Digital Network of integrated services

The integrated services digital network (ISDN) is defined by the ITU Telecommunication Standardization Sector (ITU-T) of the International Telecommunication Union (ITU), as: «Network that proceeds by evolution of the Integrated Digital Network (RDI) and that facilitates end-to-end digital connections to provide a wide range of services, both voice and other types, and to which users access through a set of standardized interfaces".

It was defined in 1988 in the CCITT Red Book. Before ISDN, the telephone system was seen as a form of voice transport, with some special services available for data. The key feature of ISDN is that it integrates voice and data on the same line and adds features that were not available in the analog telephone system.

It can be said then that the ISDN is a network that proceeds by evolution of the existing telephone network (sometimes called POTS in this context) that, by offering end-to-end digital connections, allows the integration of a multitude of services in a single access, regardless of the nature of the information to be transmitted and the terminal equipment that generates it.

In the study of the ISDN, so-called "reference points" have been defined that serve to delimit each element of the network. These are called R, S, T, U and V, where U corresponds to the pair of copper wires in the telephone loop between the exchange and the user's home, that is, between the exchange and the TR1 network termination.

Interfaces and functions

Basic Access

Basic access, also known by the acronym BRI (basic rate interface), consists of two full-duplex B channels of 64 kbit/s and a 16 kbit/s full-duplex D-channel. Then framing, synchronization, and other additional bits give a basic access point total speed of 160 kbit/s.

Primary Access

Primary rate interface, also known by the acronym PRI (primary rate interface), is intended for users with higher capacity requirements, such as offices and companies with Digital PBX or local area network. Due to differences in the digital transmission hierarchies used in different countries, it is not possible to agree on a single data rate.

The United States, Japan and Canada use a transmission structure based on 1544 Kbit/s, while in Europe the standard speed is 2048 Kbit/s. Typically, the structure for the 1544 kbit/s channel is 23 B-channels plus one 64 kbit/s D-channel, and for 2048 kbit/s speeds, 30 B-channels plus one 64 kbit/s D-channel:

• 30B(64) + D(64) + señalización + framing(64) = 2048 kbit/s (Europe. E1-PRI).
• 23B(64) + D(64) + señalización + framing(8) = 1544 kbit/s (United States, Japan and Canada. T1-PRI).

Services

Carriers

• Circuit mode: are the functions needed to establish, maintain, and close a switched circuit connection on a user channel. This function corresponds to the control of a call in existing circuit switching telecommunications networks.
• Package Mode: are the functions needed to establish a switched circuit connection in a RDSI package switching node.
  • Virtual call carrier service.
  • Permanent virtual circuit carrier service.

Teleservices

• Telephone to 7 kHz
• Facsimile groups 2 and 3 facsimile group 4
• Teletex, videotex, video phone.
• Supplementary
  • Closed user group.
  • User identification call you.
  • Restriction of user identification caller.
  • Connected user ID.
  • Restriction of connected user identification.
  • Call identification on hold.
  • Direct dialing of extensions.
  • Multiple payout numbers.
  • Abbreviated marking.
  • Conference three.
  • Call diversion.
  • Transfer of calls inside the passive bus.
  • Charging information.

Terminal adaptation

Terminal adapters (AT) are used to connect non-ISDN devices to the network and perform the following functions.

• Speed adaptation (AV).
• Signal conversion (CS).
• Conversion X.25 (AV +CS).
• Physical interface conversion.
• Digitization.

User-network interface

In order to define ISDN user access requirements, it is very important to understand the anticipated configuration of user equipment and the required standardized interfaces. The first step is to group roles that may exist on the user's computer.

• Reference points: conceptual points used to separate groups of functions.
• Functional groups: certain finite provisions of physical equipment or combination of equipment.

Terminal equipment is the subscriber equipment using ISDN. Two types are defined. Terminal equipment type 1 (ET1) are devices that support the standardized ISDN interface. For example: digital phones, integrated voice/data terminals, and digital fax machines. Terminal equipment type 2 (ET2) contemplates the existence of non-ISDN equipment. For example, host computers with an X.25 interface. Such equipment requires a Terminal Adapter (AT) to connect to the ISDN interface.

Service support

• Points 1 or 2: (T and S) basic services.
• Point 4: (R) access to other standardized services. (Interfaces X and V).
• Points 3 and 5: access to teleservices
• 3 RDSI terminals
• 5 RDSI terminals

The reference point T (terminal) corresponds to the minimum ISDN network termination of the client equipment. Separates the network provider equipment from the user equipment.

The S (system) reference point corresponds to the ISDN individual terminal interface. It separates the user's terminal equipment from the network related communication functions.

The R (rate) reference point provides a non-ISDN interface between the non-ISDN compatible user equipment and the adapter equipment.

Protocol architecture

From the point of view of the OSI standard, an ISDN stack consists of three protocols:

• Physical hood.
• Data Link Cap (data layer link, 'DLL'.
• Network shelf (network layer)the RDSI protocol itself.

From the user interface point of view, protocols for user-network interaction and protocols for user-user interaction are included on the network layer.

In the context of the OSI model, the protocols that are defined or referenced in ISDN. Since ISDN is essentially indifferent to user layers 4-7, access is concerned only with layers 1-3. Layer 1, defined in I.430 and I.431, specifies the physical interface both for basic access as primary.

The differences with the OSI model are:

• Multiple interrelated protocols.
• Multimedia calls.
• Multipoint connections.

For the D channel, a new data link layer standardization has been defined, LAPD (link access procedure on the D channel), an ISDN link layer protocol that comes from LAP -B (link access procedure, balanced). This standardization is based on HDLC, modified to meet ISDN requirements. All transmission on the D channel is in the form of LAPD frames that are incremented between the subscriber equipment and an ISDN switching element. Three applications are considered: control signaling, packet switching, and telemetry.

Channel B can be used for circuit switching, semi-permanent circuits, and packet switching. For circuit switching, a circuit is built on channel B on demand.

A semi-permanent circuit is a B-channel circuit that has been established by agreement between the connected users and the network. Both the circuit-switched connection and the semi-permanent circuit, the connected stations exchange information as if a direct full duplex link had been established.

In the packet-switched case, a circuit-switched connection is established on a B-channel between the user and the packet-switched node using the D-channel protocol.

ISDN connections

ISDN provides three types of services for end-to-end communications.

1. Switched circuits on channel B: network configuration and circuit switching protocols involve user and network setting and closing of calls, and access to network facilities
2. Permanent connections on channel B: an indefinite period of time after subscription. There is no establishment and release of call on channel D.
3. Package switching provided by RDSI.

Connector RJ-45 EIA/TIA-568B
(TIA-568A connects differently).

pine	Color	Use
1	Orange and white	Earth (power sink) 3-
2	Orange	Power source 3+
3	Green and white	Positive reception
4	Blue	Transmission potential
5	Blue and white	Negative transmission
6	Green	Negative reception
7	Brown and white	Earth (power sink) 2-
8	Brown	Power source 2+

The pinout (pinout) indicated above is the connection used between the NT1 and the ISDN device that is going to communicate. A pair of wires arrives at the TR1 from the telephone exchange, which is where the digital communication signal travels and the power supply to the TR1 itself.

Numbering

An ISDN address can be used to:

• Identify a specific terminal within a RDSI digital line.
• Identify a network service access point in an OSI environment.
• Identify a point of access to the network service in an environment not consistent with the OSI model.

Numbering (services)

• Multiple payout numbers.
• It allows terminals connected to existing networks to reach compatible terminals connected to a basic access in a passive bus type configuration.
• Minimum requirements:
  • A number will be assigned to all terminals belonging to the same service.
  • A different number will be assigned to the terminals of the following services:
    • Telephonic
    • Facsimil
    • Data series V
    • Data in package mode
• The installation of a basic RDSI access user is characterized by the existence of a network transmission equipment (TR or TR1), which makes separation between the transmission to two TR1 threads to the telephone station, the transmission to four threads between TR1 and terminal equipment (ET or TR2).
• Wiring configurations:
  • Point to point (1 ET).
  • Short passive bus (up to 8 ET).
  • Extended passive bus (up to 4 ET).