提交文件suzl2500202detailed design for蘇州軌道交通二號(hào)線

1、蘇州軌道交通二號(hào)線蘇州軌道交通 2 號(hào)線工程 Suzhou Rail Transit Line 2ed Design for DCS Wayside無線通信系統(tǒng)軌旁詳細(xì)設(shè)計(jì)SAP No: A6Z00033484847/PM1/AIC MOL RAreleasedPage 1 of 56SuZL2-5002-02© Siemens.,© Siemens.,2012 保留所2012利受限-蘇州 2 號(hào)線使用視為商業(yè)Confided as a trade secret.Restricted-For SuZL2 use only本文件 不得轉(zhuǎn)讓及 ，不得散布并/或 改造，不 或轉(zhuǎn)錄

2、其內(nèi)容，違者 並須賠償損失。保留所有的專利權(quán)及造型設(shè)計(jì)試樣和工藝 新樣品的 權(quán)。Transmittal, reproduction, dissemination and/or editing of this document as well as utilization of its contents and communication thereof to others without express authorization are prohibited. Offenders will be held liable for payment of damages.s created by

3、patent grant or registration of a utility m or design patent are.Name部門 Dept日期 Date簽字 Signature批準(zhǔn) ReleasedChen Xiao GuangIC MOL RA PM2013-10-12Sgd. CHEN_XIA核稿 CheckedMa Li HongIC MOL RA QM2013-10-12Sgd. MA_LI核稿 CheckedPhilipp KudlaIC MOL PEK2012-10-12Sgd.KUDLA_PHI擬稿 PreparedXie FanIC MOM2013-10-10Sg

4、d. XIE_FAN無線通信系統(tǒng)軌旁詳細(xì)設(shè)計(jì)ed Design for DCS Wayside保護(hù)級(jí)別classes受限 Restricted僅用于本項(xiàng)目 To be used for project purpose.簽發(fā)部門Issuing Department: IC MOL RA工程名稱：蘇州軌道交通二號(hào)線信號(hào)系統(tǒng)工程Project name: Suzhou Rail Transit Line 2 Signalling System Project客戶名稱：蘇州軌道交通二號(hào)線Customer Name: Suzhou Rail Transit Line 2 Company.工程總包方：十四

5、所Main contractor: NRIET無線子通信系統(tǒng) Digital Communication Subsystem蘇州軌道交通二號(hào)線蘇州軌道交通 2 號(hào)線工程 Suzhou Rail Transit Line 2修改Overview of Changes檢查總覽Overview of Reviewed Design for DCS Wayside無線通信系統(tǒng)軌旁詳細(xì)設(shè)計(jì)SAP No: A6Z00033484847/PM1/AIC MOL RAreleasedPage 2 of 56SuZL2-5002-02© Siemens.,© Siemens.,2012 保留

6、所2012利受限-蘇州 2 號(hào)線使用視為商業(yè)Confided as a trade secret.Restricted-For SuZL2 use only技術(shù)檢查目標(biāo)Review Target: SPM / expert- 正確性 Correctness- 完整性 Completeness- 一致性 Consistency質(zhì)量經(jīng)理檢查目標(biāo)Review Target: QMiP- 提交文件正式發(fā)布建議/確保符合審查流程Release recommendation/adherence to review process版本Version簽發(fā)日期Issue作者Author修改章節(jié)Changed C

7、hapter修改Reason for Change-2012-06-04Xie Fan全部All首次發(fā)布First DraftA2013-10-10Xie Fan所有與 21 號(hào)站站名相關(guān)的內(nèi)容和圖片All sentences and pictures refer to the station name of Station 21根據(jù)業(yè)主要求更改 21 號(hào)站站名Change the name of Station 21 as required by customer蘇州軌道交通二號(hào)線蘇州軌道交通 2 號(hào)線工程 Suzhou Rail Transit Line 2目 錄Table of Cont

8、ents..3簡介 Introduction5本文目的 Scope and Purpose5縮寫及定義 Abbreviations & Definitions.5縮寫 Abbreviations5參考標(biāo)準(zhǔn) Referenced Standards and Guidelines6參考文檔 Reference Documents.722.1概述 General8系統(tǒng)結(jié)構(gòu)System Architecture1器 Wayside Communication Controller16軌旁通信無線服務(wù)器機(jī)柜 Radio Server Ca

9、binets16正線服務(wù)器機(jī)柜（車站 21， 盤蠡路）Server Cabinet of Main Line (Station 21, Pan Li Lu) 16試車線機(jī)柜（車輛段）Cabinets of Test Track (Depot)18環(huán)網(wǎng)接入交換機(jī)和插接盤機(jī)柜 Ring Access Switch and Patch Panel Cabinets19正線光纖環(huán)的環(huán)網(wǎng)接入 Ring Access for Main Line Fiber Optic Ring20車輛段的環(huán)網(wǎng)接入 Ring Access for Depot.24接入點(diǎn)設(shè)計(jì) Access

10、 Point Design245天線 Antennas2966.16.2纜線連接概圖 General Cabling Scheme30數(shù)據(jù)連接電源連接Data Connections30Power Connections327.1.2安裝原則軌旁安裝Mounting Principles34Trackside Installation34隧道區(qū)域中的接入點(diǎn)位置 Position of Access Points in Tunnel Areas34露天軌道和高架區(qū)域接入點(diǎn)的位置 Position of Access Points in-Track and ViaductArea

11、s37天線裝置的標(biāo)準(zhǔn)安裝 Standard installation arrangement of antennas407.1.38接地與防雷 Earthing and Lightning. 4299.1位置 Location of Equipment45鏈路預(yù)算 Link Budget45ed Design for DCS Wayside無線通信系統(tǒng)軌旁詳細(xì)設(shè)計(jì)SAP No: A6Z00033484847/PM1/AIC MOL RAreleasedPage 3 of 56SuZL2-5002-02© Siemens.,© Siemens.,2012 保留所2012利受限

12、-蘇州 2 號(hào)線使用視為商業(yè)Confided as a trade secret.Restricted-For SuZL2 use only蘇州軌道交通二號(hào)線蘇州軌道交通 2 號(hào)線工程 Suzhou Rail Transit Line 210.310.1.4技術(shù)數(shù)據(jù) Technical Data48接入點(diǎn) Access Point48主板 Main Board48無線模塊（IEEE 802.11b/g） Radio Module (IEEE 802.11b/g)49電源 Power Supply50八木天線 Huber+Suhner Sencity SP

13、OT-MYagi Antenna Huber+Suhner SencitySPOT-M51天線電纜 Antenna cable5210.1.5Indoor equipment52室內(nèi)軌旁通信器（WCC）和網(wǎng)絡(luò)（NMS） Wayside Communication Controller(WCC) and Network Management System (NMS)5310.2.2網(wǎng)絡(luò)和管理計(jì)算機(jī) Network Monitoring and Management PC53圖片列表 List of Figures54表格列表 List of Tables56ed Design

14、 for DCS Wayside無線通信系統(tǒng)軌旁詳細(xì)設(shè)計(jì)SAP No: A6Z00033484847/PM1/AIC MOL RAreleasedPage 4 of 56SuZL2-5002-02© Siemens.,© Siemens.,2012 保留所2012利受限-蘇州 2 號(hào)線使用視為商業(yè)Confided as a trade secret.Restricted-For SuZL2 use only蘇州軌道交通二號(hào)線蘇州軌道交通 2 號(hào)線工程 Suzhou Rail Transit Line 21簡介 Introduction1.1本文目的 Scope and Pu

15、rpose此文件為蘇州軌道交通 2 號(hào)線無線系統(tǒng)軌旁設(shè)計(jì)的概述。此文件后面的章節(jié)內(nèi)描述了軌旁無線系統(tǒng)的總體、安裝原則、布置和鏈路預(yù)算的計(jì)算。This document summarizes the trackside Radio Design for Suzhou Rail Transit Line 2 Signaling Project. The document in its subsequent chapters gives the general information on the trackside radio system, the mounting principles, al

16、location and link budget calculations.邏輯、網(wǎng)絡(luò)和 IP 設(shè)計(jì)不是此文件的一部分，這些之后，作為最終設(shè)計(jì)的一部分提供。Logical and Network, as well as IP Design are not part of this document and will be provided as part of the final design stages after all equipment locations and numbers have been fixed.車載在另一個(gè)單獨(dú)的文件DESIGN_ONBOARD“無線系統(tǒng)車載詳細(xì)設(shè)計(jì)

17、”中進(jìn)行描述。Train side equipment is described in a separate document DESIGN_ONBOARD “蘇州軌道交通 2 號(hào)線無線通信系統(tǒng)車載詳細(xì)設(shè)計(jì)ed Design for RCS Onboard”.將在所有的位置和數(shù)量確定縮寫及定義 Abbreviations & Definitions縮寫 Abbreviationsed Design for DCS Wayside無線通信系統(tǒng)軌旁詳細(xì)設(shè)計(jì)SAP No: A6Z00033484847/PM1/AIC MOL RAreleasedPage 5 of 56SuZ

18、L2-5002-02© Siemens.,© Siemens.,2012 保留所2012利受限-蘇州 2 號(hào)線使用視為商業(yè)Confided as a trade secret.Restricted-For SuZL2 use only所寫Abbreviation英文描述English Description中文描述descriptionCOTSCommercial Off The Shelf現(xiàn)貨WCCWayside Communication Controller軌旁通信器FOFibre Optic (cable)光纖（電纜）ISMIndustrial, Scientifi

19、c and Medical (radio bands)工業(yè)、科學(xué)和醫(yī)療（無線電頻段）ISOInternational Organization for Standardization標(biāo)準(zhǔn)化國際組織KVMKeyboard,/ Visual Display Unit, Mouse鍵盤、/可視顯示器、鼠標(biāo)LLCLogical Link Control邏輯連接LPZLightningZone雷電保護(hù)區(qū)域MACMedium Access Control媒介接入NMSNetwork Management System網(wǎng)絡(luò)OBCUOn-board Control Unit (Trainguard®

20、 MT)車載單元(Trainguard®蘇州軌道交通二號(hào)線蘇州軌道交通 2 號(hào)線工程 Suzhou Rail Transit Line 21.2.2參考標(biāo)準(zhǔn) Referenced Standards and Guidelinesed Design for DCS Wayside無線通信系統(tǒng)軌旁詳細(xì)設(shè)計(jì)SAP No: A6Z00033484847/PM1/AIC MOL RAreleasedPage 6 of 56SuZL2-5002-02© Siemens.,© Siemens.,2012 保留所2012利受限-蘇州 2 號(hào)線使用視為商業(yè)Confided as a

21、 trade secret.Restricted-For SuZL2 use onlyMT)OSIOpen System Interconnection (Reference M)開放系統(tǒng)互連（參考模式）QoSQuality of Service服務(wù)質(zhì)量RASRing Access Switch環(huán)網(wǎng)接入交換機(jī)RCSRadio Communication System無線通訊系統(tǒng)RFRadio Frequency無線電頻率SNRSignal-to-noise ratio信噪比SPDSurgeDevice過電壓保護(hù)裝置TGMTTrainguard® Mass TransitTraingu

22、ard®MTTOTurn-Out道岔UPSUninterrupted Power Supply不間斷電源WCNWayside Communication Network軌旁通訊網(wǎng)絡(luò)WCUWayside Control Unit (Trainguard® MT)軌旁單元(Trainguard® MT)蘇州軌道交通二號(hào)線蘇州軌道交通 2 號(hào)線工程 Suzhou Rail Transit Line 21.2.3參考文檔 Reference Documents如果沒有特別指出，以下參考文檔都對應(yīng)其最新版本。If no specific indication, please

23、 refer to the latest version of following documents.ed Design for DCS Wayside無線通信系統(tǒng)軌旁詳細(xì)設(shè)計(jì)SAP No: A6Z00033484847/PM1/AIC MOL RAreleasedPage 7 of 56SuZL2-5002-02© Siemens.,© Siemens.,2012 保留所2012利受限-蘇州 2 號(hào)線使用視為商業(yè)Confided as a trade secret.Restricted-For SuZL2 use only參考Reference文檔Document in

24、formationALLOCATION_ PLAN蘇州軌道交通 2 號(hào)線配置計(jì)劃Allocation Plan,Suzhou Rail Transit Line 2 Signalling Project A6Z00033484889DESIGN_ONBO ARD蘇州軌道交通 2 號(hào)線無線通信系統(tǒng)車載詳細(xì)設(shè)計(jì)ed Design for RCS Onboard,Suzhou Rail Transit Line 2 Signalling Project, A6Z00033484786參考Reference文檔Document informationIEC 62305-1防雷保護(hù)-第 1 部分：通用原

25、則，日期 2006-01lightning - Part 1: General principles, dated 2006-01IEEE 802.3系統(tǒng)間通信和交換局域網(wǎng)和城域殊要求 第 3 部分：載波檢測多址存取 采用檢測(CSMA/CD)的存取和物理層規(guī)范，日期2005-01-01Information technology Telecommunications and information exchange between systems Local and metropolitan area networks; Specific requirements Part 3: Carri

26、er sense multiple access with collision detection (CSMA/CD) access method and physical layer specifications,dated 2005-01-01IEEE 802.11系統(tǒng)間通信和交換局域網(wǎng)和城域殊要求 第 11 部分： 無線 LAN 媒介接入(MAC)和物理層(PHY)規(guī)范，日期 2007-06-12 Information technology Telecommunications and information exchange between systems Local and met

27、ropolitan area networks; Specific requirements Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications, dated 2007-06-12RFC768用戶數(shù)據(jù)包協(xié)議，日期 1980-08-28User Datagram Protocol, dated 1980-08-28RFC791網(wǎng)際協(xié)議- Darpa 網(wǎng)絡(luò)程序-協(xié)議規(guī)范，日期 1981-09Internet Protocol Darpa Internet Program

28、Protocol Specification, dated 1981-09RFC793傳輸協(xié)議- Darpa 網(wǎng)絡(luò)程序-協(xié)議規(guī)范，日期 1981-09 Transmision Control Protocol Darpa Internet Program Protocol Specification, dated 1981-09蘇州軌道交通二號(hào)線蘇州軌道交通 2 號(hào)線工程 Suzhou Rail Transit Line 22General蘇州軌道交通 2 號(hào)線的 Airlink 無線系統(tǒng)是按照 IEEE 802.11b/g 標(biāo)準(zhǔn)、由標(biāo)準(zhǔn)化概述現(xiàn)貨（COTS）WLAN 部件建立的，用于Trai

29、nguard® MT 數(shù)據(jù)傳輸。此系統(tǒng)是高度冗余的，為 Trainguard® MT 的連接提供完全冗余的無線電覆蓋。它是按照項(xiàng)目使用的Trainguard® MT 規(guī)范來建立的，并具有以下特點(diǎn)：The Suzhou Rail Transit Line 2 Airlink radio system is built from standard commercial off the shelf (COTS) WLAN components according to the IEEE 802.11b/g standard. It is applied for Trai

30、nguard® MT data transmission. The system is highly redundant and allows for fully redundant radio coverage for Trainguard® MT connections. It is built according to the Trainguard® MT specifications applicable for the project. The following functions are realized:此系統(tǒng)中用于 Trainguard®

31、; MT 數(shù)據(jù)傳輸?shù)牟糠衷谄渌械膶佣际峭耆哂嗟?。還為Trainguard® MT 的數(shù)據(jù)傳輸保持了“通過空中”的冗余，是在每列車具備至少兩個(gè)無線鏈路的情況下使系統(tǒng)持續(xù)連續(xù)工作，以及通過提供完全冗余的無線信號(hào)覆蓋。The System is fully redundant with respect to Trainguard® MT data transmission on all levels of the system. Redundancy is also maintained “through the air” by continuously operating

32、the System with at least two radio links per train and by providing a fully redundant radio coverage for Trainguard® MT.系統(tǒng)提供很高的數(shù)據(jù)傳輸速率。此系統(tǒng)能為每公里高達(dá) 10 輛列車提供服務(wù)。The System provides high data throughput. It is capable of serving up to 10 trains per km.在整個(gè)蘇州軌道交通 2 號(hào)線系統(tǒng)中，在任何列車和Trainguard® MT 列車之間，

33、此系統(tǒng)以 500 毫秒的最大傳輸時(shí)間提供通信。列車啟動(dòng)程序完成后，以運(yùn)行速度從非無線電覆蓋區(qū)域進(jìn)入覆蓋區(qū)域后，Trainguard® MT 數(shù)據(jù)傳輸啟動(dòng)時(shí)間的最大值為 5 秒。The System delivers communications with aum Transmission Time of 500 msbetween any train and Trainguard® MT train control instance throughout the SuzhouRail Transit Line 2 system. The Transmission Setup

34、 Time for Trainguard® MT data after entering a radio coverage area at operational speed from a not covered area is5s atum for a train whose start-up procedure is completed.此系統(tǒng)能夠管理（邏輯尋址）與蘇州軌道交通 2 號(hào)線上配置和運(yùn)行的 160 列列車之間完全冗余的往來通信。通信管理由冗余的軌旁通信器（WCC）實(shí)現(xiàn)。The System is capable to manage (logically address

35、) fully redundant communications to and from 160 cabs configured and in operation on the Suzhou Rail Transit Line 2. Management of the communications is performed by the redundant Wayside Communication Controller (WCC).用于蘇州軌道交通 2 號(hào)線的系統(tǒng)為列車應(yīng)用配置了服務(wù)質(zhì)量（QoS）。這意味著：對此應(yīng)用來說，可以保證雙向 10 kbps 的傳輸速率（以及相應(yīng)的帶寬），且對此傳輸

36、進(jìn)行了加密。而系統(tǒng)本身有能力為 Trainguard® MT 數(shù)據(jù)傳輸提供更高的傳輸帶寬。The System for the Suzhou Rail Transit Line 2 application is configured to provide Quality of Service (QoS) for the Train Control application. This means that a transmission rate (and the respective bandwidth) of 10 kbps in both directions is guarant

37、eed and secured for the application. The System is inherently capable of delivering higher transmission rates for Trainguard® MT data transmission.ed Design for DCS Wayside無線通信系統(tǒng)軌旁詳細(xì)設(shè)計(jì)SAP No: A6Z00033484847/PM1/AIC MOL RAreleasedPage 8 of 56SuZL2-5002-02© Siemens.,© Siemens.,2012 保留所2

38、012利受限-蘇州 2 號(hào)線使用視為商業(yè)Confided as a trade secret.Restricted-For SuZL2 use only蘇州軌道交通二號(hào)線蘇州軌道交通 2 號(hào)線工程 Suzhou Rail Transit Line 2Airlink 無線系統(tǒng)為傳輸?shù)奶幚硖峁追N模式。在蘇州軌道交通 2 號(hào)線中的應(yīng)用中，系統(tǒng)配置了完全和擴(kuò)展的冗余結(jié)構(gòu)，以為列車數(shù)據(jù)傳輸提供高可用性。The Airlink radio system provides several modes for handling transmission. In case of the Suzhou Rail

39、 Transit Line 2 application the system is configured with a fully extended redundancy structure in order to provide the high availability required for Train Control data transmission.Airlink 無線系統(tǒng)數(shù)據(jù)傳輸速率受制于現(xiàn)場條件。我們，ISM 頻帶專門運(yùn)用于本應(yīng)用。必須避免在車站區(qū)域的來自公共接入點(diǎn)的任何干擾。任何諸如此類接入點(diǎn)的計(jì)劃中的安裝必須事先 并與本無線系統(tǒng)進(jìn)行協(xié)調(diào)。The Airlink radio

40、 systems data transmission rates given above are subject to conditions on site. It is assumed that the ISM band frequencies are granted for exclusive use of this application. Any interference by public access points in stations has to be avoided. Any planned installation of such access points has to

41、 be announced and coordinated with this radio system.Airlink建立在一個(gè)嚴(yán)格分層的結(jié)構(gòu)上，根據(jù)特定的項(xiàng)目進(jìn)行調(diào)整，以適應(yīng)不同的應(yīng)用、標(biāo)準(zhǔn)、技術(shù)和組件，為此，此分層結(jié)構(gòu)盡可能采用了工業(yè)標(biāo)準(zhǔn)，從而可以使用大量的標(biāo)準(zhǔn)兼容硬件和，有利于替換和升級(jí)。The Airlink product is built on a stringent layered architecture, to allow for project specific adaptation to different applications, standards, techno

42、logies, and components. The layered architecture makes use of industry standards, wherever possible, to support this. Consequently, a wide range of standard compliant hardware and software components may be used for replacement or upgrades.圖 1 顯示了從 the ISO-OSI 七層模型的角度來看Airlink 無線系統(tǒng)。Airlink統(tǒng)一定義并包含了傳輸

43、層和網(wǎng)絡(luò)層，從而形成了一個(gè)面向應(yīng)用層的統(tǒng)一接口。實(shí)際的（內(nèi)部）傳輸和網(wǎng)絡(luò)層由 Airlink有的 Airlink 系統(tǒng)功能在 Airlink處理，該安裝在軌旁通信器和列車單元上。所內(nèi)實(shí)現(xiàn)。Airlink中不包含需要使用特定硬件的目標(biāo)應(yīng)用接口和路由/網(wǎng)關(guān)功能，因此對相關(guān)硬件的更換。對 Airlink造Figure 1 shows the Airlink radio system from the ISO-OSI point of view. The Airlink product software duplicates transport and network layer to show a

44、unified interface tothe application layer. The real (internal) transport and network layers aandled bythe Airlink product software which is installed in the Wayside Communication Controller and the Train Units. All Airlink system functionalities are realized in theAirlink product software. No hardwa

45、re specific functions contribute to the Airlink target application interface and routing/gateway functions. Thus, a replacement of relevanthardwaas no impact on the Airlink product software.ed Design for DCS Wayside無線通信系統(tǒng)軌旁詳細(xì)設(shè)計(jì)SAP No: A6Z00033484847/PM1/AIC MOL RAreleasedPage 9 of 56SuZL2-5002-02

46、69; Siemens.,© Siemens.,2012 保留所2012利受限-蘇州 2 號(hào)線使用視為商業(yè)Confided as a trade secret.Restricted-For SuZL2 use only蘇州軌道交通二號(hào)線蘇州軌道交通 2 號(hào)線工程 Suzhou Rail Transit Line 2Figure 1圖 1 Airlink 分層結(jié)構(gòu) Airlink layered information technology structureAirlink的分層 IT（）結(jié)構(gòu)在上圖中從下到上具有下列詳細(xì)內(nèi)容：The Airlink products layered I

47、T (Information Technology) structure is with the following s, from bottom up in the above figure:物理層定義了的電氣和物理規(guī)格。系統(tǒng)使用一系列經(jīng)過論證的 COTS（現(xiàn)貨）組件物理層（硬件層），此組件通常用于工業(yè)通信系統(tǒng)。所有的都有備用供貨商，且都適用于鐵路領(lǐng)域。關(guān)于為蘇州軌道交通 2 號(hào)線項(xiàng)目選擇或指定的組件的詳細(xì)內(nèi)容，請參見第 10 章“技術(shù)數(shù)據(jù) ”。The Physical layer defines the electrical and physical specifications for

48、devices. The system makes use of a set of carefully selected COTS (Commercial Off-The-Shelf) components on Physical (Hardware) Layer, which are commonly in use for industrial communication systems. All equipment has a second source and is suitable for application in the rail domain. Refer to chapter

49、 10 “技術(shù)數(shù)據(jù) Technical Data” fors on the components selected or specified for Suzhou Rail Transit Line 2.ed Design for DCS Wayside無線通信系統(tǒng)軌旁詳細(xì)設(shè)計(jì)SAP No: A6Z00033484847/PM1/AIC MOL RAreleasedPage 10 of 56SuZL2-5002-02© Siemens.,© Siemens.,2012 保留所2012利受限-蘇州 2 號(hào)線使用視為商業(yè)Confided as a trade secret.Re

50、stricted-For SuZL2 use only蘇州軌道交通二號(hào)線蘇州軌道交通 2 號(hào)線工程 Suzhou Rail Transit Line 2MAC）固件層管理媒介接入對于介質(zhì)共享的操作，它檢測但不負(fù)責(zé)糾正錯(cuò)誤。MAC 層主要集中在無線單元中，為快速漫游和多樣性能力提供特殊的算法。固件是基于無線單元生產(chǎn)商所提供的商業(yè)化程序的，并得到極大增強(qiáng)，以適應(yīng)鐵路領(lǐng)域所要求的功能性。因此，各個(gè)硬件組件以后的改變將可能需要對組件進(jìn)行固件更新，且只有在西門子公司的監(jiān)督和下才能進(jìn)行此更改，或者由西門子公司直接進(jìn)行。MAC 層上面的邏輯鏈接(LLC)層于媒介，處理多通路媒介的尋址和多通道通信。The Medium Access Control (MAC) firmware layer manages the interaction of devices with a shared medium. It detects but does not correct e