GFX Sensations 1

home *** CD-ROM | disk | FTP | other *** search

/ GFX Sensations 1 / Graphic Sensations - Volume 1.iso / com_net / bbs / 4dbbsd / docs / text / 12.modem_setup < prev next >

Wrap

Text File | 2000-01-01 | 20.1 KB | 430 lines

Chapter 12: Modem Setup 12.1 Overview In order for 4D-BBS to operate correctly, it must interact with your modem correctly. This includes configuring 4D-BBS to know what commands to send to your modem, and what commands your modem returns. If these two options are not setup correctly, 4D-BBS will not be able to accept calls. This chapter is set up into two parts. The first part is a terminology section designed to give novice users information about modem terms and operations. The second section goes into detail applying these terms to 4D-BBS and explaining what the requirements for a modem are and how it should be setup. If you are familiar with modems, you may wish to skip the first section, and go directly to section 12.4, which is the first section specific to 4D-BBS. 12.2 Modems and Terminology The word modem is an abbreviation of "MOdulator-DEModulator", and stands for the action of taking data, changing it into an audible signal, and then reconverting it back into data. This is a, of course, a simple overview of what a modem really does. There are several different types of modems available today. Although most modems are compliant with the original Hayes "AT" command set, there are several modems which use a derivative of the "AT" command set or use their own proprietary method. Because of this when a command is talked about in this chapter, both the long description and the "AT" equivalent will be given. You may have to look up the appropriate command in your modem's manual if it differs from the "AT" command given. 12.2.1 Speeds There are basically two classifications of modems used today. A modem of 9600 baud or faster is considered a "high speed" modem, whereas a modem less than 9600 baud is considered a normal modem. Although this is certainly not a clear line between the two, the major determining factor is the speed at which the modem and computer communicate with each other. There are two types of speeds (or data transfer rates) which need to be clarified. The first is called the "serial" rate. This is the communication rate at which the modem and computer talk to each other. The other rate is called the "carrier" rate, and is the communication rate at which the modem talks to the other modem. The two rates do NOT have to be the same, and as we will see later, normally the serial rate must be faster than that carrier rate when using advanced features of your modem. As mentioned above, the two modem classifications are not clearly defined. They can overlap, with the main determining factor being whether the serial rate and the carrier rate are required to be the same. For high speed modems, the serial rate is normally "locked" (or set to and never changed) at a rate which is faster than the highest possible carrier rate. With earlier modems, the serial rate had to "float" or change whenever the carrier rate changed. Since this meant that the computer and modem had to be constantly switching the rate at which they were communicating data, it was easy to get them communicating at different rates causing a BBS not to work properly. Some modems however have the ability to work both ways. If they receive a call at 2400 baud or less, the modem can change the serial rate to the incoming baud rate. If the modem receives an incoming call above 2400, it will keep its serial rate locked at a designated rate. As a result of this the user will see a more responsive system. Another point to make is when two computers are connected using modems, the serial rate of one computer can be different than the serial rate of another computer (assuming high speed modems are being used). It is the modem's responsibility to act as a transport between the computer and the other modem. Below is a chart of common serial and carrier baud rates. Note that after 9600, the rates differ. This is roughly the distinction of the high speed modem. The third column below shows the CCITT designated name for that CARRIER rate. Serial Carrier CCITT 300 300 1200 1200 V.22 2400 2400 V.22bis 4800 4800 9600 9600 V.32 19,200 12,000 14,400 V.32bis 16,800 (ZyXEL/HST) 19,200 (ZyXEL) 38,400 28,800 V.Fast 57,600 76,800 115,200 The CCITT protocols run the SAME speed on both channels the modem can communicate on. Theoretically, this means that a bi-directional transfer could send up to 14.4kbits/second each way when using V.32bis. Of course these are in theory and actual transfer speeds would be lower. It should also be noted that several modems offer a faster or different carrier rate then those mentioned above. This is because there are several proprietary protocol modems available. Some of these include ZyXEL, USR, Compucom, Telebit, and Telcor. Normally these modems will only communicate with each other using the CCITT 2400 baud protocol, unless they also support a faster CCITT standard or they are connecting with another modem made by the same manufacturer. For example, an HST modem by USR can connect to a ZyXEL only at 2400 baud, but can connect to another HST using the HST protocol at 9600 or 14.4k/sec. A USR Dual standard can connect to a ZyXEL at 9600 or 14.4k/sec, since it supports either the V.32 or V.32bis protocol (depending on the model of the Dual Standard). An interesting note on the HST protocol is how it is designed. The CCITT V.32 and V.32bis protocols, as a noted above, can transfer at full speed BOTH WAYS. The HST protocol can only send full speed ONE WAY, while the other channel is only able to send data at 450bits/second. This is a definite caveat to the HST protocol. On the other hand, some protocols, like the ZyXEL protocol can transfer on both channels (like the CCITT protocols). A ZyXEL modem using the ZyXEL 19.2k protocol can transfer information at 19.2kbits/second BOTH ways. Please consult your modem's user guide for information on which protocols it supports and which modems it can connect to. 12.2.2 Error Correction and Data Compression There are several different types of error correction techniques used by modems today. The most common two are MNP2-4 and LAP-M. One of the main advantages of using error correction is that by itself it can give you a faster transfer rate. The normal way of transferring information requires two extra bits besides the normal 8 bits for the data to handle error detection. Since the modems themselves handle the error correction, these extra two bits are not needed any more and stripped by the modems. This means a theoretical 20% increase in transfer speed. Similar to error correction, there are several different types of data compression used in today's modems. These vary by the maker, but usually offer similar throughput. Since data compression and error correction are normally bundled together (most data compression modes require an error correction mode to be enabled in order for data compression to operate). We will discuss them together in their CCITT proposals. MNP2-4 was the first error correction protocol to be widely accepted. It was mostly used with 2400-9600 baud modems. Together with MNP2-4, was MNP5. MNP5 is a data compression technique which allows up to a 2:1 compression ratio. MNP5 does not have the ability to detect data which is already compressed, and can result in a degradation of throughput when transferring pre-compressed data. These two together are called by CCITT "v.42". LAP-M was the next error correction protocol to be adopted by the CCITT. It had better detection and correction methods than MNP. It also has data compression capabilities of up to 4:1. This is twice that of MNP5, and LAP-M is intelligent enough to detect a stream of compressed information and not try and compress it any further. When CCITT adopted LAP-M, they packaged it together with V.42, calling it V.42bis. V.42bis includes BOTH LAP-M and MNP2-4. The preferred protocol to use when two V.42bis modems connect is LAP-M (because of its better features and high intelligence), but this is normally configurable in the modem. 12.3 Computer-and-Modem Operations The folloiwng mainly covers high speed modems which are utilizing data compression. If you are using a non-high speed modem, you may skip this section. Now that you understand some of the different speeds and protocols, let's talk about what this all means to the computer. Earlier it was noted that the computer can talk to the modem at a DIFFERENT rate than the modem is actually talking to the other modem. There is a definite reason why this is needed. When a modem is using a data compression technique, its needs to get information from the computer FASTER than it is sending it out to the modem. This is only obvious when you think about what data compression means. If the computer is NOT getting data faster than it is sending it to the other modem, then data compression is useless. Lets look at the below illustration for example: __________ _____________ \ / \_____________________________________/ Computer 1 Modem 1 -> Carrier Width <- Modem 2 Computer _____________________________________ / \ __________/ \_____________ Now let's pretend the above is a water pipe, and you are trying to get the maximum amount of water through the pipe. In order to get the maximum amount of flow, you would have to AT LEAST have the same amount of pressure in the area where Computer 1 is, as is allowed to travel through the carrier width. If you do not, then you are not fully utilizing the maximum bandwith of the carrier. What does this all mean when related to modems? Computer 1 must be able to send information to modem 1 FASTER than modem 1 can send information to modem 2. This way modem 1 has a chance to COMPRESS the data, and send it to modem 2. When modem 2 gets the information, it will UNCOMPRESS the data, giving computer 2 information FASTER than the allowed carrier bandwidth. This same process can be reversed to show information flow from computer 2 to computer 1. What will happen if the computer-to-modem speed is the same as the modem-to-modem speed? If data compression is not being used, this will not cause a problem. If data compression IS being used, then some conflicts can arise. First of all, if the computer only sends data to the modem at the rate the modem is able to transfer it to the other modem, no problems arise. But what about when only ONE computer's serial rate is faster than the carrier rate, or when another's serial rate and carrier rate are the same? An overrun will occur on the computer which has its serial rate the same as the carrier rate (again, assuming data compression is being used). This is where flow control comes into effect. 12.3.1 Flow Control Flow control is the action of being able to start and stop the flow of data between to communication devices. Here we will talk about flow control and how it affects modem-to-computer and modem-to-modem communications. When a computer is sending information to the modem faster than the modem can send it out to the other modem (even if data compression is being used) then the possible situation of the modem's buffer getting full can occur. If the computer is not told to stop sending information until the modem is able to send data out to the other modem, then data can be lost. This situation can also be reversed to describe the flow of data from the modem to the computer. There are two types of common flow control. The first is called Xon/Xoff and the other is called RTS/CTS. Xon/Xoff flow control is two characters, specifically Control-Q and Control-S. When a device wants the other device to stop sending information, they will send an Xoff to that device. The device who receives the Xoff will stop sending data until it receives an Xon. This method works well in environments which do not have extra cabling to handle hardware hand-shaking, as we will see below. A problem to this method is that it is possible to get garbage characters, and misinterpret an Xon or Xoff character, causing flow control to not work properly. Futhermore, when transferring binary files, it is required by the transfer protocol to escape or change these characters, so that they are not misinterpretted. This means extra overhead for the protocol. Since Xon/Xoff has some draw backs, a new flow control method was designed. This method used two wires in the cable between the two devices to signal when a device was able to receive information. This is commonly called "hardware" flow control. The two signals which go between the two devices are labeled Read To Send and Clear To Send (RTS and CTS). When talking about computer-to-modem communications, the RTS signal is high when the computer wants to send to the modem, and the CTS signal is high when the modem is ready to accept data. So when the modem's buffer fills up, it sets the CTS pin to low, temporarily stopping the computer from transmitting. Since RTS/CTS is hardware based, protocols do not need to escape the Xon/Xoff characters (which are normally ignored by the hardware when RTS/CTS is enabled). There is one more thing to remember when talking about flow control. All devices must be using the SAME type of flow control. It IS possible to have both RTS/CTS and Xon/Xoff enabled, but this is not normally needed. If one device is using JUST RTS/CTS and another is using Xon/Xoff flow control will NOT work. Not only does this apply to your computer and modem, is also applies to the OTHER computer and modem you wish to connect to. For example: Computer 1 <---> Modem 1 <----------> Modem 2 <---> Computer 2 RTS/CTS RTS/CTS RTS/CTS RTS/CTS If just Computer 1 and Modem 1 are using RTS/CTS, then it's possible for Computer 2 and Modem 2 to loose information. If a user calls your BBS with a 2400 baud modem, and you have a 14.4k modem, they MUST be using some type of hand-shaking if your are locking your serial rate. Why would this be? Your computer is sending information at 19.2k (assuming your serial rate is locked at 19.2k) to your modem. Your modem can only send information to the other modem at 2400. If the other modem can not tell your modem to pause, then your modem will overrun the other modem. If you are running a high speed modem, a nice bulletin warning your users they need to use RTS/CTS is very helpful to prevent data loss. 12.4 Modem Requirements Below are some of the requirements of a modem in order for it to operate with 4D-BBS properly. This is assuming the modem can connect to the Amiga (whether via the serial port or internal) and either works with the "serial.device", or has its own "serial.device" compatible device driver (i.e. "supra.device", "gvpser.device", etc.). * Must support the AT command set, or at least the ATA command to answer the modem. * Must be able to return the following result codes: RING When a call is coming in. CONNECT XXXXX Connection to another modem. NO CARRIER Aborted connect. * Should be able to report the CARRIER speed when reporting the connect string (i.e. CONNECT 14400, not CONNECT 19200). * Must be able to report via the DCD pin whether a carrier is present or not. * Should be able to drop carrier when the DTR pin goes high-to-low or be able to hangup using the "+++,,,ATH" command. * Should be able to support RTS/CTS hardware hand-shaking. * Should be able to return verbose commands, not terse. * Should be able to not echo commands sent to it. 12.5 Configuring the Modem The first step is to configure how 4D-BBS will interact with your modem. There are several settings which can be altered, along with different options you may select. In the Configuration Editor on the Modem Setup screen, there are 13 settings which need to be talked about. They are as listed below: High Baud: This is the highest possible serial rate both your computer and the modem can handle. This is also the baud rate at which the BBS will initially setup the modem. If your modem has non-volatile memory and you are saving settings to your modem for the BBS, then you should have your terminal set at this baud rate before you save the settings into the modem's memory. This is also the baud rate the BBS will try to re-initialize the modem at after every call. If the reset baud option is set to Yes (see below) then this is the baud rate which the BBS will try and reset to. The default for this depends on your modem. A high speed modem should use 19200 or 38400, while a 2400 should use 2400. Low Baud: This is the lowest available baud rate which the BBS supports. If a connection comes in slower than this rate, the BBS will simply hangup and go back to the wait screen. Ser Device: This is the serial device driver which the BBS will try and use. This file must exist in your "Devs:" directory. The default for this is "serial.device". Auto Detect: When the auto detect mode is enabled, the BBS will expect to receive standard verbose information from your modem. The two main result codes are "RING" for an incoming call and "OK" when a good command was sent. Drop DTR: The BBS and your modem must be setup to support the same mode for hanging up. There is three different settings for this: 0 The BBS will not try to hangup the modem. This should only be used inconjunction with the "/o" parameter, to allow the calling program to either continue using the current connection, or allow the calling program to hang up the modem itself. 1 The BBS will close the serial device, pause, and then re-open the serial device in order to drop the DTR. 2 The BBS will use the escape sequence method of "+++", 2 second pause, "ATH" in order to hangup the phone. The default for this is 1. Local Off: When you log onto the BBS with either the F1 or F10 keys (or equivalent pull down menu options) this option will instruct the BBS to send a "ATH1" option, to send the modem off-hook. This will give your BBS a busy signal while you are on the BBS. You can also run a second copy of the BBS to logon, in order to avoid this. The default is No. Reset Baud: When the "/b" command line parameter is NOT given to the BBS, its serial rate will "float" to match the incoming carrier rate. Set this option to Yes if you want to have your modem return to the serial rate specified by the high baud option above. If you wish for your modem to stay at the incoming caller's baud rate until a new incoming call changes it, specify No here. The default is No. Ser Unit: This is the unit of the serial device specified in the "Ser Device" option which the BBS will use. The default is 0. Connect Str: If the Auto Detect option is turned on, this is the string the BBS will look for to determine when a connection has been made. Your modem must return this as the FIRST part of the connect string. The BBS will parse the number IMMEDIATELY following the string and assume that is the carrier rate. The default for this is "CONNECT". Buffer Size: This is the buffer size for the serial port. Normally a buffer size of 4k is sufficient. High speed modems might want to use a larger buffer size to make transfers less choppy. The default is 4096. ARQ Str: When a reliable connection is made, the BBS needs to know this in order to calculate transfer times accurately. This string can appear ANYWHERE in the string which begins with the connect string specified above. The default is "ARQ". Fax String: If your modem has fax capabilities and is able to distinguish between a fax and a data call, set this to the string which your modem will return when it detects an incoming fax. This is NOT the complete string, and the BBS will search for this string inside of the result string. The default is "FAX". Init String: This is the string which the BBS will send out to the modem when starting up, and after each call. The BBS expects to receive either an "OK" or a "0" back from the modem after sending this string. Certain characters in this string are translated by the BBS. They are: ~ Pause for 1.5 seconds | Send a Return ^ Hangup the modem (Defined by drop DTR setting) The default string is "~|AT|" (Wait 1.5 seconds, send a return, AT, and another return). 12.6 YSMTG (Yet Still More TO Go!)