mdb is a data record file in the database access
Protecting the workbook can only protect the window or structure of the worksheet! The protection of the worksheet to protect a lot of items, and the functions of the various versions of different, with the upgrading of the version has been strengthened, especially from 2000 upgraded to the XP version, the function has increased a lot of cell data, formatting, and even the operation of the worksheet (such as adding/deleting rows, whether or not you can choose to protect the cell) to protect.
Windows support for computer communication protocols
A communication protocol
The formation of a network, you must choose a network communication protocols, so that users can communicate with each other "communication". Protocol (Protocol) is a set of rules used by network devices to communicate, this set of rules can be understood as a common language that can be understood by each other. The protocols in the network can be summarized into two categories: "internal protocols" and "external protocols" are described below.
1. Internal protocols
In 1978, the International Organization for Standardization (ISO) developed a standard model for network communications, called OSI/RM (Open System Interconnect/Reference Model, Open System Interconnect Reference Model) architecture. The structure*** is divided into seven layers, from low to high are the physical layer, data link layer, network layer, transport layer, session layer, representation layer and application layer. In this, there is a specific form of protocol between the upper and lower layers of any network device, while there is also a protocol convention used between the same layers of two devices (e.g., workstations and servers). Here, we define all such protocols between the upper and lower layers and between the same layers as "internal protocols". Internal protocols are rarely used in networking, and are mainly used by network developers. If you're just setting up a network, you don't need to worry about internal protocols.
2. External protocols
External protocols are the protocols that we must choose when networking. Since it is directly responsible for the communication between computers, it is often called the network communication protocol. Since the introduction of networking, a number of companies have invested in the development of communication protocols, such as IBM, Banyan, Novell, and Microsoft. The protocols developed by each company are generally initially designed to fulfill their own network communications, but as network applications become more popular, the requirement for interconnecting different networks becomes more and more urgent, and thus communication protocols become the key technology to solve the problem of interconnecting networks. Just as people who use different native languages need a common language to talk to each other, communication between networks also needs a common language, and this common language is communication protocols. At present, the communication protocols (external protocols) commonly used in LANs are mainly NetBEUI, IPX/SPX and its compatible protocols and TCP/IP.
3. Principles of selecting network communication protocols
We should generally follow the following principles when selecting communication protocols:
First, the selected protocols should be consistent with the network structure and function. If your network exists more than one network segment or to be connected through a router, you can not use the NetBEUI protocol that does not have routing and cross-segment operation functions, and you must choose protocols such as IPX/SPX or TCP/IP. In addition, if your network size is small, and at the same time just for simple file and device **** enjoyment, this time you are most concerned about the network speed, so in the choice of protocols should be selected to take up less memory and bandwidth utilization of protocols, such as NetBEUI. when your network size is large and the network structure of the complexity, you should choose the better manageability and expandability of the protocol, such as TCP/IP.
Second
In order to use the protocols, you need to choose the protocols that do not have routing and cross-segment operation. /p>
Second, except for special circumstances, a network try to choose only one communication protocol. In reality, the practice of many people is to choose more than one protocol at a time, or select all the protocols provided by the system, in fact, this is very undesirable. Because each protocol takes up the computer's memory, the more protocols you choose, the more memory resources taken up by the computer. On the one hand, it affects the running speed of the computer, and on the other hand, it is not conducive to the management of the network. In fact, a network in general a communication protocol can meet the needs.
Third, pay attention to the version of the protocol. Each protocol has its own development and improvement process, resulting in different versions, each version of the protocol has its most suitable network environment. As a whole, the function and performance of the higher version of the protocol is better than the lower version. Therefore, when choosing, under the premise of meeting the requirements of network functions, you should try to choose the higher version of the communication protocol.
Fourth, the consistency of the protocol. If you want two interconnected computers to talk to each other, they both use the same communication protocol must be. Otherwise, there is a need for a "translator" for the conversion of different protocols, which not only affects the speed of communication, but also is not conducive to the safety of the network and stable operation.
The three communication protocols commonly used in LANs
1. NetBEUI protocol
■NetBEUI communication protocol features. NetBEUI (NetBIOS Extended User Interface) was developed by IBM in 1985, which is a small size, NetBEUI is also one of Microsoft's favorite communication protocols, so it is called the "mother tongue" of all Microsoft products. In its early products, such as DOS, LAN Manager, Windows 3.x and Windows for Workgroup, Microsoft mainly chose NetBEUI as its communication protocol. In Microsoft's mainstream products today, such as Windows 95/98 and Windows NT, NetBEUI has become its inherent default protocol. Some people position WinNT as a low-end network server operating system, which is directly related to the fact that Microsoft's products rely too much on NetBEUI. NetBEUI is specially designed for single-segment departmental-level small LANs made up of a few to more than a hundred PCs, and it doesn't have the ability to work across segments, i.e., NetBEUI doesn't have a routing function. If you have installed more than one NIC on a server, or if you want to use a router or other devices to interconnect two LANs, you will not be able to use the NetBEUI communication protocol. Otherwise, communication will not be possible between devices connected to different NICs (each NIC is connected to a network segment), and between different LANs.
While NetBEUI leaves a lot to be desired, it also has advantages that the other protocols do not have. Of the three communication protocols, NetBEUI takes up the least amount of memory and requires essentially no configuration on the network. Especially in today's world, where Microsoft products almost exclusively dominate PC operating systems, it is well suited for a wide range of network beginners.
■ Relationship between NetBEUI and NetBIOS. Attentive readers may have found that NetBEUI contains a network interface standard NetBIOS. NetBIOS (Network Basic Input/Output System) is a set of standards developed by IBM in 1983 to enable PCs to communicate with each other, with the goal of developing a communication specification for use only on small LANs. The purpose was to develop a communication specification for use only on small local area networks (LANs). The network consists of PCs, the maximum number of users does not exceed 30, which is characterized by highlighting a "small" word. Later, IBM found many defects in NetBIOS, so it improved it in 1985 and introduced the NetBEUI communication protocol. Immediately, Microsoft took NetBEUI as the basic communication protocol for its client/server network system and further expanded and improved it. Most notably, a component called SMB (Server Message Blocks) was added to NetBEUI to reduce communication congestion on the network. For this reason, the NetBEUI protocol is sometimes referred to as the "SMB protocol".
People often confuse NetBIOS and NetBEUI, in fact, NetBIOS can only be regarded as a network application interface specification, is the basis of NetBEUI, it does not have a strict communication protocol function. NetBEUI is a network transport protocol built on top of NetBIOS.
2. IPX/SPX and its compatible protocols
■ Characteristics of the IPX/SPX communication protocols. IPX/SPX (Internetwork Packet eXchange/Sequences Packet eXchange) is a communication protocol from Novell Inc. set. The obvious difference with NetBEUI is that IPX/SPX appears to be larger and is highly adaptable in complex environments. Because, IPX/SPX was designed from the beginning with multiple network segments in mind, it has powerful routing capabilities and is suitable for large networks. IPX/SPX and its compatible protocols are the best choice when the user side is accessing a NetWare server. However, IPX/SPX is generally not used in non-Novell network environments. especially in Windows NT networks and peer-to-peer networks consisting of Windows 95/98, it is not possible to use the IPX/SPX communication protocol directly.
■ How the IPX/SPX protocol works. IPX/SPX and its compatible protocols do not require any configuration, and they identify themselves by their "network address. The network address in a Novell network consists of two parts: the "network ID" that identifies the physical network segment and the "node ID" that identifies the special device. The network ID is centralized in the NetWare server or router, and the node ID is the ID number of each NIC (NIC card number). All network IDs and node IDs are unique "internal IPX addresses". It is the uniqueness of the network address that gives IPX/SPX its strong routing capabilities.
In the IPX/SPX protocol, IPX is the lowest layer of NetWare, it is only responsible for the movement of data in the network, and does not guarantee the success of the data transmission, and does not provide error correction services.
In IPX is responsible for the transmission of data, if the receiving node is in the same segment of the network, it will be transmitted directly according to the node's ID of the data; if the receiving node is a remote (not in the same segment, or is located in a different network), it is not possible to use the network address of the receiving node. If the receiving node is remote (not in the same network segment, or located in a different LAN), the data will be handed over to the network ID in the NetWare server or router for the next transmission of the data.SPX is responsible for the error-free processing of the transmitted data throughout the entire protocol, which is why IPX/SPX is also called the "Novell's protocol set".
■NWLink communication protocol. Windows NT provides two IPX/SPX-compatible protocols: "NWLink SPX/SPX Compatible Protocol" and "NWLink NetBIOS", collectively called "NWLink NetBIOS". The NWLink protocol is Novell's implementation of the IPX/SPX protocol in Microsoft networks, which inherits the advantages of the IPX/SPX protocol while adapting to Microsoft's operating system and network environment. Windows 95/98 users can utilize the NWLink protocol to obtain the services of the NetWare server. If your network is moving from a Novell environment to a Microsoft platform, or if both platforms ****exist, the NWLink communication protocol is the best choice. However, when using the NWLink protocol, in which the "NWLink IPX/SPX Compatible Protocol" is similar to the "IPX/SPX Compatible Protocol" in Windows 95/98, it can only be used as a client protocol to realize access to the NetWare server. It can only be used as a client protocol to realize the access to the NetWare server, and this compatibility protocol will be useless after leaving the NetWare server; whereas the "NWLink NetBIOS" protocol can not only transfer the information between the NetWare server and Windows NT, but also can be used in Windows NT, Windows 95/98 to communicate with each other.
3. TCP/IP protocol
TCP/IP (Transmission Control Protocol/Internet Protocol) is one of the most commonly used communication protocols in the computer world. In LANs, TCP/IP first appeared in Unix systems, and now almost all vendors and operating systems have begun to support it. Also, TCP/IP is the base protocol for the Internet.
■ Characteristics of the TCP/IP communication protocol.TCP/IP is highly flexible, supporting networks of any size and connecting almost all servers and workstations. However, its flexibility also brings a lot of inconvenience for its use, in the use of NetBEUI and IPX/SPX and its compatible protocols do not need to be configured, while the TCP/IP protocol in the use of the first to carry out complex settings. Each node needs at least one "IP address", one "subnet mask", one "default gateway" and one "host name". hostname". Such complex settings, for some users who are new to the network does bring inconvenience. However, in Windows NT provides a tool called Dynamic Host Configuration Protocol (DHCP), which can automatically assign clients the information they need to connect to the network, reducing the burden of networking and avoiding errors. Of course, a DHCP server is required for DHCP to work.
Like IPX/SPX and its compatibles, TCP/IP is a routable protocol. However, there are some differences.TCP/IP addresses are hierarchical, which makes it easy to identify and locate users on the Web and also improves network bandwidth utilization. Servers running the TCP/IP protocol (such as Windows NT servers) can also be configured as TCP/IP routers when needed. Unlike TCP/IP, the IPX/SPX protocol in the IPX/SPX protocol uses a broadcast protocol, which is often clogged with broadcast packets and therefore does not allow for optimal network bandwidth.
■ TCP/IP in Windows 95/98 Users of Windows 95/98 can not only use TCP/IP to form a peer-to-peer network, but also easily access other servers. It is worth noting that if a Windows 95/98 workstation has only the TCP/IP protocol installed, it cannot directly join a Windows NT domain. Although the workstation can access the Internet through a proxy server (such as Proxy Server) running on the Windows NT server, it cannot log on to the Windows NT server's domain through it. For a Windows 95/98 user with only the TCP/IP protocol installed to join a Windows NT domain, the NetBEUI protocol must also be installed on Windows 95/98.
■ TCP/IP protocol configuration on a LAN. When it comes to the TCP/IP protocol, there are many users who are troubled by its complex description and configuration, and do not dare to use it with confidence. In fact, as far as LAN users are concerned, as long as you have some knowledge about TCP/IP, it is very easy to use.
● IP address basics. In the previous IPX/SPX protocol, we already know that the IPX address consists of two parts: the "Network ID" (NetWork ID) and the "Node ID" (Node ID), and the IPX/SPX protocol relies on the IPX address for the identification of online users. The IPX/SPX protocol relies on the IPX address to identify users on the Internet. Similarly, the TCP/IP protocol also relies on its IP address to identify the location and identity on the Internet, IP address is also composed of "Network ID" and "Node ID" (or HOST ID, host address) two parts. A complete IP address with 32 bits (bit) binary number composition, every 8 bits (1 byte) for a segment (Segment), **** 4 segments (Segment1 ~ Segment4), segment with the segment between the "." sign between segments. In order to facilitate the application, the IP address is not used directly in binary, but with the familiar decimal number, such as 192.168.0.1, etc. The complete composition of the IP address: "Network ID" and "Node ID" are contained in the 32-bit binary number. 32-bit binary number.
Twisted-pair cable is called Twist-Pair in English, and it is one of the most commonly used transmission media in integrated wiring projects.
Twisted-pair cable uses a pair of insulated metal wires twisted together to protect against a portion of external electromagnetic interference. The two insulated copper wires are twisted together with each other at a certain density to reduce the level of signal interference, and the waves radiated by each wire in transmission are canceled out by the waves emitted from the other wire. This is where the name "twisted pair" comes from. Twisted-pair wires generally consist of two 22-26 gauge insulated copper wires twisted around each other; in practice, twisted-pair wires consist of multiple pairs of twisted-pair wires wrapped together in an insulated cable casing. Typical twisted-pair cables come in four pairs, or more pairs in a single cable casing. These we call twisted pair cables. Within a twisted pair cable (also known as a twisted pair cable), different pairs have different twist lengths, generally speaking, the twist length is between 38.1cm and 14cm, twisted in an anti-clockwise direction. The adjacent pairs of twisted length of 12.7cm or more, generally the denser the twisted line of its anti-jamming ability is stronger, compared with other transmission media, twisted-pair cable in the transmission distance, channel width and data transmission speeds, etc. are subject to certain limitations, but the price is relatively inexpensive.
Twisted-pair cable is commonly found in Category 3, Category 5 and Super Category 5 cable, as well as the latest Category 6 cable, the former wire diameter is thin and the latter wire diameter is thick, the models are as follows:
1) a class of wire: mainly used for transmission of voice (a class of standards are mainly used in the early 1980s, before the telephone cables), different from the data transmission.
2) Category 2 cable: Transmission frequency of 1MHZ, used for voice transmission and data transmission at a maximum transmission rate of 4Mbps, commonly used in the old token network using the 4MBPS specification of the token passing protocol.
3) Category 3 cable: refers to the cable currently specified in the ANSI and EIA/TIA568 standards, the cable has a transmission frequency of 16MHz and is used for voice transmission and data transmission at a maximum transmission rate of 10Mbps is mainly used for 10BASE--T.
4) Category 4 cable: the cable in this category has a transmission frequency of 20MHz and is used for voice transmission and data transmission at a maximum rate of 16 Mbps is mainly used for token-based LANs and 10BASE-T/100BASE-T.
5) Category 5 cable: this category of cable has an increased winding density and jackets a high-quality insulating material, with a transmission rate of 100 MHz, is used for voice transmission and data transmission at a maximum rate of 10 Mbps, and is mainly used for 100BASE-T and 10BASE-T networks. This is the most commonly used Ethernet cable.
6) Super Category 5 cable: Super Category 5 has a small attenuation, less crosstalk, and has a higher attenuation to crosstalk ratio (ACR) and signal-to-noise ratio (Structural Return Loss), smaller time-delay difference, the performance has been greatly improved. Super Category 5 cable is primarily used for Gigabit Ethernet (1000Mbps).
7) Category 6 cable: the transmission frequency of this type of cable for 1MHz ~ 250MHz, Category 6 cabling system at 200MHz when the integrated attenuation crosstalk ratio (PS-ACR) should have a large margin, which provides two times the bandwidth of the Super Category 5. The transmission performance of Category 6 cabling is much higher than that of the Ultra Category 5 standard and is best suited for applications with transmission rates higher than 1Gbps. An important difference between Category 6 and Category 5 is the improved performance in terms of crosstalk as well as return loss, which is extremely important for the new generation of full-duplex, high-speed network applications. Category VI standard canceled the basic link model, cabling standards using a star topology, the required cabling distance: the length of the permanent link can not exceed 90m, the channel length can not exceed 100m.
Currently, twisted-pair cable can be divided into unshielded twisted-pair (UTP=UNSHILDED TWISTED PAIR) and shielded twisted-pair (STP=SHIELDED TWISTED PAIR). SHIELDED TWISTED PAIR). The outer layer of shielded twisted pair cable is wrapped by aluminum platinum to reduce radiation, but it does not completely eliminate radiation. Shielded twisted pair cable is relatively expensive and more difficult to install than unshielded twisted pair cable. Unshielded twisted-pair cable has the following advantages:
(1) no shielded jacket, small diameter, saving the occupied space;
(2) light weight, easy to bend, easy to install;
(3) minimize or eliminate crosstalk;
(4) flame retardant;
(5) independence and flexibility, applicable to the structured integrated cabling.
In these two categories are divided into 100 ohm cables, dual-body cables, large logarithmic cables, 150 ohm shielded cables and so on.