Communication cable for junction c1 and. Algorithm for working with the C1-I interface. Parameters of electrical signals in circuits
GOST 22937-78
Group E55
STATE STANDARD OF THE USSR UNION
CIRCUITS OF LOCAL TWO-POLE TELEGRAPH SYSTEMS
COMMUNICATIONS AND DATA TRANSMISSION
Types and main parameters
Circuits local bipolar for telecommunication and data transferring systems.
Types and basic parameters
Valid from 01/01/79
until 01/01/84
By Resolution of the State Committee of Standards of the Council of Ministers of the USSR dated January 27, 1978 N 245, the validity period was established from 01/01/1979 to 01/01/84*
_______________
* The validity period was lifted according to Protocol N 5-94 of the Interstate Council for Standardization, Metrology and Certification (IUS N 11-12, 1994). - Note "CODE".
INTRODUCED: Change No. 1, put into effect by Resolution of the USSR State Committee on Standards dated 04.25.84 N 1421 from 11/01/84, Change No. 2, approved and put into effect by Resolution of the USSR State Committee on Standards dated 06/27/88 N 2363 from 01.12. 88
Amendments No. 1, 2 were made by the legal bureau "Code" according to the text of IUS No. 8, 1984, IUS No. 11, 1988
This standard applies to local two-pole information circuits of telegraph communication systems and data transmission A unified automated communication network designed for transmitting signals with nominal speeds up to 200 Baud, and establishes the types and basic parameters of local two-pole information circuits used to interface telegraph equipment (TGA) with TGA and data transmission equipment (ADT) with TGA, signal parameters in local two-pole information circuits, parameters for pairing equipment at the interface with telegraph network channels (C1-TG interface).
The standard does not apply to circuits at the interface with external circuits of cable and overhead communication lines. When working on external circuits, matching devices or telegraph channel-forming equipment must be used.
Definitions of terms used in the standard are given in the reference appendix.
(Changed edition, Amendment No. 1).
1. TYPES OF CIRCUITS
1. TYPES OF CIRCUITS
1.1. Local two-pole information circuits TGA and APD (Fig. 1) are divided into the following types:
“transmitted (received) data” - for transmitting discrete signals between mating equipment;
"signal grounding" - to establish the common potential between mating equipment. If it is necessary to interface equipment using a two-wire (symmetrical) circuit, the “signal grounding” circuit is replaced by a return wire.
E.m.f. source of positive polarity; - e.m.f. source of negative polarity; - resistance of the output device of the TGA, APD to direct current, defined as the ratio of the difference between the open circuit voltage and the voltage at a load resistance of 1000 Ohms to the current flowing in the load; -
resistance of the TGA, APD input device to direct current, defined as the ratio of the input voltage to the load current; - DC resistance of the local information circuit; - insulation resistance of the local information circuit; - capacitance of the “transmitted (received) data” circuit relative to the signal ground; - input resistance of the control device of the switching station; - input capacity of the control device of the switching station; - input resistance of the control and measuring device; - input capacitance of the instrument
The “signal grounding” circuit (return wire) should not have a permanent connection with the housing of the TGA, ADF*.
______________________
* The requirement applies to equipment whose development begins after 01/01/88.
(Changed edition, Amendment No. 2).
1.2. The TGA and ADF must provide connection points for the “transmitted (received) data” and “signal grounding” circuits (Figure 1).
1.3. Interfacing of a TGA or ADF through a switching station that does not convert signals must be done by galvanically connecting the circuits according to Figure 1.
When connecting a TGA or ADF through a switching station that converts signals, the latter must be equipped with input and output devices that comply with this standard.
With a non-switched connection, the switching station is excluded from the circuit and the pairing of the TGA with the TGA or the TGA with the ADF is carried out directly using connecting wires.
To monitor and measure signal parameters, it must be possible to connect instrumentation at points in the local information circuit.
2. BASIC PARAMETERS OF CIRCUITS
2.1. The parameters of the circuits with positive and negative polarities of the parcels and a rated voltage of ±20 V must correspond to the following:
output device resistance | |
TGA, APD, Ohm, no more | |
resistance of the input device TGA, APD | |
DC current, Ohm | |
loop resistance of the connecting circuit | |
direct current, Ohm, no more: | |
in an asymmetrical circuit | |
in a symmetrical scheme | |
insulation resistance of the local information circuit and the "Signal grounding" circuit relative to the housing of the TGA, ADF, ,
MOhm, not less | |
input resistance of the control device of the switching station, kOhm, not less | |
input resistance of the control and measuring device, kOhm, not less | |
local information circuit capacity, µF, no more | |
equivalent input capacitance of the control device of the switching station, μF, no more | |
equivalent input capacitance of the control and measuring device, μF, no more. |
Note. Allowed 3000±300 Ohm.
(Changed edition, Rev. N,).
2.2. The input device response voltage for the positive and negative polarities of the input signal in absolute value should be no more than 3 V (Fig. 2).
-
signal voltage at the input of TGA, ADF;
, - input device response voltage for positive and negative signal polarities;
- rated signal voltage at the input of the TGA, ADF;
- doubled signal amplitude.
The absolute value of the algebraic sum of the input device response voltages should not exceed 1 V.
(Changed edition, Amendment No. 1).
2.3. When the input voltage drops to a value less than 1.5 V in absolute value, the input device must go into a state corresponding to receiving the start signal. The transition to this state must be carried out in one of the modes: in the interval from 1 to 100 ms or in the interval from 1 to 50 ms after a sudden drop in voltage. The second mode is preferable.
No more than 15 ms after the voltage step increases to a value greater than 3 V absolute value, the input device must be capable of receiving signals in accordance with the stated sensitivity requirements.
Note. The specified requirements do not apply to the final and control-measuring TGA and APD.
(Changed edition, Amendment No. 1).
3. PARAMETERS OF ELECTRICAL SIGNALS IN CIRCUITS
3.1. Signals in local two-pole information circuits must represent two-pole messages direct current.
The positive polarity of the signal must correspond to a “binary one” (stop parcel), and the negative polarity of the signal must correspond to a “binary zero” (start parcel).
3.2. The duration of signal edges in local information circuits should be no more than 0.5 ms in the range from 0.1 to 0.9 of the voltage drop when the voltage polarity changes (Fig. 2).
3.3. The duration of the edges at the output of the output device with an active load resistance of 1000±100 Ohms should not exceed 0.3 ms.
3.4. The voltage of bipolar parcels in local information circuits must be within the limits:
at the "Exit" point:
16-30 V - when operating in an asymmetrical circuit;
14-30 V - when operating in a symmetrical circuit;
at the "Entry" point:
14-30 V - when operating in an asymmetrical circuit;
10-30 V - when operating in a symmetrical circuit.
(Changed edition, Amendment No. 1).
3.5. The difference between the absolute values of the voltages of the parcels of positive and negative polarities in local information circuits should not exceed 10% of their average value. In this case, the average voltage value should be determined as the arithmetic mean of the absolute values of the voltages of the parcels of positive and negative polarities.
3.6. The voltage of two-pole signals at the output of the equipment with an active load resistance of 1000 Ohms, taking into account operating conditions, must be within the limits:
17-25 V - when operating in an asymmetrical circuit;
15-25 V - when operating in a symmetrical circuit.
In this case, the difference between the absolute values of the voltages of the positive and negative polarities should not exceed 7% of their average value.
3.7. The output current of the TGA, ADF during a short circuit and reverse connection should be no more than 100 mA.
3.8. The effective value of the ripple voltage at the “Input” and “Output” points for any signal polarity should not exceed 3% of the constant voltage component.
APPENDIX (reference). TERMS USED IN THE STANDARD AND THEIR DEFINITIONS
APPLICATION
Information
1. Local information circuit - a circuit that serves to interface telegraph or data transmission equipment inside a building and does not have a direct connection with external lines.
Notes:
1. Local information circuits are divided into one- and two-pole.
2. The local two-pole information circuit includes output and input devices, a “transmitted (received) data” circuit and a “signal ground” circuit or return wire.
2. Telegraph equipment (TGA) - equipment intended for the formation and control of a telegraph circuit.
Note. Telegraph equipment is, for example, a calling device, a switching station, as well as channel-forming equipment and telegraph channel monitoring equipment, designed for use only in local telegraph circuits.
The text of the document is verified according to:
official publication
M.: Standards Publishing House, 1978
Legal Bureau "Code" in
text of the document included: Amendments No. 1, 2,
adopted by the Resolution
GOST 27232-87
Group P85
STATE STANDARD OF THE USSR UNION
CONNECTION OF DATA TRANSMISSION EQUIPMENT WITH PHYSICAL LINES
Main settings
Interface of data transmission equipment with physical lines.
Basic parameters
OKP 66531
Valid from 01/01/88
until 01.01.93*
______________________________
* Validity limit removed
according to the protocol of the Interstate Council
on standardization, metrology and certification
(IUS No. 2, 1993). - Note "CODE"
INFORMATION DATA
1. PERFORMERS:
B.P.Kalmykov, Ph.D. tech. Sciences (topic leader); E.A. Kolganov; L.A.Kuznetsov, O.I.Muchenikova
2. APPROVED AND ENTERED INTO EFFECT by Resolution of the USSR State Committee on Standards dated March 25, 1987 N 914
3. Date of the first inspection - 1991. Inspection frequency - 5 years.
5. INTRODUCED FOR THE FIRST TIME
This standard establishes the parameters for pairing signal conversion devices (SCDs) with physical lines (PL) with two-wire and four-wire ends at the S1-FL interface with a two-way simultaneous or two-way sequential method of organizing data transmission at speeds up to 480,000 bps (Figure 1).
Damn.1
1. The S1-FL joint includes the following chains:
transmitted data;
received data;
transmitted and received data (in the case of using a two-wire connecting line).
2. Linear transmission and reception circuits at the S1-FL junction must be symmetrical with respect to the grounding circuits and galvanically isolated from the rest of the UPS circuits (in the case of using a four-wire line).
3. The asymmetry attenuation of linear transmission and reception circuits at the points of connection to the line must be at least 43 dB at a frequency numerically equal to maximum speed UPS work.
4. Short circuit between the circuits of the S1-FL interface and the grounding circuit should not cause damage to the UPS.
5. The exchange of data signals at the S1-FL interface during asynchronous transmission should be carried out with bipolar direct current messages in the primary code (low-level signals) at speeds up to 19200 bps.
The timing diagram of the data signal and the corresponding low level signal is shown in Figure 2.
Damn.2
6. The exchange of data signals at the S1-FL interface during synchronous transmission should be carried out by bipolar parcels with redundant recoding into a bipulse signal in the speed range from 1200 to 144000 bps, at information transmission speeds over 144000 bps - by three-level parcels with redundant recoding into quasi-ternary signal with bursts shortened in duration.
The algorithm for converting a data signal into a quasi-ternary signal must proceed according to the following rules: with each subsequent transmission of the “1” symbol, the polarity of the converted signal pulse changes to the opposite one compared to the previous pulse. The character "0" is transmitted as a space in the converted signal.
The timing diagram of the data signal and the corresponding quasi-ternary signal (QTS) is shown in Figure 3.
Damn.3
Duration of a single data signal interval; - signal amplitude
7. The algorithm for converting a data signal into a bipulse signal must proceed according to the following rules: the symbols “0” and “1” of the data signal are transmitted at the clock interval by two pulses of equal duration and opposite polarity.
The order of alternating pulse polarity compared to the previous clock interval does not change when transmitting the “1” symbol and changes when transmitting the “0” symbol.
The timing diagram of the data signal and the corresponding bipulse signal is shown in Figure 4.
Damn.4
Duration of a single data signal interval; - signal amplitude
8. As an additional method of encoding the original sequence of binary symbols in the speed range from 1200 to 480000 bps, it is allowed to use the Miller code.
9. The algorithm for converting a data signal into a signal in the Miller code should occur according to the following rules: the transition from one level to another occurs at the center of the unit interval corresponding to the symbol “1”, and at the end of the unit interval corresponding to the symbol “0”, only in that case when the next character is also "0".
The timing diagram of the data signal and the corresponding signal in the Miller code is shown in Figure 5.
Damn.5
Duration of a single data signal interval; - signal amplitude
10. Electrical parameters coupling of the UPS with the FL at the S1-FL junction must comply with the standards given in the table.
Parameter name | Signal standards |
|
low level | bipulse signal in Miller code, KTS |
|
Nominal value of output resistance at points of connection to the line at frequency, Ohm | From 20 to 150 | |
Deviation of output resistance from the nominal value, %, no more | ||
Amplitude value of the transmission signal at the points of connection to the line at a load resistance of 150 Ohm, mV | 300, 600, 900 | 400, 1000, 3000*** |
Deviation of transmission signal amplitude from the nominal value, %, no more | ||
Nominal value of input impedance at points of connection to the line at frequency, Ohm | From 50 to 300 | |
Deviation of input resistance from the nominal value, %, no more | ||
Range of amplitude values of the receiving signal at the points of connection to the line, mV | From 20 to 900 | From 20 to 1000 |
Transmission signal shape at points of connection to the line at a load resistance of 150 Ohms | Rectangular | Rectangular |
Overshoot relative to signal amplitude during transmission, %, no more** | ||
Rise and fall time between 10 and 30% of signal swing, no more** | ||
________________
* It is allowed to use spectrum-limited signals with cutoff frequencies:
6 kHz - at speeds 1200-2400 bps;
24 kHz - at speeds 4800-9600 bps;
120 kHz - at a speed of 48000 bps.
** Parameters are checked only with a square waveform.
*** Parameters for quasi-ternary signal only.
Note. The frequency value (), Hz, is numerically equal to the data transmission rate, bit/s, for a bipulse signal and a signal in the Miller code and half the transmission rate for a low-level signal and CTS; - duration of a single data signal interval.
The text of the document is verified according to:
official publication
M.: Standards Publishing House, 1987
CHAPTER 1 TELECOMMUNICATIONS BASICS
1. 1. Typical data transmission system
Any data transmission system (DTS) can be described through its three main components. These components are the transmitter (or the so-called "source of information transmission"), the data transmission channel and the receiver (also called the "receiver" of information). In two-way (duplex) transmission, the source and destination can be combined so that their equipment can transmit and receive data simultaneously. In the simplest case, SPD between points A and B (Fig. 1. 1) consists of the following main seven parts:
> Data terminal equipment at point A.
> The interface (or interface) between the data terminal equipment and the data link equipment.
> Data channel equipment at point A. > Transmission channel between points A and B. > Data channel equipment at point B. > Interface (or junction) of data channel equipment.
> Data terminal equipment at point B.
Data terminal equipment(DTE) is a generic concept used to describe a user terminal equipment or part thereof. OOD
Rice. 1.1. Typical data transmission system: A - block diagram of the data transmission system;
b - real data transmission system
may be a source of information, its recipient, or both at the same time. The DTE transmits and/or receives data through the use of data link equipment (DCH) and a transmission channel. The corresponding international term is often used in the literature - DTE (Data Terminal Equipment). Often the DTE can be Personal Computer, mainframe computer (mainframe computer), terminal, data acquisition device, cash register, GPS receiver, or any other equipment capable of transmitting or receiving data.
Data link equipment is also called data communication equipment (DTE). The international term DCE is widely used (Data Communications Equipment), which we will use further. The function of a DCE is to enable the transfer of information between two or more DTEs over a specific type of channel, such as a telephone channel. To do this, the DCE must provide a connection to the DTE on one side, and to the transmission channel on the other. In Fig. 1. 1, A The DCE may be an analog modem if an analog channel is used, or, for example, a channel/data service unit (CSU/DSU - Channel Seruis Unit/Data Service Unit), if a digital channel like E1/T1 or ISDN is used. Modems, developed in the 60s and 70s, were devices exclusively with signal conversion functions. However, in recent years, modems have acquired a significant number of complex features, which will be discussed below.
Word modem is an abbreviated name for the device that performs the MODulation/DEModulation process. Modulation is the process of changing one or more parameters of the output signal according to the law of the input signal. In this case, the input signal is, as a rule, digital and is called modulating. The output signal is usually analog and is often called a modulated signal. Currently, modems are most widely used for transmitting data between computers through switched telephone network common use (PSTN, GTSN - General Switched Telephone Network)
An important role in the interaction of DTE and DCE is played by their interface, which consists of incoming/outgoing circuits in DTE and DCE, connectors and connecting cables. The term is also often used in domestic literature and standards joint
The connection between DTE and DCE occurs via one of the C2 type joints. When DCE is connected to a communication channel or distribution medium, one of the C1 type joints is used.
1. 2. Communication channels
1. 2. 1. Analog and digital channels
Under communication channel understand the totality of the distribution medium and technical means of transmission between two channel interfaces or joints of type C1 (see Fig. 1-1). For this reason, the C1 joint is often called a channel joint.
Depending on the type of transmitted signals, two large classes of communication channels are distinguished: digital and analog
A digital channel is a bit path with a digital (pulse) signal at the input and output of the channel. A continuous signal is received at the input of the analog channel, and a continuous signal is also removed from its output (Fig. 1 2). As is known, signals are characterized by the form of their representation
Fig 1 2 Digital and analog transmission channels
Signal parameters can be continuous or take only discrete values. Signals can contain information either at each moment in time (continuous in time, analog signals), or only at certain, discrete moments in time (digital, discrete, pulse signals).
Digital channels include PCM systems, ISDN, T1/E1 channels and many others. Newly created SPDs are trying to be built on the basis of digital channels, which have a number of advantages over analogue ones.
Analog channels are the most common due to their long history of development and ease of implementation. A typical example of an analog channel is a voice-frequency channel (VFC), as well as group paths with 12, 60 or more voice-frequency channels. A PSTN telephone circuit typically includes numerous switches, splitters, group modulators, and demodulators. For PSTN, this channel (its physical route and a number of parameters) will change with each next call.
When transmitting data, there must be a device at the input of the analog channel that converts the digital data coming from the DTE into analog signals sent to the channel. The receiver must contain a device that converts the received continuous signals back into digital data. These devices are modems. Similarly, when transmitting over digital channels data from DTE has to be converted to the form accepted for this particular channel. This conversion is carried out by digital modems, very often called ISDN adapters, E1/T1 channel adapters, line drivers, and so on (depending on the specific type of channel or transmission medium).
The term modem is used widely. This does not necessarily imply any modulation, but simply indicates certain operations of converting the signals coming from the DTE for their further transmission over the channel being used. Thus, in a broad sense, the terms modem and data circuit equipment (DCE) are synonymous.
1. 2. 2. Switched and dedicated channels
Switched channels are provided to consumers for the duration of the connection upon their request (call). Such channels fundamentally contain switching equipment of telephone exchanges (PBX). Conventional telephones use PSTN circuits. In addition, switched circuits provide digital network with integration of services(ISDN - Integrated Services Digital Network).
Dedicated (leased) channels are leased from telephone companies or (very rarely) laid by the most interested organization. Such channels are fundamentally point-to-point. Their quality is generally higher than the quality of switched channels due to the lack of influence of the switching equipment of the telephone exchange.
1. 2. 3. Two- and four-wire channels
Typically, channels have a two-wire or four-wire termination. For brevity, they are called two-wire and four-wire, respectively.
Four-wire channels provide two wires for transmitting a signal and two more wires for receiving. The advantage of such channels is the almost complete absence of influence from signals transmitted in the opposite direction.
Two-wire channels allow the use of two wires for both transmitting and receiving signals. Such channels allow you to save on the cost of cables, but require more complex channel-forming equipment and user equipment. Two-wire channels require solving the problem of separating the received and transmitted signals. Such decoupling is realized using differential systems that provide the necessary attenuation in opposite directions of transmission. The imperfection of differential systems (and nothing is ideal) leads to distortions in the amplitude-frequency and phase-frequency characteristics of the channel and to specific interference in the form of an echo signal.
1. 3. Seven-layer OSI model
In order to interact, people use a common language. If it is not possible to talk to each other directly, auxiliary means are used to transmit messages. One such means is the postal system (Fig. 1. 3). In its composition, certain functional levels can be distinguished, for example, the level of collection and delivery of letters from mailboxes to the nearest postal communication nodes and in the opposite direction, the level of sorting of letters at transit nodes, etc. d. Various standards adopted in the postal service for the size of envelopes, the procedure for registering addresses, etc. allow you to send and receive correspondence from almost anywhere in the world.
A similar picture occurs in the field of electronic communications, where the market for computers, communication equipment, information systems and networks is unusually wide and varied. For this reason, the creation of modern information systems is impossible without using common approaches in their development, without unifying the characteristics and parameters of their constituent components.
The theoretical basis of modern information networks is determined by the Basic Reference Model of Open Systems Interconnection (OSI - Open Systems Interconnection) International Standards Organization (ISO - International Standards Organization). It is described by the ISO 7498 standard. The model is an international standard for data transmission. According to the reference
Table 1. 1. Functions of the levels of the open systems interaction model
| Level | Functions |
| 7. Applied | Interface with application processes |
| 6. Representative | Coordination of presentation and interpretation of transmitted data |
| 5. Session | Support for dialogue between remote processes; ensuring the connection and disconnection of these processes; implementation of data exchange between them |
| 4. Transport | Ensuring end-to-end data exchange between systems |
| 3. Network | Routing; segmentation and merging of data blocks; data flow management; error detection and reporting |
| 2. Channel | Data transmission channel management; personnel formation: control of access to the transmission medium; data transmission over the channel; detection of errors in the channel and their correction |
| 1. Physical | Physical interface with data transmission channel; bit modulation and line coding protocols |
The OSI interaction model identifies seven levels that form the area of open systems interaction (Table 1. 1).
The main idea of this model is that each level is assigned a specific role. Thanks to this, the overall task of data transmission is split into individual specific tasks. The functions of a level, depending on its number, can be performed by software, hardware, or firmware. As a rule, the implementation of functions of higher levels is of a software nature; the functions of the channel and network levels can be performed both in software and in hardware. The physical layer is usually implemented in hardware.
Each level is defined by a group of standards that include two specifications: protocol and provided for higher level service. A protocol means a set of rules and formats that define the interaction of objects at the same level of the model.
The application layer is closest to the user. Its main task is to provide already processed (accepted) information. This is usually handled by system and user application software, such as a terminal program. When transferring information between different computer systems, the same code representation of the alphanumeric characters used must be used. In other words, the application programs of interacting users must work with the same code tables. The number of characters represented in the code depends on the number of bits used in the code, that is, on the base of the code. The most widely used codes are those given in Table. 1. 2.
Rice. 13. Functional levels of the postal system
Table 1. 2. Main characteristics of common character codes
Various national extensions of the listed codes are often used, for example, the main and alternative Cyrillic encodings for the ASCII code. In this case, the code base is increased to 8 bits.
The functions of modern modems belong to the levels that are “farthest” from the user - physical and channel.
1. 3. 1. Physical layer
This level defines the interfaces of the system with the communication channel, namely, mechanical, electrical, functional and procedural parameters of the connection. The physical layer also describes the procedures for transmitting signals to and receiving signals from the channel. It is designed to carry a stream of binary signals (a sequence of bits) in a form suitable for transmission over the particular physical medium being used. Such a physical transmission medium can be a voice-frequency channel, a connecting wire line, a radio channel, or something else.
The physical layer performs three main functions: establishing and releasing connections; signal conversion and interface implementation.
Establishing and releasing a connection
When using switched channels at the physical level, it is necessary to carry out a preliminary connection of the interacting systems and their subsequent disconnection. When using dedicated (leased) channels, this procedure is simplified, since the channels are permanently assigned to the corresponding communication directions. In the latter case, data exchange between systems that do not have direct connections is organized by switching flows, messages or data packets through intermediate interacting systems (nodes). However, the functions of such switching have been performed for more than high levels and have nothing to do with the physical level.
In addition to the physical connection, interacting modems can also “agree” on an operating mode that suits both of them, that is, modulation method, transmission speed, error correction and data compression modes, etc. d. Once the connection is established, control is transferred to a higher data link layer.
Signal conversion
To match the sequence of transmitted bits with the parameters of the analog or digital channel used, it is necessary to convert them into an analog or discrete signal, respectively. This group of functions includes procedures that implement the interface with a physical (analog or digital) communication channel. This junction is often called environment-dependent interface and it can correspond to one of the guest channel joints C1. Examples of such C1 joints can be: S1-TF (GOSTs 23504-79, 25007-81, 26557-85) - for PSTN channels, S1-TC (GOSTs 23475-79, 23504-79, 23578-79, 25007-81, 26557-85) - for dedicated voice frequency channels, S1-TG (GOST 22937-78) - for telegraph communication channels, S1-ShP (GOSTs 24174-80, 25007-81, 26557-85) - for primary broadband channels, S1 -FL (GOST 24174-80, 26532-85) - for physical communication lines, S1-AK - for acoustic coupling of DCE with a communication channel and a number of others.
The signal conversion function is the most important function of modems. For this reason, the first modems, which did not have intellectual capabilities and did not perform hardware compression and error correction, were often called signal conversion devices(OOPS).
Interface Implementation
Implementing the interface between DTE and DCE is the third critical function of the physical layer. Interfaces of this kind are regulated by relevant recommendations and standards, which, in particular, include V. 24, RS-232, RS-449, RS-422A, RS-423A, V. 35 and others. Such interfaces are defined by domestic GOSTs as converter joints C2 or junctions independent of the environment.
Standards and recommendations for DTE-DCE interfaces define general characteristics (transmission speed and sequence), functional and procedural characteristics (nomenclature, category of interface circuits, rules of their interaction); electrical (voltage, current and resistance values) and mechanical characteristics (dimensions, distribution of contacts in circuits).
At the physical level, a certain class of faults is diagnosed, for example, wire breaks, power failure, loss of mechanical contact, etc. P.
A typical protocol profile when using a modem that supports only physical layer functions is shown in Fig. 1. 4. In this case, it is assumed that the computer (DTE) is connected to the modem (DCE) via the RS-232 interface, and the modem uses the V modulation protocol. 21.
Fig 1 4 Protocol profile for a modem with physical layer functions only
The noise immunity of a communication channel consisting of two modems and the transmission medium between them is limited and, as a rule, does not meet the requirements for the reliability of transmitted data. For this reason, the physical layer is considered an unreliable system. The problem of correcting distorted bits in the transmission channel is solved at higher levels levels, in particular at the data link level
1. 3. 2. Link layer
The data link layer is often called the data link control layer. The tools at this level implement the following main functions
> formation of data blocks of a certain size from the transmitted sequence of bits for their further placement in the information field of frames, which are transmitted over the channel,
> encoding the contents of the frame with an error-resistant code (usually with error detection) in order to increase the reliability of data transmission,
> restoration of the original data sequence on the receiving side,
> ensuring code-independent data transmission in order to provide the user (or application processes) with the ability to arbitrarily select a data presentation code;
> data flow control at the channel level, that is, the rate at which it is issued to the recipient's DTE;
> elimination of the consequences of losses, distortions or duplication of frames transmitted in the channel.
ISO recommends HDLC as a standard for layer 2 protocols. (High Level Data Link Control). It has become extremely widespread in the world of telecommunications. Based on the HDLC protocol, many others have been developed, which are essentially some adaptation and simplification of a number of its capabilities in relation to a specific application area. This subset of HDLC includes the commonly used SDLC protocols (Synchronous Data Link Control), LAP (Link Access Procedure) LAPB (Link Access Procedure Balanced), LAPD (Link Access Procedure D-channel), LAPM (Link Access Procedure for Modems), LLC (Logical Link Network), LAPX (Link Access Procedure eXtention) and a number of others. For example, the LAPB and LAPD protocols are used in ISDN digital networks (Integrated Services Digital Network)," LAPM is the basis for the V error correction standard. 42, LAPX is a half-duplex variant of HDLC and is used in terminal networks and systems operating in the Teletex standard, and the LLC protocol (Link Logic Control) implemented in almost all multi-access networks (for example, wireless local networks). In Fig. 1. Figure 5 shows the HDLC protocol family and its application areas.
Rice. 1. 5. HDLC protocol family
Figure 1 6. Protocol profile for modem with physical and link layer functions
A possible protocol profile for a modem that supports the functions of the physical and data link layers is shown in Fig. 1. 6. It is believed that the computer connects to the modem via the RS-232 interface, and the modem already implements the V 34 modulation protocol and hardware error correction according to the V 42 standard
Rice. 1 7 Protocol Profile for DCE with Multiple Access
In some networks based on multipoint links, the signal received by each DCE is the sum of the signals transmitted from a number of other DCEs. The links in such networks are called multi-access links or mono links, and the networks themselves are called multi-access networks. Such networks include some satellite networks, terrestrial packet radio networks, and local wired and wireless networks.
The corresponding layers of the OSI model when transmitting in multiple access mode are somewhat different from those used in point-to-point links. The second layer must provide the upper layers with a virtual channel for error-free packet transmission, and the physical layer must provide the bit path. There is a need for an intermediate layer to manage the multi-access link so that each DCE can transmit frames without constant conflicts with other DCEs. This layer is called the MAC media access control layer (Medium Access Control). It is usually considered the first sub-level of level 2, i.e. i.e. level 2. 1. The traditional link layer in this case turns into the LLC logical channel control layer (Logical Link Control) and is sublevel 2. 2. In Fig. 1. 7 shows the interconnection of the second layer and the LLC and MAC sublayers.
1. 4. Facsimile
1. 4. 1. Sending a fax image
Facsimile communication is a type of documentary communication designed to convey not only the content, but also the appearance of the document itself. The essence of the facsimile transmission method is that the transmitted image (original) is divided into separate elementary areas, which are scanned at a scanning speed of 60, 90, 120, 180 or 240 lines/min. The brightness signal proportional to the reflectance of such elementary areas is converted into digital view and is transmitted over a communication channel using one or another modulation method. At the receiving side, these signals are converted into image elements and reproduced (recorded) on the receiving form.
The block diagram of fax communication is shown in Fig. 1. 8. The image (original) to be transmitted is scanned with a light spot of the required size. The spot is formed by a light-optical system containing a light source and an optical device. The movement of the spot along the surface of the original is carried out by a spreading device (RU). Part luminous flux, incident on the elementary area of the original, is reflected and supplied to a photoelectric converter (PC), in which it is converted into an electrical video signal. The amplitude of the video signal at the output of the photoconverter is proportional to the magnitude of the reflected light flux. Next, the video signal enters the input of an analog-to-digital converter (ADC), where it is converted into digital code. From the output of the ADC, the digital code is fed to the input of a signal conversion device (SCD), that is, a modulator, where, through the use of one of the modulation protocols, the spectrum of the digital video signal is transferred to the frequency range of the communication channel used.
Rice. 1. 8. Block diagram of fax communication
At the receiving side, the modulated signal coming from the communication channel sequentially enters the UPS and DAC for demodulation and digital-to-analog conversion, respectively. Next, the video signal enters the reproducing device (RD), where, as a result of the action of the reaming device, a copy of the transmitted image is reproduced on the form. The process of obtaining the final fax copy, the reverse of the scanning process, is called replication. To ensure synchronization and in-phase sweeps, synchronization devices (SD) are used on the transmitting and receiving sides.
Thus, a facsimile communication machine (fax) is very similar to a photocopier, in which the original and the copy are separated by many kilometers.
Modern fax modems include all the components of fax machines with the exception of scanning and reproducing devices. They “know how” to communicate with ordinary faxes, and the received information about the transmitted image is sent to a computer, where the fax message program is converted into one of the common graphic formats. In the future, the document obtained in this way can be edited, printed out, or sent to another correspondent who has a fax machine or a computer with a fax modem.
1. 4. 2. Fax standards
According to recommendations Standardization Sectors of the International Telecommunication Union(ITU-T - International Telecommunications Union - Telecommunications) Depending on the type of modulation used, faxes are distinguished into four groups. The first facsimile standards, classified as Group 1, were based on the analogue method of transmitting information. Group 1 faxes transmitted a page of text in 6 minutes. Group 2 standards have improved this technology to increase transmission speed, resulting in a reduction in transmission time per page to 3 minutes.
The Group 3 fax standard was originally defined by the ITU-T Recommendation T. 4 1980. This standard was reissued twice, first in 1984 and again in 1988. The 1990 modification of this standard adopted encoding schemes developed for Group 4 facsimile machines, as well as the higher bit rates defined by the V standards. I 7, V. 29 and V. 33. The radical difference between Group 3 fax machines and earlier ones is the fully digital transmission method with speeds of up to 14,400 bps. As a result, using data compression, a Group 3 fax transmits a page in 30-60 seconds. When communication quality deteriorates, Group 3 faxes go into emergency mode, slowing down the transmission speed. According to the Group 3 standard, two levels of resolution are possible: standard, providing 1728 dots horizontally and 100 dpi vertically; and high, doubling the number of vertical dots, which gives a resolution of 200x200 dpi and halves the speed.
Fax machines of the first three groups are focused on the use of analogue PSTN telephone channels. In 1984, ITU-T adopted the Group 4 standard, which provides resolutions up to 400x400 dpi and increased speed at lower resolutions. Group 4 faxes provide very high quality resolution. However, they require the high-speed links that ISDN networks can provide and cannot operate over PSTN links.
Almost all faxes currently sold are based on the Group 3 standard. Fig. 1. 8 illustrates the operation of just such faxes.
1. 5. Flow control
1. 5. 1. Need for flow control
In any system or data transmission network, situations arise when the load entering the network exceeds the capacity to service it. In this case, if no measures are taken to limit incoming data (graphics), the size of the queues on the network lines will grow without limit and will eventually exceed the size of the buffers of the corresponding communication means. When this happens, data units (messages, packets, frames, blocks, bytes, characters) arriving at nodes for which there is no free buffer space will be discarded and later retransmitted. The result is an effect when, as the incoming load increases, the actual throughput decreases and transmission delays become extremely high.
The means to combat such situations are flow control methods, the essence of which is to limit incoming traffic to prevent overloads.
A flow control circuit may be needed at a transmission link between two users ( transport layer), between two network nodes (network layer), between two neighboring DCEs exchanging data via a logical channel (data link layer), as well as between terminal equipment and data link equipment interacting via one of the DTE-DCE interfaces (physical layer).
Transport layer flow control schemes are implemented in file transfer protocols such as ZModem; flow control circuits network layer- as part of the protocols of X. 25 and TCP/IP; Link layer flow control circuits - as part of assurance protocols such as MNP4, V. 42; Flow control at the physical layer is implemented within the function set of corresponding interfaces, such as RS-232. The listed three levels of control circuits are directly related to the hardware and software of modems, and their specific implementations will be discussed in the relevant sections of the book.
1. 5. 2. Window method
Consider a class of flow control methods often used by link, network, and transport protocols called windowed flow control. A window refers to the largest number of information units that can remain unacknowledged in a given direction of transmission.
During transmission between the transmitter and the receiver, windowing is used if an upper limit is set on the number of data units that have already been transmitted by the transmitter, but for which an acknowledgment has not yet been received from the receiver. Upper bound, specified as a positive integer and is the window or window size. The receiver notifies the transmitter that it has received a unit of data by sending special message to the receiver (Fig. 1. 9). This message called an acknowledgment, authorization, or receipt. Confirmation may be positive - ASC (ACKknowledgement) signaling the successful reception of the corresponding information unit, and negative - NAK (Negative AcKnowledgement), indicating that the expected piece of data was not received. After receiving the receipt, the transmitter can transmit another unit of data to the receiver. The number of receipts in use should not exceed the window size.
Rice. 1. 9. Windowed flow control
Receipts are either contained in special control packages or added to regular information packages. Flow control is used when transmitting over one virtual channel, a group of virtual channels; the entire flow of packets occurring in one window and addressed to another node can be controlled. The transmitter and receiver can be two network nodes or a user terminal and an input node of the communication network. The data units in a window can be messages, packets, frames, or characters.
There are two strategies: end-to-end window control and node-based control. The first strategy refers to controlling the flow between the input and output nodes of a network for some transfer process and is often implemented as part of file transfer protocols. The second strategy relates to flow control between each pair of serial nodes and is implemented as part of link layer protocols such as SDLC, HDLC, LAPB, LAPD, LAPM and others.
1. 6. Classification of modems
There is no strict classification of modems and, probably, cannot exist due to the great diversity of both the modems themselves and the areas of application and modes of their operation. Nevertheless, it is possible to identify a number of characteristics according to which a conditional classification can be made. Such characteristics or classification criteria include the following: scope of application;
functional purpose; type of channel used; design; support for modulation, error correction and compression protocols data. Many more detailed technical features can be identified, such as the modulation method used, interface to DTE, and so on.
1. 6. 1. By area of application
Modern modems can be divided into several groups:
> for dial-up telephone channels;
> for dedicated (leased) telephone channels;
> for physical trunks:
Low level modems (line drivers) or short distance modems (short range modems)",
- baseband modems (. baseband modems);
> for digital systems transfers (CSU/DSU);
> for cellular communication systems;
> for packet radio networks;
> for local radio networks.
The vast majority of modems produced are intended for use on dial-up telephone channels. Such modems must be able to work with automatic telephone exchanges (PBX), distinguish between their signals and transmit their own dialing signals.
The main difference between physical line modems and other types of modems is that the bandwidth of physical lines is not limited to 3. 1 kHz, typical for telephone channels. However, the bandwidth of a physical line is also limited and depends mainly on the type of physical medium (shielded and unshielded twisted pair, coaxial cable, etc.) and its length.
From the point of view of signals used for transmission, modems for physical lines can be divided into low level modems(line drivers) using digital signals, and modems from the "baseband" (baseband), which use modulation methods similar to those used in modems for telephone channels.
Modems of the first group usually use digital methods of bi-pulse transmission, which make it possible to generate pulse signals without a constant component and often occupy a narrower frequency band than the original digital sequence.
Modems of the second group often use different kinds quadrature amplitude modulation, allowing to radically reduce the required frequency band for transmission. As a result, on identical physical lines such modems can achieve transmission speeds of up to 100 Kbps, while low-level modems provide only 19. 2 Kbps.
Modems for digital transmission systems resemble low-level modems. However, unlike them, they provide connection to standard digital channels, such as E1/T1 or ISDN, and support the functions of the corresponding channel interfaces.
Modems for cellular communication systems are distinguished by their compact design and support for special modulation and error correction protocols, which allow efficient data transmission in conditions of cellular channels with a high level of interference and constantly changing parameters. Among these protocols, ZyCELL, ETC and MNP10 stand out.
Packet radio modems are designed to transmit data over a radio channel between mobile users. In this case, several radio modems use the same radio channel in multiple access mode, for example, carrier sense multiple access, in accordance with ITU-T AX. 25. The radio channel is close in its characteristics to the telephone channel and is organized using standard radio stations tuned to the same frequency in the VHF or HF range. The packet radio modem implements modulation and multiple access techniques.
Local radio networks are a fast-growing, promising network technology that complements ordinary local networks. Their key element is specialized radio modems (local radio network adapters). Unlike the previously mentioned packet radio modems, such modems provide data transmission over short distances (up to 300 m) at high speeds (2-10 Mbit/s), comparable to transmission speeds in wired local networks. In addition, radio modems of local radio networks operate in a certain frequency range using signals of complex shapes, such as signals with pseudo-random tuning of the operating frequency.
1. 6. 2. By transmission method
Based on the transmission method, modems are divided into asynchronous and synchronous. When talking about synchronous or asynchronous transmission methods, they usually mean transmission over a communication channel between modems. However, transmission over the DTE-DCE interface can also be synchronous or asynchronous. The modem can work with the computer in asynchronous mode and simultaneously with the remote modem - in synchronous mode or vice versa. In this case, they sometimes say that the modem synchronous-asynchronous or it works in synchronous-asynchronous mode.
Typically, synchronization is implemented in one of two ways, related to how the sender and receiver clocks work:
independently of each other (asynchronously) or in concert (synchronously). If Since the transmitted data is composed of a sequence of individual characters, then, as a rule, each character is transmitted independently of the others and the recipient is synchronized at the beginning of each received character. For this type of communication, asynchronous transmission is usually used. If the transmitted data forms a continuous sequence of characters or bytes, then the clock generators of the sender and receiver must be synchronized over a long period of time. In this case, synchronous transmission is used.
Asynchronous transmission mode is used mainly when the transmitted data is generated at random times, for example by the user. In such a transmission, the receiving device must reset the clock at the beginning of each character received. To do this, each transmitted character is framed by an additional start bit and one or more stop bits. This asynchronous mode is often used when transmitting data over the DTE-DCE interface. When transmitting data over a communication channel, the possibilities of using the asynchronous transmission mode are largely limited by its low efficiency and the need to use simple modulation methods, such as amplitude and frequency. More advanced modulation methods, such as OFM, QAM, etc., require maintaining constant synchronism of the reference clock generators of the sender and receiver.
In the synchronous transmission method, a large number of characters or bytes are combined into separate blocks or frames. The entire frame is transmitted as a single string of bits without any delay between eight-bit elements. In order for the receiving device to provide different levels of synchronization, the following requirements must be met.
> The transmitted bit sequence must not contain long sequences of zeros or ones so that the receiving device can stably allocate the synchronization clock frequency.
> Each frame must have reserved sequences of bits or symbols marking its beginning and end.
There are two alternative methods for organizing synchronous communication: character- or byte-oriented, and bit-oriented. The difference between the two is how the start and end of the frame are determined. With the bit-based method, the recipient can determine the end of the frame with precision down to a single bit, rather than a byte (character).
In addition to high-speed data transmission over physical channels, synchronous mode is often used for transmission over the DTE - DCE interface. In this case, additional interface circuits are used for synchronization, through which a clock signal is transmitted from the sender to the recipient.
1. 6. 3. By intellectual capabilities
Modems can be classified according to their intellectual capabilities:
without control system;
> supporting a set of AT commands;
> with support for V commands. 25bis;
> with a proprietary command system;
> supporting network management protocols.
Most modern modems are equipped with a wide range of intelligent capabilities. The de facto standard has become a set of AT commands, developed at one time by Hayes, which allow the user or application process to fully control the characteristics of the modem and communication parameters. For this reason, modems that support AT commands are called Hayes-compatible modems. It should be noted that AT commands support not only PSTN modems, but also packet radio modems, external ISDN adapters, and a number of other modems with narrower applications.
The most common set of commands that allows you to control the connection establishment and auto-call modes are the ITU-T V recommendation commands. 25bis.
Specialized modems for industrial use often have a proprietary command system that is different from the AT command set. The reason for this is the large difference in operating modes and functions performed between widely used modems and industrial (network) modems.
Industrial modems often support the SMNP network management protocol (Simple Manager Network Protocol), allowing the administrator to manage network elements (including modems) from a remote terminal.
1. 6. 4. By design
Modems are classified according to their design:
> external;
> internal;
> portable;
> group.
External modems are stand-alone devices that connect to a computer or other DTE via one of the standard DTE-DCE interfaces. An internal modem is an expansion card that is inserted into the corresponding slot on the computer. Each design option has its own advantages and disadvantages, which will be discussed below.
Portable modems are designed for use by mobile users in conjunction with Notebook-class computers. They are small in size and high in price. Their functionality, as a rule, is not inferior to that of full-featured modems. Often portable modems are equipped with a PCMCIA interface.
Group modems are a collection of individual modems combined into a common unit and having a common power supply, control and display devices. A separate modem of a group modem is a board with a connector installed in the unit and is designed for one or a small number of channels.
1. 6. 5. To support international and proprietary protocols
Modems can also be classified according to the protocols they implement. All protocols regulating certain aspects of the operation of modems can be classified into two large groups:
international and branded.
International level protocols are developed under the auspices of the ITU-T and adopted by it as recommendations (formerly ITU-T was called International Telephony and Telegraph Advisory Committee - CCITT, international abbreviation - CCITT). All ITU-T recommendations regarding modems are in the V series. Proprietary protocols are developed by individual modem companies in order to outperform the competition. Often proprietary protocols become de facto standard protocols and are adopted in part or in full as ITU-T recommendations, as happened with a number of Microcom protocols. Such well-known companies as AT&T, Motorolla, U. are most actively developing new protocols and standards. S. Robotics, ZyXEL and others.
From a functional point of view, modem protocols can be divided into the following groups:
> Protocols that define the standards for interaction between the modem and the communication channel (V. 2, V. 25):
> Protocols regulating the connection and algorithms for interaction between the modem and DTE (V. 10, V. 11, V. 24, V. 25, V. 25bis, V. 28);
> Modulation protocols that determine the main characteristics of modems designed for dial-up and dedicated telephone channels. These include protocols such as V. 17, V. 22, V. 32, V. 34, HST, ZyX and a large number of others;
> Error protection protocols (V. 41, V. 42, MNP1-MNP4);
> Transmitted data compression protocols, such as MNP5, MNP7, V. 42bis;
Rice. 1. 10. Classification of modem protocols
> Protocols defining procedures for diagnosing modems, testing and measuring parameters of communication channels (V. 51, V. 52, V. 53, V. 54, V. 56).
> Protocols for coordinating communication parameters at the stage of its establishment (Handshaking), for example V. 8.
The prefixes "bis" and "ter" in protocol names denote, respectively, the second and third modification of existing protocols or a protocol related to the original protocol. In this case, the original protocol, as a rule, remains supported.
Some clarity among the variety of modem protocols can be brought about by their conditional classification shown in Fig. 1. 10. CHAPTER 8 DATA COMPRESSION PROTOCOLS
Algorithm for working with the C1-I interface
Vagin Fedor Anatolievich,
Evdokimov Alexander Vladimirovich,
Knol Maxim Gennadievich,
Knol Dmitry Gennadievich,
master's students of Omsk State Technical University.
Interface is a concept that is used to describe a set of circuitry means and functions that ensure direct interaction of the constituent elements of data processing systems (DPS), networks, data transmission systems (DTS), and peripheral equipment subsystems.
The definition of “joint” (according to GOST - 23633-79) is the connection point of data signal transmission devices included in data transmission systems.
The main purpose of the joints is to unify intra- and intersystem, intra- and inter-network connections in order to effectively implement methods for designing functional elements (FE) of computer systems, data access systems and networks.
The main function of the joints is to ensure informational, electrical and structural compatibility between PV systems and networks.
At the C1-I junction, the symbol “1” of the input information sequence corresponds to bipulse 10 or 01, coinciding with the previous one, and the symbol “0” corresponds to bipulse 10 or 01, inverse with respect to the previous bipulse. In other words, this code is relative, similar to that used in OFM. Relative coding allows us to solve the problem of bipulse phase uncertainty on the receiving side. As a result of this, the C1-I interface is not afraid of errors such as “mirror reception” or “reverse operation” (inversion of signs) and reversal of the polarity of the contacts of the physical line or used connectors.
Algorithm No. 1 (Using a capture block)
The implementation of this algorithm is carried out by means of measuring the duration of the pulses of the processed signal. When using microcontroller tools, the least resource-intensive way is to use a capture unit,which remembers the state of the counter when an external event occurs, thereby determining the time of its occurrence. An external signal acts as an event/events.
The algorithm is based on dividing the input signal into two types of pulses: long and short. The type is selected by comparing the processed pulse with the standard calculated for a given speed (the ratio of the frequency of the quartz oscillator to the speed of the received signal) of a long and short pulse. By long we mean two pulses of equal duration, by short – one.
The main problem of this method is the lack of pulses of the same type that are equal in duration. This problem is explained by the imperfection of the timing characteristics of the input signal and the instability of the microcontroller's quartz oscillator, which results in the impossibility of direct comparison with the standard. The solution to this problem is to introduce an additional variable depending on the speed of the received signal, which takes into account the likelihood of inaccurate counting of the number of clock cycles during a pulse.
Using a timer-counter prescaler allows you to reduce the number of processing operations and the time to determine the type of pulse.
Figure 1 shows an illustration of the algorithm in the form of a block diagram, which uses the following abbreviations:
A. imp. – analyzed impulse;
Dl. – the number of clock cycles corresponding to a long pulse;
Cor. – the number of clock cycles corresponding to a short pulse;
T. bit – bit value determined in accordance with the types of the previous and analyzed pulse;
Sl. bit – the bit following the current bit;
Pogr. – an additional variable depending on the speed of the received signal, which takes into account the likelihood of inaccurate counting of the number of clock cycles during a pulse.
Rice. 1. Illustration of the algorithm.
Literature
1. Bulatov V.N. Elements and components of information and control systems (Fundamentals of theory and synthesis): Textbook. – Orenburg: State Educational Institution of Higher Professional Education OSU, 2002. - 200 p.
2. GOST 23633-79. Joints in data transmission systems [Text]: terms and definitions. – Moscow: USSR State Committee for Standards, 1979. – 28 p.
3. GOST 27232-87. Interface between data transmission equipment and physical lines [Text]: basic parameters. – Moscow: USSR State Committee for Standards, 1987. – 8 p.
TO physical level This also applies to the interface between the DCE and the communication channel (physical communication line or transmission medium), which must comply with international standards. In our country, this interface is called the C1 interface, which has its own designations and GOST standards for different channels. Thus, for analog telephone channels, the C1 joints are divided into C1-TF in the case of using a switched PSTN network and C1-TC for non-switched TC channels. These joints correspond to GOST standards: 23504-79, 25007-81, 26557-85, and for S1-TC also 23475-79. To operate via the TC radio channel, the C1-TChR interface was introduced (GOST 23578-79). If the transmission is carried out through a telegraph network, then the C1-TG interface is used (GOST 22937-78). In case of direct access, i.e. when connecting to a network node with a dedicated line, modems for physical lines (for example, from Zelax) are used with S1-FL joints (GOSTs 24174-80, 26532-85), which have three types of signals: low level signal (S1-FL-NU) , bipulse signal (S1-FL-BI) and quasi-ternary signal (S1-FL-KI). The bipulse signal (Manchester code) is widely used in local networks, and the quasi-ternary signal is used in the channels of digital transmission systems (international interface G.703), where the AMI signal (with alternating pulse polarities - PPI) or a modified HDB3 signal is used, in which long series are eliminated zeros.
All C1 joints and the corresponding GOST standards are developed based on international standards IOC and ITU-T recommendations.
Exchange at the joints S1-TF and S1-TC is carried out by modulated signals in the operating frequency band of voice-frequency channels. V series modems act as ADCs. When transmitting over a radiotelephone channel, the C1-TCR interface is used. The parameters of these joints are presented in table. 2.4 and 2.5.
Table 2.4 Parameters of joints S1-TF and S1-TC
Table 2.5 Parameters of the S1-TCR interface
Joints S1-FL
Data transmission in the S1-FL interface circuits is carried out using pulse signals at speeds of up to 480 kbit/s. The nomenclature of the circuits at the C1-FL junction and the requirements for them are the same as at the C1-TF and C1-TC junctions. In all three types of C1-PL junction, the ratio of the amplitude of a pulse of positive polarity (+U) to the amplitude of a pulse of negative polarity (-U) should be within 0.95? 1.05.
The parameters of the S1-FL joints are presented in Table. 2.6.
Table 2.6 Main parameters of S1-FL joints
For the S1-FL-NU interface, low-level digital signals of different polarity (LL) are used without returning to zero (NRZ - NonReturntoZero).
The NRZ method is simple to implement, has relatively high noise immunity (due to two sharply different potentials), but does not have the property of self-synchronization. When transmitting a long sequence of ones or zeros, the signal on the line does not change, so the receiver cannot determine from the input signal the moments in time when it is necessary to read the data again. Even with a highly stable clock generator, the receiver may make a mistake with the moment of data collection, since the frequencies of the two generators are never completely identical. Therefore, when high speeds data exchange and long sequences of ones or zeros, a small mismatch in clock frequencies can lead to an error of a whole clock cycle and, accordingly, the reading of an incorrect bit value.
Another serious disadvantage of the NRZ method is the presence of a low-frequency component that approaches zero when transmitting long sequences of alternating ones or zeros. Because of this, many communication channels that do not provide a direct galvanic connection between the receiver and the source do not support this type of coding. As a result, the NRZ code in its pure form is not used in networks. However, various modifications are used that eliminate the above disadvantages. The attractiveness of the NRZ code lies in the fairly low frequency of the fundamental harmonic f0, which is equal to N/2 Hz (where N is the data bit rate).
For the joint S1-FL-KI a quasi-ternary pulse code with alternating pulse polarity is used - PPI (AMI–BipolarAlternateMarkInversion).
This method uses three potential levels - negative, zero and positive. To encode a logical zero, for example, a zero potential is used, and a logical unit is encoded either by a positive potential or a negative one, with the potential of each new unit being opposite to the potential of the previous one.
The AMI code partially eliminates the problems of the presence of a constant component and the lack of self-synchronization inherent in the NRZ code. This occurs when transmitting long series of "units". In these cases, the signal on the line is a series of alternating heteropolar pulses with the same spectrum as the NRZ code, transmitting alternating zeros and ones, that is, without a constant component and with a fundamental harmonic of N/2 Hz (where N is the bit rate of data transfer ). Long series of “zeros” are just as dangerous for the AMI code as for the NRZ code - the signal degenerates into a constant potential of zero amplitude.
In general, the AMI code results in a narrower signal spectrum than the NRZ code, and therefore higher bandwidth lines. For example, when transmitting alternating ones and zeros, the fundamental harmonic has a frequency of N/4 Hz. The AMI code also provides some capabilities for recognizing erroneous signals. Thus, a violation of the strict alternation of signal polarity indicates a false pulse or the disappearance of a correct pulse from the line. A signal with incorrect polarity is called a prohibited signal. signal violation.
A modified AMI code (HDB-3) is often used, in which each series of 4 zeros is converted into a non-zero combination according to a certain rule, which increases the stability of the clock synchronization system.
Joint S1-FL-BI uses bipulse codes. With bipulse coding, each clock cycle is divided into two parts. Information is encoded by potential drops that occur in the middle of each clock cycle. Since the signal changes at least once per transmission cycle of one data bit, the bipulse code has good self-synchronizing properties. In a simple bipulse code, “1” is encoded by an edge from a low signal level to a high one, and “0” is encoded by a reverse edge.
The most common bipulse code is Manchester Code, which is used in local networks.
The difference between the Manchester code and a simple bipulse code is that each subsequent logical “0” changes the phase of the bipulse to the opposite, and “1” maintains the phase of the previous bipulse.
The Manchester code also does not have a constant component, and the fundamental harmonic in the worst case (when transmitting a long sequence of ones or zeros) has a frequency of N Hz, and in the best case (when transmitting alternating ones and zeros) it is equal to N / 2 Hz. The Manchester code has another advantage over the AMI code in that it uses not three signal levels, but two, for data transmission.
G.703 interface
The G.7O3 standard is based on the following ITU-T recommendations: G.702 “Digital Hierarchy Rates” (we are talking about Plesiosynchronous Digital Hierarchy - PDH); G.704 “Structure of synchronous frames based on primary and secondary hierarchical levels”; I.430 “ISDN Basic Rate User Interface – Level 1 Specification (D-Capala Signaling Protocol).”
This standard is intended for use in networks not only with the PDH hierarchy, but also with the synchronous digital hierarchy SDH (the transmission rates and frame structure of the latter are given in ITU-T Recommendations G.708 and G.709). It was originally developed as a basic interface for systems using pulse code modulation (PCM).
Physical and electrical characteristics. The standard regulates the physical and electrical characteristics of the G.703 interface for the primary data rate of 64 kbit/s and the series generated by the primary (North American with speeds of 1544, 6312, 32064, 44736 kbit/s) and secondary (European 2048, 8448, 34368, 139264 kbit/s) PDH hierarchies, as well as for an additional speed of 97728 kbit/s. Let us list the main ones: equipment interaction diagram; data transfer rate and clock signal frequency; type of code and algorithm for its generation; pulse shape (mask) and corresponding tolerance field; the type of cable pair used for each transmission direction; load impedance; rated peak pulse voltage; peak voltage in the absence of a pulse; nominal pulse width; the ratio of the amplitudes of the positive and negative pulses to the width of the negative; maximum phase jitter (jitter) at the output port.
Let's look at some of these characteristics in more detail.
Equipment interaction diagram. The standard provides three interaction schemes between two terminal devices (controlling - controlled or receiving - transmitting): co-directional interface, SNI, (Correctional Interface). Information and clock (timing or synchronizing) signals are transmitted from one terminal to another, and the terminals are equal and symmetrical; multidirectional interface, RNI, (Contradi-rectional Interface). Here the terminals are unequal: one of them is the manager, the other is the controlled one. Clock signals are directed only from the control terminal to the controlled one, and information signals are symmetrical. interface with the central clock generator, TsGI, (Centralized Clock Interface). Clock signals are directed from the central master oscillator to both terminals, and information signals are symmetrical.
Data transfer rate and clock frequency. These parameters specified in the standard basically correspond to the PDH hierarchy. The clock (synchronizing) signal comes from a separate source or is formed from a transmitted encoded information signal. The clock frequency may or may not be the same as the data transfer rate. In the latter case, it can be two, four or eight times less, depending on the data encoding method used. For example, for a speed of 64 kbit/s the nominal speed is clock frequency 64 kHz, but 8 kHz (octet clock) generated by the PCM multiplexer control unit or an external source can also be used.
Type of code (algorithm for its generation). Depends on the data transfer rate and the interaction scheme of the interface equipment. If the code is not standardized separately, then a description of the algorithm for its generation is given in the G.703 standard itself, as is done for the speed of 64 kbit/s with a co-directional scheme. If the code is standardized, then only its name and features are indicated.
Pulse shape and corresponding tolerance range. These characteristics are specifically specified for each transmission speed and interface hardware interaction scheme. The single pulse mask for 64 kbit/s is shown in Fig. 2.7. At a speed of 2048 kbit/s and its derivatives, the shape of the mask remains virtually unchanged.
Rice. 2.7. Pulse shape for G.703 interface and tolerance limits
Type of line used and load impedance. Typically, coaxial cable pairs, balanced pairs, or a combination of both are used. The load impedance of a balanced pair varies from 100 to 120 ohms.
Maximum pulse voltage and signal level during pause. These parameters depend on a number of factors, including transmission speed and noise level, which may be specified separately.
Connecting user equipment to a network with a G.703 interface. The connection diagram depends on the type of transmission line (coaxial or symmetrical pair) and its impedance (75 or 100-120 Ohms), the presence of an input with the G.703 interface and the propagation medium (electrical or fiber optic cable).
This scheme will be simple if an electrical cable is used for the backbone connection, and the equipment has an input with a G.703 interface. For connection, RG-59 connectors (coaxial pair with an impedance of 75 Ohms) or DB-15, RJ-11, RJ-48X (symmetrical pair with an impedance of 100-120 Ohms) are used. It is permissible to connect a symmetrical pair to the patch panel “with a screw” without a connector. If the equipment input impedance does not match the line impedance, then a matching transformer is used (for example, 120 ohm balanced pair / 75 ohm coaxial pair for 2048 kbps).
When propagating along a fiber-optic cable, the light signal is converted into an electrical signal (at the input of the user equipment) and back (at its output) using a special optoelectronic converter. In this case, various types of optical connectors (connectors), for example SC, SMA, ST types, are installed at the optical inputs and outputs.