larry@uunet.uu.net (Larry Lippman) (07/16/90)
In article <9156@accuvax.nwu.edu> the Telecom Moderator writes: > [Moderator's Note: Loops are used to test circuits from a remote > location. As an example, a tester in Kansas City wants to check a > circuit coming from Chicago. He calls the incoming side of a loop in > Chicago and 'loops-around' to make an outgoing call back to Kansas > City. I've never encountered any loop-around circuits which placed *outgoing* calls on the second termination port of the circuit. All loop-around circuits that I am familiar with serve to connect two incoming calls, whether the loop around is terminated as directory numbers in a end office CO, or as a trunk termination code in a tandem office (common codes were 663 for first port, and 667 for second). The use of loop-around circuits in tandem offices has generally been replaced by 104-type (also the dial code) test lines. Loop-around circuits are intended for one craftsperson to make two-way transmission loss measurements on four-wire interoffice trunks or two-wire interoffice trunks with hybrid repeaters. The transmission frequency used is almost always 1,000 or 1,004 Hz. Note that I said two-way transmission measurements because the direction of transmission is changed during the test to obtain two sets of measurements. Two-way measurements obviously cannot be made using a milliwatt test circuit. Two-way measurements are important to assure that far-to-near and near-to-far losses are the same. There are three common variations in loop-around test lines: (1) The two terminations are simply connected together, one after the other. (2) The first termination provides a milliwatt test line, which is disabled when the second number is called, thereby connecting the two terminations together. (3) The first termination provides a milliwatt test line, if it is called by itself, and the second termination provides a balanced termination for noise measurements if called by itself. When both terminations are called at the same time, they are connected together. This is often called a CLA line (Combined Loop-Around). To discourage unauthorized persons from using loop-around lines for "talking purposes", some loop-around circuits are provided with guard circuits which detect speech energy (as opposed to a single tone used in transmission measurement), and force a disconnect. > Other loops may allow a telco employee working outside his/her > regular district to access special codes which only work from one > central office rather than everywhere. As an example, certain loops in > Chicago receive calls on one line, and immediatly grab the outgoing > side and place a call to '611' (repair service). The 611 you get is > obviously not the 611 I get, so if I want to get yours (as a telco > employee authorized to do so), I have to get a line from your central > office. The loop provides this access to dial tone in another office > when it is needed. PT] I've not encountered any of the loop-around variations that are described above. Quite frankly, I am at a loss [>pun alert<] as to why such circuits would be implemented. Transmission measurements using loop-around circuits are *invariably* made from the CO. There are innumerable ways in which a craftsperson in a CO can reach other CO's or test facilities through direct access to an outgoing tandem trunk or other test trunk circuit. Larry Lippman @ Recognition Research Corp. "Have you hugged your cat today?" {boulder||decvax||rutgers||watmath}!acsu.buffalo.edu!kitty!larry VOICE: 716/688-1231 || FAX: 716/741-9635 {utzoo||uunet}!/ \aerion!larry [Moderator's Note: For many years, we had loop arounds here in Chicago numbered XXX-9954 and XXX-9955. Call in on 54, it answered and extended dial tone from 55. An auto-dialer would grab the line and dial out '611', and 9955 would place the call for them. Then one day a couple of phreaks ruined it by learning that if they dialed out *before* the auto dialer got started, they could call wherever they pleased, and having satisfied the equipment accepting the outcall from 9955, it would ignore the 611 request by the autodialer. PT]