characteristics that are relevant when supporting CES. 1. Bandwidth-The ATM interface must provide ade- Dynamic bandwidth CES (DB-CES). quate bandwidth to carry n * 64 or unstructured ATM trunking of narrowband services using AAL2. traffic after segmentation. 2. Timing-The ATM interface can be used to convey timing traceable to a primary reference source from Circuit Emulation Services the ATM network to the CES inter working function, where the external connection to network timing is Circuit Emulation allows the user to establish an AALI not supported. connection to support a circuit, such as a full T1 or E1, over the ATM backbone. In using CES, the ATM network An ATM network providing CES should also consider provides a transparent transport mechanism for various QoS parameters such as Peak Cell Rate (PCR) and CDV. CBR services based on AAL1. It specifically covers the There are standards that specify what values are opti- following types of CBR services: mum for these parameters. 1. Structured DS1/E1 n * 64 kbps (fractional DS1/E1) CES's advantage is the simplicity of implementation. service. The ATM network is used to provide virtual replacements for physical links in an existing network. Still, CES has 2. Unstructured DS1/E1 (1.544 Mbps, 2048 Mbps) two limitations. First, it is unable to provide any statistical service. multiplexing. It does not differentiate between idle and 3. Unstructured DS3/E3 (44,736 Mbps, 34,368 Mbps) active timeslots. This means all idle traffic is carried. service. Therefore, CES voice transport consumes 10 percent ATM CBR virtual channel CBR service interface CBR service interface ATM network ATM CES interworking (public or private) ATM CES equipment interworking equipment (e.g., PBX) function function (e.g., PBX) UNI ATM access interface Figure 3.2-3 ATM circuit emulation. CHAPTER 3.2 Network infrastructure more bandwidth than would be required to transfer the a) AAL2 Common Part Sub-layer same voice traffic over leased circuits. Second, it is often CPS provides the basic structu