Access control is decentralized

2.

1. Collision detection of carrier sense multiple access method: applicable to all LANs

2. Token ring access control method: only suitable for ring topology LAN

3. Token bus access control method: mainly used in bus or tree network structure

extended data

token bus access control mode is similar to token ring, but each workstation in bus or tree network is arranged in a certain order, such as by the size of interface address, to form a logical ring. Only the token holder can control the bus and have the right to send information. Information is two-way transmission, each station can detect the information sent by the station

The other main problem that CSMA / CD should solve is how to detect conflicts. When the network is idle for a moment, there are two or more workstations to send information at the same time

3. Although SS7 technology can be applied in a wide range of potential areas, it has only been applied in a few exclusive areas: telephone network, application and infrastructure based on operators, and application and infrastructure based on large enterprises. The following table lists some applications of SS7<

application examples of SS7 applicability in application field
enhanced services are most suitable for network (based on central switching system) applications; The applicability of enterprise application is poor· Voice station
· unique number / phone following service
· wireless prepaid platform
VoIP gateway is very suitable for network (operation level) application · VoIP to PSTN network connection (call control)
wireless network is very suitable · mobile application (such as supporting GSM map)
· call control for MSc: establishment, disconnection, etc<

service control point is very suitable · VLR / HLR server in wireless network
advanced intelligent network / intelligent network is absolutely necessary · LNP (local number portability)
· 800 route search
· enhanced intelligent network service
call Center medium application · large capacity call center
SS7 switching and gateway application · SS7 Mini STP
· SS7 / MF gateway
· through Data network transmission SS7

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1. Enhanced services

the scope of enhanced services is very wide, including: voice, fax messages, single number / phone following services, and prepaid services provided by operators for users. The main reason why enhanced services are widely used in various operators is that it can bring them an important source of income and make operators have their own characteristics. Enhanced services are generally provided by the platform connected with the central exchange. These platforms can be provided by the system configured as a point of service or as intelligent peripherals. In both cases, SS7 is used as the call control protocol between the central exchange and the point of service or peripherals

Figure 6 shows an operation level voice platform, which is an open architecture enhanced service platform for the public. The system is composed of several computers (or nodes). The nodes are connected by Ethernet to provide rendancy function. It is a very standard and highly reliable system. In this example, two SS7 nodes are used to provide signaling for applications, while several other media nodes are used to provide channel connection and call processing functions with PSTN, including all audio or sound conversations with users and information playback. Each media node usually uses a board with a certain number of lines and DSP resources, such as NMS AG series 4E1 board, for hundreds or even thousands of simultaneous mixed media (voice, fax, etc.) calls. Fault tolerant disk server is used as storage medium to provide voice mail message retrieval function to media nodes

the two SS7 nodes are configured in a rendant way, sharing the same signaling point code to provide call control for all media nodes. All SS7 links shown in the figure are backed up in the rendancy mode of single signaling point coding. Even if one of the SS7 nodes fails, the signaling system can continue to provide uninterrupted voice mailbox access for incoming calls

the upgrade and expansion of this information system is very convenient. As long as the media nodes are added, the system capacity can be easily expanded. Since it is rare for two signaling points to collapse at the same time, this structure has high reliability

Figure 6 example of operation level structure of enhanced service system

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2. VoIP gateway

e to the rapid growth of voice transmission market through IP network, it is necessary to introce the VoIP gateway launched by many equipment suppliers. VoIP gateway is used to connect telephone network (circuit based PSTN) and packet based IP network, so that calls can be seamlessly connected to PSTN telephone through IP network. At first, the gateway only supports a few calls, generally only supports a single T1 or E1, and can only support hundreds of voice circuits at most. This early gateway usually uses ISDN protocol or signaling protocol to connect to PSTN. However, with the rapid growth of IP based voice applications and their adoption by operators, the capacity of the gateway has been graally expanded to support thousands of calls. With the expansion of gateway scale and the requirement of operation level gateway, SS7 has become a necessary signaling protocol to connect IP gateway and PSTN< The VoIP gateway supported by SS7 is generally connected to the central office switch of PSTN, and ISUP signaling is used to provide call control. In many cases, the calling function of any PSTN obviously has to be realized across the network, such as calling number information display, calling number from PSTN through the gateway into the IP network, and then through the gateway to the other end of the PSTN users<

1) the progress of VoIP standards

the VoIP community's interest in SS7 continues to rise. The main reason is that in order to make packet based VoIP network access to ubiquitous telephone services, it must be connected to circuit switched PSTN, which uses SS7 as signaling mechanism, so VoIP gateway can only support SS7 and has no choice

with the continuous development of VoIP standards, an obvious trend is the separation of media (voice, fax, etc.) and signaling in the gateway (see Figure 8). In addition, although a variety of signaling methods have been proposed, so far, it seems that SS7 or its variants have played an important role in forming the VoIP standard. At present, the two key problems to be solved urgently are: transmitting SS7 through IP network, and connecting PSTN circuit and packet network<

figure 8 breakdown diagram of media and signaling gateway in VoIP network

2) SS7 is transmitted through IP network

that is, two PSTN networks are connected through IP network, and SS7 signaling information between PSTN networks is transparently transmitted to each other through IP network. The purpose is to provide users with the same access and service function set anytime and anywhere, no matter whether the call is through circuit based PSTN or VOIP network. In addition, the cost of basic transmission equipment needed to transmit SS7 through IP network is much lower than that needed to transmit traditional SS7 Refer to figure 9)

3) connect PSTN circuit and packet switching network

when VoIP network needs to process or generate SS7 information such as establishing call connection between PSTN and VoIP, how to process SS7 signaling information is the content of this problem. An example of this problem is: when a call from PSTN arrives at a device in the VoIP network, the two networks use different signaling, but no matter which network is used, users can still access the same functions and services, just as they used to access pure PSTN services
in the above two problem areas, NMS company actively participates in and is a leader in various standardization and instrialization organizations, including the relevant working groups of IETF and in forum<

Figure 9 SS7 on IP network

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3. Wireless network applications

in addition to the air interface, supporting the mobility of wireless users in the design of wireless system is the main challenge. The signaling requirements of wireless system are much more stringent than those of ground wired system. As a result, the wireless network must combine more advanced and comprehensive signaling control system. Even if the mobile unit is in its home network, the system should track its location at any time, because the call in, authentication and delivery functions need to know its current location. The mobile level provided by system operators to users is becoming more and more complex, which requires the application of complex signaling between systems to support roaming, registration, routing and other functions. This is why SS7 is widely used as signaling protocol in wireless systems

Fig. 10 wireless roaming supported by SS7

SS7 is also widely used as interface signaling between wireless network and PSTN, as well as between increasing wireless network subsystems. In GSM system, SS7 is also used as signaling between base station control center (BSC) and mobile switching center (MSc)

as shown in Figure 10, each system manages its own HLR (home location register) and VLR (visit location register) databases. All the data of a user is managed by a single HLR of its own system, and the data of external users is managed by the VLR of the system. When a user travels to another service area and wants to make a call, the visited system will first make a confirmation registration (when the mobile unit is powered on for the first time), confirm that the user is a remote visiting user, and use SS7 to have a dialogue with the user's home system to make a temporary registration for the visiting mobile unit in its VLR. At the same time, the home system modifies its HLR to correspond to the data of the user, so that when calling the user, the call will be transferred to the visited system

there are other important examples of SS7 application in wireless infrastructure, as shown in Figure 11, which shows how SS7 transmitted by T1 / E1 between BSC and MSc supports is-634, which is the interface protocol between MSC and common 800m wireless communication base station<

Fig. 11 SS7 bridges BS and MSc in 800m system

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4. Service control point / VLR / HLR application

service control point is an intelligent network element that determines how to handle calls. It uses TCAP protocol to provide transmission and necessary (low-level) application instructions. The performance requirements of SCP will vary greatly with different applications. Some SCP systems will have a large scale, such as the 800 conversion database of the main operator. Some will be very small and used in very professional applications, such as VLR / HLR in a distributed wireless network like a wireless office environment (in a building). However, in each case, SCP must be connected to SS7 network and provide database and service control program through the network. Another important requirement for SCP is to centralize business programming or business design environment

Figure 12 shows a conceptual model of SCP connecting to an intelligent network using TCAP

Figure 12 service control point (SCP)

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5. Local number portability (LNP)

here, LNP refers to mobile number portability (MNP) of wireless network. LNP is realized by ISUP and TCAP when users change services (service portability) When changing the service provider (carried by the service provider), or even changing the geographical location (carried by the location), the original telephone number is still retained. Number portability is being accepted by various communication specification systems, such as the FCC (Federal Communications Commission) of the United States

the LNP function is usually implemented by TCAP request. The source switching system detects the number dialed by a user. If the number is registered for carrying transfer, the switching system initiates a TCAP transaction with the LNP database (SCP) to translate the number. Once the translation is completed, the call is controlled by ISUP just like the normal call. There is one difference: the ISUP message now contains the flag information indicating that the called number has been translated, so as to avoid being used in later call processing

4. The problem is chaotic. I'll give you an answer. If you don't understand, you can continue to ask

access a class. That is to use properties, methods, etc. in a class

for example. If there is a student class
student student = new student()// Create a class instance< br />student.name ; // Access variables in a class
studnet. Learn()// Access methods in a class

restricting access to a class means. There are some methods that I don't want to call for others. Just to serve the methods in my class

for example,
public class student {
private void think() {} / / think / / this method can only access
public void learn() {/ / public methods inside the class, and can be called outside the class to access
think()// I can call the private method defined in this class
}
}

/ / assuming that this is the main method
student = new student()// Create a class instance< br />
student.learn(); // Access public methods in a class< br />
//student.think(); This method can't be called. The call will report an error, because this method can't be called outside the student class

5. I want to add a new user ~ ~ 1. Can't open disk d
2. Can't install and uninstall software ~ ~
please advise ~ ~ ~ solution:
1 in the control panel - user account
remove everyone, user and other unnecessary users and groups, leaving only administrator and other necessary users, then user can't access disk D. Of course, you can also leave user in the security option of disk D, but you can limit the user to read-only and run permissions, and restrict the user's access
additional: 1) run the Group Policy editor program
2) in the left window of the editor window, expand computer configuration → Windows settings → security settings → local policy → user permission assignment branch step by step
3) double click the user rights to be changed, click add user or group, then double-click the user account you want to assign the rights to, and finally click OK to exit
click Start - control panel - management tools - Computer Management - open local users and groups - select users - right-click to select new user, create a new user, and set a password for him, so that other people can log in to your computer with this user, then you can't install the program
the default user created by the system belongs to the user group and has no permission to install the program

6. My understanding is that Ping means sending packets to the other party and receiving the other party's response to indicate connectivity. Obviously, the finance department sent it to the logistics department, and the response from the logistics department was blocked. If the finance department could not wait for a response, it would be overtime
in addition, can the ACL reject one item and then allow others?

7. 9. Access control what is the use of access control
this is how it is used in
software companies. Let's imagine a scenario. In your class, you make three private methods, mark to win. Of course, no one else can use them, but you can't use them outside the class. You have to make another public method in the class, and others can use them freely! However, if your public method calls the three private methods in the order of 132, people all over the world can only call the public method and use the three private methods in the order of 132. If they want to call the three private methods in the order of 123, they can't do it. Because you have the right to publish this class, people all over the world will only download it from your website. If
people have a need, they suggest that you change the order to 321, and they have to get your consent to add the public method in the order of 321 to your class again, and then package and publish it on your own website, so your
intellectual property rights are protected
how and to what extent a class and its members can be accessed depends on the access specifier it declares
java access indicator classification 1. Public 2. Private 3. Protected 4. Default access level
public: when a class member is modified by the public indicator, it can be accessed by any other code in your program
private: when a class member is designated as private, it can only be accessed by other members in its class
main() is always decorated with the public indicator. It is called by code outside the program, that is, by the Java runtime system
default access level: if the access indicator is not used, the default access of the class member is set to be public in its own package, but it cannot be accessed outside its package
source code of this chapter
class test3 {
int a// Default access
public int b// public access
private int c; // private access
// methods to access c
void setc(int i) { // set c' s value
c = i;< br /> }
int getc() { // get c' s value
return c;< br /> }
}

public class Test {
public static void main(String args[]) {
Test3 ob = new Test3();< br /> Find "mark to win" on the Internet for details, and refer to his website or his cyberspace: Chapter 2 of Java