Switches

• An $n_1 \times n_2$ switch consists of $n_1$ input ports, $n_2$ output ports,
• links connecting each input to every output, control logic to select a specific connection, and internal buffers.
• Although $n_1$ and $n_2$ do not have to be equal, in practice and in most cases they have the same value, which is usually power of two.
• A switch is used to establish connections from the input ports to the output ports.
• These connections may be
  • one-to-one, which represent point-to-point connections, or
  • one-to-many, which represent multicast or broadcast.
• The case of many-to-one should cause conflicts at the output ports and therefore needs arbitration to resolve conflicts if allowed.
• When only one-to-one connections are allowed, the switch is called crossbar.

• An $n \times n$ crossbar switch can establish $n!$ connections (to allow only one-to-one connections,
  • the first input port should have $n$ choices of output ports,
  • the second input port will have ($n-1$) choices,
  • the third input port will have ($n-2$) choices, and so on.
  • Thus, the number of one-to-one connections is $n * (n-1) * (n-2)\ldots* 2 * 1=n!$
• For example, a binary switch has two input(output) ports.
• The number of one-to-one connections in a binary switch is two (straight and crossed),
• while the number of all allowed connections is four (straight, crosses, lower broadcast, and upper broadcast).

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Routing can be achieved using two mechanisms:

1

Source-path, the entire path to the destination is stored in the packet header at the source location.

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When a packet enters the switch, the outgoing port is determined from the header.

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Used routing data is stripped from the header and routing information for the next switch is now in the front.

2

Table-based routing, the switch must have a complete routing table that determines the corresponding port for each destination.

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When a packet enters the switch, a table lookup will determine the outgoing port.

Figure 1: Source-path routing vs. table-based routing.

• Figure 1 illustrates the difference between source-path routing and table-based routing in the case when a packet enters an 8-port switch at port 0.
• In the source-path case, the header contains the entire path and the next port is port 6.
• In the table-based case, the destination address dest-id is looked up in the routing table and port 6 is followed.