Application of VoIP Technology in Wireless LAN
With the development of Internet technology and the multimedia of computers, VoIP has developed rapidly. VoIP is a transmission technology that implements voice communication using a router-based IP packet switching network. The biggest advantage of this technology is that IP telephony uses advanced speech coding technology, which requires only 8 to 12 kbit/s of bandwidth and can be transmitted over a much smaller bandwidth than the traditional circuit-switched network of 64 kbit/s.
1. Basic principles and framework of VoIP
VoIP is a technology that can transmit voice signals over an IP network. It uses a series of transcoding, encoding, compression, packing and other procedures to make the voice data transmitted to the destination end on the IP network, and then through the reverse procedure, the voice signal is restored to the listener for reception. The basic structure of the VoIP model is shown in Figure 1.
The transmission of voice signals over an IP network requires a basic VoIP architecture consisting of roughly four basic elements: Media Gateway, Media Gateway Controller, voice server, and Signaling Cateway. They are mainly responsible for IP packets of voice signals, signal transmission and conversion, providing voice response, related exchange control and value-added services.
Figure 1 VoIP model structure
The VoIP transmission process can be roughly divided into five phases:
The first is to encode the analog voice signal, and the 8-bit or 6-bit quantization of the analog voice signal enables the voice signal to be transmitted in the IP network, and the digitization process can be implemented by using multiple voice coding schemes;
The second is to compress and encode the voice packet with a specific frame length, and add addressing and control information, so that the data can be forwarded to the destination through the network;
The third is the transmission of the IP packet, and according to the destination address of each data packet, the routing completes the transmission of the packet from the source address to the destination address;
The fourth is the data processing of the IP packet, to the destination address, the destination VoIP device receives the IP data and starts processing, removes the addressing and control information, retains the original original data, and then provides the original data to the decoder;
The fifth is to restore the digital voice back to analog voice and broadcast it through the speaker at a specific frequency.
2. VoIP application in WLAN
As 802.11-based wireless data networks become more common around the world, end users are looking for more ways to expand their application models. VoWLAN (Voice over WLAN) is one of the emerging applications.
Unlike wired VoIP, VoWLAN is a wireless VoIP calling capability that utilizes a WLAN network. It gives users access to voice, email, and other connected resources at any time, while increasing network efficiency and reducing the cost per call, reducing overall IT costs.
(1) The challenge of VoWLAN - QoS guarantee
In the VoWLAN system, crosstalk and multipath propagation introduced by the wireless link will cause fading and dispersion, causing additional delay and jitter of the system. Voice services are very sensitive to delay and jitter, so it is very important to provide a QoS guarantee technology in VoWLAN systems. The IEEE 802.11 standard defines two different channel access mechanisms: one is the Point Coordination Mechanism (PCF), which is based on the CSMA/CA method;
The other is the Distributed Collaboration Mechanism (DCF), which is based on polling. However, neither of these priorities is prioritized. Therefore, as the number of users increases, the MAC cannot guarantee reliable packet transmission for real-time voice services and the transmission delay and jitter are within the specified range.
To this end, the Media Access Control (MAC) Improvement Task Group (ie, E Task Group) of the IEEE 802.11 Working Group improved the 802.11 MAC layer protocol to support applications with QoS requirements, namely the IEEE 802.11e standard. In IEEE 802.11e, MAC access uses a hybrid cooperative function (HCF) control mechanism.
HCF is directly compatible with PCF and DCF and can support prioritized and parameterized media access services. HCF combines two mechanisms of contention and polling. The contention-based access mechanism is called Enhanced Point Coordination Function (EPCF), and the non-contention access mechanism is called Enhanced Distribution System Function (ED-CF).
The EDCF classifies the services first, and provides different service output queues for different priority data. Each service queue uses the EDCF method to compete for transmission resources. The main performance is that the minimum idle time (DIFS) of different priority queues is different from the competition window. Parameters can be used to change the size of the contention window, so that different retransmission waiting times can be obtained, which ensures that the real-time service has higher service priority. level.
The EPCF channel access method uses a QoS-related point coordination function called a hybrid synthesizer (HC). The HC uses point coordination to assign priority channels to wireless terminals for transmitting data with QoS requirements to meet predefined transmission priorities, service rates, delays, and jitter.
A mobile terminal having QoS requirements can send a reservation request (RR) to the HC. The mobile terminal may transmit the RR in the EDCF mode or the EPCF mode, or may transmit in the controlled contention interval (CGI).
It can be seen from the above that with the deepening of QoS research, the IEEE 802.11e protocol is gradually improved. On the one hand, the distributed and centralized coordination mechanism is modified within the original framework, and the compatibility with the traditional protocol is maintained;
On the other hand, some unique solutions have been proposed, such as batch response and admission control. According to research and simulation reports made abroad, IEEE 802.11e can achieve good QoS performance.
(2) VoIP application in WLAN - mobility
Mobility is also a big challenge. Wireless phone users may roam between access points more frequently, which requires a seamless, low latency transition between access points. When the movement of the VoIP node crosses different subnets, that is, moves to different IP network segments, the link layer switching in the previous section cannot solve the problem.
Currently, VoIP phone mobility support includes the Mobile Layer protocol and the application layer signaling protocol of the network layer. SIP. SIP signaling enables the caller to transparently dial the VoIP call when the location of the VoIP node changes, such as a VoIP node. You can use the same phone number to make calls in Zone A and Site B.
In addition, when the user who is on the call moves from A to B, Mobile IP technology can ensure uninterrupted calls.
At present, the biggest problem with Mobile IP technology is the triangular routing problem. When the location of the VoIP node changes, the voice data sent by the communication partner to the VoIP node must still be sent to the home agent first, and then the home agent to the VoIP node. Point forwarding.
The direct communication between the two parties will increase the delay of the call between the two parties. The triangular route is shown in Figure 2. In order to eliminate triangle routing, route optimization techniques such as Mobile IPv6 can be used.
Figure 2 Triangle routing problem
In addition, due to the limited coverage of the IEEE 802.11 network, when the VoIP node moves out of the WLAN, the call is interrupted. For this problem, the current dual-mode phone is a good method, as shown in Figure 3.
When the user is in the WLAN range, an IEEE802.11-based VoIP phone is used, and if it is removed, a VoIP phone composed of technologies such as GPRS is used. Of course, if you want to make the call uninterrupted and achieve smooth switching, you can consider using Mobile IP technology.
Figure 3 Dual-mode mobile phone application model
(3) VoIP application in WLAN - security
Finally, security is a complex issue, but it must be addressed so that users can confidently communicate voice or data. From a security perspective, voice and data communications are exactly the same. Security algorithm solutions, such as the current WPA and the upcoming IEEE 802.11i standard (TKIP), will enhance the security of WLAN networks.
WPA includes IEEE 802.1X-based authentication and TKIP.TKIP uses RCA encryption algorithms and various mechanisms such as Michael's message integrity code, per-packet key per packet, and extended initialization vector (IV). Eliminate potential security vulnerabilities in the WEP protocol.
IEEE 802.11i will also include an AES-based encryption/data authentication protocol CCMP (CBC-MAC Protocol). With the development and approval of the 802.11i standard, VoWLAN security will provide the highest level of protection, making it widely available.
3, VoWLAN development prospects
There are many obstacles to the implementation and operation of VoWLAN, but the demand for this application is very strong. In the enterprise sector, especially in vertical industries such as healthcare, VoWLAN will continue to grow stronger.
Cellular-WLAN dual-mode mobile phones will gradually become popular, and once the price drops to a certain level, WLAN will become a universal function of mobile phones, so VoLAN has great growth potential in the consumer field.
Lightsaber could be come true, this light stick design we make the light soft and nice. One way light saber or two ways light saber is available. The light sick could be install on floor and wall, indoor and outdoor are available. Different color could be provided. Just make the life more and more interested.
Fiber Optic Glow Stick Light,Fiber Optic Glow Sticks,White Fiber Optic Wands,Fiber Optic Sticks
ZHONGSHAN G-LIGHTS LIGHTING CO., LTD. , https://www.glightsled.com