CHAPTER VII. The Switched Communication Network Simple 2 wire Local Transmission Line Echo Path ÚÄÄÄÄÄÄÄ¿ -1- ÚÄÄÄÄÄÄÄ¿ ³ Trans ³ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄij Rec ³ ³ ³ \ \ ³ ³ ³ ³  -2-  ³ ³ ³ ³ ÄÄ / ÄÄ / ³ ³ ³ Rec ³ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄij Trans ³ ÀÄÄÄÄÄÄÄÙ Ä Ä Ä Ä Ä Ä ÀÄÄÄÄÄÄÄÙ -3- -1- represents the transmit carrier from one modem. -2- represents the echoing back of the transmit signal to the other wire. (Talkers echo) -3- the echo on the 2nd wire cause another echo on the other direction (Listeners echo) For short distance communication, your two wire line goes in and out of Telco Offices with any major transformations. Among the first things we encounter is the ECHO. Irregularities inherent in transmission cause a portion of the signal energy to be reflected back toward the originating end. This is referred to as Talkers Echo. When using a Phone, you can here your own Voice on the receiver but at a much lower Level. NOW get this !!! "If the Talkers Echo encounters Irregularities (by the way I am quoting a Bell System Manual here) on its way back to the originating end, still another echo is produced which will propagate in the same direction as the desired signal" but on the other line. Think about it for a minute. You MODEMS receiver might see its own TRANSMIT Carrrier !! AND !! Meanwhile the 2nd echo is aimed at the other modems TRANSMITTER. ³ 500mv ³ ³ xxxxxxxxxxxxxxxxxx Normal Modem Receive Level 400mv ³ ³ 300mv ³ ³ 200mv ³ xxxxxxxxxxxxxxxxxx Nominal Echo Receive Level ³ 100mv ³ xxxxxxxxxxxxxxxxxx Background Noise Receive Levels ³ 0 voltsÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ Normally, the echo's are not a problem. Their strength is usually so low that they can not be picked up by the receiver. However should these Phone Line Irregularities become so prominent that they generate loud echo's, Data WILL BE interfered with. Loud echo are obvious. Connecting a handset to a line and checking for echo's is easy. Remember This !!!!! Transmit ³ ³ ³ ³ Receive Frequency ----- ³ ³ ³ ³ ----Frequency Range ³ ³ ³ ³ Range . . . . . . . . . . . x . . . . x x . . . . x x . . . . x x . . . . x ³ x . . . . x ³ x . . . . . x ³ x . . x ³ x . . x ³ x . . x ³ x . ... x x ³ x . . . ³ ³ 1170 Hz 2125HZ This kind of effect is know as "Distortion" Distortion occurs when the loss or delay of a transmission line varies as a function of frequency. In the above example, we see that we can pass certain frequencies but can not pass other frequencies. If we were to transmit a test signal at one end and monitor the receive level at the other end, we would see a sharp loss when we attempt to transmit through the BAD area. As an example, if at 1170Hz we were to receive the test signal at 400 mv (milli volts) or (400 x 10 -3) or (.4 volts) then at the 2125Hz in the Bad area, the receive level might be 100 to 200 mv. The most common used term in the communications industry is the DB (not to be confused with PCB's). The Decibel (DB) is a unit for measuring the relative loudness of sound OR unit for expressing the ratio of two amounts of electric or acoustic signal power. Since we are after all dealing with Sound transmissions, the DB is apropriately used. Dont let it confuse you. Decibels may be a distinct unit of measure however it can be converted to Voltage potential (Volts and milli volts). You should think of it as an alternative language. It is important you learn DB's for in the world of Data Comm it separates the people who know a little from the people who know even less. We transmit at a certain DB level,we receive at a certain DB level, Interferring noise is measured in DB's and Frequency response is also measured in DB's. For this reason, next week I will included a chart showing DB to Voltage conversion with respect to Power & impedance of the voice grade line. With this chart, you will be able to measure the actual voltage level of received or transmitted signals and by referring to the chart, know what DB level you are dealing with. The opposite will also be true. If someone tells you to look for a specific DB level, you can look at the chart and know what voltage level you are looking for. When you get this chart; Keep it. Copy it. Laminate it. Dont ever lose it. DECIBELS were designed to help determine the rate of change of sound. In the Data Comm world, that use is not consistent. We use the DB in the comm world as a reference point. A voltage level actually. However since the DB is the language of the Phone company, we are forced to accomodate them. In the DB world a 2volt peak to peak signal (2.2 actually) is the ZERO reference point for our application. It represents the maximum permissible signal level that can be or should be put on a Telco line. From there, all voltage levels get smaller. However since ZERO is as HIGH as we can go, we represent these smaller voltages in NEGATIVE DB numbers or MINUS DB. The following table will not be found in a library or bookstore. I tried. I got it from General Data Comm Tech support. These levels represent DB's of a Balanced Telephone Line Load. P-P DB P-P DB P-P DB Voltage Level Voltage Level Voltage Level 2.2 0 120 mv -17 16 mv -34 2.0 -1 100 mv -18 14 mv -35 1.8 -2 90 mv -19 13 mv -36 1.6 -3 80 mv -20 11 mv -37 1.4 -4 70 mv -21 10 mv -38 1.2 -5 60 mv -22 9 mv -39 1.1 -6 55 mv -23 8 mv -40 1.00 -7 50 mv -24 4.5 mv -45 900 mv -8 45 mv -25 2.5 mv -50 800 mv -9 40 mv -26 700 mv -10 35 mv -27 640 mv -11 30 mv -28 570 mv -12 28 mv -29 500 mv -13 25 mv -30 450 mv -14 22 mv -31 400 mv -15 20 mv -32 350 mv -16 18 mv -33 If you were to measure with an oscilloscope, the p-p voltage at the Telco D mark, and see a signal that is 1 volt from peak to peak, you would say your signal strength is Minus 7 DB. On the other hand, if you were to use a VF MOnitor that registers DB's and it registered -7DB you would know that you have present a 1 Volt Peak to Peak signal. (Peak to Peak is refering to the wave form I displayed as carrier. From the most positive voltage to the most negative voltage. --------------------------------------------------------------------- The Telephone Company provides Voice Grade lines for telecommunication use. We use these lines for our Personal Computer Modems. You can also get condition Voice Grade Lines for Data use commonly referred to as Switched Network Lines. These lines operate the same way as the Voice Grade lines, but they are conditioned for lower noise levels and have a Fixed Loss Loop. Line Loss can be said to include the impedance of your "In House wiring" added to the distance of the telephone pole outside your house to the nearest Phone Company Office. Fixed Loss Loop (FLL) is a preset factor which the Telephone Company creates by taking the already exisiting Line Loss and adding or subtracting impedance to insure that your modems transmit carrier arrives at the Phone Company office at a Level they prefer to see. Each Phone Company office may vary as to what it wants to see. However by using a predetermined Modem Transmit level, any phone company can adjust the FLL to suit its own need. Normally the FLL is set such that if your Modem were to transmit at -11 db it would reach the Phone Company office at the correct Level. If your modem were to transmit at a higher level (-9 db -8 db etc) you begin to risk jamming the phone company with a signal that is to strong. If you transmit at a lower level (-13db -15 db etc.) you risk your signal being to weak to be received. Of course there is a tolerance factor. Most Modems in the PC community are designed to transmit at -9 db. There are few problems with that level. The alternative to the FLL is the Programable Jack. The Phone Company creates sort of a bridge effect. It puts into your Phone Jack a resistor that allows your MOdem to determine a balance of impedance with the phone line. The Modem can thus adjust its transmit level as the Phone line impedance changes insuring a more accurate transmit voltage. Electrical properties are Non-stable. The impedance of Lines can vary by their length (distance), by their gauge (width of wire), by the temperature and other outside influences. If we expect to Transmit in the -9 DB range, then what do we expect to receive our incoming signal at ?? The phone company is required by Tariff to insure no greater that a 16 db loss from tranmitter to receiver. If we transmit at -9 db that we expect the other modem to receive the signal at -25 db. Modems are capable of receiving downward to -40 db, However since Line noise does exist, any receive level below -30DB can be suspect as a problem child. These levels are true for Dial Up Modems over switched Networks ONLY. ---------------------------------------------------------------- In the world of Industry, Modem transmit levels must be set by the customer or installer. The installer must know if the Jack being connected too is a FLL Jack or programmable. Some modems have an option for Programmable & Permissive to allow the modem to make the automatic adjustment. Others assume Permissive and give you NO option. Home PC modems do not have these options YET but they will have as they attempt higher and higher speeds. When our switched carrier phone line connects us long distance, we no longer operate over just a switched network. Our call is placed on a trunk or multiplexer for lengthy travel. Since noise and deterioration would occurr normally, our signals are separated and individually handled allowing for better amplification or regeneration without loss. 2 Wire to 4 wire Transmission Line ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿ ³ Modem ³ ÀÄÄÄÄÄÂÄÄÂÄÄÄÄÄÄÙ ³ ³ Local Phone Company ³ ³ Wires ³ ³ ÚÄÄÄÄÄÄÄÄÄÄijÄijÄÄÄÄÄÄÄÄÄÄÄÄ¿ ³ ÚÄÄÄÄÄÙ ÀÄÄÄÄÄ¿ ³ ³ ³ ³ ³ ³ ÚÄÄÄÁÄÄÄÄ¿ ÚÄÄÄÁÄÄÄÄ¿ ³ Phone Company Converts 2 Wire to ³ ³ Hybrid ³ ³ Hybrid ³ ³ 4 wire. A separate 2 wire Line ³ ³ Trans ³ ³ Trans ³ ³ for the transmitter and a ³ ÀÄÄÄÒÄÄÄÄÙ ÀÄÄÄÒÄÄÄÄÙ ³ separate 2 wire line for the ³ º º ³ Receiver ³ º º ³ Four Wire Trunk Line Or Multiplexed On a T1 Facility ³ º º ³ ³ º º ³ ³ º º ³ ³ ÚÄÄÄÐÄÄÄ¿ ÚÄÄÄÐÄÄÄ¿ ³ ³ ³ Hybrid³ ³ Hybrid³ ³ ³ ³ Trans ³ ³ Trans ³ ³ ³ ÀÄÄÄÂÄÄÄÙ ÀÄÄÄÂÄÄÄÙ ³ ÀÄÄÄÄijÄÄÄÄÄÄÄÄÄÄÄÄÄijÄÄÄÄÄÄÙ ÀÄÄÄÄ¿ ÚÄÄÄÄÄÙ ³ ³ ³ ³ Telephone Line ³ ³ ÚÄÄÄÄÄÁÄÄÄÁÄÄÄÄÄÄ¿ ³ Modems ³ ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ Most of our work has been involving dial up Modems over switched Networks. Dedicated Lines are also used, mostly by Industry. These lines run from point A to point B. They are not accessible by outsiders. No dialing is required. The line is always connected to the modems and the modems are always connected to each other. These dedciated 4 wire lines are Two pairs of phone lines. One used exclusively for the Transmitter and the other used exclusively for the Receiver. Each tranmitter now having a full line, a full band spread, of its own not needing to share it with anyone. ÚÄÄÄÄÄÄÄ¿ ÚÄÄÄÄÄÄÄ¿ ³ M TxÆÍÍÍÍÍÍÍÍÍÍÍ͵Rec M ³ ³ O ³ ³ O ³ ³ D RecÆÍÍÍÍÍÍÍÍÍÍÍ͵Tx D ³ ³ E ³ ³ E ³ ³ M ³ ³ M ³ ÀÄÄÄÄÄÄÄÙ ÀÄÄÄÄÄÄÄÙ Since these lines are DEDICATED, they can be shared with other specific locations HARD WIRED into the Line. Thus we get our two definitions of DEDICATED LINES. 1. POINT to POINT - A four wire line running from point A to point B 2. MultiDrop - A 4 wire line connected from a Master Modem to 2 or MORE remote modems (drops) ÚÄÄÄÄÄÄÄÄ¿ ÉÍÍÍ͵Rec M ³ º ³ O ³ º ³ D ³ º É͵Tx E ³ º º ³ M ³ º º ³ ³ º º ÀÄÄÄÄÄÄÄÄÙ ÚÄÄÄÄÄÄÄÄ¿ º º A 2 "DROP" M ³ M TxÆÍÍÍÍÍÍÍÎ º A ³ O ³ º º Multidrop Line S ³ D ³ º º T ³ E RecÆÍÍÍ» º º E ³ M ³ º º º ÚÄÄÄÄÄÄÄÄ¿ R ³ ³ º ÈÍÍÍ͵Rec M ³ ÀÄÄÄÄÄÄÄÄÙ º º ³ O ³ º º ³ D ³ ÈÍÍÍÍÍÍÊ͵Tx E ³ ³ M ³ ³ ³ ÀÄÄÄÄÄÄÄÄÙ Wait a second. Is This possible ?? Three Modems on a Line ????? Wont the transmitters interfer with each other. This and other exciting information in Chapter ??, The Wonders of the Multidrop Line. When you utilize a Dedicated Line, you are always using the same circuitry. If the line goes bad, it must be repaired. On a dial up line, if it goes bad, you hang up and call again. When you call locally, you carrier goes through the local phone comapny switch to its destination. When you dial non local but no too far, you call goes via microwave or trunk to the proper Phone company office onto its destination. When you call very long distance, or coast to coast, there is now a good possibility that your call will be via satellite. Satellites make for poor Data Comm facilities. There are massive delays in transmission, horrible echos, and something called the DOPPLER effect. The DOPPLER effect is simply a reference to the fact that the satellite is in motion and that during prolonged transmissions, this motion can cause the carrier to become distorted. --------------------------------------------------------------------- Bear with me on this one. You have a dedicated circuit (this problem occurs on dial up also but since you can dial a new line, you will never know it) that wont let you pass data. The Phone company has swept the line and found no problem. Your Data Comm engineer has such a sincere face, you know he is right. What might the problem be ?? PROPAGATIONAL DELAY ...... In a switched network, the amplifiers on the phone circuit, sees the carrier turned on and then they turn on to allow the signal through. HOWEVER it is possible that some circuit, switch, amplifier or whatever "turns on toooo slow". You Transmit a Data Block. An amplifier turns on toooooo slow and the beginning of you carrier gets clobbered turning your Data Block to mince meat. The phone company can not detect this problem because they use a Continuous Tone to sweep the line. Only an engineer or a good guesser can determine that problem. The biggest threat to Data Circuits is NOISE. There are a number of different types of Noise. IMPULSE NOISE is sporadic, Low Frequency Voltage spikes caused by older type Phone Company Equipment. There are still many many parts of the country with Non-state of the art Phone Equipment. A VF line monitor or telephone handset can pick up impulse noise. It sounds like a pop, or a crackle. (sorry Kellogg, no Snap) BACKGROUND NOISE is present in every circuit. Usually it is filtered to such a degree that it is rendered harmless. It becomes harmful when its signal strength increases to a point where it can compete with your carrier. It usually has the same sound you here when you have your radio or TV tuned to NO station. When you amplify your Data Signal you amplify the background noise with it. If the Data Signal is weak and Telco tries to compensate by increasing the circuit Gain, they will increase the noise level as well. INTERFERENCE - Outright interference is a real problem. It comes from many sources sneaking its way in at the Phone Company Office, along the telephone line route or from the wiring in your house. I have heard Military CW, Radio Stations, Foreign Exchange lines and engine noise interfere with Data Communications. Idealy your Computer and Modem should be on its own circuit breaker (Mine is). An Air Conditioner turning on and off can do incredible things to your data. Your computer and Modem should share a Common Ground. DO NOT USE a Ground Eliminator Adapter. This can be hazardous to the health of your Hardware. Interference must be tracked down and eliminated. If the source is the Phone company, it is their problem to correct. Otherwise it is your problem. SHIELDED TWISTED PAIRS are the best Data Comm cable routing. Another interesting device is called an echo suppressor. When making a long distance call, the phone company will sometimes insert an echo suppressor designed to control the amount of echo on the line. This has the effect of shutting down one side of the line. This of course is only used in the 4 wire state after your call has been transformed from a 2 wire state. The echo suppressor, depending on the type, can have a 100 millisecond delay in getting out of the circuit. Carrier generated during that time may be destroyed. Some commercial modems have what is called a "Dither Tone". The purpose of the dither tone is to deactivate the echo suppressor by keeping both side of the line in constant use with a low frequency continuous wave outside the carrier band width. -------------------------------------------------------------- Every Modem or similar device that is designed to connect to a Phone line must conform to FCC standards. One of those standards is the amount of load placed on the line by the device. Most Modems and related devices use a Transformer to not only provide for a balanced load but also to act as an isolator to separate potentially damaging voltages from passing back and forth. These transformers are designed to provide a 600 ohm Balanced load at the line. A 600 ohm load will 1. Help minimize echos 2. Provide for proper Gain of the received signal 3. Insure a proper Transmit Level to the line. Improper impedance WILL adversely effect Data Communications. Impedance is not controllable by the customer (You) in most cases. More advanced Modems and Telephone equipment do provide such optioning. -------------------------------------------------------------------- Please remember that Commercial Modems are far superior that Modems designed for the Home PC and thus are capable of doing far greater things. One of those things is called the "Clear To Send Delay" This is how it works. Your computer raises RTS (Request to Send). In preparation for sending Data, the Modem responds by turning carrier on, HOWEVER the Modem does not provide the Computer with Clear To Send. You have carrier being transmitted without any Data being sent. If there are any TURN ON problems, echo suppressors or whatever, they will be clobbering carrier without Data. After a desired delay, CTS is then turned on and your Computer begins sending Data, ONTO a clean, cleared line. The Clear To Send Delay can be optioned from a fast 5 to 10 milliseconds to cope with minor problems or expanded to 500 milliseconds (1/2 a second) for sever problems such as those created by satellite transmissions. While we are on the subject of telephone lines...... It is ILLEGAL to connect and Non-Registered Device to a Phone Line. The connecting of "home made" filters, equalizers or amplifiers is subject to severe Penalties. Should you desire to connect an Unregistered device to a Phone Line, you must request the Phone Company to install DAA Data Access Arrangement or equivalent. This device protects the Phone Line should your equipment not perform as you expect it to.