Date: Sun, 14 May 1995 21:21:14 -0400 From: Dave Emery Reply to: Radio Scanner Discussion Forum To: Multiple recipients of list SCAN-L Subject: Re: Motorola Sabers > > >From what I understand regarding a digital radio system the whole thing > is digital both into and outof the repeater. > Motorola digital radios such as the Sabre use FSK modulation at 12 kbs both to and from the radio when in digital mode. Most Motorola radios will also do FM voice on the same channel. The Astros have the ability to do 12.5 or 6.25 khz channel spacing digital voice at a lower bit rate (possibly using GMSK). The Sabre DVP/DES radios use CVSD voice encoding at 12 kbs, I beleive the much newer Astros use LPC at a lower rate. The Sabres come with a couple of digital security modules; one which supports an easily crackable Motorola proprietary code that is allowed for export called DVP, and a couple of versions used by the Federal government which use DES in what I beleive is CBC mode. The current federal DES version is called DES-XL. In this mode the initialization vector for DES on each bit is derived from the previous 64 bits of ciphertext. Repeaters for radio systems using either DVP or DES detect the presence of the FSK rather than FM (presumably by looking for the 12 kbs data clock related spectral components) and do a classic regenerative repeating of the data, reclocking it with a 2 or 3 bit delay and modulating the transmitter on the output freq with the reclocked data. Thus the repeaters do not necessarily ever decode the secure voice to clear voice (red) but merely repeat patterns of bits from the input channel. Larger systems with linked repeaters use modems to send the digital DES data from the receiving site to all the transmitters. A specific tone is used on the linking audio channel to indicate the presence of clear rather than secure traffic and switch the transmitter to transmit FM rather than FSK. It has been rumored (but not confirmed) that in a few places when the Federal DES radio systems (such as Customs, FBI, Secret Service and so forth) were first installed, they linked some of the repeaters together over an analog in the clear channel on microwave links rather than using the modems to carry the secure voice in digital form. This of course meant that anyone with the right microwave receivers could listen to the secure traffic in the clear !! It is known that some federal wide area systems are linked together on microwave (much of it around 1.8 and 2.2 ghz) without the digital link and can only work secure within the range of one transmitter receiver pair. Dave Emery N1PRE