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Local area networks have come into widespread use for computer communications. Together with the trend towards digital transmission of telephone signals, this has sparked interest in the use of computer networks for the transmission of integrated voice/data traffic. This work addresses two related aspects of local area network performance, a detailed characterization of the performance of Carrier Sense Multiple Access with Collision Detection (CSMA/CD), and the comparative performance of several broadcast bus networks with voice/data traffic. While prior analysis of CSMA/CD has shown that the protocol achieves good performance with data traffic over a range of conditions, the widely used IEEE 802.3 (Ethernet) implementation of the protocol has several aspects that are not easily amenable to mathematical analysis. These include the binary-exponential back-off algorithm used upon collision, the number of buffers per station, and the physical distribution of stations. Performance measurements on operational 3 and 10 Mb/s networks are presented. These demonstrate that the protocol achieves high throughput with data traffic when the packet transmission time is long compared to the propagation delay, as predicted by analysis. However, at 10 Mb/s, with short packets on the order of 64 bytes, performance is poorer. The inflexibility of measurement leads to the use of simulation to further study the behaviour of the Ethernet.