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Fig. 6.17 An example of routing a multicast cell.
interconnection network, such as the binary network, the Clos network, and so on. The unequal length of the interconnection wires increases the diffi-
culty of synchronizing the signals, and, consequently limits the switch fabric’s size, as with the Batcher᎐banyan switch. The SWEs in the switch modules
only communicate locally with their neighbors, as do the chips that contain a two-dimensional SWE array. The switch chips do not need to drive long wires
to other chips on the same printed circuit board. Note that synchronization of data signals at each SWE is only required in each individual switch
module, not in the entire switch fabric.
6.3.3 Translation Tables
Ž .
The tables in the IPC, MTT, and the OPC Fig. 6.18 contain information necessary to properly route cells in the switch modules of MGN1 and MGN2,
and to translate old VCI values into new VCI values. As mentioned above, cell routing in the MGNs depends on the multicast pattern and priority
A TWO-STAGE MULTICAST OUTPUT-BUFFERED ATM SWITCH
161
Fig. 6.18
Translation tables in the IPC, MTT, and OPC.
values that are attached in front of the incoming cell. To reduce the translation table’s complexity, the table contents and the information at-
tached to the front of cells are different for the unicast and multicast calls. In a point-to-point ATM switch, an incoming cell’s VCI on an input line
can be identical with other cells’ VCIs on other input lines. Since the VCI translation table is associated with the IPC on each input line, the same VCI
values can be repeatedly used for different virtual circuits on different input lines without causing any ambiguity. But cells that are from different virtual
circuits and destined for the same output port need different translated VCIs.
In a multicast switch, since a cell is replicated into multiple copies that are likely to be transmitted on the same routing link inside a switch fabric, the
Ž .
switch must use another identifier, the broadcast channel number BCN , to uniquely identify each multicast connection. In other words, the BCNs of
multicast connections have to be different from each other, which is unlike the unicast case, where a VCI value can be repeatedly used for different
connections at different input lines. The BCN can either be assigned during call setup or be defined as a combination of the input port number and the
VPIrVCI value.
For the unicast situation, upon a cell’s arrival, its VCI value is used as an index to access the necessary information in the IPC’s translation table, such
as the output addresses in MGN1 and MGN2, a contention priority value, Ž .
and a new VCI, as shown in Figure 6.19 a . The output address of MGN1,
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162
Ž . Ž .
Fig. 6.19
Unicast and multicast cell formats in a MGN1, b MGN2.
A1, is first decoded into a flattened address, which has K bits and is put into Ž .
the MP1 field in the cell header, as shown in Figure 6.19 a . The MP1 and P are used for routing cells in the MGN1, and A2 is used for routing cells in
the MGN2. The I bit is the multicast indication bit, which is set to 0 for unicast and 1 for multicast. When a unicast cell arrives at the MTT, the A2
field is simply decoded into a flattened address and put into the MP2 field as the routing information in MGN2. Thus, no translation table in the MTT is
needed for unicast cells. Note that A2 is not decoded into a flattened address
Ž until it arrives at the MTT. This saves some bits in the cell header e.g., 27
. bits for the above example of a 1024 = 1024 switch and thus reduces the
required operation speed in the switch fabric. The unicast cell routing format Ž .
in MGN2 is shown in Figure 6.19 b . For the multicast situation, besides the routing information of MP1, MP2,
and P, the BCN is used to identify cells that are routed to the same output group of a switch module. Similarly to the unicast case, an incoming cell’s
VCI is first used to look up the information in the translation table in the Ž .
IPC, as shown in Figure 6.18 b . After a cell has been routed through MGN1, MP1 is no longer used. Instead, the BCN is used to look up the next routing
Ž . information, MP2, in the MTT, as shown in Figure 6.18 c . The cell formats
Ž . Ž .
for a multicast call in MGN1 and MGN2 are shown in Figure 6.19 a and b . The BCN is further used at the OPC to obtain a new VCI for each
duplicated copy that is generated by a cell duplicator at the OPC. The entry Ž .
for the multicast translation table in the OPC is shown in Figure 6.18 d .
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163
Note that the in MTTs that are connected to the same MGN2 contain identical information, because the copy of a multicast cell can appear
randomly at any one of L M output links to MGN2. As compared to those in
1
w x
18, 26 , the translation table size in the MTT is much smaller. This is because the copy of a multicast cell can only appear at one of L M output
1
w x
links in the MOBAS, vs. N links in 18, 26 , resulting in fewer table entries in the MTT. In addition, since the VCI values of replicated copies are not
stored in the MTT’s, the content of each table entry in the MTT is also less.
6.3.4 Multicast Knockout Principle
