th connections. Currently, the end users have three options: multiple T-1 Frame Relay ports, a high-speed Frame Relay port, or an ATM port. End users running voice, video, and data and having already invested in ATM customer premises equipment at the head- quarters may benefit most from deploying ATM at this location. With ser- vice interworking, the network is responsible for protocol conversion, enabling a Frame Relay site to communicate with an ATM site and vice versa. ETSI is expected to support ATM in the future as the broader band data needs are satisfied. In the interim, as the users and operators combine their needs, the pos- sibility is also one that X.25 and Frame Relay interworking may also exist. Figure 9-16 shows this possibility as the network provider provides the interfaces through the routers. This is not an ETSI specification, but a hypothetical possibility because in many parts of the world, X.25 is still the primary data access mechanism from the carriers and end-user interfaces alike. Network Service Sublayers The GPRS Layer 2 interface for the SGSN and Packet Control Unit Support Node (PCUSN) is shown in Figure 9-17 using the NS layer, which is com- posed of two parts: The NS Subnetwork part that defines the Layer 2 protocol that will be used (Frame Relay, ATM). In the beginning, Frame Relay will be used. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PCUSN-to-SGSN Interface (Gb) PCUSN-to-SGSN Interface (Gb) Figure 9-16 Frame and X.25 interworking. Switch Switch Relay Switch Router Figure 9-17 The NS layer. BSSGP Independent from FR or ATM NC Control Part NS Layer NS Subnetwork Part Physical Now Frame Relay (in the future ATM) The NS Control part creates circuit identifiers and defines procedures to manage them. Each NS virtual circuit that is created is associated to a Frame Relay virtual circuit. Downloaded from Digi