The Asynchronous Operation

Firewire bridges are considered inbound portals, due to the fact that they examine the bus to detect primary asynchronous packets (that start the protocol of a communication) sent by other buses. But the portal that transmits the primary packet is called outbound portal. Basically, there are 6 types of primary packets:

•  Writing Request;
•  Response to a writing request;
•  Reading request;
•  Response to a reading request;
•  Bus protection request;
•  Response to the bus protection request.

On an inbound operation, the bridges are constantly monitoring the bus searching for primary packets. At the moment that it finds a primary packet, the bridge examines the virtual node ID contained in the packet and verifies, on the network topology calculated after the reset, if the destination node of the transaction is "hanging" on one of the bridges' portal buses. If that happens, the destination portal receives the packet and starts an outbound operation to retransmit the detected primary packet to the buses hierarchically connected to it.

As an example, we take the topology shown in Figure 3. Five portals are represented in it, with the references "a", "b", "c", "d" and "e". The portal "a" in question is the "a". We suppose that N1 node wants to communicate with N2 node. Observing the figure, we conclude that the following steps are taken:

•  Node "N1" transmits a request packet through FireWire of portal "b"
•  Portal "b", implemented in bridge "a-b", detects a request and verifies that it is needed to retransmit the packet through "a" portal for it to be able to reach its destination (inbound operation);
•  Packet travels through bridge "a-b" and arrives at portal "a";
•  Portal "a" retransmits the packet through the FireWire bus that unites "a" and "c" (outbound operation);
•  Portal "c", implemented on bridge "d-c-e", detects a request and verifies that it is need to retransmit the packet through portal "e", for it to be able to reach its destiny (inbound operation);
•  Packet travels over the bridge "c-e" and arrives at portal "e";
•  Portal "e" retransmits the packet through the FireWire bus, where node "N2" is connected (outbound operation);
•  Node "N2" receives packet;
•  Node "N2" starts the opposite process, that is, sends response packet to request. 

Figure 3: Example of topology with Firewire buses, portals, bridges and nodes.

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