CATMES - Cellular ATM Evaluation System M. Bergenwall 1 , T. Kaaresoja 2 , Y. Raivio 1 , S. Uskela 1 1 Nokia Telecommunications, Espoo, Finland, Email: Martin.Bergenwall@ntc.nokia.com, Yrjo.Raivio@ntc.nokia.com, Sami.Uskela@ntc.nokia.com 2 Nokia Research Center, Helsinki, Finland Email: Topi.Kaaresoja@research.nokia.com Abstract: During the recent years more and more attention has been focused on the usage of Asynchronous Transfer Mode (ATM) in the wireless environment. Third generation mobile network standards like UMTS and IMT-2000 have chosen ATM as a transport technology candidate for terrestial circuits in cellular networks. Why ATM? A prototype system called CATMES, Cellular ATM Evaluation System, was created to investigate this question. As a background the current transport problems are shown. CATMES system is described in detail, and the major system impacts are shown. The new ATM Adaptation Layer type 2 (AAL2) is explained and compared to other alternatives. Finally some interesting results concerning transmission efficiency, AAL2 details and synchronisation aspects are evaluated. 1. Introduction As the dust has settled after the ETSI air interface decision for the next generation mobile network system, discussions about the network solutions have started. Several alternatives can be considered including current TDM networks and some more sophisticated choices such as IP, Frame Relay and ATM. Currently both ITU-T and ETSI are focusing on ATM in their third generation mobile networks, but no decisive choices have been made yet. ATM seems to be a feasible technology for data transmission and switching, but how does it fit into the mobile world? As far as speech only has been the major element of the cellular networks, there has been no great interest in ATM. Fixed cells provide very suitable technology for data applications but until today low speed and delay sensitive speech transportation has been seen as a problem area. The latest development on the ATM field has changed this picture. Furthermore, new demands arise when cellular operators extend their business from traditional mobile services to more comprehensive service and solution offerings, especially for corporate customers. Easy access to IP networks will be essential for any operator in the future. This study concentrates on the usage of ATM in the fixed part of cellular networks. The main difference between this view (Cellular ATM) and Wireless ATM (WATM) is the air interface. In WATM, ATM cells are sent over air too, but in Cellular ATM not. The term Mobile ATM furthermore defines some enhancements for ATM signalling protocols to support mobility. In Cellular ATM mobility is handled by GSM Mobile Application Protocol (MAP) or by Intelligent Network (IN). The efficient transmission of short speech packets forms the most critical challenge for the ATM evolution into cellular networks. The starting point in this paper has been the current GSM system where only the TDM based transmission network on layers 1 and 2 has been replaced with ATM and equivalent layer 2 protocol. Higher layers are still using the current GSM signalling stacks. The system architecture of the trial equipment called Cellular ATM Evaluation System, CATMES, is described in addition to the main results and observations. 2. Architecture CATMES consists of a GSM system with an ATM-based radio access network. The trial system includes the basic GSM elements like basestations (BTS), Base Station Controller (BSC), Mobile Switching Centre (MSC) and Transcoder (TC). The BTSs are standard GSM basestations with an E1 interface. The BTSs are connected to PCs which offer the interworking between TDM and ATM. The