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2.2 | |
WAN Technologies | |
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ISDN |
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The internal connections, or trunks, of
the PSTN have changed from carrying analog frequency-division
multiplexed signals, to time-division multiplexed (TDM) digital
signals. An obvious next step
is to enable the local loop to carry digital signals that result in
higher capacity switched connections.
Integrated Services Digital Network (ISDN) turns the local loop into a TDM digital connection. The connection uses 64 kbps bearer channels (B) for carrying voice or data and a signaling, delta channel (D) for call set-up and other purposes. Basic Rate Interface (BRI) ISDN is intended for the home and small
enterprise and provides two 64 kbps B channels and a 16 kbps D
channel. For larger installations, Primary Rate Interface (PRI) ISDN
is available. PRI delivers twenty-three 64 kbps B channels and one 64
kbps D
channel in North America, for a total bit rate of up to 1.544 Mbps.
This includes some additional overhead for synchronization. In Europe,
Australia, and other parts of the world, ISDN PRI provides thirty B
channels and one D channel for a total bit rate of up to 2.048 Mbps,
including synchronization overhead.
The BRI D channel is underutilized, as it has only two B channels to control. Some providers allow the D channel to carry data at low bit rates such as X.25 connections at 9.6 kbps. For small WANs, the BRI ISDN can provide an ideal connection
mechanism. BRI has a call setup time that is less than a second, and
its 64 kbps B channel provide greater capacity than an analog modem
link.
Another common application of ISDN is to provide additional capacity as needed on a leased line connection. The leased line is sized to carry average traffic loads while ISDN is added during peak demand periods. ISDN is also used as a backup in the case of a failure of the leased line. ISDN tariffs are based on a per-B channel basis and are similar to those of analog voice connections. With PRI ISDN, multiple B channels can be connected between two end points. This allows for video conferencing and high bandwidth data connections with no latency or jitter. Multiple connections can become very expensive over long distances.
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In North
America PRI corresponds to a T1 connection. The rate of international
PRI corresponds to an E1 connection.
If greater capacity is required, a second
B channel can be
activated to provide a total of 128 kbps. Although inadequate for video, this
would permit several simultaneous voice conversations in addition to
data traffic.
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