Untitled

such as 1B+D and 0B+D for applications where only a single B channel or a single
D channel is required.
7.6.9 Always On/Dynamic ISDN
An interesting variation on the ISDN theme is Always On/Dynamic ISDN (AODI),
which enables the user to establish a LAN - like always - on ISDN BRI connection to
an ISP server, corporate intranet server, or corporate video server, for example,
through special equipment and using only the D channel. The D channel maintains
the always - on logical link between the client and the server systems, enabling the
transfer of data (e.g., e - mail, stock quotes, or news bulletins) at rates of up to
9.6 kbps over an X.25 switched virtual circuit and without the call set up time
required for a circuit - switched connection. As illustrated in Figure 7.26 , a portion
of the D channel on the client side operates as a link to a X.25 packet network, with
the Multilink Point - to - Point Protocol (MPPP) used for access to the network from
the TA. The D - channel AODI service employs the X.25 packet protocol that is
used for signaling and control purposes in the SS7 network and establishes an SVC
over the packet network. On the client side, the TCP/IP suite is encapsulated within
the X.25 logical channel carried by the D channel, in support of connectionless data
transfer. Packet handlers route the data packets around the circuit - switched network
and over the X.25 packet - switched network, thereby maximizing effi ciency and
avoiding the unnecessary use of the circuit - switched PSTN for such an inappropriate
application. On the server side, the SVC can be terminated on a BRI D channel, a
designated PRI B channel, or a high - speed serial link. Should the need arise, the
equipment automatically activates one or both B channels for transfer of large sets
of data or to establish a videoconference. Once the need for the B channel(s) has
ceased, the equipment automatically terminates those channel connections, the cost
of which typically is usage sensitive. The cost of D - channel usage in an AODI application
generally is based on a fl at rate per month, with surcharges for kilopackets
or megapackets applying above a defi ned usage threshold [28, 29] .
REFERENCES
ISDN
CO
2B + D X.25 23B + D
Figure 7.26
ISDN
Always - on/dynamic ISDN.
1. Standage , Tom. The Victorian Internet . Berkley Books , 1998 .
2. Casson , Herbert N. , The History of the Telephone . A. C. McClurg & Co ., 1910 .
3. Engineering and Operations in the Bell System . Bell Telephone Laboratories, 1977 .
4. Brooks , John. Telephone: The First Hundred Years . Harper & Row , 1976 .
ISDN
CO
REFERENCES 351