Uzaktan denetim sistemlerinin veri bağlantı kontrolü

Abstract

İletişim alanında ortaya çıkan yoğun yapılanmanın sonucunda, en uzak yerleşim birimlerine kadar hizmet vermek üzere, çok sayıda haberleşme istasyonları kurulmaktadır. Bu istasyonlarda, sürekli olarak eleman bulundurmak yerine, uzaktan izleme imkanı sağlamak çok daha ekonomik bir çözümdür. İstasyonları arasında, karşılıklı olarak veri iletiminin yapıldığı bir sistemin, tasarımı aşamasında göz önünde bulundurulması gereken en önemli unsurlardan bir tanesi, iletim sırasında oluşan periyodik veri bozulmalarının üstesinden gelecek bazı yöntemlerin geliştirilmesidir. Sisteme bağlı bulunan istasyonlardan herhangi biri, diğer bir istasyona veri gönderebilmeli ve bu verinin karşı tarafa hatasız olarak ulaştığından emin olmalıdır. Karşılıklı olarak haberleşen istasyonlar, gönderdikleri mesajların tam kontrolünü ellerinde tutabilmeli ve verinin bozulması durumunda, mesajın tekrar gönderilmesi veya hataların düzeltilmesi sağlanabilmelidir. Gönderici ve alıcı taraflar, iletilen mesajların tanımını ve sıralamasını bilmeli ve hattın çok sayıda kullanıcı tarafından paylaşılmasını sağlayacak yöntemler bulunmalıdır. Bu tezin ikinci bölümünde, bu ihtiyaçlara cevap veren veri bağlantı kontrol yöntemlerinin temel unsurları olan, mesaj yapılan, bağlantı kontrol yöntemleri, hata sezme ve düzeltme yöntemleri ve akış kontrol yöntemleri incelenmiştir. Daha sonraki bölümlerde ise, Alcatel Teletaş'ta geliştirilmekte olan SR2810A Sayısal Radyo Link Sistemi'nin, Uzaktan Denetim (Uzdenetim) Birimi için geliştirilen veri bağlantı kontrol yöntemini oluşturan, donanım ve yazılım birimlerinin tanıtımı yapılmıştır. Uzdenetim Birimi, aynı haberleşme hattına bağlı, terminal ve tekrarlayıcı olarak çalışan Radyo Link (R/L) sistemlerinin, çalışmalarını denetlemek, ve alarm ve gösterge bilgilerini izlemek için tasarlanmış olan, mikrodenetleyici tabanlı bir devredir.

Since certain errors are inevitable in a system, a method must be provided to deal with the periodic distortions that occur within a transmission. The data communications system must provide each site with the capability to send data to another site. The sending site must be assured that the data arrive error free at the receiving site. The sending and receiving sites must maintain complete accountability for all messages. In the event the data are distorted, the receiving site must have the capability of notifying the originator to resend the erroneous message or otherwise correct the errors. The movement of traffic to and from the many points within the network must flow in a controlled and orderly manner. This means that the sending and receiving sites must know the identification and sequencing of the messages being transmitted among all users. The connection path between sites is usually shared by more than one user (as in a multidrop configuration); consequently, procedures must provide for the allocation and sharing of the path among the many users. Unfortunately, communication circuits make errors occasionally. Furthermore, they have only a finite data rate, and there is a nonzero propagation delay between the time a bit is sent and the time it is received. These limitations, coupled with the finite processing speed of the machines, have important implications for the efficiency of the data transfer. The protocols used for communications must take all these factors into consideration. Data link controls (DLC's), or line protocols, provide for these needs. They manage the flow of data messages across the communication path or link and can be designed to offer various services. Three reasonable possibilities are, unacknowledged connectionless service, acknowledged connectionless service and connection-oriented service. Unacknowledged connectionless service consists of having the source machine send independent frames to the destination machine without having the destination machine acknowledge them. No connection is established beforehand or released afterwards. If a frame is lost due to noise on the line, no attempt is made to recover in the data link protocol. This class of service is appropriate when the error rate is very low and recovery is left to a higher level of control. It is also appropriate for real time traffic, such as speech, in which late data are worst then bad data. Many LAN's use unacknowledged connectionless service in the data link layer. The next step up in terms of reliability is acknowledged connectionless service. When this service is offered, there are still no connections used, but each frame sent is individually acknowledged. In this way, the sender knows whether or VI not a frame has arrived safely. If it has not arrived within a specified time interval, it can be sent again. The most sophisticated service the data link layer can provide to the network layer İs connection-oriented service. With this service, the source and destination machines establish a connection before any data are transfered. Each frame sent over the connection is numbered, and the data link protocol guarantees that each frame sent is indeed received. Furthermore, it guarantees that each frame is received exactly once and that all frames are received in the right order. With connectionless service, in contrast, it is conceivable that a lost acknowledgement causes a frame to be sent several times and thus received several times. Connection-oriented service, in contrast provides the higher level controls with the equivalent of a reliable bit stream. DLC's consist of a combination of software and hardware and are located at each site in the network. The DLC is concerned with providing the following functions to the network; synchronizing the sender and receiver, controlling the sending and receiving of data, detecting and recovering transmission errors between two points, maintaining awareness of link conditions. The DLC does not provide the user with end-to-end accountability. Since data link controls do not provide for end-to-end access and flow control, a higher level of control is required to provide for session-to-session accountability and control. Data link controls can be described and classified by (a) message format, (b) line control method, (c) error-handling method, and (d) flow control procedure. Asynchronous message formats originated with older equipment with limited capability, but are still widely used due to their simplicity. Their main disadvantage is the overhead of the control bits. Notwithstanding, many variations and improvements have been made to asynchronous protocols and, in spite of the overhead, DLC's with asynchronous formats still remain one of the dominant data link controls. Asynchronous techniques are found in practically all systems that use teleprinter and teletype terminals, and constitude the format for the vast majority of personal computers. The four commonly used synchronous formats are, character count, character stuffing, bit stuffing and physical medium coding violations. The character count framing method uses a field in the header to specify the number of characters in the frame. When the data link layer at the destination sees the character count, it knows how many characters follow, and hence where the end of the frame is. The trouble with this algorithm is that the count can be garbled by a transmission error causing the destination to get out of synchronization, and to be unable to locate the start of the next frame. Even if the checksum is incorrect so the destination knows that the frame is bad, it still has no way of telling where the next frame starts. Sending a frame back to the source asking for retransmission does not help either, since the destination does not know how many characters to skip over to get to the start of the retransmission. vn The synchonous byte-oriented DLC was developed in 1960s but is still widely available. It uses the same string of bits and bytes to represent data characters and control characters. In this case the DLC logic can mistakenly interpret the data as control information. Most versions of synchronous byte techniques have logic provisions to handle this problem. Byte protocols also use a format in which the control fields occur in variable locations within the frame. The more advanced synchronous DLC s use the bit-oriented approach. In this method, line control bits are always unique and cannot occur in the user data stream. The logic of the DLC examines the data stream before it is transmitted and alters the user data if it contains a bit configuration that could be interpreted as a control indicator. Of course, with this approach, the receiving DLC has the capability to change the data stream to its original contents. The bit-oriented DLC achieves code transparency. Furthermore, bit protocols use the individual bits for line control, not the full character itself. This means that the logic is not dependent on a particular code such as ASCII or EBCDIC. The control fields/bits usually reside in fixed locations within the frame. The last method of framing is only applicable to networks in which the encoding on the physical medium contains some redundancy. For example, Manchester encoding encodes each 1 bit as a high-low pair and each each 0 bit as a low-high pair. The combinations high-high and low-low are not used for data. However, some protocols use an invalid sequence as high-high-low-low for framing. While this technic is a little like cheating, it has the clear advantage that no stuffing is required. At the broadest level, data link control methods are classified as (a) primary/secondary, (b) peer to peer, or (c) a combination of these. Primary/secondary (or master/slave) protocols provide for one station (such as a computer) to manage all traffic on the channel. The other stations must obtain permission from the primary station before they can transmit data. Peer-to-peer protocols have no master station. In this situation all stations are equal and use the channel under some form of contention or negotiation. The most common method of line primary/secondary control is through the use of polling/selection techniques. A site in the network is designated as the master or primary station. This site is responsible for the sending and receiving of messages between all secondary or slave sites on the line. In fact, the secondary sites cannot send any messages until the master station gives approval. The polling/selection protocol is widely used for several reasons. The centralized approach allows for hieararchical control. Traffic flow is directed from one point, which provides for simpler control than a noncentralized approach. Priorities can be established among the users. Certain computers and terminals can be polled or selected more frequently than others, thus giving precedence to certain users and their applications. Sites (terminals, software Vlll applications, or computers) can be readily added by changing polling/selection tables within the DLC logic. There is a price to pay for all these features. The polling/selection DLC incurs a substantial amount of overhead due to the requirement for polling, selection and control messages. On some networks, the negative responses to polls can consume a significant portion of the network capacity. More recent implementations of the polling/selection DLC (in SDLC) use some clever methods to reduce the number of overhead messages. Timeouts allow the link control station to check for errors or questionable conditions on the line. A timeout occurs when a polled station does not respond within a certain time. The nonresponse condition evokes recovery action on the part of the controlling station. The timeout threshold is dependent on signal propagation delay to and from the polled station, processing time at the polled station and turn-around delay at the polled station on a half-duplex line. These factors are highly variable and depend on the line type, line length, modem performance, and processing speed at the polled terminals' site. A variation of polling/selection is known as hub polling. This approach is used on a multipoint line to avoid the delay inherent in the polled terminals turning around a half-duplex line to return a negative response to the poll. In this operation, the master station sends a poll to a terminal on the line. This terminal turns the line around with a message to the master station if it has data to send or, under another version, it piggybacks the data onto the poll and forwards both to the next station on the line. If it has no data, it sends the polling message to the next terminal on the line. If this terminal is busy, idle or has nothing to send, it relays the polling message to the next appropriate station. The transmission of this poll continues in one direction without additional turnarounds. Eventually, a terminal will be found that has data for the master station. Thus hub polling eliminates the line turnaround time that occurs if each terminal receives a poll from the host. Contention is a widely used peer-to-peer link control method. It differs significantly from polling/selection, since there is no master station. With contention, each site has equal status on the line, and the use of this path is determined by the station that first gains access during an idle line period. Contention DLC's must provide for a station to relinquish the use of the path at an appropriate interval of time in order to prevent line domination from one site. Contention control is widely used today, primarily because of its relative simplicity and the absence of a master station. The polling/selection approach suffers from the vulnerability of the primary site to failure, which could bring down all sites on the link. Since the contention method does not rely on a controling site, a failed site does not prevent the other sites from communicating with each other. The method of detecting and correcting errors in a message is a key selection criterion for a data link control. Certainly, one option is to ignore errors. However, IX many applications cannot afford errors. Financial data systems, such as an electronic transfer of funds between customers' bank accounts, must have completely accurate data messages arrive at the end point. Most methods used to provide for data error detection entail the insertion of redundant bits in the message. The actual bit configuration of redundant bits is derived from the data bit stream. The vertical redundancy check (VRC) is a simple technique, which consists of adding a single bit (a parity bit) to each string of bits that comprise a character. The longitudinal redundancy check (LRC) is a refinement of the VRC approach. Instead of a parity bit on each character, LRC places a parity on a block of characters. Echoplex technique is used in many asynchronous devices, notably personal computers. Each character is transmitted to the receiver, where it is send back or echoed to the original station. The echoed character is compared with a copy of the transmitted character. If they are the same, a high probability exists that the transmission is correct. Another technique that is quite widely used is the cyclic redundancy check (CRC). The CRC approach entails the division of the user data stream by a predetermined binary number. The remainder of the number is appended to the message as an FCS (frame check sequence) field. The data stream at the receiver site has another calculation performed and compared to the FCS field. If the remainder is zero, the message is accepted as correct. A DLC must manage the transmission and receipt of perhaps thousands of messages in a short period of time. Communication lines should be evenly used, and no station should be unnecessarily idle or saturated with excessive traffic. Thus, flow control is a critical part of the network. The stop-and-wait DLC allows one message to be transmitted, checked for errors, and an appropriate message returned to the sending station. No other data messages can be transmitted until the receiving station sends back a reply. It is well suited to half-duplex transmission arrangements, since it provides for data transmission in both directions, but only in one direction at a time. Moreover, it is a simple approach requiring no elaborate sequencing of messages or extensive message buffers in the terminals. In the newer data link controls the data and control signals flow from sender to receiver in a more continuous manner, and several messages can be outstanding at any time. These DLC s are often called sliding windows because of the method used to synchronize the sending sequence numbers in the headers with appropriate acknowledgements. The transmitting station maintains a sending window that delineates the number of messages it is permitted to send. The receiving station maintains a receiving window that performs complementary functions. The two sites use the windows to coordinate the flow of messages between each other. In essence, the window states how many messages can be outstanding on the line or at the receiver before the sender stops sending and awaits a reply. The supervisory unit, introduced in this thesis, is a 80C51 FA 8-bit microcontroller based circuit, which is designed to control the operation of SR2810A 8 Mbit/s R/L systems, operating as terminals or repeaters, connected to the same transmission line, and to monitor the alarms and indicators of these systems. The SR2810A is a Digital Microwave Radio Link System customized for 2, 8 or 4x2 Mbit/s data transmission using QPSK modulation at 10.5 GHz frequency band. The channel frequency arrangement is according to CCIR Report No. 607-4, Item 3.8. The two different basic systems available are, SR2810A S2 and SR2810A S8. S2 is a low cost system for 2 Mbit/s data transmission. It is available in (1+0) unprotected configuration only. Additional features such as supervisory or service channel, are not available. S8 is a low cost basic system for 8 Mbit/s data transmission. S8 has a complete range of additional options. Optional equipment for S8 are: Protection, Supervisory Unit, Digital Service Channel, 4x2 Mbit/s Mux and Hand-Set. SR2810A system satisfies public and private requirements in the field of telecommunication and as communications media for railways, power line transmission systems, oil and gas pipelines, etc. The alarms and indicators, gathered from various points of a R/L system, are read and evaluated by the supervisory unit. This information is saved in different forms such as urgent alarm, alarm list, past alarm list and indicator list. A hand-held terminal can be connected to the supervisory unit, through an RS232 interface, in order to learn local and remote alarm information and to change the system settings if the system password is known. The system settings include link name, system name, number of systems, system address, system type, date and time. If an operator wants to learn the alarm information of a specific remote system or all of the remote systems, using the hand-held terminal, a modem communication is performed between two systems. The program, running on the supervisory unit, is written in C language. This program gathers the alarms and indicators of the system, records this information in various forms, provides a menu structure and a communication protocol for the hand-held terminal and also a data link control protocol between the systems connected to the same link. The compilation, linking and locating operations are performed using the Keil Elektronik C51 package, and ICE-51/PC emulators are used during program debugging. The data link protocol, used for modem communication, makes use of asynchronous message format, contention and timeout line control method, LRC error-handling method and stop and wait flow control procedure.