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Tip of the Day
Language: Web Development
Expertise: Beginner
Feb 26, 1997

ATM: Connection-Oriented Switching

Question:
What does it mean that ATM switching is connection-oriented, and what are the benefits of this?

Answer:
ATM, or Asynchronous Transfer Mode, through its connection-oriented protocol, offers a level of scaleable bandwidth, high speed performance and Quality of Service (QoS). ATM is very well suited for some of tomorrow's bandwidth-hungry applications such as multimedia, distributed databases, server mirroring, etc.

Through the course of this discussion we will focus on the ATM protocol and how it delivers what it promises. The ATM network fundamentally is comprised of ATM switches that are interconnected. There two primary interfaces with the realm of ATM -- User Network Interfaces (UNIs) and Network Network Interfaces (NNIs).

UNIs are designed to connect end nodes such as user workstations, servers, routers, etc. to an ATM switch. An NNI interface will connect ATM switches together.

Since ATM is connection oriented, circuit types have to be defined. Again, there are two kinds: Virtual Circuits and Virtual Paths. Using ATM terminology, these are called Virtual Circuit Identifier (VCI) and Virtual Path Identifier (VPI). A VPI is a bundle of VCIs.

The fundamental operation of an ATM switch is as follows:

Information in an ATM network is transferred using cells. When an ATM switch receives a cell it also has access to the VCI and VPI information for that cell. The logic in the switch now looks up a translation table and will retransmit the cell on an new VCI/VPI (as defined by the cross-reference table) on the outgoing link.

Why is the operation of the ATM switch so simple? The answer lies in the various mechanisms that help realize this simplicity.

Let's now look a the ATM connections that help make things simple for switches.

A Permanent Virtual Circuit (PVC) is set up by a mechanism that is external to the switch; for instance, network management software. A PVC is a set of VCI/VPI values that help connect a source with its destination. On the other hand, a Switched Virtual Circuit (SVC) is set up automatically via signaling mechanism and associated protocol. Since SVC require no manual interaction like their PVC cousins, they tend to be more popular with the higher level protocols that operate over ATM.

Consider the following example, in which Station Alpha wants to communicate with Station Beta:

The first step: Alpha issues a request to connect to Beta to switch A. A well-known virtual channel is used for signaling packets, VPI=0/VCI=5. This signal travels through the network from switch to switch until the destination is reached. As it travels from Alpha to switch A and then to switch B and so on, and finally to station Beta, the connection identifiers are being set up. Along the way, a switch can either confirm a connection request or reject it.

ATM Connection Setup using SVCs

Once the VPI/VCI definitions are established, Alpha can start communicating with Beta via Switch A and Switch B. The data also flows along the same path. When the communications session is complete, the connection is terminated and links are torn down.

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