Enterasys Networks CSX7000 User Manual download pdf (Page 254)

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USER’S GUIDE

254 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

DCE

DTE

= X.25 Virtual Circuits

CSX5500

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Enterasys Networks CSX7000 User Manual Page 254

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