Virtual Channels/Virtual Paths

The term "Virtual Circuit" is (almost) never used in ATM.

ATM Terminology

VC = Virtual Channel

• Generic term for unidirectional transport of cells from a source to a destination.

VCC = Virtual Channel Connection.

• A single unidirectional connection between 2 points in an ATM network where the AAL (ATM Adaptation Layer) is accessed.
• Between 2 users (data transfer).
• User to Network (signaling).
• Network to Network (network management).
• Basic unit of switching.

VP = Virtual Path

• Grouping of Virtual Channels

VPC: Virtual Path Connection

• Bundle of VCC's which follow same route throughout the VPC

Why Use Virtual Paths?

• Allows switching of VP's without looking at the VCI (Virtual Channel Indicator) field in ATM header.
• Manage and allocate resources according to VP.
• Simplify connection setup.

• Preserves cell order.
• Allows Switched Virtual Circuit (SVC) ("connection on demand")
  or
  Semi-Permanent Virtual Circuit (PVC).
• Quality of Service (QOS) can be specified in terms of cell loss probability or delay (mean, variance). (What the network guarantees to the user.)
• Traffic Parameter Negotiation: Between user and network (e.g. average rate, peak rate) (What the user is allowed to send.)
• Usage Monitoring: Network ensures negotiated parameters are not violated. (Policing of the user by the net.)
• More on these 3 properties in Chapter 6 on Traffic Management.

VP and VC in ATM Switching

VP (Only) Switch

• Looks only at VPI (Virtual Path Indicator) in order to switch.
  VCI must remain the same.

VP / VC Switch

• Looks at both VPI and VCI to switch.
• Can act like a VP switch for some VPC's.
• Can terminate a VP and switch on a VCI.

Tubes and Strings Analogy

• VCC = String passing through the network with each end tied to a user.
• VPC = Tube connecting 2 points each of which can be an end point or a VP/VC Switch.

Fig. The tubes and strings analogy for a VP switch.

Fig. The tubes and strings analogy for a VP/VC switch.

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Broadband Networks Lecture 12 Notes taken by Brutus Youn from Prof. K.S. Vastola on March 19, 1997.