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USER’S GUIDE
208 CyberSWITCH
a virtual path, although it appears that a real circuit exits, in reality, the network routes the device’s
information packets to the designated designation. Any given path may be shared by several
devices.
When the virtual circuit is established, a logical channel number is assigned to it at the originating
end. A logical channel number is also assigned to the virtual circuit at the destination end, such that
at each end there is a one to one correspondence between logical channel number and the virtual
circuit. However, the logical channel numbers at each end of a virtual circuit are different.
Two types of virtual circuits can be used: a permanent virtual circuit (PVC) or a switched virtual
circuit (SVC). A PVC is similar to a dedicated line. At subscription time, the subscriber gives the
network the address to be associated with that virtual circuit. A logical channel is permanently
assigned. From that point on, no call set up is needed. Data to be sent to that destination is simply
sent in data packets using the assigned logical channel.
A SVC is similar to a dial-up connection. A call origination packet called a Call Request packet,
containing the address of the called party, must be given to the network to cause the establishment
of the virtual circuit.
As is specified by X.25, multiple logical connections can be multiplexed over a single physical
channel. In the case where an ISDN basic rate line is providing the physical channel to a PPSN,
multiple X.25 virtual circuits can be present on a single B-channel. The following diagram
illustrates that point.
Note: In the illustration, the DTEs are all CyberSWITCH systems. Throughout the X.25 Access
section, the term “DTE” can be interchanged with “CyberSWITCH”.
Public Packet Switched Networks are typically more cost effective for users who transmit data in
the mid-traffic range. Low volume users can incur lower costs using public telephone dial-up than
a comparable session in a packet network. At the other end of the spectrum, high volume users are
better served with leased lines.
Public Packet
Switched Network
D
T
E
D
C
E
D
C
E
D
T
E
DCE
DTE
D
B1
B2
D
B1
B2
D
B1
B2
= X.25 Virtual Circuits
POWER
SERVICE
TX
RX
10BASE - T
LAN B-CHANNELS E1 ONLY
B2 B4
B6 B8
B26 B28
B22 B24
B18 B20
B14 B16
B10 B12
B30 L1
B1 B3
B5 B7
B25 B27
B21 B23
B17 B19
B13 B15
B9 B11
B29 B31
E1
D
T1
D
POWER
SERVICE
TX
RX
10BASE - T
LAN B-CHANNELS E1 ONLY
B2 B4
B6 B8
B26 B28
B22 B24
B18 B20
B14 B16
B10 B12
B30 L1
B1 B3
B5 B7
B25 B27
B21 B23
B17 B19
B13 B15
B9 B11
B29 B31
E1
D
T1
D
POWER
SERVICE
TX
RX
10BASE - T
LAN B-CHANNELS E1 ONLY
B2 B4
B6 B8
B26 B28
B22 B24
B18 B20
B14 B16
B10 B12
B30 L1
B1 B3
B5 B7
B25 B27
B21 B23
B17 B19
B13 B15
B9 B11
B29 B31
E1
D
T1
D
CSX1200 CSX1200
CSX1200