OK...you're closer than Garlic Whatziname...but still not quite there. :) I wish, I wish, I wish I knew of an online source for this printout that I have in my hand...its *very* instructive about how T1's work...but alas, you all will have to deal with my summarization. :) This just in <sound of teletype here>...found it online :) http://duracef.shout.net/~wildixon/telecom/t1/t1.html Thus spake Aaron Nabil
Second problem with the signalling bits is that they only occur every 6th and 12th frames. So even if they were always ones, they would only affect ones density during those frames.
Correct so far (assuming D4 framing of course...which is what was being discussed, so a good assumption :)
So "lots" (8) consecutive zero can and do occur on a CT1 line.
Depends on the application. With voice calls in the DS0's, the encoding of the voice data into the DS0's actually enforces one's density inherently. The encoding algorithm won't generate bit patterns that don't meet one's density requirements.
Without B8ZS, the framer has to enforce ones density by stuffing ones, this causes a drop in the S/N ratio. (I seem to remember 4db as the figure, but I don't have any reference material in front of me to back that up.)
I don't know what it translates into as far as a db loss, but it eats up one out of every eight bits of data meaning a loss of 192kbps on a T1 total...dropping the total useable bandwidth to 1.344Mbps
B8ZS is able to handle these strings of zeros by inserting a bipolar violation that the receiver knows to remove.
Actually, its two bipolar violations in a specific pattern...but you had the right idea. :) The specific pattern is 00011011, with BPV's at the 4th and 7th bits.
Since B8ZS doesn't have to force ones, it doesn't have the associated S/N degradation of AMI only.
Actually...AMI doesn't have any S/N degradation. I think you're thinking of ZCS, or Zero Code Substitution (I've also seen it referred to as simply "Ones Insertion"). Basically, with ZCS, the equipment jams a bit to one to enforce the one's density...of course the receiving equipment doesn't have any way to tell whether this bit was on because of the data or because of the need to maintain one's density, so you basically just cannot put any data into the bit used like this...this drops the data rate down to 1.344Mbps as above. Again, with voice encoding, neither ZCS or B8ZS is really needed as the encoding won't generate codes that don't meet one's density requirements...its when you get into data transmision that you have to be concerned about one's density since that can generate all zero's. AMI, for what its worth, is pretty much universal on all T1's. Basically, all AMI (Alternate Mark Inversion) means is that a "mark" or one bit, has the opposite voltage polarity from the previous mark or one bit. If two successive marks (with 0 or more interveaning spaces or zero bits) have the same voltage polarity, then you have a BiPolar Violation (BPV). As you can see...since B8ZS uses BPV's to indicate an all zero timeslot, B8ZS assumes AMI. :)
So try and get B8ZS if you can.
If you're only running voice calls...B8ZS isn't really needed...now, having ZCS enabled (as our telco did on some of their trunks at one point) can cause some problems since that fairly significantly cuts down the total bandwidth...you're gonna see speed drops in that case. But as long as ZCS is off, you should be OK as far as transmission speeds even without B8ZS. As someone else mentioned...if you have problems B8ZS and ESF will be nice to have as they do have some trouble-shooting capabilities...but in general operation...its not really a problem if all you're doing is voice calls. Now, when you go to ISDN PRI, you have to have B8ZS with ESF because at that point you're encoding data onto the circuit which can result in all-zero timeslots. -- Jeff McAdams Email: jeffm@iglou.com Head Network Administrator Voice: (502) 966-3848 IgLou Internet Services (800) 436-4456 - To unsubscribe to usr-tc, send an email to "majordomo@xmission.com" with "unsubscribe usr-tc" in the body of the message. For information on digests or retrieving files and old messages send "help" to the same address. Do not use quotes in your message.