channels to be operated simultaneously through the same regular phone wiring used for analog lines. The change comes about when the telephone company's switches can support digital connections. Therefore, the same physical wiring can be used, but a digital signal, instead of an analog signal, is transmitted across the line. This scheme permits a much higher data transfer rate than analog lines. BRI ISDN, using a channel aggregation protocol such as BONDING or Multilink-PPP, supports an uncompressed data transfer speed of 128 kb/s, plus bandwidth for overhead and signaling. In addition, the latency, or the amount of time it takes for a communication to begin, on an ISDN line is typically about half that of an analog line. This improves response for interactive applications, such as games.

Multiple Devices¶

Previously, it was necessary to have a separate phone line for each device you wished to use simultaneously. For example, one line each was required for a telephone, fax, computer, bridge/router, and live video conference system. Transferring a file to someone while talking on the phone or seeing their live picture on a video screen would require several potentially expensive phone lines.

ISDN allows multiple devices to share a single line. It is possible to combine many different digital data sources and have the information routed to the proper destination. Since the line is digital, it is easier to keep the noise and interference out while combining these signals. ISDN technically refers to a specific set of digital services provided through a single, standard interface. Without ISDN, distinct interfaces are required instead.

Signaling¶

Instead of the phone company sending a ring voltage signal to ring the bell in your phone ("In-Band signal"), it sends a digital packet on a separate channel ("Out-of-Band signal"). The Out-of-Band signal does not disturb established connections, no bandwidth is taken from the data channels, and call setup time is very fast. For example, a V.90 or V.92 modem typically takes 30-60 seconds to establish a connection; an ISDN call setup usually takes less than 2 seconds.

The signaling also indicates who is calling, what type of call it is (data/voice), and what number was dialed. Available ISDN phone equipment is then capable of making intelligent decisions on how to direct the call.

znači mi od telecom-a dobijamo S/T interface, i na njega možemo zakačiti više ISDN terminalne opreme TE1, ili starih uređaja TE2 preko terminalnih adaptera

ono što mi zovemo NT je precizno rečeno NT1 uređaj, uređaj koji U interface prebacuje na S/T ili skraćeno na S interface

Layer 1 - fizički layer¶

The ISDN Physical Layer is specified by the ITU I-series and G-series documents. The U interface provided by the telco for BRI is a 2-wire, 160 kb/s digital connection. Echo cancellation is used to reduce noise, and data encoding schemes (2B1Q in North America, 4B3T in Europe) permit this relatively high data rate over ordinary single-pair local loops.

2B1Q echoding schema - North America¶

2B1Q (2 Binary 1 Quaternary) is the most common signaling method on U interfaces. This protocol is defined in detail in 1988 ANSI spec T1.601.

• Two bits per baud
• 80 kilobaud (baud = 1 modulation per second)
• Transfer rate of 160 kb/s

Bits     Quaternary
Symbol     Voltage
Level
00     -3     -2.5
01     -1     -0.833
10     +3     +2.5
11     +1     +0.833

This means that the input voltage level can be one of 4 distinct levels (note: 0 volts is not a valid voltage under this scheme). These levels are called Quaternaries. Each quaternary represents 2 data bits, since there are 4 possible ways to represent 2 bits, as in the table above.

Frame Format¶

Each U interface frame is 240 bits long. At the prescribed data rate of 160 kb/s, each frame is therefore 1.5 ms long.

• Frame overhead - 16 kb/s
• D channel - 16 kb/s
• 2 B channels at 64 kb/s - 128 kb/s

Sync: 18 bits, 12 * (B1 + B2 + D) = 216 bits, Maintenance: 6 bits

irb(main):001:0> 18+216+6 => 240

• The Sync field consists of 9 Quaternaries (2 bits each) in the pattern +3 +3 -3 -3 -3 +3 -3 +3 -3.
• (B1 + B2 + D) is 18 bits of data consisting of 8 bits from the first B channel, 8 bits from the second B channel, and 2 bits of D channel data.
• The Maintenance field contains CRC information, block error detection flags, and "embedded operator commands" used for loopback testing without disrupting user data.

Data is transmitted in a superframe consisting of 8 240-bit frames for a total of 1920 bits (240 octets). The sync field of the first frame in the superframe is inverted (i.e. -3 -3 +3 +3 +3 -3 +3 -3 +3).

Layer 2 - Data Link Layer¶

The ISDN Data Link Layer is specified by the ITU Q-series documents Q.920 through Q.923. All of the signaling on the D channel is defined in the Q.921 spec.

LAP-D - (ovo je protokol koji se koristi u US u euroisdn koristi DSS1, pogledaj dole)¶

Link Access Protocol - D channel (LAP-D) is the Layer 2 protocol used. This is almost identical to the X.25 LAP-B protocol.

Here is the structure of a LAP-D frame:

LAP-D/a) Flag (1 octet) - This is always 7E16 (0111 11102)

LAP-D/b) Address (2 octets)

1 2 3 4 5 6 - SAPI (6 bits) ; 7 8 - C/R EA0 ; TEI (7 bits) EA1

• C/R (Command/Response) bit indicates if the frame is a command or a response
• EA0 (Address Extension) bit indicates whether this is the final octet of the address or not
• TEI (Terminal Endpoint Identifier) 7-bit device identifier (see below)
• EA1 (Address Extension) bit, same as EA0

LAP-D/c) Control (2 octets) - The frame level control field indicates the frame type (Information, Supervisory, or Unnumbered) and sequence numbers (N(r) and N(s)) as required.

LAP-D/d) Information - Layer 3 protocol information and User data
LAP-D/e) * CRC (2 octets) - Cyclic Redundancy Check is a low-level test for bit errors on the user data.
LAP-D/f) * Flag (1 octet) - This is always 7E16 (0111 11102)

SAPIs¶

The Service Access Point Identifier (SAPI) is a 6-bit field that identifies the point where Layer 2 provides a service to Layer 3.

See the following table:

• SAPI Description
• 0 Call control procedures
• 1 Packet Mode using Q.931 call procedures
• 16 Packet Mode communications procedures
• 32-47 Reserved for national use
• 63 Management Procedures

Others Reserved for Future Use

TEIs¶

Terminal Endpoint Identifiers (TEIs) are unique IDs given to each device (TE) on an ISDN S/T bus. This identifier can be dynamic; the value may be assigned statically when the TE is installed, or dynamically when activated.

• TEI Description
• 0-63 Fixed TEI assignments
• 64-126 Dynamic TEI assignment (assigned by the switch)
• 127 Broadcast to all devices

Establishing the Link Layer¶

The Layer 2 establishment process is very similar to the X.25 LAP-B setup, if you are familiar with it.

1. The TE (Terminal Endpoint) and the Network initially exchange Receive Ready (RR) frames, listening for someone to initiate a connection
2. The TE sends an Unnumbered Information (UI) frame with a SAPI of 63 (management procedure, query network) and TEI of 127 (broadcast)
3. The Network assigns an available TEI (in the range 64-126)
4. The TE sends a Set Asynchronous Balanced Mode (SABME) frame with a SAPI of 0 (call control, used to initiate a SETUP) and a TEI of the value assigned by the network
5. The network responds with an Unnumbered Acknowledgement (UA), SAPI=0, TEI=assigned.

At this point, the connection is ready for a Layer 3 setup.

Layer 3 - Network Layer¶

The ISDN Network Layer is also specified by the ITU Q-series documents Q.930 through Q.939. Layer 3 is used for the establishment, maintenance, and termination of logical network connections between two devices.

SPIDs¶

Service Profile IDs (SPIDs) are used to identify what services and features the telco switch provides to the attached ISDN device. SPIDs are optional; when they are used, they are only accessed at device initialization time, before the call is set up. The format of the SPID is defined in a recommendation document, but it is only rarely followed. It is usually the 10-digit phone number of the ISDN line, plus a prefix and a suffix that are sometimes used to identify features on the line, but in reality it can be whatever the telco decides it should be. If an ISDN line requires a SPID, but it is not correctly supplied, then Layer 2 initialization will take place, but Layer 3 will not, and the device will not be able to place or accept calls. See ITU spec Q.932 for details.

Information Field Structure¶

The Information Field is a variable length field that contains the Q.931 protocol data.

• Protocol Discriminator (1 octet) - identifies the Layer 3 protocol. If this is a Q.931 header, this value is always 0816.
• Length (1 octet) - indicates the length of the next field, the CRV.
• Call Reference Value (CRV) (1 or 2 octets) - used to uniquely identify each call on the user-network interface. This value is assigned at the beginning of a call, and this value becomes available for another call when the call is cleared.
• Message Type (1 octet) - identifies the message type (i.e., SETUP, CONNECT, etc.). This determines what additional information is required and allowed.
• Mandatory and Optional Information Elements (variable length) - are options that are set depending on the Message Type.

Layer 3 Call Setup¶

These are the steps that occurs when an ISDN call is established. In the following example, there are three points where messages are sent and received; 1) the Caller, 2) the ISDN Switch, and 3) the Receiver.
1. Caller sends a SETUP to the Switch.
2. If the SETUP is OK, the switch sends a CALL PROCeeding to the Caller, and then a SETUP to the Receiver.
3. The Receiver gets the SETUP. If it is OK, then it rings the phone and sends an ALERTING message to the Switch.
4. The Switch forwards the ALERTING message to the Caller.
5. When the receiver answers the call, is sends a CONNECT message to the Switch
6. The Switch forwards the CONNECT message to the Caller.
7. The Caller sends a CONNECT ACKnowledge message to the Switch
8. The Switch forwards the CONNECT ACK message to the Receiver.
9. Done. The connection is now up.

Q.931 Disconnect Reasons

Code    Cause
    0        Valid cause code not yet received
    1        Unallocated (unassigned) number
    2        No route to specified transit network (WAN)
    3        No route to destination
    4        send special information tone
    5        misdialled trunk prefix.
    6        Channel unacceptable
    7        Call awarded and being delivered in an established channel
    8        Prefix 0 dialed but not allowed
    9        Prefix 1 dialed but not allowed
    10        Prefix 1 dialed but not required
    11        More digits received than allowed, call is proceeding
    16        Normal call clearing
    17        User busy
    18        No user responding
    19        no answer from user
    21        Call rejected
    22        Number changed
    23        Reverse charging rejected
    24        Call suspended
    25        Call resumed
    26        Non-selected user clearing
    27        Destination out of order
    28        Invalid number format (incomplete number)
    29        Facility rejected
    30        Response to STATUS ENQUIRY
    31        Normal, unspecified
    33        Circuit out of order
    34        No circuit/channel available
    35        Destination unattainable
    37        Degraded service
    38        Network (WAN) out of order
    39        Transit delay range cannot be achieved
    40        Throughput range cannot be achieved
    41        Temporary failure
    42        Switching equipment congestion
    43        Access information discarded
    44        Requested circuit channel not available
    45        Pre-empted
    46        Precedence call blocked
    47        Resource unavailable - unspecified
    49        Quality of service unavailable
    50        Requested facility not subscribed
    51        Reverse charging not allowed
    52        Outgoing calls barred
    53        Outgoing calls barred within CUG
    54        Incoming calls barred
    55        Incoming calls barred within CUG
    56        Call waiting not subscribed
    57        Bearer capability not authorized
    58        Bearer capability not presently available
    63        Service or option not available, unspecified
    65        Bearer service not implemented
    66        Channel type not implemented
    67        Transit network selection not implemented
    68        Message not implemented
    69        Requested facility not implemented
    70        Only restricted digital information bearer capability
            is available
    79        Service or option not implemented, unspecified
    81        Invalid call reference value
    82        Identified channel does not exist
    83        A suspended call exists, but this call identity does not
    84        Call identity in use
    85        No call suspended
    86        Call having the requested call identity has been cleared
    87        Called user not member of CUG
    88        Incompatible destination
    89        Non-existent abbreviated address entry
    90        Destination address missing, and direct call not subscribed
    91        Invalid transit network selection (national use)
    92        Invalid facility parameter
    93        Mandatory information element is missing
    95        Invalid message, unspecified
    96        Mandatory information element is missing
    97        Message type non-existent or not implemented
    98        Message not compatible with call state or message type
            non-existent or not implemented 
    99        information element nonexistant or not implemented
    100        Invalid information element contents 
    101        Message not compatible with call state
    102        Recovery on timer expiry
    103        parameter non-existent or not implemented - passed on
    111        Protocol error unspecified
    127        Internetworking, unspecified

http://www.56k.com/reports/bonding.shtml

još jedan dobar materijal o ISDN-u http://ckp.made-it.com/isdn.html, čini mi se evropljanin je pisao

s0 bus¶

Da razjasnim termin koji sam kod nas često čuo "S0 bus".

The switches are part of the telco network from which the U-interface enters the customer premisis. Most US telcos leave it to that, where the European telcos do supply the NT-1 (Network Termination unit 1) and leave the customer with the S/T interface, also called S-bus or S0-bus.

Znači precizni isdn termin je S/T interface, a kod nas se često koristi S0 bus.

http://en.wikipedia.org/wiki/DSS1

Digital Subscriber Signalling System No. 1 (DSS1), also known as Euro-ISDN or E-DSS1 (European DSS1), is a digital signalling protocol used for the ISDN. The interface is also called NET3 for BRI and NET5 for PRI lines. Can also be called CTR4. It is defined by ITU-T I.411 (ETS 300 102). It supports Bearer Capability, Low Level Compatibility and High Level Compatibility, ANI, DNIS and redirected number signaling in both directions.

DSS1 is a pan-European standard developed by ETSI. In 1989, 26 network operators from 20 European countries decided to develop this standard to replace earlier national protocols (such as FTZ 1 TR 6 or VN3). DSS1 has been one of the keys to the success of the ISDN within European countries (as compared to, for example, the U.S.).

Non-European countries using DSS1 include Australia, Brazil, India, Israel, New Zealand, Pakistan, Peru, Singapore, South Africa, Sri Lanka, Taiwan and the United Arab Emirates.

The DSS1 protocol knows four different codesets for information elements. Codeset 0 regards to Q.931, Codeset 5 to the ETSI standard, Codeset 6 for national applications and Codeset 7 for PBX applications. The most common Codeset is 0/Q.931.
Retrieved from "http://en.wikipedia.org/wiki/Digital_Subscriber_System_No._1"

lista isdn standarda

http://www.zytrax.com/tech/protocols/isdn/isdn_stds.htm

• Q.930 March 1993 Digital Subscriber Signalling System No. 1 (DSS1) - ISDN user-network interface layer 3 - General Aspects
• Q.931 May 1998 Digital Subscriber Signalling System No. 1 (DSS1) - ISDN user-network interface layer 3 - specification for basic call control
• Q.932 June 2000 Digital subscriber Siganlling System No. 1 (DSS1) - Generic Procedures for the control of ISDN Supplementary Services

Q.931 Protocol Overview, lincoln.edu¶

http://www.lincoln.edu/math/rmyrick/ComputerNetworks/InetReference/126.htm

Q.931 is ISDN's connection control protocol, roughly comparable to TCP in the Internet protocol stack. Q.931 doesn't provide flow control or perform retransmission, since the underlying layers are assumed to be reliable and the circuit-oriented nature of ISDN allocates bandwidth in fixed increments of 64 kbps. Q.931 does manage connection setup and breakdown. Like TCP, Q.931 documents both the protocol itself and a protocol state machine.

...

vrlo dobar dokument, ovaj lincoln.edu je baš dobar resurs izgleda za sve protokole