Cabletron Systems CSX1000, CSX1200 manual B1 B2

USER’S GUIDE

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.

D
	D
T	B1
E
	B2

CSX1200

= X.25 Virtual Circuits

	Public Packet
D	Switched Network		D
C			C
E			E
		D C E

D B1 B2

D T E

CSX1200

D

B1

B2

D

T

E

CSX1200

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.

208 CyberSWITCH