###### Abstract  
Microwave line-of-sight radio relay (RR) systems are a constitutive part of a telecom operator transport network, as an alternative to optical transmission systems when the latter are not technically possible or rational to implement. Nowadays, RR links are quite often used in the access network for connecting mobile radio base stations, thus also enabling traffic aggregation, and so on. In this paper, we focus on a practical, real-life, five-section heterogeneous RR network, comprising classic synchronous digital hierarchy (SDH) and SDH new generation network (NGN) architecture, hybrid parallel and mutually independent transmission of native Ethernet and TDM services, and all-IP network parts. Specifically, the main task of this work is to answer whether such a diverse RR system could satisfy the quality norms for Ethernet-based services, meaning whether a tolerable RR unavailability will necessarily imply the according Ethernet quality of service (QoS) degradation. This question is addressed by the comprehensive in-service and out-of-service testing of an operational hybrid RR transmission system. After extensive practical testing and appropriate analysis of the achieved results, it came out that the impact of RR-level impairments that determine the performance prediction affected the Ethernet QoS to the extent that BER values increased to the acceptability threshold values. We believe that the preliminary results reported here could serve as a hint and a framework for a more comprehensive cross-layer test strategy in terms of both test diversity and repeating rate, which contemporary network operators need to implement in order to enable the appropriate quality of experience for users of their services. radio relay networks; carrier Ethernet; performance; QoS; cross-layer testing 202  
Locally, RR systems are used for aggregation with IP/Multiple Protocol Label Switching (MPLS) nodes, as well as for cross-connects of transmissions to multiple base stations. Moreover, RR links are also used for back-up transmission at the trunk level [5]. State-of-the-art RR systems are heterogeneous, encompassing classical (and outgoing) synchronous digital hierarchy (SDH) and SDH new generation networks (NGN), hybrid parallel and mutually independent transmission of native Ethernet data, and time-division multiplex (TDM) services, as well as all-IP networks [6]. In this regard, as an RR link is sensitive to fading and radio interference, its transmission performance is determined by the order of the chosen modulation scheme and protective coding. The choice is fixed for SDH NGN technologies, whereas adaptive coding and modulation (ACM) is applied in hybrid and all-IP RR systems, depending on instantaneous propagation conditions [7]. Furthermore, as Ethernet technology spreads from public wideband access networks to transport ones, the classic transmission technologies, namely SDH and wavelength-division multiplex (WDM), are still also used with Ethernet services (based on TCP/IP protocol stack) as well. The NGN devices transmit packets over the network initially predetermined for voice transmission, but now the Carrier Ethernet encapsulates all TDM services in its frames. Of course, the price for this is solving synchronization problems and clock distribution, as well enabling appropriate quality of service (QoS) for TDM services transversing through Ethernet networks, which adds another degree of freedom or uncertainty in this regard [8]. Moreover, the most important goal of a network operator is to provide an appropriate level of services so that its end users can achieve the best possible quality of experience (QoE), and the prerequisite for that is to enable the according QoS throughout all relevant network segments, as a common set of services' performances that determine the end user's QoE, such as loss of information, delay, and delay variation in particular [9].