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 scaleablebandwidth, high speed performance and Quality of Service (QoS). ATM isvery well suited for some of tomorrow’s bandwidth-hungry applications suchas multimedia, distributed databases, server mirroring, etc.

Through the course of this discussion we will focus on the ATM protocol andhow it delivers what it promises. The ATM network fundamentally is comprisedof ATM switches that are interconnected. There two primary interfaceswith the realm of ATM — User Network Interfaces (UNIs) and Network NetworkInterfaces (NNIs).

UNIs are designed to connect end nodes such as userworkstations, servers, routers, etc. to an ATM switch. An NNI interfacewill 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 ATMterminology, these are called Virtual Circuit Identifier (VCI) and VirtualPath 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 ATMswitch receives a cell it also has access to the VCI and VPI informationfor that cell. The logic in the switch now looks up a translation tableand will retransmit the cell on an new VCI/VPI (as defined by thecross-reference table) on the outgoing link.

Why is the operation of theATM switch so simple? The answer lies in the various mechanisms that helprealize 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 isexternal to the switch; for instance, network management software. A PVCis 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 upautomatically via signaling mechanism and associated protocol. Since SVCrequire no manual interaction like their PVC cousins, they tend to be morepopular with the higher level protocols that operate over ATM.

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

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

ATM Connection Setup using SVCs

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

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