•    ISDN provides variety of services, supporting existing voice and data application.
•    There are two ways to classify these services
•    The I.200 series of ITU-T recommendation referred to as service capabilities.
•    This provides a classification and method of description of the classification and method of description of the telecommunication service supported by ISDN.
•    There are three types of services that are defined by ITU-T, they are

(a)    Bearer Services:
•    Bearer services provide the mean to convey information(speech, data, video  etc.,)between users without alteration of the content of the message.
•    These functions corresponds to the lower three layer of the OSI(1 to 3)

(b)    Tele services:
•    This combine the transportation function with the information processing function.
•    They employ bearer services to transport data and provide a set of higher layer functions 4 to 7 of OSI layer. E.g., Telephony, videotex and message handling.

(c)    Supplementary services:
•    This may used in conjunction with one (or more) of the bearer (or) teleservice.
•    It can be used alone., Eg.,Reverse charging. This can be used to reverse charge on a circuit switched call (or) a packet switched virtual call

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