Information Technology Reference
In-Depth Information
can be switched in a virtual path switch (b). Individual channel switching requires a virtual channel switch which is
more complex and causes more delay.
When a route is set up, at each switch a table is created. When a cell is received at a switch the VPI and/or VCI
code is looked up in the table and used for two purposes. First, the configuration of the switch is obtained, so that
this switch will correctly route the cell. Second, the VPI and/or VCI codes may be updated so that they correctly
control the next switch. This process repeats until the cell arrives at its destination.
In order to set up a path, the initiating device will initially send cells containing an ATM destination address, the
bandwidth and quality of service required. The first switch will reply with a message containing the VPI/VCI codes
which are to be used for this channel. The message from the initiator will propagate to the destination, creating
look-up tables in each switch. At each switch the logic will add the requested bandwidth to the existing bandwidth in
use to check that the requested quality of service can be met. If this succeeds for the whole channel, the
destination will reply with a connect message which propagates back to the initiating device as confirmation that
the channel has been set up. The connect message contains an unique call reference value which identifies this
transaction. This is necessary because an initiator such a file server may be initiating many channels and the
connect messages will not necessarily return in the same order as the set-up messages were sent. The last switch
will confirm receipt of the connect message to the destination and the initiating device will confirm receipt of the
connect message to the first switch.
7.10 ATM AALs
ATM works by dividing all real data messages into cells of 48 bytes each. At the receiving end, the original
message must be re-created. This can take many forms and
Figure 7.33
shows some possibilities. The message
may be a generic data file having no implied timing structure, a serial bitstream with a fixed clock frequency, known
as UDT (unstructured data transfer) or a burst of data bytes from a TDM system.
Figure 7.33:
Types of data which may need adapting to ATM.
The application layer in ATM has two sub-layers shown in
Figure 7.34
. The first is the segmentation and
reassembly (SAR) sub-layer which must divide the message into cells and rebuild it to get the binary data right. The
second is the convergence sublayer (CS) which recovers the timing structure of the original message. It is this
feature which makes ATM so appropriate for delivery of audio/visual material. Conventional networks such as the
Internet don't have this ability.
Figure 7.34:
ATM adaption layer has two sublayers, segmentation and convergence.
