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With ISDN, voice and data are carried by bearer channels (B channels) occupying a bandwidth of 64 Kbps (bits per second). Some switches limit B channels to a capacity of 56 Kbps. A data channel (D channel) handles signaling at 16 Kbps or 64 Kbps, depending on the service type. 

There are two basic types of ISDN service: Basic Rate Interface (BRI) and Primary Rate Interface (PRI).  BRI consists of two 64 kbps B channels and one 16 kbps D channel for a total of 144 kbps. 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 30 B channels plus one 64 kbps D channel for a total of 1984 kbps. In USA , PRI consists of 23 B channels plus one 64 kbps D channel for a total of  1536 kbps. It is also possible to support multiple PRI lines with one 64kbps D channel using Non-Facility Associated Signaling (NFAS). 

H channels provide a way to aggregate B channels. They are implemented as: 

     H0=384 kbps (6 B channels) 
     H10=1472 kbps (23 B channels) 
     H11=1536 kbps (24 B channels) 
     H12=1920 kbps (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. 
 

Speed

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 kbps.
Commonly available modems have a maximum speed of 56 kbps, but are limited by the quality of the analog connection and routinely go about 45 kbps. Some phone lines do not support 56K connections at all, and there were currently 2 competing, incompatible 56K standards (X2 from US Robotics (recently bought by 3Com), and K56flex from Rockwell/Lucent). This standards problem was resolved when the ITU released the V.90 standard for 56K modem communications. 
 

Multiple Devices

Previously, it was necessary to have a 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. 
 

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, and call setup time is very fast. For example, a V.34 modem typically takes 30-60 seconds to establish a connection; an ISDN call 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. 
 
 
 
 

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