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This paper outlines some of the different types of digital radio relay systems that are now in worldwide service, identifies the factors which contribute to their performance, and illustrates the performance standards that are currently being achieved. The CCIR performance standards for high capacity digital radio systems are discussed, together with some of the system planning guidelines in current use, and these are compared with worldwide results from a number of system field evaluations. The comparison shows that the CCIR standards can be achieved on established microwave networks, using advanced equipment designs which incorporate diversity and adaptive equalizers to overcome the effects of dispersive fading. The performance and availability of medium and lowcapacity systems are also briefly reviewed, and predictions of non-availability due to rainfall fading are given for some typical systems. The paper concludes that the performance standards for digital radio systems will be in accordance with the highest quality grades required for transmission media in an Integrated Services Digital Network.