"In 1998 CBS did a 50th anniversary special , claiming that 1948 was the beginning of the TV network..."
In a way that was true, since April of 1948 was the time CBS began to send programming to other cities and other affiliates beyond the New York market (anything they produced before April of '48 was seen only on Channel 2 in New York). But WCBS-TV was on the air from the beginning day of commercial TV in 1941 (with the callsign WCBW until 1946)--they just didn't feed programming anywhere else, and most of their key early affiliates in towns like Buffalo, Boston, Detroit, Philly, Baltimore, Cleveland and DC didn't get up and running until the winter of 1947 or the spring of 1948.
NBC, on the other hand, was sending shows on a spot basis to other stations from the start, even in the months before WPTZ in Philly (fall '41) and WRGB in Schenectady/Albany (spring '42) had their full commercial licenses in hand. NBC had a regular schedule of prime-time newscasts and news/feature magazines fed to those three cities as early as the middle of 1944, when all three had their full commercial tickets.