Path: utzoo!utgpu!water!watmath!clyde!rutgers!gatech!hao!ames!pasteur!ucbvax!CGL.UCSF.EDU!tef From: tef@CGL.UCSF.EDU Newsgroups: comp.sys.proteon Subject: Re: T1 converters - long (intro to T1 + T1 CSUs) Message-ID: <8802170244.AA22772@socrates.ucsf.edu> Date: 17 Feb 88 02:44:36 GMT Sender: daemon@ucbvax.BERKELEY.EDU Organization: The Internet Lines: 127 Thanks to Marty Schulman and Eugene Hastings for the excellent introduction to T1 signaling technology and CSU equipment. Their writeup removes much of the "black magic" surrounding T1. At BARRNet we have built our regional network based entirely on T1 circuits and hence have gained a wealth of experience with T1 equipment and common carrier providers during the past 1.5 years. With this preface, I'd like to share some T1 knowledge and correct a few errors in Marty's and Eugene's writeup. The T1 Carrier standard specified in Bell Pub 62411 specifies minimum ones density in two ways (both minimums must be met) (a) an average ones density of not less than 12.5%, and (b) no more than 15 consecutive zeros between one bits. In North America, a bit is "robbed" in each DS0 subchannel every sixth frame to carry circuit signaling information (e.g. on-hook and off-hook indications). Thus a bit is NOT stolen from every byte in a DS0 signal, but rather only from every 6th byte of any particular channel. However, since data terminal equipment (DTE) has no easy way of determining which byte will have have a bit robbed from it, it is simplier just to have a 56 kbps clock (7/8 * 64 kbps) for all DS0 data circuits. If a T1 formatted bit stream does not represent 24 DS0 channels, then there is no need to do anything funny with one of the bits from each channel every sixth frame. In other words, it is NOT necessary to force every 8th bit of user data to be a one. This just needlessly decreases usable bandwidth (more on this below). The reason it is done so often in clear channel CSUs is because it is easy to implement and clearly meets (actually far exceeds) the ones density requirements listed above. More sophiscated CSUs (such as the Verilink 551VCC) do not treat the user's bit stream as 24 7-bit bytes, but rather operate on a larger group of bytes in a more intellegent manner (hence their higher cost). The ones density requirements, as Marty says, is to keep the T1 line repeaters operating properly. Note that the 62411 standard was developed when analog repeaters were the only ones available. Today's digital repeaters can operate on a much lower ones density; some military spec repeaters operate with up to 50 or 60 consecutive zeros. Unfortunately you have no way of knowing what kind of repeaters are in any particular T1 circuit and hence all commerical CSUs are built to the 62411 standard. Other minor discrepancies in the writeup: 1) the "I" in ZBTSI stands for interchange, not insertion. The algorithm exchanges the "time slot" occupied by a byte of all zeros with another non-zero byte. The position of the zero byte in the data stream is indicated by a 7 bit (inherently non-zero) index value with the 8th bit indicating if there are additional zero bytes present. Either framing bits or data bits (see #5 below) are used to flag the fact that the data stream has been encoded. 2) ZBTSI is now a Bell standard and is not proprietary to Verilink. The Verilink encoding scheme is actually slightly different from the ZBTSI standard. 3) ZBTSI has nothing to do with extended superframe format (ESF). Both ZBTSI and Verilink's proprietary clear channel technique work independently of ESF. 4) CSUs should not be configured to generate their own clock, rather they should always recover the clock from the network. Common carriers have gone to GREAT lengths to insure synchronized clocking. In the USA, there is a nominal USA-wide master clock generated from an atomic time source located (I think) in Atlanta. Obviously the phase of this clock varies from location to location across the USA, but the frequency should always be 1,544,000 Hertz EXACTLY. 5) Because of the different encoding schemes, there are actually several options for getting the highest effective user bandwidth on a T1 channel. The data rates that commonly come up are: 1.544 Mbps - The total bit stream including both user data and framing bits. The standard framing bit format today is D4 and includes both "T" (terminal) framing bits and "S" (multiframe alignment) framing bits. 1.536 Mbps - The total bit stream rate less the framing bits. I.E. the maximum usable user bandwidth on a T1 channel. It is this bit stream that is usually modified to meet the ones density requirements (this is because the framing bits must conform to the D4 standard and hence cannot be modified, although one of the Verilink 551VCC products does modify the framing bits). 1.528 Mbps - A DACS-compatable (digital access and cross-connect system) clear channel bit stream. Some telco central offices contain DACS equipment which strips the framing bits off from T1 bit streams, then reframes the stream later on. Since some encoding methods (e.g. Verilink VCC) purposely inject errors into the "T" framing bits on a T1 signal, these bit streams are not compatable with DACS equipment. To make these encoding schemes compatable with DACS, an 8 kbps "channel" is used for the encoding control information. 1.344 Mbps - (= 24 * 56 kbps). This is the user data rate obtained when using the brute force method of insuring minimum ones density. As Marty and Eugene point out, the method may be so crude as to clock the user data in "gapped" form, essentially stalling the DTE data clock while the CSU inserts its own bits for ones density and framing. CSUs which operate in this mode essentially "throw away" 192 kbps of user bandwidth by robbing every 8th bit position in the user's data stream. Not so much a discrepancy, but something that should be pointed out is the fact that very little of today's installed telco equipment (~1%) is capable of working with B8ZS (bit 8 zero substitution) signal format. I'm told by telco personnel that within 10 years 90% of all T1 equipment will be B8ZS compatable. What this means to you, the user, is that it is unlikely that any B8ZS CSUs you buy today will work today. If you buy a clear channel CSU that works via B8ZS encoding be sure you test it with the telco T1 circuit before you commit your dollars. If your T1 channel is multiplexed and demux'ed by the common carrier, B8ZS can't work until all the mux'ing equipment is upgraded to understand receiving intentional bipolar violations and regenerating them at the far end. ZBTSI, on the otherhand, will work with all of today's equipment. As VLSI circuits are developed to implement the ZBTSI algorithm (it requires buffering 96 bytes of data and encoding these as a unit), more manufactures will offer ZBTSI equipment. The only manufacturer I know of currently offering a ZBTSI clear channel product is Verilink. --tom ferrin