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Communication modem

In the early 70s, in the United States, the first modem in the market. For dial-up speed was low, ²300 bps and performance even lower. A need existed to communicate between computers and telephone lines was the obvious media. The speeds increased Eventually, and in the late 70th century appeared the first modems are up to 1200 bps full-duplex operation over dial-up connection. Unfortunately, there were standards established in the United States and Europe that were not compatible. An organization was formed early in order to standardize different kommunikationsmoder.och procedures, CCITT. CCITT has members from all the world's telecommunications administrations and even the major manufacturers of equipment for data communications and recommendations have often V.nn term which describes in detail how the equipment to communicate with other devices that follow a specific recommendation.

One can say that as of that speed is 2400 bps and above have a common standard is established, ie, V.22bis (2400 bps) and up. To achieve greater certainty over the line developed several algorithms for companies ARQ, retransmission of erroneous data, and data compression. Market leader became Microcom, Inc., which developed the MNP protocols which also covered the data compression. CCITT is behind a recommendation called for ARQ V.42 and V.42bis compression. Today, a supplier must have both the MNP and V.42 implemented in their products in order to be competitive.

Solid lines, 2-wires for transmission and 2-wires for reception could easily be adapted to data transfer speeds and the line reached already in the mid 70's 9600 bps. 1974 offered Codex, USA Pools for 9600 bps leased line. Several competitors followed the next few years and in 1978 established a recommendation as specified in CCITT V.29 CCITT as the standard for fixed connection to 9600 bps. Several standards followed for lower speeds as wise V.27 4800 bps.

Data throughput and line speed. It is important that we distinguish between the concepts bps and baud. More of our competitors do not differentiate between the two concepts. Bps is the abbreviation for bits per second and refers to the speed between the computer and modem. The speed of the phone line listed in Baud and definition changes per unit time. On the line, the communication with modulated tones and the bandwidth we have available is 2400 Hz, called a voice channel. A telephone can not accept more than 2400 Hz or 2400 changes per unit of time (1 second). Thus, we must resort to some tricks to transmit high bit rates on a line that has limited space. This is done in modem that transmits coded signals, each signal contains information on a number of bits. In order to transfer additional higher speeds between computers kodkomprimering developed.

Modem technology. For lower speeds, ²2400 bps, parts of the modem line circuits up the phone line in a transmission channel and a receiver channel. These are referred to ORG and ANS, the originate and answer. We then transfer the data in each direction on the completely independent. At higher speeds 9600 and 14400, this technique can not be used. The capacity of the channels is simply not to. The technology developed is called Echo Cancelling and places very high demands on the processing capability of the modem. Simply described, the following occurs during communication between two Pools of type V.32 or V.32bis.

A modem dials and establishes a connection with the modem B. Modems are shaking hands. A modem transmits a test signal which takes the whole of the connection's capacity. Modem A measure which any echoes that occur on the line and stores these values ​​in its memory. Modem A finishes his test. Modem B performs the same procedure as modems A.

Both modems have now details his memories of the line characteristics and modem are now simultaneously sends out information that takes the whole band utilized. The information on the line now consists of information from both modems transmitters and echoes from the line. Modems are talking in unison!

How to be as wise modem A to know what is information from modem B? Well, the information on the line consists of A's transmitter, B's transmitter and echoes from the line. If A removes its own transmit information, and the echoes that it stored in its memory, the resulting information may be B's send data!

Error correction, compression and interface speed. A dialup connection is often exposed to various disturbances which means that the transmitted data arrives incorrect. By block the data and provide the transmit blocks with checksum can any incorrect data is detected and retransmitted. Two different ARQ (Automatic Repeate Request) algorithms exist. Partly MNP 4, and V.42. These are not compatible with each other but the automatic choice can be made between them at the speed of the modem connection.

To achieve greater efficiency and higher speeds of transmission to gripes data compression. Two different algorithms exist. Partly MNP 5, V.42bis both. These algorithms can only compress data asynchronously. Compression of synchronous data requires knowledge of the used communication protocol. It should be mentioned that V.42bis is more effective than MNP5. These are not compatible with each other but the automatic choice can be made between them at the speed of the modem connection. In principle, these algorithms are based on the most frequent characters are encoded into a shorter and less frequent signs of a longer code. In the ASCII alphabet, we have 1 start, 7 information bits, 1 stop bit and 1 parity. A total of 10 pieces. Start bit, stop bit and parity bit is removed by sending modem and still we have 7 pieces of information. The transmitting modem analyzes information bits and ex way, we can encode the most common signs that

A 1111 T 0111 E 0011 S 0001 R 11001 01100 .. U .. Z 111 001 011 for all longer codes for less frequent characters. The transmitting modem built up a code table that is changing dynamically and transmitted to the receiving modem for decoding. The result of this coding gives a compression of a factor of 2 - 3 That we may send data to the modem with 2 - 4 times higher speed than the maximum transfer rate. A factor of 4 is sometimes given, but occurs in extreme cases only.

Buffered interface for the PC to be able to accommodate varying speeds from the modem dependent transfer required a buffered interface to the modem with a fixed rate regardless of transmission speed. The interface can usually be set for speeds of 2400, 4800, 9600, 19200, 38400, 57600 and 76800 bps. The last two 57600 and 76800 are not actually useful when we V.32 Pools and achieves a compression on the 2 - 3 At high speeds are also limitations in the PC's hardware and software.

Standards for dial-up connections.

CCITT V.21 Speed ​​Note 300 Asynchronous data V.23 75-1200 Asynchronous data These standards have no practical significance today, and low speeds both poor performance.

V.22 1200: Asynchronous and synchronous data V.22bis 2400 asynchronous and synchronous data of the standards is being replaced by higher speeds, but they occur in a large number of Pools in the market. Low prices together with compression makes them still attractive.

9600 V.32 asynchronous & synchronous data V.32bis 14400 Asynchronous and synchronous data A V.32 modem has always V.22 standard built-in, you get it for free, and a V.32 modem automatically adapts to the dial-up modem standards. Communication is established automatically at the highest rate.

Another standard, probably the last one for dial-up lines, is under preparation and will be fixed in the middle of 1984th standard during the project termed V.Fast and will be when it determined that termed V.34. Here is the maximum speed 28800 over the line, and with good compression, we may in some 100 000 Kbs. Great demands are imposed on the line kvalitŽ.

Market demands for a dial-up connection. Here are some concepts and features that are more or less a must for the modem to be competitive:

T-marking. To get connect equipment to Televerkets lines required to meet certain requirements modem. These requirements are set for the connected equipment does not interfere telefonätets function or other subscribers to communicate across the network. The modem is T-marked does not mean that the modem has undergone performance measurement or otherwise are better than others Pools; T-marking only means that the modem is approved for connection to the public telephone network.

Auto dialer - AT commands or Hayes compatible. Hayes Inc. in the United States was the first to define a command set for Pools. Commands always start with the characters AT (attention) followed by the characters for various functions, such as wise ATDT123456 relates D = Dial and T = tone, that ring up number 123456 with tonsignallering. Note there is no connection between a modem performance and functionality and to be Hayes AT compatible. It covers only compatibility with the command set. Most modems have an expanded command set that differs between modems. For dialing with synchronous terminals, or about the same number always ringes this can be done by the modem monitors the status of DTR leader in the PC one and automatically dials a preselected number when DTR goes high, called the 108.1 dial. Another option is to call a pre-stored number from the front panel.

MNP4 / V.42, MNP5 / V.42bis & MNP10 These relate to the functions of error correction and compression. Specifically, the compression may be more or less effectively implemented. This is difficult for a customer to measure, then the interface speed is usually set to the highest modem and accepts the user believes he is transferring data at this speed; it seems that the data is transmitted by the interface speed, but in practice, the effective transfer rate is often lower. MNP10 developed for modem transmission via NMT or Mobitex that call for maintaining synchronization at bärvågsförlust

Buffered interface. Fast interface transfer speed regardless of the line is a must. Ex wise is the speed between the PC and the modem is set to 38400 bps connection to irrespective 14400 or 2400 modem.

Number list. List of the modem where the user can store phone numbers and names for quick and easy of calls either by specifying the position in the list, or by mentioning names.

Automatic Callback. In the event that the number is busy, the modem to redial up to nine times with a 1-minute intervals, according to the Swedish tele maintenance rules. This function is often rendered useless when the communication software can redial almost immediately after busy signal and repeat callback indefinitely. Callback with passwords. Defendant exact modem dial-up modem users password. Dial-up modem checks the entered password against a stored list. A password is in the list have a telephone number where the caller can be reached. When properly specified password, disconnect dial-up modem connection is down and RPS to the number in the list .. To a hacker or other unauthorized person is able to establish connection is required and that the correct password is entered, and that this person is in the correct phone number. Indication of incorrect password causes disconnect. In more extensive installations with multiple defendants modem is often installed as a network management under software control gives statistics on the number of the calling user, incorrect password, etc.

Remote configuration. The calling modem via special password will be able to monitor and configure the far-end parameters. This saves time and above all the costs associated with troubleshooting and service trips.

FAX communication. Many Pools can now send and receive faxes. The modem will support Group 3 fax machines, ie, 9600 bps.

SNA communications. In Sweden, the 3270 SNA communications widespread and minicomputer systems AS 400 with a 5250 communication säljsuccŽ. Communication over dialup connection in these environments are complex and require either plug-in card in the PC for synchronous communication or connection with protocol converter. A third alternative is available with special software on the PC and a modem LTI.

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