Information Technology Reference
In-Depth Information
As all cells are identical in size, a multiplexer can assemble cells from many transactions in an arbitrary order. The
exact order is determined by the quality of service required, where the time positioning of isochronous data would
be determined first, with asynchronous data filling the gaps.
Figure 7.30 shows how a broadband system might be implemented. The transport network would typically be
optical fibre based, using SONET (Synchronous optical network) or SDH (synchronous digital hierarchy). These
standards differ in minor respects. Figure 7.31 shows the bit rates available in each. Lower bit rates will be used in
the access networks which will use different technology such as xDSL.
Figure 7.31: Bit rates available in SONET and SDH.
SONET and SDH assemble ATM cells into a structure known as a container in the interests of efficiency.
Containers are passed intact between exchanges in the transport network. The cells in a container need not belong
to the same transaction, they simply need to be going the same way for at least one transport network leg.
The cell-routing mechanism of ATM is unusual and deserves explanation. In conventional networks, a packet must
carry the complete destination address so that at every exchange it can be routed closer to its destination. The
exact route by which the packet travels cannot be anticipated and successive packets in the same transaction may
take different routes. This is known as a connectionless protocol.
In contrast, ATM is a connection oriented protocol. Before data can be transferred, the network must set up an end-
to-end route. Once this is done, the ATM cells do not need to carry a complete destination address. Instead they
only need to carry enough addressing so that an exchange or switch can distinguish between all of the expected
transactions.
The end-to-end route is known as a virtual channel which consists of a series of virtual links between switches. The
term virtual channel is used because the system acts like a dedicated channel even though physically it is not.
When the transaction is completed the route can be dismantled so that the bandwidth is freed for other users. In
some cases, such as delivery of a TV station's output to a transmitter, or as a replacement for analog cable TV the
route can be set up continuously to form what is known as a permanent virtual channel .
The addressing in the cells ensures that all cells with the same address take the same path, but owing to the
multiplexed nature of ATM, at other times and with other cells a completely different routing scheme may exist.
Thus the routing structure for a particular transaction always passes cells by the same route, but the next cell may
belong to another transaction and will have a different address causing it to routed in another way.
The addressing structure is hierarchical. Figure 7.32 (a) shows the ATM cell and its header. The cell address is
divided into two fields, the virtual channel identifier and the virtual path identifier. Virtual paths are logical groups of
virtual channels which happen to be going the same way. An example would be the output of a video-on-demand
server travelling to the first switch. The virtual path concept is useful because all cells in the same virtual path can
share the same container in a transport network. A virtual path switch shown in Figure 7.32 (b) can operate at the
container level whereas a virtual channel switch (c) would need to dismantle and reassemble containers.
Figure 7.32: The ATM cell (a) carries routing information in the header. ATM paths carrying a group of channels
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