WORLD METEOROLOGICAL ORGANIZATION
TECHNICAL OPTIONS OF COMPUTERZED WORLD WEATHER WATCH BASIC SYSTEMS AT NMCs IN RA I
prepared by
E. NYONI
(Data Processing Consultant, Dar-es-Salaam, Tanzania)
July 2000
Secretariat of the World Meteorological Organization
Geneva - Switzerland
NOTE
The designations employed and the presentation of material in this publication do not imply the expression of any opinion whatsoever on the part of the Secretariat of the World Meteorological Organization concerning the legal status of any country, territory, city or area, or of its authorities, or concerning the delimitation of its frontiers or boundaries.
The report has been produced without editorial revision by the WMO Secretariat. It is not an official WMO publication and its distribution in this form does not imply endorsement by the Organization of the ideas expressed.
Table of Contents
Page No.
1.INTRODUCTION1
2.DATA COMMUNICATION TECHNOLOGIES2
2.1Networking2
2.1.1The OSI Model 2
2.1.1.1Connection Services: Layers 1-23
2.1.1.2Transport Services: Layers 3-66
2.1.1.3Applications Services: Layer 77
2.1.2Network Classification7
2.1.2.1Geographical Area Coverage7
2.1.2.2Resource Distribution7
2.2The WMO GTS8
2.2.1Satellite -based Platform9
2.2.2Ground-based Platform9
2.3The Internet9
2.3.1Internet Resources10
2.3.2Resource Transport11
2.3.2.1IP Address11
2.3.2.2Domain Name11
2.3.2.3Universal Resource Locator (URL)11
2.3.3Resource Storage12
2.3.3.1Server Type12
2.3.3.2Server Connection to Internet12
2.3.4User Access to resources13
2.3.4.1Dial-up Terminal-Type Connection13
2.3.4.2Dial-up TCP/IP Connection13
2.3.4.3Connection Over Internet Network14
2.4Internet Security14
2.4.1Cryptographic 14
2.4.2Firewall15
2.4.2.1Proxy Servers15
2.4.2.2Routers16
2.4.3Physical Isolation16
2.4.4Protocol Isolation16
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2.4.5Protocol Isolation with Server Replication17
2.4.6Multi-homed System with Routing Disabled17
2.4.7Tunnelling Through the Internet18
3.SYSTEM VISION18
3.1System Concept18
3.2Considerations18
4.SYSTEM DESIGN20
4.1Communication Link Sub-System20
4.1.1Satellite Communication Links20
4.1.2Terrestrial Communication Links20
4.1.3The X.25 and IP Protocol21
4.1.4The Role of Internet in the GTS22
4.1.4.1Application of Internet Technology within GTS23
4.1.4.2The GTS Completely turned over to Internet23
4.1.4.3A Mix of GTS and Internet23
4.2The AMSS Sub-System23
4.2.1AMSS Data/Products Reception23
4.2.2AMSS Data/Products Transmission24
4.3Database Management System (DBMS)24
4.4Products Generating System25
4.4.1The Plotting Module25
4.4.2The Climatological and Database Management Module25
4.4.3The Media Module26
4.4.4Operational Weather Forecasting Module26
4.4.5Numerical Weather Prediction (NWP) Module28
4.4.6Internal Internet Web Server Module29
4.5Protecting The GTS29
5.THE RECOMMENDED COMPUTERIZED DATA HANDLING SYSTEM31
5.1The Basic Computerized GDPS System31
5.1.1Hardware31
5.1.2Software32
5.1.3DBMS system32
5.1.4Training32
5.2Migrating Towards The Desirable GDPS System32
5.2.1The GTS Circuits33
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5.2.2The Application sub-system34
5.3The Basic Internet System34
5.4The Desirable Internet Connection34
5.5Remote Access Services (RAS)35
6.COSTS36
7.TRAINING36
8.CONCLUDING REMARKS37
9.REFERENCES39
10.ANNEXES
Annex 1The Basic NMC Computerized Data Handling System40
Annex 2A Simple Remote Access Services (RAS) Configuration41
Annex 3The Desirable NMC Computerised Data Handling System42
Annex 4The Multi-User Dial-up Internet System43
Annex 5The Desirable Internet System44
Annex 6Estimated Costs of Typical Packages45
Annex 7PC System Specifications46
Annex 8Some OSI Model Implementation49
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1.INTRODUCTION
For the NMCs to carry out their work they need data and/or products. The data is obtained from their national observing stations, the Regional Telecommunication Hubs (RTHs), meteorological Satellites and through bilateral arrangements with neighbouring NMCs and other Institutions. The products are mainly received via meteorological satellites but can also be received via fax and Internet.
The data so collected is quality controlled and further processed to produce national products. The national collectives are also injected into the WMO GlobalTele-communication System (GTS) for onward transmission to the RTH and/or other NMCs. The NMCs prepared products may also be communicated to national “zonal offices.” With regard to received products, the NMC would further process these to derive value-added products which are used at the NMC and at its zone offices.
In pursuit of carrying out these functions NMCs in RA I have, over the years, invested heavily to implement a robust operational regional data handling infrastructure - an infrastructure which includes the data observing system, data collection, tele-communications and data processing facilities. The National Meteorological Centres (NMCs) expected returns from this investment includes the timely preparations of national products from local data and/or imported products to service the national needs and the capacity to transmit or re-transmit such data and/or products to other NMCs.
However national and international requirements are constantly expanding in volume and sophistication; the NMC have to respond to these changes, national financial constraints notwithstanding, by further investments in:
- Observational network expansion
- Data and products transmission capacity
- Data and products handling
The twelfth Session of RA I (Arusha, 14-23 October 1998), tasked the Working Group on Planning and Implementation of World Weather Watch (WWW), among other things are the following:
- Review of the Regional Telecommunication Networks (RTNs), and monitoring their implementation and operation.
- Monitoring the status of implementation and operation of Regional Data-processing Centres, emerging Centres and National Meteorological Centres including pilot projects on computerization.
Pursuant to these tasks, a detailed specification for organizational and implementation aspects of the GDPS infrastructure at NMCs in RA I which include suggestions of cost effective systems for the modernization of data processing facilities, related telecommunication interface facilities for data collection , quality control of data and post processing of imported products to generate value added products meeting national and other users requirements are being provided in this report.
2.DATA COMMUNICATION TECHNOLOGIES
Presently meteorological data/products can be communicated between centres using the GTS or the Internet.
2.1Networking
A network is a collection of computers and other devices along with cabling and the network interface controllers that are inside the computer and the software.
The principal aim of networking is to have a sharing ofdata and information resources. The WMO GTS has the same main function of ensuring global sharing of meteorological data and information. The Internet is also a data/information sharing system. It seems natural therefore to briefly discuss networking before discussing the GTS and the global network of networks (Internet), and how the GTS can interact with Internet and benefit from its technology.
List of some network hardware and peripherals:
-Computers: PCs, Workstations, Clients, Servers, etc.
-Printers of different types
-Cash Registers and Point of Sale Devices
-Cables and wires, fibre optics, Microwaves, etc.
-Hubs, Routers, Gateways, Terminal Servers, Modems, etc.
Two principal types network software:
a.Network Operating Systems (NOS)
-Novel NetWare
-Windows NT
b.Network Applications (2 types)
-Pure Network Applications which include E-mail, FTP (File Transfer Protocol), Terminal Emulation, Web Browsers
-Stand-alone Network Applications which include Word processing, Spread sheets, Databases, etc.
2.1.1The OSI Model
If every network vendor were to build a network in ones own way, the situation would be extremely complicated and chaotic. The International Standard Organisation (ISO) proposed a model in which network communication is divided into seven (7) layers. The model is called, The Open System Interconnection (OSI), which provides a framework and guidelines for network communication. The philosophy of “divide and conquer “ is used in this system, the complicated system is divided into smaller manageable layers. There are set of rules that has to be followed to implement any of the layers, called PROTOCOLspecifications. The rules are like house drawings, which, if given to different contractors will result into similar but not identical houses. The drawings are the protocols and the houses are implementation of the protocols. The functions of the layers are summarized in Annex 8.
The 7 layers of OSI model are sometimes grouped into 3 broad categories based on broad functionality or services they offer, these are:
- Connection Services:Layers 1-2
- Transport Services: Layers 3-6
- Application Services:Layer 7
2.1.1.1Connection Services (Layers 1 – 2)
This group is concerned on how information gets from one computer in to the network connection medium (e.g. cable) and how the information moves from the medium into the computer at the other end. The implementation rules (protocols) for these two layers have to consider:
- Types of transmission media
- Data transmission rates in the media
- The physical layout of the media and devices – topology
- Methods to access various media
There are number of set of protocols in which vendors can adhere to when designing the connection services such as, the IEEE (Institute of Electrical and Electronic Engineers) suite, Ethernet, Token Ring, ARCNET, etc. To have a good understanding of these sets of protocols, it requires some basic knowledge on the four considerations upon which the rules are based:
a.Medium Types (Types of transmission media)
Data can be transmitted using either bound or unbound medium.
- Bound Mediais based on cable of many different types, such as thin coaxial, thick coaxial, optical fibre, etc.
- UnboundMediais based on air and implementations including microwave, laser, infrared and radio.
There are many in determining which medium implementation should be used. In a Local Area Network (LAN) cabling would be used but to connect two LANs which are far apart, microwave could be chosen. There are different accessories required to build network and interconnect them, e.g. hubs, repeaters, connectors, terminal plugs, modems, routers, etc., which all go into preparing the road on which data will travel.
b.Transmission Data Rates (Data transmission rates in the media)
How much data can be transmitted:
- From one device to another over the medium/cable
- Between networks connected in a Wide Area Network ( WAN)
- Over the Internet.
It could be as little as a few thousand bits per second or as much as hundreds of millions of bits per second. In the Internet situation the “last longest mile“ phenomenon is the most speed restricting factor to end users. It should also be remembered that graphic files are generally huge that requires high data transmission speed.
c.Network Topologies (The physical layout of the media and devices)
Describes the physical layout of the network medium and attached devices. (DVC=Device)
- Star
- BUS
- Ring
- Mesh
d.Access Methods (Methods to access various media)
Access methods are the rules that govern how the various devices that make up the network communicates, either using a common medium or on point-to-point links.
- Contention - CSMA/CD
CSMA/CD stands for Carrier Sense, Multiple Access with Collision Detection. This is an access method in which any device wishing to transmit data/products listen to the medium, when it is idle the device transmits. If more then one device transmits at the same time the data packets will collide to produce noisedetectable by ALL network devices. Transmission will terminate and the transmitting devices will choose a RANDOM time interval to re-schedule retransmission of the collided frame.
- Token Ring
In this access method, the devices do not share a common medium but instead have a point-to-point link which form a closed loop (ring). A special bit pattern called the TOKEN is generated and circulates on the ring and a device can only transmit if it has the token.
- Polling
In this system one computer designated as controller will poll the others to find out whether they have something to transmit.
Each of these access methods has their own advantages and disadvantages. Ethernet and Token Ring are very popular. Ethernet is the most common for small networks because it is cheaper.
There are many connection services protocols. The three most common protocols that many vendors use to implement the connection services (Physical and Data Link layers) of the OSI Model are:
- The IEEE Protocol Suite
The Institute of Electrical and Electronic Engineers (IEEE) developed the so called “802 Series” protocol suite which deals with the Physical and Data Link layers. The 802 series has several components, 802.2 through 802.5.
- Ethernet (standard)
-All attached devices operate independently (peer to peer)
-All devices are attached to a shared medium (bus)
-Medium access control is by CSMA/CD
-Various kinds of Coaxial cables are used
Ethernet is considered simple, fairly robust, inexpensive and is therefore the most popular. Ethernet resembles the IEEE 802,3 standard in many ways.
- Token Ring (standard)
Does not have a shared medium, each device is connected to the next one in a ring formation
Has a controller to manage the token
Uses token ring as its medium access control
Has star or ring as its topology
Is an IBM standard which resembles 802.5
2.1.1.2Transport Services: Layers 3 – 6
These services augment the connection services to provide reliable communications between computers. The services ensures that:
- the two communicating devices are properly connected.
- the data packets are properly addressed and checked to make sure that no packet is lost or damaged.
- the data format is in conformity with the application requirements – where necessary format conversion is performed.
The most popular protocols to operate within the Transport Services are the TCP and IP.
The Origin of TCP/IP
The US government funded the development of suite protocols which collectively has come to be known as the INTERNETProtocol, essentially to enable the networking ofdissimilar computer systems the government had. The two best known components of the Internet Protocol suite are the IP and TCP. TCP fulfilling the functionality of the OSI Transport Layer and IP fulfilling the functionality of the OSI Network Layer.
The TCP/IP Implementation
Various vendors have used the ISO OSI model to produce various networks: NetWare, SNA Network, DECnet, AppleTalk, TCP/IP, etc. Annex 8 to this report shows the “NetWare” and “TCP/IP” implementation of the model. The WMO GTS had decided to use TCP/IP for a number of reasons, not least because Internet also uses it. There is more to TCP/IP than just TCP and IP. TCP/IP is a SUITE containing many different protocols that work together. Brief description of the two most popular components of the suite.
- IP Protocol (OSI Layer 3)
This protocol takes care the packaging of data for delivery. It defines an address scheme that UNIQUIELY identify devices on private networks and on the Global Internet (IP address). The data packets contain IP addresses of the sender and the recipient. The ROUTERS determine exactly what to do with the packet used in this information.
- TCP Protocol (OSI Layer 4)
Error checking and sequence numbering of the data packets are two main functions of TCP, telling the sending device to retransmit any lost or erroneous data packets.
Annex 8 shows the TCP/IP OSI model implementation has the top four layers bundled together so that, for example, the File Transfer Protocol (FTP) carries out all the functions of the OSI model layers 5 to 7.
2.1.1.3Application Services: Layer 7
Services in this layer depend on the other two services. The Application Services let an application on one computer talk to a similar application on the other computer in order to perform functions like copying files, etc.
2.1.2Network Classifications
Networks can be classified either by the area they span or by the way the resources are distributed on the PCs that are connected on the network.
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2.1.2.1 Geographical Area Coverage
These networks are based on the size of the geographical area they span:
a.Local Area Network (LAN)
This network spans one office or one floor of a building or the whole building, or could even several neighbouring buildings. LANs have high data transfer, low error rates and inexpensive media.
b.Wide Area Network (WAN)
Normally, WAN is the result of interconnecting several LANs across different cities or countries, e.g. Internet. Communication over WAN takes place via telephone lines, satellites or terrestrial microwave systems.
2.1.2.2Resources Distribution
Networks can also be classified based on the way resources are distributed. Resources may be located on each of the computers that are on the network (peer to peer architecture), or may be placed on a few specific computers called “Servers”. Other computers called “Clients”, would access the resources stored in the servers (client/server architecture).
a.Peer-to-Peer Architecture
In this architecture, computers on the network may share its resources with others. All computers are “peers” (are equal) and there is no centralised resource management. While this architecture may work well for small networks, the lack of central resource control complicates resource access for networks with a large number of users.
b.Client/Server Architecture
In this architecture, the resources are placed on the server where all users can obtain them. There are e-mail servers, file servers, print servers, web servers, etc. The management of the resources (servers) is centralised and carried out by one computer - Network Server. Depending on the volume of the resources, more then one resources can be co-located on one physical computer; e.g. one PC can store e-mail and file resources.
Notes:
- Client/Server implementation is by software where there is a “server” version of the software on the server computer and a “client” version of the samesoftware on the client computer.
- Even though essentially a peer-to-peer implementation may not require centralised resource management, it can work better with servers.
2.2THE WMO GTS
In a sense the WMO GDPS GTS all along has been a closed private communication system, much like America On Line (AOL), CompuServe, etc. That is, the GTS can be visualised as an “INTRANET” albeit with old technology and whose subscriber have, so far, been the world’s national meteorological services.
Specifically, the GTS consists of an integrated network circuits which interconnect meteorological telecommunication centres. The circuits of the GTS are composed of a combination of terrestrial and satellite communication links. They comprise of point-to-point circuits, point-to-multi-point circuits for data distribution, multi-point to point circuits for data collection, as well as two-way multi-point circuits. And just like the Internet the GTS has a hierarchical structure:
- The Main Telecommunication Network (MTN)
- The Regional Meteorological Telecommunication Network (RMTN)
- The National Meteorological Telecommunication Network (NMTN)
The MTN is the backbone of the GTS. It links together three World Meteorological Centres and 15 RTHs. In this hierarchical structure RA I has four RTHs, namely Algiers, Cairo, Dakar, and Nairobi. These RTHs connect to the MTN via other RMTN or directly. The MTN has the main function of providing an efficient and reliable communication service between its centres, in order to ensure rapid and reliable global and inter regional exchange of observational data, processed information and other data required by Members