The Integrated Services model has an initial deployment scenario of routers connected together by point to point links. In that situation, packet scheduling for service classes needs only to be deployed within the routers, to provide the service overall.
However, many parts of the Internet involve other interconnection technologies between routers. Two common, but extremely different situations are:
The Integrated Services over Specific Lower Layers, Working Group of the IETF has defined the mapping of some of the IP level services onto services provided at the lower layer.
In some cases, the Link Layer as currently deployed cannot support the upper layer services with any reasonable guarantee, and so some enhancement is typically called for - one such case is Ethernet where the shared Media Access is so non-deterministic, that an enhancement based on some form of distributed bandwidth manager is required to provide anything beyond best effort IP service over an Ethernet hop.
In the case of NBMA networks, particularly ATM, a much richer variety of services is available at the lower layer for IP to utilise. In fact, there are multiple possible mappings from the high level IP service, to the proposed ``Bearer Service Classes'' of ATM - for example:
Again, this area is very active, and simplification will no doubt be sought and found by the market. In the next 3 subsections, we briefly describe the ``pure'' IP approach.