<P> At these lower speeds, ATM provides a useful ability to carry multiple logical circuits on a single physical or virtual medium, although other techniques exist, such as Multi-link PPP and Ethernet VLANs, which are optional in VDSL implementations . DSL can be used as an access method for an ATM network, allowing a DSL termination point in a telephone central office to connect to many internet service providers across a wide - area ATM network . In the United States, at least, this has allowed DSL providers to provide DSL access to the customers of many internet service providers . Since one DSL termination point can support multiple ISPs, the economic feasibility of DSL is substantially improved . </P> <P> ATM operates as a channel - based transport layer, using virtual circuits (VCs). This is encompassed in the concept of the virtual paths (VP) and virtual channels . Every ATM cell has an 8 - or 12 - bit virtual path identifier (VPI) and 16 - bit virtual channel identifier (VCI) pair defined in its header . The VCI, together with the VPI, is used to identify the next destination of a cell as it passes through a series of ATM switches on its way to its destination . The length of the VPI varies according to whether the cell is sent on the user - network interface (on the edge of the network), or if it is sent on the network - network interface (inside the network). </P> <P> As these cells traverse an ATM network, switching takes place by changing the VPI / VCI values (label swapping). Although the VPI / VCI values are not necessarily consistent from one end of the connection to the other, the concept of a circuit is consistent (unlike IP, where any given packet could get to its destination by a different route than the others). ATM switches use the VPI / VCI fields to identify the virtual channel link (VCL) of the next network that a cell needs to transit on its way to its final destination . The function of the VCI is similar to that of the data link connection identifier (DLCI) in frame relay and the logical channel number and logical channel group number in X. 25 . </P> <P> Another advantage of the use of virtual circuits comes with the ability to use them as a multiplexing layer, allowing different services (such as voice, frame relay, n * 64 channels, IP). The VPI is useful for reducing the switching table of some virtual circuits which have common paths . </P>

Why gfc field is not included in nni header