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Last update: 03.12.95 kexx
The user has access to the ISDN by means of a local interface to a digital pipe of a certain bit rate. Pipes of various sizes will be available to satisfy differing needs. At any given point in time, the pipe to user's premises has a fixed capacity, but the traffic on the pipe may be a variable mix up to the capacity limit. Thus a user may access circuit-switched and packet-switched services, as well as other services, in a dynamic mix of signal types and bit rates. To provide these services, the ISDN will require rather complex control signals to instruct it how to sort out the time-multiplexed data and provide the required services. These control signals will also be multiplexed onto the same digital pipe.
The integration of voice and a variety of data on a single transport system means that the user does not have to buy multiple services to meet multiple needs. The requirements of various users can differ greatly in a number of ways; information volume, traffic pattern, response time and interface types.
The ISDN provides a variety of services, supporting existing voice and data applications as well as facsimile, teletex and videotex.
The digital pipe between the central office and the ISDN user will be used to carry a number of communication channels. The capacity of the pipe may vary from user to user. The transmission structure of any access link is constructed from the following types of channels:
The B-channel is the basic user channel. It can be used to carry digital data, PCM-encoded digital voice, or mixture of lower-rate traffic, including digital data and digitized voice encoded at a fraction of 64 kbps. In the case of mixed traffic, all traffic must be destined for the same endpoint.
The D-channel serves two purposes, First, it carries signaling information to control circuit-switched calls on associated B-channels at the user interface. In addition, the D-channel may be used for packet-switching or low-speed telemetry at times when no signaling information is waiting.
H-Channels are provide for user information at higher bit rates. The user may use such a channel as high-speed trunk or subdivide the channel according to the user's own TDM scheme. Examples of applications include fast facsimile, video, high-speed data, high-quality audio and multiple information streams at lower data rates.
The basic channel structure consists of two full-duplex 64 kbps B-channels and a full-duplex 16 kbps D-channel. Each frame of 48 bits includes 16 bits from each of the B-channels and 4 bits from the D-channel.
CPI -
Customer Premises Installation - the phone network within the home or business.
CSD - Circuit Switched Data. Phone calls, with data
only.
CSV - Circuit Switched Voice. This is traditional
phone service.
DOSBS - Data Over Speech Bearer Service. A
configuration where the phone company expects you to be using the channel
for voice, but you can send data. Also called DOVBS, V for voice.
DS1 service -
A standard service in the U.S. phone network, running at approximately
1.544 Mbps, capable of handling 24 standard audio phone calls.
E1 service -
A standard service in the European phone network, running at approximately
2.048 Mbps, capable of handling 31 standard audio phone calls.
IDN - Integrated Digital Network, the phone company's
digital network. It provides digital service within and between all phone
companies, but not to the cutomer's door.
POTS - Plain Old Telephone System. Pre-ISDN equipment.
PPP - Point to Point Protocol. An IETF standard for
transmitting IP traffic across dialup connections. PPP has been accepted
as the standard for IP over ISDN.
PSD - Packet Switched Data. For using ISDN with X.25, or
things like that. This won't be very common.
National ISDN - A set of interoperability standards
being developed in the U.S. so that all the different vendors and phone
companies can make equipment that works together.
NI-1(or NI-2, NI-3) - See National ISDN
N-ISDN - More commonly stands for
National ISDN, less commonly for Narrowband ISDN.
SDH -
is the European equivalent of SONET. They are not the same,
but they are close enough that the hardware manufacturers can make equipment
that conforms to both standards at the same time.
Signaling -
the exchange of information for call setup and control.
Sends information on the phone network regarding phone numbers, busy status,
call interruptions, hangups, etc. The grating tones that a touch tone
phone makes are part of signalling - they are interpreted by the local
phone company as a phone number.
SONET -
is a standard that defines how data flows across fiber obtics.
The phone companies uses SONET internally to move lots of data around. They
also use SONET to provide service to customers. If you get a SONET link
from the phone company, it looks like they've given you your very own dark
fiber. In fact, you may be sharing a physical fiber with other people, but
the way SONET is defined, you'll never know.
SPID - Service Profile ID. These identify to the
phone company what types services and features are supported for a
given device. SPIDs are optional in the ISDN standard, but
usually required in North America.
Switching - the process of connecting a phone
call by connecting lines, or logical channels to each other to form a
complete end-to-end connection.
TEI - Terminal Equipment ID. These uniquely identify
each each TE device. This ID is dynamically allocated when the TE is attached
to the network.