19-11-2012, 03:39 PM
ISDN
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INTRODUCTION
ISDN is a type of digital phone/data and internet service that preceded ADSL (asynchronous digital subscriber line) and has for the most part been superceded by it.
Normal telephone lines carry analog signals that must be amplified and converted to digital signals by the phone company. This process introduces not a slight lag time, but also distortion in the signal. Dial-Modems and telephone are examples of equipments that use analog signals.ISDN makes use of digital signals running along existing copper lines to increase the data throughout, reduce line noise and enhance signal quality. In the mid 1990s, ADSL was very expensive not widely available. Companies and individuals wanted a faster way to connect to the internet; but the technology behind dial-up modems had reached its threshold. ISDN became a viable alternative to provide speeds of up to 128 kilobits per second, versus the standard connection of 30-53 kbps with a dial-up modem.
The most common type of ISDN service for internet onnection is the Basic Rate Interface, ISDN BRI. This technology creates two B-channels on the existing copper lines of 64 kbps each, along with a single 16 kbps D-channel for the hone line. This separates data channel from the voice channel, allowing telephone or fax use while online.
ISDN CHANNEL
ISDN is based on a number of fundamental building blocks. First, there are two types of isdn channels or communication paths:
1. B-channel
The bearer (“B”) channel is a 64 kbps channel which can be used for voice, video, data, or multimedia calls. B channels can be aggregated together for even higher bandwidth application.
2. D-channel
The data (“D”) channel can be either a 16 kbps or 64 kbps channel used primarily for communication (or signaling ) between switching equipment in the isdn network and the ISDN equipment at your site .
These isdn channels are delivered to the user in one of the two predefined configurations:
• Basic rate interface (BRI)
• primary Rte interface (PRI)
ISDN SERVICES
There are two types of services associated with ISDN:
• BRI
• PRI
ISDN BRI SERVICES
The ISDN Basic Rate Interface (BRI) service offers two B channels and one D channel (2B+D). BRI Bchannel service operates at 64 kbps and is meant to carry user data; BRI D-channel service operates at 16 kbps and is meant to carry control and signaling information, although it can support user data transmission under certain circumstances. The D channel signaling protocol
comprises Layers 1 through 3 of the OSI reference model. BRI also provides for framing control and other overhead, bringing its total bit rate to 192 kbps.
The BRI physical layer specification is International Telecommunication Union-Telecommunications Standards Section (ITU-T) (formerly the Consultative Committee for International Telegraph and Telephone [CCITT]
DESIGN ISDN INTERNETWORKING
The public telephone network (PSTN) has been transformed into an integrated system digital network (isdn) .implementing of signaling system 7 (s7)in the PSTN backbone has made possible such widespread services as caller-id and dialednumber delivery, 800 directory number lookup ,calling card services ,and digital data service . Using BRI and PRI services, isdn call switching can be extended to customer premises equipment (CPE ) and provide end-to-end digital paths . Previous to isdn available, data connectivity over the public switched telephone network (PSTN) was via plain old telephone services (pots) using analog modems. Connectivity over isdn offers the internetworking designers
increased bandwidth, reduced call. Beyond the TE1 and TE2 devices, the next connection point in the ISDN network is the network termination type 1 (NT1) or network termination type 2 (NT2) devices. These are network-termination devices that connect the four-wire subscriber wiring to the conventional two-wire local loop. In North America, the NT1 is customer premises equipment (CPE) device. In most other parts of the world, the NT1 is part of the network
provided by the carrier. The NT2 is a more complicated device that typically is found in digital private branch exchanges (PBXs) and that performs Layer 2 and 3 protocol functions and concentration services. An NT1/2 device also exists as a single device that combines the functions of an NT1 and an NT2. Te