Virtual Paths and Virtual Channels
Key to understanding how ATM transfers information through an ATM network are the concepts of the transmission path, virtual path and virtual channel, illustrated in Figure 14.7.
Figure 14.7 Channels Within a Path Within the Transmission Medium
The transmission path consists of the physical cable connected to a particular port of an ATM switch. The cable has a defined bandwidth, such as 155 megabits per second for an Optical Carrier-3 (OC-3) optical fiber link.
The bandwidth of the transmission path is logically divided into separate virtual paths and identified using the VPI in the ATM header. Each virtual path is allocated a fixed amount of bandwidth. Virtual paths do not dynamically vary their bandwidths beyond what has been allocated.
The bandwidth of a virtual path is logically divided into separate virtual channels using a virtual channel identifier in the ATM header. Unlike virtual paths, virtual channels share the bandwidth within a virtual path dynamically.
The transmission path to virtual path to virtual channel hierarchy is the basis for ATM switching. ATM can switch cells at the transmission path, virtual path and virtual channel level.
Switching at the Transmission Path Level
Switching at the transmission path level allows an ATM switch to determine which output port to use to forward the cell.
Switching at the Virtual Path Level
Switching at the virtual path level allows entire groups of virtual channels to be switched at the same time. Virtual path switching is similar to the telephone system cross-connect switching of entire groups of telephone calls based on the area code of the phone number. The switching occurs based on the area code, not the 7-digit individual phone number.
When performing virtual path switching, an ATM switch looks only at the virtual path identifier in the ATM cell header. This ability to ignore the rest of the header makes virtual path switching faster than virtual channel switching.
ATM virtual path switching most often occurs within the public networks of ATM service providers, because this virtual path switching allows ATM service providers to aggregate bundles of virtual channels along high speed backbone links. These aggregate channels create trunk line structures very similar to those used in telephone networks.
Switching at the Virtual Channel Level
Switching at the virtual channel level allows for a granularity of switching and bandwidth allocation. Virtual channel switching resembles switching a phone call to its final 7-digit location; that is, ATM switching is based on the entire VPI/VCI, just as the final phone switching is based on the entire 10-digit phone number (3-digit area code and 7-digit individual phone number).
ATM virtual channel switching occurs within both private and public networks. A switch must analyze both the virtual path identifier and the virtual channel identifier to make a switching decision.