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Currently the demand for mobile services is not limited to traditional voice services, but also a strong and growing demand for services for mobile data services, especially mobile Internet access, making manufacturers and network operators to search for a new network architecture based on traditional architecture infrastructure to reduce costs and simplify the development of multi-service networks, multimedia.

Technology and Wi-fi service

I. Introduction Currently the demand for mobile services is not limited to traditional voice services, but also a strong and growing demand for services for mobile data services, especially mobile Internet access, making manufacturers and network operators to search for a new network architecture based on traditional architecture infrastructure to reduce costs and simplify the development of multi-service networks, multimedia.

The 2G has served quite well for normal voice and a 2-generation applications such as SMS and Internet access with WAP technology. But when users require services such as video telephony and high-speed Internet access, they can not be fully met. Besides, the mobile communication system is now the narrowband system only provides limited information quality, especially for data transmission needs, but high rates.

The development of mobile device technology such as laptop laptop, palmtop handheld computers, devices that support PDA Personal Digital ... boost demand more data while mobile network mobile third generation (3G) which is considered to be the solution for future communication services to mobile multimedia are not ready to enter the contest. This context has led to a widely used class new wireless mobile networks. It's wireless local area network WLAN (Wireless Local Area Network). WLAN will promptly meet the demand for wireless data transmission speed with low cost and high flexibility, rapid deployment. The Wi-Fi technology based on the IEEE 802.11b standard for WLAN is built is the key to help developing countries leapfrog, catch up with developed countries in the field of information technology. Recognizing the inevitable development of Wi-Fi technology, in Vietnam VDC companies actively deploying hotspots (hotspot) access Wi-Fi services at the center and will post upcoming VDC Register to be named on the map Wi-fi world. This event demonstrates Vietnam are actually integrated and developed in conjunction with the breakthrough advancesof the high-tech world. 

OVERVIEW WLAN 

LAN is a data communication system flexibility is done on the extension of a wired LAN. WLANs use radio waves to transmit and receive data, minimizing the use of wire connections, so WLAN connectivity combined with the data transmission of mobile users. Using the 2.4 GHz ISM band. WLAN coverage within the university campus or office building.

The introduction of the WLAN bridge has brought many benefits to mobility and network operators flexibility. With WLAN, users can access shared information without the need to find a place to plug in the device and network managers can set up or augment networks without installing or moving wires system . WLAN traffic increased productivity, convenience, cost advantage compared to wired networks tradition. WLAN has the advantages:

- Mobility increases efficiency and service

- Quick and simple installation

- Reduced cost of operation

- Ability to upgrade


WLAN network configurationWLANs typically have two basic configurations:
- Network independence WLAN aka Ad Hoc network (Figure 1): the mobile node (laptop computer) gathered in a small space to form connections across levels (peer-to-peer) between them. The mobile nodes communicate with each other directly through the radio changes, the network administrator does not need to. Since ad-hoc networks can be implemented quickly and easily, so they are often established without a tool or special skills so it is very suitable for use in commerce or in conference temporary working groups. However, there can be disadvantagesof the coverage is limited, people are used to hearing each other.


- WLAN base (Figure 2) consists of access points AP (Access Point) attached to a wired backbone network and communicate with mobile devices within the coverage area of ​​a cell. AP cell role and control to network traffic control. Mobile devices do not communicate directly with each other but with the AP.Cac cell communication can overlap at 10-15% allowing mobile stations to move without losing the wireless connection and provide coverage for the lowest cost. The mobile station will select the best AP to connect. An access point can be located in the center ofand distributed access control for nodes disputes, provide appropriate access to the backbone network, assign the address and prioritize, monitor network traffic, move management and maintenance packages in monitoring network configuration. However, multi-protocol access concentrator allows mobile nodes transmit directly to other nodes in the same region as the access points in the WLAN configuration independent. In this case, each package will have to be played by 2 times (from the original play button and then the access point) to the destination node before it, this process will reduce transmission efficiency and increase the transmission delay

The WLAN standard


Radio frequency is used for transmission is another major factor for WLAN. WLAN ISM band is allocated in three frequency bands do not need to register to use the following: 902 MHz, 2.4 GHz, and 5 GHz. Currently there are a number of popular WLAN standards worldwide using 3 frequency bands as shown in Table 1. 

Standard is most popular IEEE 802.11b. 802.11b Standards Association Electrical and Electronics Engineers (IEEE) released in September 1999. WLAN IEEE 802.11b high-speed operation in 2.4 GHz band and can provide transmission speeds up to 11 Mbps. IEE 802.11b only identified last two layers of the OSI reference model: the physical layer (PHY) and the data link layer (layered access control MAC) (Figure 3).

NETWORK COMPONENTS OF THE IEEE 802.11B
IEEE 802.11b network includes the following components interact with each other (Figure 4):- The BSS service basis: the most fundamental building blocks in 802.11b WLANs. Figure 4 shows 2 BSS, each with 2 stations STA BSS components.- DS delivery system: a component architecture to link between the BSS.- Access Points AP is a STA outside the station acts as a normal station provides access to the distribution system by providing DS services delivery system DSS. 
- The service extended ESS: BSS is a collection of many interconnected and can be integrated with the LAN as an independent BSS.

IEEE 802.11b physical layerThe physical layer refers to the physical interface between devices and the transmission of raw bits over communication channels. IEEE 802.11b offers 3 types of speed: the basic rate (1 Mbit / s) transfer using binary phase modulation DBPSK differential, extended speed (2 Mbit / s) using quadrature phase modulation transfer differential DQPSK and enhanced speed (5.5 Mbps and 11 Mbit / s) modulation using CCK additional code key.
Also IEEE 802.11b defines the shift speed automatic, which allows automatic adjustment of the transfer ratenoisy conditions, ie devices will transmit at lower speeds (1, 2, 5.5 Mbps) in noisy conditions and equipment move to the higher-speed transmission, the connection will automatically increase the speed up.


The physical layer of the IEEE 802.11b divided into 2 parts: the convergence protocol physical layer (PLCP) and layered physical medium dependent (PMD). PMD perform the encoding and decoding radio PLCP presents common interface to distributed MAC layer and provides the sensor carrier and channel set free (CCA).
GRADE DISTRIBUTION OF MAC Access Control IEEE 802.11b
Classification MAC (Medium Access Control) function as the interface between the physical layer and upper layer devices. MAC Classification defines a basic access mechanism (based on the multi-touch access carrier CSMA) for mobile nodes to access the wireless medium. The two main functions of the MAC layer cycle redundancy check CRC errors and fragmented packets (Packet Fragmentation). Each packet will be calculated and assigned a value and the CRC to ensure that no packets will be sent when the transmission process. The package will cut fragment large packets into smaller packets for transmission in space. This will yield two benefits: First, reduce the playback requirements for packages greater the ability to package the greater wrong, the second benefit is in the wrong case, the node package just passed a package small, so the faster the transmission.


SERVICE OF WIFI @ VNN VDC
Wifi service of VDC @ VNN be deployed based on wireless technology Wi-Fi IEEE 802.11b standard, providing users the ability to access high-speed Internet using personal computers equipped laplop more Wi-Fi card. Users can use the Wi-Fi hotspots. Hotspot is the wireless Internet access points are installed where the AP, may be airports, railway stations, hotels ...To use Wi-Fi to 3 factors:

- Broadband connection from the Internet service provider (ISP)

- A hotspot with at least one AP is installed

- A Wi-Fi card for laptop or desktop computer to connect to the APThe characteristics of WiFi @ VNN service provided by VDC The radius of each Hotspot spanning less than 300 m

- Frequency of use: 2.4 GHz (ISM band), bandwidth is 22 MHz

- Output power less than 100 mW

- Access speed is 11 Mbit / s (shared bandwidth)

- How is the security protocol for wired equivalent privacy WEP (Wire Equivalent Protocol)

- Systems management: operations RADIUS protocol (AAA)

- Charging method: charging by time, by size or bulk freight rates (flat rate).
Configuring the Wi-Fi systemFigure 5 shows a fully configured Wi-Fi system. AP Coverage of creating a hot spot hotspot, users with devices such as laptops equipped with wi-fi card can access the network through communication with the AP. The AP in a hot spot is focused on the switch and connected to the network through the router (the router). The router is connected to a wired network with a transmission mode depending on the particular service provider.

In its implementation plan, VDC has selected transmission mode SHDSL (Single-Pair High-Speed ​​Digital Subscriber Line). SHDSL solution using balanced data transmission rate can reach 192 Kbit / s to 2.3 Mbit / s on a single fiber pair. In addition, SHDSL signaltransmission capable of far more than connections using ADSL and SDSL technologies, allowing service providers to satisfy customer needs in the distance. Using this technology, at every pointhotspot access to an SHDSL router, providing broadband transmission on the Internet.

For Wi-Fi systems: the medium is the wave environment, 802.11b communications. In fact it can be considered a broadcast environment (broadcast), all workstation (client) standing in the coverage area can catch the signal, the AP is less likely to access controls. The proposed solution is to use the port facilities subscribers. This device will stand out of the block at the AP to the Internet, the environment will always be the AP waves provide any workstation in the standing wave propagation environment. But when users log into the AP's broadcast environment, the port immediately subscribers will proceed to the subscription receipt. Users will be automatically controls access to a Web page authentication has been integrated in the construction of subscriber ports. Here, the username / password (user name / password) will be required to enter into. Portal subscription contacted AAA Server centralized management center

RADIUS protocol network operator to obtain information about the customer in the system database.

 If authentication is successful, the user is allowed to go through the Internet Subscriber Gateway, and accounting information will be sent to the subscriber portal AAA Server. Portal subscribers have the ability to control access in real time, allowing the supply of various services. Based on the network model is built and the project's technical and RADIUS billing systems, the requirements for IP address connected to the SHDSL router is required to address the public IP range (Public_IP) and to static granted by the hotspot system will be connected to the center via the Internet so that the address must be understood on the Internet. Because the subscriber port to conduct communications with RADIUS AAA server located in the network management center should address SHDSL connection to the router must be of static, not changing after each system restart.By design, there are 2 network center 75 Dinh Tien Hoang, Hanoi and 125 Hai Ba Trung Dist. Ho Chi Minh City. (In case you need to deploy quickly using the Wireless Bridge, the Tay Son 292 VDC can be used as a center).

According to technical SHDSL connection, the center will be connected to the DSLAM with a Fast Ethernet connection is made on the central switch. Connecting to the Internet is controlled by two parallel ports subscriber load distribution. As wireless systems, characterized by high bandwidth, multi-access environment it is necessary to have appropriate protection measures to avoid forms of network attacks on VNN. Appropriate method of protection is to use a firewall (firewall) in the center of the two models above. 

SOME WI-FI DEVELOPMENT BASED ON EXISTING INFRASTRUCTURE

The following presents two development proposals for Wi-Fi, combined with the utilization of existing infrastructure:

- Implementing new network (roaming) between the service provider Wi-Fi

- Combines Wi-Fi and GPRS networks.

Roaming between the service provider Wi-Fi

Roaming service allows a customer to provide Wi-Fi service can access the services they signed up through a network of service providers other Wi-Fi using the information received real as in their home network. Roaming services described in the contract between the provider roaming services. The service provider network to the subscriber's subscription billing, and network service providers will charge providers of network services.


To implement roaming, need to establish a network between the service provider. That's because the RADIUS server in the network Roaming customers should be able to connect to the RADIUS server network roaming the home network because the server is always responsible for subscriber authentication whether subscribers are roaming or not done. In Figure 7 is the panorama roaming network, which is the solution for GRX network between the service provider by the GSM Association (GSMA) recommends.      
Currently in Vietnam VDC is the only service provider Wi-fi. In the future, when demand for mobile Internet access increases, along with the advantages of rapid and simple deployment, large capital investment, Wi-Fi service has the potential to grow and there will be many service provider and the service provider. When providing roaming that will create a huge convenience for customers just register with a service provider and the service can be used in manyregion and profitable for operators.

Combining Wi-Fi and GPRS networks

While Wi-Fi service provider of high-speed data, but only in limited areas, the radius GPRS offers high security services, have mobility over an area that can not be accomplished with WLAN. This means that the integration Wi-Fi/GPRS is needed to allow users access to wireless data services on an ongoing basis, utilizing the advantages of the high speed of Wi-Fi in where services will switch to GPRS when the user moves orare in areas without Wi-Fi coverage. There are two main ways to integrate Wi-fi/GPRS as follows:

Combining cutting
Wifi networks are tightly associated illustrated in Figure 8. Network has incorporated the use of system resources available for active GPRS network, which provides AAA functionality, security and other functions, these functions are integrated into systems Wifi, Wifi users will access most of the GPRS service. Functional interactions GIF (GPRS Interworking Function) is used to communicate with the wireless network. All traffic will be routed through the serving GPRS support node SGSN.

Combine liquid

Figure 9 is a wifi network model is loosely coupled. Loosely coupled mechanism will be based on Mobile IP. For wireless networks with just minor changes to components operate additional charge under the agreement and mutual support between Wi-Fi and GPRS. The user will receive an invoice billing combination of both networkIn loosely coupled systems act as the Internet backbone. The advantage of this model is WiFi and GPRS networks can be designed independently.

CONCLUSION

WLAN will promptly meet the needs wireless data transmission speed with low cost and high flexibility, rapid deployment. With the advantages of speed, cost, scalability and future development, Wi-Fi service promises success in the future when the use of mobile devices or tech should be more popular.

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