Roger Fajman <RAF@cu.nih.gov> (09/24/90)
I would appreciate a short definition of exactly how AMI works on T1 lines. I took a short course from Datatech Institute recently on T1/T3 technology. (I liked the course, by the way.) Now I would like to compare AMI to B8ZS for meeting one's density requirements on our clear channel T1s, but can find only the definition of B8ZS in the course notes. AMI is mentioned as being less preferable, but is not defined. A reference to look it up in would be helpful too, especially if it's a publication likely to be found in a computer-oriented library.
dave o'leary <oleary@noc.sura.net> (09/24/90)
Roger Fajman wrote: >I would appreciate a short definition of exactly how AMI works on T1 >lines. I took a short course from Datatech Institute recently on >T1/T3 technology. (I liked the course, by the way.) Now I would like >to compare AMI to B8ZS for meeting one's density requirements on our >clear channel T1s, but can find only the definition of B8ZS in the >course notes. AMI is mentioned as being less preferable, but is not >defined. AMI is Alternate Mark Inversion. Basically it means that each ones bit is opposite in polarity from the ones bit proceeding it - this is used to deal with capacitive effects, etc. If two consecutive ones bits are received on the line with the same polarity, it is called a bipolar violation. These could be due to noise or a bunch of other factors. One's density is defined in different ways, with a certain number of ones bits necessary to maintain line energy and keep the repeaters in sync. FCC Part 68 and the AT&T 62411 specs disagree on what ones density means. I can provide more details if anyone cares. AMI does nothing to meet one's density requirements by itself - it just takes a synchronous bit stream and flips the ones bits. B8ZS is a technique used to maintain ones density - when the bit stream contains 8 consecutive zeroes, the "Binary 8 Zero Substitution" code is inserted in place of the 8 zeroes. This code intentionally contains a bipolar violation. This is why you have to work it out with the telco when you want to run B8ZS - their repeaters have to pass the bipolar violations (BPV's) rather than "fixing" them. Another technique for meeting ones density is called bit stuffing, i.e. you simply clock the DTE slower than 1.536 Mb/s and but a one bit into each byte, which is stripped out at the other end. When you work out the arithmetic it comes out to a DTE bit rate of 1.344 M/s, your ones density is insured, and AMI works as usual. The bit stuffing is not standardized between CSU's :-( so if you want different CSU manufacturer's equipment to talk to each other on different ends of the T1 (and not lose the extra bandwidth) then you need to run B8ZS. However, not all phone company equipment does B8ZS, although this seems to be getting a lot better. I guess this is why they say that B8ZS is preferable. Our C&P sales guy told us that B8ZS costs more and that we need to run ESF to use it (which I didn't understand...if anyone can explain that one I'd appreciate it). So basically you are always kind of running AMI, its just that B8ZS allows the special BPV's to get through. >A reference to look it up in would be helpful too, especially if it's >a publication likely to be found in a computer-oriented library. I've seen a bunch of books on the digital hierarchy around - however the ones I've purchased aren't with me right now. Two that I would recommend are a book by Bernard Keisler, which I can't remember the title of (something like Digital Transmission Systems, it is yellow with black lettering) and another book called something like "Megabit Communications Systems", I can't remember the author (it is orange and black). The Keisler book is older and is more technical/mathematical. The other book is new, from this year I think, and covers a wider range of material. I should have these books back by later this week, let me know if you can't find them. I saw the "Megabit.." book at the Maryland Book Exchange and at Reiter's downtown today. Good luck...drop me a line or give me a call if you have other questions... dave o'leary oleary@noc.sura.net SURAnet NOC Mgr. (301)982-3214
bakerj@ncar.ucar.edu (Jon Baker) (09/25/90)
In article <12480@accuvax.nwu.edu>, RAF@cu.nih.gov (Roger Fajman) writes: >I would appreciate a short definition of exactly how AMI works on T1 >lines. Now I would like >to compare AMI to B8ZS for meeting one's density requirements on our >clear channel T1s, but can find only the definition of B8ZS in the >course notes. AMI is mentioned as being less preferable, but is not >defined. From ANSI T1.403-1989, Carrier-To-Customer Installation - DS1 Metallic Interface : AMI, Alternate Mark Inversion: A pseudoternary signal, conveying bindary digits, in which successive "ones" (marks, pulses) are of alternating, positive (+) and negative (-) polarity, equal in amplitude, and in which a "zero" (space, no pulse) is of zero amplitude. In AMI, one's density is maintained by converting a zero sample (i.e. eight consecutive zero-bits) into a '2'. Thus, clear-channel is not possible. This method of maintaining one's density is called Zero Code Suppression (ZCS). B8ZS: Bipolar with 8-Zero Substitution. A code in which eight consecutive "zeros" are replaced with the sequence 000+-0-+ if the preceding pulse was +, and with the sequence 000-+0+- if the preceding pulse was -, where + represents a positive pulse, - represents a negative pulse, and 0 represents no pulse. B8ZS basically inserts deliberate bi-polar violations when transmitting a zero; the far-end is expected to detect the deliberate BPV's and unencode them into the zero-sample without reporting an actual BPV. JB
BRUCE@ccavax.camb.com (Barton F. Bruce) (09/25/90)
In article <12480@accuvax.nwu.edu>, Roger Fajman <RAF@cu.nih.gov> writes: > I would appreciate a short definition of exactly how AMI works on T1 > lines. I took a short course from Datatech Institute recently on > T1/T3 technology. (I liked the course, by the way.) Now I would like > to compare AMI to B8ZS for meeting one's density requirements on our > clear channel T1s, but can find only the definition of B8ZS in the AMI simply means Alternate Mark Inversion, and translated to english means that each one bit will have the opposite polarity of the preceeding one regardless of how many zero bits are in between. In fact if a plus follows a plus or a minus follows a minus, that is a bipolar violation and is an error in a pure AMI system. The dumb repeaters need to see some ones to keep their clock in step, so long strings of zeroes are to be avoided. All B8ZS does is replace a string of zeroes with a recognisable pattern providing some needed ones that could not be normal data, and that is itself balanced around zero (same number of plus 1s as minus 1s so there is no net DC component). This recognisable pattern is simply replaced with the equivalent number of zeroes at the far end of the span, but the repeaters see the ones they need to keep their clocks in sync. The special pattern B8ZS uses has two bipolar violations. There will be a plus followed by a plus and a minus followed by a minus. The B8ZS notes you found probably show that special string being substituted for eight zeroes. If you simply DON'T do that substitution, you have a vanilla AMI line. A vanilla AMI line can't guarantee ones density if all bits are to be available for your random use. If you rob a bit fron each DS0 you get your 56kb rate rather than the 64 you should have. Some T 1/2 fractional services provided by a LEC may give you clear channel on 'your' 12 DS0s, but they can simply ram ones on in 'their' 12 DS0s. Theirs will be every other time slot. You get the odd and they get the even, or vice-versa. That way there can NEVER be long strings of zeroes no matter what you send, and they can use most of their OLD hardware to provide you the service. YUK. Clear as mud, right?
bakerj@ncar.ucar.edu (Jon Baker) (09/25/90)
In article <12512@accuvax.nwu.edu>, oleary@noc.sura.net (dave o'leary) writes: > However, not all phone company equipment does B8ZS, although this > seems to be getting a lot better. I guess this is why they say that It should gradually become more widespread. B8ZS is the preferred long-term method of providing Clear Channel. > B8ZS is preferable. Our C&P sales guy told us that B8ZS costs more > and that we need to run ESF to use it (which I didn't understand...if > anyone can explain that one I'd appreciate it). Of course B8ZS will cost more, because it requires special equipment to handle it. There is no particular reason why B8ZS can not be used in an SF format T1. If they only support B8ZS on ESF, it is due to limitations particular to the equipment they are using, or due to their own (marketing?) restrictions. JB