3Com 3C63400-3AC-C - PathBuilder S700 Switch, Reference Manual, Page: 90 / 272

76 C
HAPTER
4: P
ATH
B
UILDER
S700 M
ODULE AND
A
PPLICATION
O
VERVIEW
Figure 46 Virtual Circuit Scheme
Each ATM cell (a fixed-length unit of data over ATM) is assigned to a virtual circuit
by including the circuit's VPI/VCI in the cell's header. This is then used to steer the
cell through an ATM unit and the ATM network.
Figure 47 illustrates virtual circuits on a simple network using PathBuilder S700s.
Figure 47 Virtual Circuits in a Simple Network
Suppose workstation 1 on LAN AA wants to send data to workstation 2 on LAN
BB. It transmits Ethernet packets which include its MAC address and that of
workstation 2. PathBuilder S700 A groups the packets into cells and, since it does
not know where workstation 2 is, broadcasts the cells on all VPI/VCIs.
PathBuilder S700 B learns that workstation 1 transmits to its slot 4 port 2 over
VPI/VCI 1/33 and to slot 5 port 2 over VPI/VCI 2/35. This information is then stored
in forwarding tables in PathBuilder S700 B.
When workstation 2 responds, it includes its MAC address and that of workstation
1 in the packets. PathBuilder S700 B looks in the appropriate forwarding table and
finds that it can reach workstation 1 on VPI/VCI 2/35. When the first response cell
on 2/35 is formatted back into packets at PathBuilder S700 A, PathBuilder S700 B
learns how to reach workstation 2 without broadcasting.
VPI 1
VPI 2
VPI 3
{
{
VCIs
VCIs
VCI 1 (Transmit)
VCI 1 (Receive)
{
Virtual
Circuit
Transmission
Path
LAN
AA
LAN
AB
LAN
BA
LAN
BB
VPI/VCI
2/35
VPI/VCI 1/33
S4P2
S5P2
PathBuilder S700 A
S4P1
S5P1
DS3 UNI
Workstation 1
PathBuilder S700 A
PathBuilder
S700 B
Workstation 2