Two-color one-way frequency transfer through one strand of optical fiber is an alternative method to two-way frequencytransfer, and is useful if unknown asymmetries exist in the link due to different paths for each direction. The term “twocolor”refers to the ability to send signals at two different wavelengths utilizing the same fiber in one direction. The methodis suitable for implementation in existing urban Single Mode Fiber networks, for instance in networks that are utilized for dataand television communication. It is therefore able to coexist with data channels in wavelength-division multiplexing (WDM)systems. It performs as a dynamical control of transit time and simultaneously enables a real-time phase stabilized output signal.This paper presents results from a comparison of two cesium beam frequency standards separated by about 3 km over an opticalfiber network located in a metropolitan area in Sweden. The cesium standards were simultaneously compared to each other witha Global Positioning System (GPS) satellite link and over optical fibers, so that the optical fiber technique could be evaluatedwith respect to the GPS technique. The difference in frequency stability between the two methods is shown to be about 3 × 10-15over an averaging interval of 10 000 s.1. IntroductionThe need for high accuracy time and frequencytransfer has significantly increased over thelast decade. Users with the most demandingrequirements for the characterization of remotelylocated time and frequency standardsare investigating the use of optical fibers, asthe accuracy and stability of methods based onGNSS (Global Navigation Satellite Systems)such as Global Positioning System (GPS) appearto be reaching their limits. Recent workin the field of optical frequency transfer showsresults with potential accuracies below 10-17obtained in less than a day, demonstrating thatthe technique is useful for the comparison ofoptical frequency standards [1, 2]. Most of theoptical frequency transfer methods or laserstabilizations [3] focus on two-way frequencytransfer [4], whether it uses the optical phase[5-10] or intensity modulation at microwavefrequencies [11], dark fiber, or data protocolsutilized for timing in wavelength division multiplexingchannels [12-14]. When two-wayfrequency transfer methods are used, there is apresumption that the signals delays are equivalent(symmetrical) in both directions, but inmost cases the two counter wise transmissionpaths are asymmetrical and this introduces a