om) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Transport Technologies Chapter 7 Forum categorizes them as real-time and non-real-time services. Under the real-time category, they define constant bit rate services that demand fixed resources with guaranteed availability. They also define real-time, variable bit rate (VBR) service, which provides for statistical multi- plexed, variable-bandwidth service allocated on demand. A further subset of real-time VBR is peak-allocated VBR, which guarantees con- stant loss and delay characteristics for all cells in that flow. Under the non-real-time service class, unspecified bit rate (UBR) is the first service category. UBR is often compared to IP in that it is a "best effort" delivery scheme in which the network provides whatever band- width it has available, with no guarantees made. All recovery functions from lost cells are the responsibility of the end user devices. UBR has two subcategories of its own. The first, non-real-time UBR (NRT-UBR), improves the impacts of cell loss and delay by adding a net- work resource reservation capability. Available bit rate (ABR), UBR's other subcategory, makes use of feedback information from the far end to manage loss and ensure fair access to and transport across the network. Each of the five classes makes certain guarantees with regard to cell loss, cell delay, and available bandwidth. Furthermore, each of them takes into account descriptors that are characteristic of each service described. These include peak cell rate (PCR), sustained cell rate (SCR), minimum cell rate (MCR), cell delay variation tolerance (CDVT), and burst tolerance (BT). ATM Forum Specified Services The ATM Forum has identified a collection of services for which ATM is a suitable, perhaps even desirable, network technology. These include cell relay service (CRS), circuit emulation service (CES), voice and telephony over ATM (VTOA), frame-relay bearer service (FRBS), LAN emulati