A dry pair is a pair of telephone wires that does not have regular voice on it, thus no dialtone. It is a dedicated line usually used for internet. Dry pairs are now found in SDSL, ISDN, IDSL, or a T-1 type service.
The following clarification submitted by NickNielsen
It is true a dry pair does not have dial tone, but a dry pair does not have any battery on it by definition. A wet pair has battery present on it.
There are several applications for a dry pair. A T1 is not a dry pair in the sense used. Indeed, a T1 opposite of the customers side of the network is hardly ever dry. It will commonly have 120VDC which is used to power remote repeaters for the T1 circuit depending on its length. The T1 network interface will isolate this high voltage and bring the signal to a much safer level. This is the reason the CSU/DSU has a Line Build Out (LBO) setting maximum of just over 600' from DMARC. It may not reliably work past that distance from the CSU/DSU.
This point might have been confused with the fact that any T1 provisioning requires removal of load coils from the existing cable, which are used to balance the freq response for voice freqs. These coils also block the higher freq that T1 uses. So when one asks for a "dry pair" for data application, it should also be "unloaded", otherwise it will not pass the high freq for the data.
A DSLR user adds this note:
In southwestern Ontario, dry loops (or pairs) do have a dial tone. This is so we can call the Bell# that identifies the number you are calling from. It's important to make sure you have the right pair when a company has 10+ copper lines. You can always try calling a regular local number and if it is a dry loop, it will tell you that the line must be activated. These lines are owned by Bell Canada.
last modified: 2007-10-11 13:15:33