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I have omitted the section that discussed pricing and availability of T1 CSU's as it is over three years out of date. -- Evan Wetstone SesquiNet Network Support ============================================================================== Date: Tuesday, 16 February 1988 01:28:34 EST From: Eugene.Hastings@morgul.psc.edu To: ronr@sphinx.uchicago.edu Cc: cisco@spot.colorado.edu, p4200@devvax.tn.cornell.edu Subject: Re: T1 converters - long (intro to T1 + T1 CSUs) Message-Id: <1988.2.16.6.12.36.Eugene.Hastings@morgul> Status: RO Apologies to those on both lists, but this seems of sufficient interest to spead widely.. Enclosed is a description of T1 principles and our own experience evaluating CSUs as of roughly August. It represents collective experiance of everybody at PSC involved in communications at PSC. The writeup is by Marty Schulman, so the improvements in intelligibility are his, and the inaccuracies are shared by the rest of us :-) (Marty is out of the country right now, and so in no position to post it himself.) I have omitted notes on our local configurations and settings as a perhaps fruitless attempt at brevity. Gene ---------------------------------------------------------------------- T1 Clear Channel CSU/DSU A. Expository on T1 Service T1 for Computer Networks The Bell System's Digital Signal Hierachy ----------------------------------------- To improve signal/noise ratio on multi-line phone trunks, Bell began converting some frequency division multiplexing (FDM) lines to time division multiplexing (TDM) back in the 1960's. The digitization technique chosen was pulse code modulation (PCM), taking 8000 samples/second of the analog waveform and quantizing it to 8 bit precision with an analog to digital (A/D) converter. When the bits are serially shifted out, the signal source is called a "DS0" by the phone company. Including several DS0 channels in one TDM bit stream requires the addition of framing bits, so the individual channels can be identified on recovery. A "DS1" is composed of 24 byte-wise interleaved 8-bit samples (from 24 different DS0's) and one framing bit. The total bit rate is: total rate = 8000 samples/sec * [(8 bits/sample * 24 samples) + 1 frame bit] = 1.544 Mbps a1 a2 a3 a4 a5 a6 a7 a8 b1 b2 b3 b4 b5 b6 b7 b8 ... x6 x7 x8 f0 | | | | | sample from 1st DS0 sample from 2nd DS0 ..... frame bit Sample Bit Frame Four DS1's can be combined into a DS2; 7 DS2's compose a DS3. There are also DS4's and DS5's, used for long-distance trunks often running on optical fiber. T1 Framing Patterns ------------------- To synchronize with the bit stream, the receiver picks a random bit, and then examines every 193rd bit for the presence of the special framing pattern. If too many received bits differ from the pattern, it delays one bit and begins the search again. At 1.544 Mbps, it does not take long to synchronize. The Bell company first used a "D1" framing pattern when T1 began. The pattern was so simple that putting a 1 KHz tone (the standard Bell test frequency) on one of the DS0 channels would cause the circuitry to synchronize on the wrong bit. They changed to 1004 Hz test tones, and later changed to D2 framing patterns. D4 framing is the current most common framing pattern, but recent advances in signal processing make it slightly more redundant than necessary. "ESF" refers to a framing pattern in which three of every four framing bits (8000 frame bits/s * 0.75 = 6 kbps) are used for control and error-checking information. Modulating the 1.544 Mbps bit stream ------------------------------------ While framing patterns facilitate timing recovery at the receiver, special encoding techniques must be used for operation with T1 line "repeaters", the T1 signal amplifiers and conditioners located about every 6000 feet in Bell intracity wiring. Zeros or spaces in the bit stream correspond to periods of zero volts, while ones or marks are converted to 2.7 to 3.3 volts. Using alternate mark inversion (AMI), all adjacent marks are of opposite polarity. When adjacent marks have the same polarity, a "bipolar violation" (BPV) has occurred. Some telephone repeaters can tolerate these. Repeaters also require a "minimum ones density", a pulse at least every 8 bits, in order to recover timing information. For voice channels, forcing one bit in each 8 bit sample to a mark does not seriously degrade quality. Thus, most voice channels actually occupy 56 kbps. This is also why digital dataphone services (DDS) offered by the phone company comes in 56 Kbps chunks. 1 DS0 = ((8 bits - 1 bit) * 8000 samples/s ) = 56,000 "bits per sec" = 56 Kbps Connecting to a T1 line ----------------------- As private branch exchanges (PBX) and computer networking became more popular, phone companies began offering end-to-end digital lines. Equipment connected to these lines which insures proper signal levels, protects against surges, and cleans up BPV's are called "channel service units" (CSU's). Together with PBX's, the equipment is sometimes called "customer premises equipment" (CPE). The specific CSU's intended for T1 lines are referred to as "T1 CSU's", but may be abbreviated as just "CSU". The actual wires bringing the T1 service onto the customer premises may provide a DC current source (60 or 140 mA) for powering the CSU. Such a T1 line is called "wet"; lines not providing this "span power" are called "dry". Though most CSU's allow for use with or without span power, be careful when touching T1 lines, or servicing CSU equipment even when powered down; the constant current source may provide several hundred DC volts without a load. The reason for powering CSU's is to insure a "keep alive" signal of all marks sent on the T1 line, even if main electrical power at the customer site is removed. Without ones density, a repeater can oscillate, affecting communications on adjacent T1 lines. Formally, the phone company is to be notified whenever a T1 CSU is connected or disconnected, but recent advances in T1 repeaters make this "not always necessary" (I didn't say it.) Official T1 line specifications are available in Bell publication 62411. The repeater nearest to the CSU is guaranteed to be within 3000 feet. The CSU provides enough drive to operate that far, and often includes a switchable attenuator known as "line build-out" (LBO) in case it is much closer. The optimum setting of this switch should be provided to the customer by the phone company. Using T1's to carry computer data --------------------------------- Clearly, the 56 kbps and T1 data rates and formats were not chosen with computer data in mind. But if we don't violate the specifications for our applications, the phone company does not care about the type of information source we use. Interfacing computers to T1 lines requires a special formatter to clock serial serial data from a computer (on an RS-422 or V.35 interface, for example) at one rate, insert framing patterns and ones density bits as needed, and then shift out the data at another (possibly different) rate. Also needed is a CSU to properly interface to the line. Of course, the reverse operations need be done at the receiving end. An integrated piece of electronics to perform both these functions is called a "clear channel CSU". If we are using T1 modems "in house", over our own wires in a building less than 6000 feet apart, we can run them at full 1.544 Mbps; no framing bits are needed. It's possible for the phone company to provide a T1 line between two locations in the same city. If told so by the phone company, then only the repeater requirements need be met; the framing bits are irrelevent. However, in most cases the framing bits are included by the equipment, anyway. For intercity T1 lines, framing bits must almost always be added. Adding the framing bits is straight forward. The formatter, or the clear channel CSU, inserts them into the bit stream. Note that right away, available data bandwidth is reduced to 1.536 Mbps: effective rate = 8000 samples/sec * [(8 bits/sample * 24 samples)] = 1.536 Mbps Meeting repeater requirements of one's density are more difficult, and several approaches are available. They include: 1) B8ZS Standing for "bit 8 zero substitution", this technique transmits data at 1.536 Mbps by inserting the pattern 00011011, with BPV's in the fourth and seventh positions, wherever ones density requirements are not met by the unmodified data. It requires the CSU to not remove BPV's, and works only where the phone company equipment can tolerate them. 2) Clever encoding If we know enough about the format or information content of our bit stream, we could perform some clever conversion to suppress strings of eight consecutive zeros. Such techniques rely on the actual information rate being less than 1.536 Mbps, even though that is the final clocking rate of bits onto the line. Three possible specific applications include: a) Run Length Encoding By looking for all consecutive strings of eight or more zeros, and encoding them in a special way within the data stream, ones density can be met. Such an approach is often used to encode image data (often with long stretches of zeros or ones), and is very similar to... b) ZBTSI "Zero Byte Time Slot Insertion" is a proprietary technique used by Verilink in their 551VCC/U clear channel CSU, where long strings of zeros are encoded, and the decoding information is inserted within the framing pattern. (Remember how ESF makes available 6 Kbps for special functions). It offers the most generalized scheme of increasing throughput, at the correspondingly highest price. c) HDLC/SDLC If we understand the protocol enough to know where ones must be, we can scramble the bits and spread them out evenly, satisfying ones density. The Digital Link DL551 offers this approach, and eventually Proteon gateways are to use HDLC or SDLC. 3) Ones Insertion Just as the phone company sacrifices one bit in eight for each DS0, so can we force every eighth bit to a mark, and reduce computer link bandwidth to 1.344 Mbps. Whether such a reduction is tolerable depends on the specific application being considered. effective rate = 56 Kbps * 24 = DS0 rate * 24 = 1.344 Mbps T1 Testing ---------- The phone company often guarantees service performance in terms of "percentage error free seconds per month", though actually measuring that quantity is difficult. In order of increasing thoroughness, some techniques for testing include: 1) Loop Up/Loop Down The most primitive indication of line operation is to attempt to "loop up" the remote CSU, by sending the standard remote analog loopback pattern of "10000". The remote end should return the signal within five seconds of application. Looping down with "100" pattern may take slightly less time. This test, often built into CSU's, takes the line out of service, but is usually only done to determine whether complete link outages are due to the line or computer. 2) Passive monitoring By using the MON jacks available on some CSU's, you can watch the incoming bit stream and check for proper D4 framing bits. If any of these are in error, you can assume a line error occurred, and multiply the frequency of framing bit errors by 193 to estimate total line errors. The FIREBERD bit error rate tester does this. It can test continuously, with no interruption to service. 3) ESF The extended superframe officially divides the 6 Kbps bandwidth scavenged from the D4 framing pattern into a supervisory channel of 4 kbps, (for interogating remote equipment, for example), and 2 kbps for a cyclic redundancy check (CRC). The standard specifies that this be computed using all bits, including data, so it has a better statistical chance of catching line errors than examination of framing bits only. Some companies offer conversion equipment which takes D4 framed signals and adds ESF functions to them. It provides continuous testing while the line is in service. 4) Bit Error Rate Tester For suspected line quality problems, a bit error rate tester (BERT) is usually put on one end of the line, with the other end looped back. Whether it provides a useful measure may depend on: whether gapped clocks are used (as with the DL551V), and whether loopback is analog or digital. You should ask the manufacturer under what conditions this technique is appropriate with a given CSU. It requires taking down the link, and is therefore usually only done when quality is so poor as to significantly impede link utilization. Appendix 1: What's a Gapped Clock? ---------------------------------- A clear channel CSU, or CSU-formatter combination, usually provides the serial data clock to the computer equipment. Depending on the type of encoding, the clock may be 1.544, 1.536, or 1.344 MHz. However, that's given in clock transition rate; they are not necessarily evenly spaced. For example, the Digital Link DL551 clear channel CSU provides a 1.344 MHz clock like: _ _ _ _ _ _ _ _ _ _ _ |_| |_| |_| |_____| |_| |_| |_| |_| |_| |_| |_____| |_| ^ ^ missing missing transition transition If the transitions had been included, the total rate would be 1.544 MHz. But everywhere the CSU inserted a ones density or framing bit, it simply gapped the clock to the computer. This clock is incompatible with some BERTs. Appendix 2: Digital vs. Analog Loopback --------------------------------------- Remote loopback of CSU's is an analog loopback, as it basically sends the same incoming voltage back out the line. However, you can also provide digital loopback, either by placing a loopback connector on the digital signal interface to the computer, or sometimes by configuring the computer interface a certain way. Either digital approach may indicate BERT errors even with a good line. The reason is that each modem may independently determine the rate at which it sends data out on the T1 line. For example: ---------- ------- ------- ---------- | |-----TxD---->| |\/\/\/\/\/\/\/| |-----RxD---->| | |Computer|<----TxC-----| |/\/\/\/\/\/\/\| |-----RxC---->|Computer| | #1 | | CSU | | CSU | | #2 | | |<----RxD-----| |\/\/\/\/\/\/\/| |<----TxD-----| | | |<----RxC-----| |/\/\/\/\/\/\/\| |-----TxC---->| | ---------- ------- ------- ---------- TxD refers to transmitted data, and TxC is the clock for this data; similarly for received data. Note how the CSU provides each clock to its associated computer. As is usally the case, each CSU determines the rate at which it transmits data from an internal osciallator. It must be 1.544 MHz, +/- 75 Hz. The rate at which it clocks in the received data is of course equal to the rate of the other CSU's transmission. Thus, if the clocks are the slightest bit off (and they usually are), digital loopback produces a skewed return signal, producing bit errors at a rate related to the beat frequency of the two oscillators. Sometimes CSU's can be configured to adjust their transmit clock to match the rate of the receiver clock, or even to lock transmission rate to an external clock. This may make remote digital loopback work for a BERT, but has the disadvantage of requiring different hardware configurations for each end of the link. Appendix 3: Foreign Standards ----------------------------- In case it comes up in conversation, European phone networks space repeaters somewhat closer than every 6000 feet, allowing them to use a 2.048 Mbps stream for their equivalent "T1" trunks. Some vendors produce multiplexing equipment capable of connecting countries of different systems. From cisco@spot.colorado.edu Tue Feb 16 21:24:12 1988 Received: from rice.edu by iapetus (AA27799); Tue, 16 Feb 88 21:24:02 CST Received: from spot.Colorado.EDU by rice.edu (AA05988); Tue, 16 Feb 88 21:22:59 CST Received: by spot.Colorado.EDU (1.2/Ultrix2.0-B) id AA21713; Tue, 16 Feb 88 19:50:32 mst Received: from cgl.ucsf.edu (cgl.ucsf.edu.ARPA) by spot.Colorado.EDU (1.2/Ultrix2.0-B) id AA21691; Tue, 16 Feb 88 19:50:18 mst Received: by cgl.ucsf.edu (5.54/GSC4.5) id AA24314; Tue, 16 Feb 88 18:44:39 PST Received: by socrates.ucsf.edu (5.54/GSC4.5) id AA22772; Tue, 16 Feb 88 18:44:36 PST Date: Tue, 16 Feb 88 18:44:36 PST From: tef@cgl.ucsf.edu Message-Id: <8802170244.AA22772@socrates.ucsf.edu> To: cisco@spot.colorado.edu, p4200@devvax.tn.cornell.edu Subject: Re: T1 converters - long (intro to T1 + T1 CSUs) Cc: eugene.hastings@morgul.psc.edu, ronr@sphinx.uchicago.edu Status: RO 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 From cisco@spot.colorado.edu Wed Feb 17 16:52:59 1988 Received: from rice.edu by iapetus (AA29740); Wed, 17 Feb 88 16:52:53 CST Received: from spot.Colorado.EDU by rice.edu (AA08771); Wed, 17 Feb 88 16:51:30 CST Received: by spot.Colorado.EDU (1.2/Ultrix2.0-B) id AA08242; Wed, 17 Feb 88 15:44:49 mst Received: from cgl.ucsf.edu (cgl.ucsf.edu.ARPA) by spot.Colorado.EDU (1.2/Ultrix2.0-B) id AA08215; Wed, 17 Feb 88 15:44:31 mst Received: by cgl.ucsf.edu (5.54/GSC4.5) id AA01903; Wed, 17 Feb 88 13:18:58 PST Received: by socrates.ucsf.edu (5.54/GSC4.5) id AA28228; Wed, 17 Feb 88 13:18:56 PST Date: Wed, 17 Feb 88 13:18:56 PST From: tef@cgl.ucsf.edu Message-Id: <8802172118.AA28228@socrates.ucsf.edu> To: cisco@spot.colorado.edu, p4200@devvax.tn.cornell.edu Subject: Re: T1 converters (correction) Cc: heker@jvnca.csc.org, ronr@sphinx.uchicago.edu Status: RO I stand corrected on a couple of points in my recent email message: 1) AT&T's USA-wide master T1 clock is located in Hillsboro, Missouri, not Atlanta. Hillsboro was chosen because it is the "geographic center" of the country. (Does this mean if there was a giant H-bomb it would be dropped there? Never mind...) There are several backup master clocks arranged in a hierarchal fashion in case of failures in the primary synchronization system. An article about this recently appeared in Data Communications. I'm told it is not necessarily easy to slave CSUs to the master clock. The clock is used by telco COs, but it may not be easy for you to get at it. 2) The ZBTSI ANSI standard is part of T1X1 committee and will be balloted on shortly. Several telco's are already using the current ZBTSI document as a defacto standard. The standard does, in fact, require ESF. It uses 2 kbps of the 4 kbps ESF data channel for transmitting "Z" control bits used to flag encode control information. The properitary Verilink 551VCC product does not require ESF. Other differences between ZBTSI and Verilink VCC are more substantial than I orignally implied. They include: (a) 500 microsecond delay on xmit and recv for Verilink, 500 microsecond delay on xmit only with ZBTSI, (b) no modification of the T1 bit stream if it already meets density requirements for Verilink, channel 96 time slot always exchanged with channel 1 time slot for ZBTSI, (c) no bit scrambling with Verilink, 5-bit scrambler added to ZBTSI data stream to minimize error multiplication.