Information Technology Reference
In-Depth Information
Figure 7.28: FireWire uses twin twisted pairs and a power pair.
Figure 7.29: The strobe signal is the X-OR of the data and the bit clock. The data and strobe signals together form
a self-clocking system.
This signalling technique is subject to skew between the two twisted pairs and this limits cable lengths to about 10
metres between nodes. Thus FireWire is not a long-distance interface technique, instead it is very useful for
interconnecting a large number of devices in close proximity. Using a copper interconnect, FireWire can run at 100,
200 or 400 megabits/s, depending on the specific hardware. It is proposed to create an optical fibre version which
would run at gigabit speeds.
7.9 Broadband networks and ATM
Broadband ISDN (B-ISDN) is the successor to N-ISDN and in addition to providing more bandwidth, offers practical
solutions to the delivery of any conceivable type of data. The flexibility with which ATM operates means that
intermittent or one-off data transactions which only require asynchronous delivery can take place alongside
isochronous MPEG video delivery. This is known as application independence whereby the sophistication of
isochronous delivery does not raise the cost of asynchronous data. In this way, generic data, video, speech and
combinations of the above can co-exist.
Figure 7.30: Structure and terminology of a broadband network. See text.
ATM is multiplexed, but it is not time-division multiplexed. TDM is inefficient because if a transaction does not fill its
allotted bandwidth, the capacity is wasted. ATM does not offer fixed blocks of bandwidth, but allows infinitely
variable bandwidth to each transaction. This is done by converting all host data into small fixed size cells at the
adaptation layer. The greater the bandwidth needed by a transaction, the more cells per second are allocated to
that transaction. This approach is superior to the fixed bandwidth approach, because if the bit rate of a particular
transaction falls, the cells released can be used for other transactions so that the full bandwidth is always available.
 
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