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In the setup phase, a switch creates an entry for a virtual circuit For example, suppose source A needs to create a virtual circuit to B Two steps are required: the setup request and the acknowledgment
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Switch and tables in a virtual-circuit network
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Source-to-destination data transfer in a virtual-circuit network
Incoming Outgoing Port VCl Port VCl I 14 3 66 Incoming Outgoing Port VCl Port VCI 2 22 3 77
Incoming Outgoing Port VCI Port VCl 1 66 2 22
Setup Request A setup request frame is sent from the source to the destination Figure 814 shows the process a Source A sends a setup frame to switch 1 b Switch 1 receives the setup request frame It knows that a frame going from A to B goes out through port 3 How the switch has obtained this information is a point covered in future chapters The switch, in the setup phase, acts as a packet switch; it has a routing table which is different from the switching table For the moment, assume that it knows the output port The switch creates an entry in its table for
SECTION 83
VIRTUAL-CIRCUIT NETWORKS
Setup request in a virtual-circuit network
Incoming Outgoing Port IVCI Port IVCI 2 122 3 I
Incoming Outgoing Port IvcI Port IVCI 1 I 14 3 I
VCI=77
Incoming Outgoing Port IVCI Port IVCI 2 I I I 66
this virtual circuit, but it is only able to fill three of the four columns The switch assigns the incoming port (1) and chooses an available incoming VCI (14) and the outgoing port (3) It does not yet know the outgoing VCI, which will be found during the acknowledgment step The switch then forwards the frame through port 3 to switch 2 c Switch 2 receives the setup request frame The same events happen here as at switch 1; three columns of the table are completed: in this case, incoming port (l), incoming VCI (66), and outgoing port (2) d Switch 3 receives the setup request frame Again, three columns are completed: incoming port (2), incoming VCI (22), and outgoing port (3) e Destination B receives the setup frame, and if it is ready to receive frames from A, it assigns a VCI to the incoming frames that come from A, in this case 77 This VCI lets the destination know that the frames come from A, and not other sources
Acknowledgment A special frame, called the acknowledgment frame, completes the entries in the switching tables Figure 815 shows the process
a The destination sends an acknowledgment to switch 3 The acknowledgment carries the global source and destination addresses so the switch knows which entry in the table is to be completed The frame also carries VCI 77, chosen by the destination as the incoming VCI for frames from A Switch 3 uses this VCI to complete the outgoing VCI column for this entry Note that 77 is the incoming VCI for destination B, but the outgoing VCI for switch 3 b Switch 3 sends an acknowledgment to switch 2 that contains its incoming VCI in the table, chosen in the previous step Switch 2 uses this as the outgoing VCI in the table c Switch 2 sends an acknowledgment to switch 1 that contains its incoming VCI in the table, chosen in the previous step Switch 1 uses this as the outgoing VCI in the table d Finally switch 1 sends an acknowledgment to source A that contains its incoming VCI in the table, chosen in the previous step e The source uses this as the outgoing VCI for the data frames to be sent to destination B
SWITCHING
Setup acknowledgment in a virtual-circuit network
Incoming Outgoing Port IVCI Port IVCI I I 14 3 166 Incoming Outgoing Port IVCI Port IVCI 2 122 3 I 77
VCI == 14
VCI == 77
A 1--==--;
Incoming Outgoing Port IVCI Port IVCI I I 66 2 122
Teardowil Phase In this phase, source A, after sending all frames to B, sends a special frame called a teardown request Destination B responds with a teardown confirmation frame All switches delete the corresponding entry from their tables
Efficiency
As we said before, resource reservation in a virtual-circuit network can be made during the setup or can be on demand during the data transfer phase In the first case, the delay for each packet is the same; in the second case, each packet may encounter different delays There is one big advantage in a virtual-circuit network even if resource allocation is on demand The source can check the availability of the resources, without actually reserving it Consider a family that wants to dine at a restaurant Although the restaurant may not accept reservations (allocation of the tables is on demand), the family can call and find out the waiting time This can save the family time and effort
In virtual-circuit switching, all packets belonging to the same source and destination travel the same path; but the packets may arrive at the destination with different delays if resource allocation is on demand
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