Integrated Services in IP-ATM Networks

      Experts of the IETF and of the ATM Forum are developing techniques for running IP on top of local area ATM networks. The proposed techniques are named  Classical IP and  LAN Emulation, respectively.

The problems of integrating real-time services in the IETF Classical IP architecture are summarised in [RFC1821]. The main issues are the service model, the reservation model, and QoS-aware routing.
As described above, the service model of Integrated Services Internet architecture used service classes---such as best effort, guaranteed, predictive delay, and controlled delay---for QoS specification. ATM in turn has five service classes, which are not in direct correspondence with the IP classes:

• CBR (constant bit rate), which emulates a leased line, providing very tightly constrained delay and designed for applications which can use a fixed bandwidth pipe.
• VBR (variable bit rate)--real-time, which attempts to constrain delay for applications whose bandwidth requirements vary.
• VBR--non--real-time, intended for variable bandwidth applications without tight delay constraints.
• UBR (unspecified bit rate), which most closely approximates the best effort service of traditional IP.
• ABR (available bit rate), which uses a complex feedback mechanism to control data loss.

 
Table:  Differences between RSVP and ATM UNI 3.0.