Time servers come in several shapes and sizes. The primary difference between most dedicated time servers is in the way they receive a timing source.
Some time servers utilise national time and frequency transmissions that are broadcast on long wave while other use the GPS network.
Some time servers are designed to be rack-mountable perfect for the average U system of racks allowing the sever to be snugly fitted into your existing rack.
Other time servers are nothing more than small boxes that can be discretely hidden.
Here is a list of top time server manufacturers: