Because of the best-effort service in Internet, direct routing path of Internet may not always meet the VoIP quality requirements. Thus, many researches proposed Peer-to-Peer VoIP systems such as SIP+P2P system, which...Because of the best-effort service in Internet, direct routing path of Internet may not always meet the VoIP quality requirements. Thus, many researches proposed Peer-to-Peer VoIP systems such as SIP+P2P system, which uses relay node to relay RTP stream from the source node to the destination node and uses application-layer routing scheme to lookup the best relay nodes. The key of those systems is how to lookup the appropriate relay nodes, which we call relay lookup problem. This paper presents a novel peer relay lookup scheme based on SIP+P2P system. The main ideas are to organize the P2P network using a Cluster overlay and to use topology-aware to optimize relay selection. We introduce the mechanism in detail, and then evaluate this mechanism in NS2 network simulation environment. The results show that our scheme is scalable and can get high relay hit ratio, which confirm the feasibility of a real system. We also make comparison with traditional schemes and the results show that our scheme has good path quality.展开更多
VoIP (Voice over IP) is a rapidly growing area with great market potential. To promote it for both commercial and research purposes, a prototype VoIP system based on state-of-the-art Motorola communication techniques ...VoIP (Voice over IP) is a rapidly growing area with great market potential. To promote it for both commercial and research purposes, a prototype VoIP system based on state-of-the-art Motorola communication techniques has been developed. It is a gateway system integrating a PBX and a VoIP module. All components that H.323 defines to support VoIP are implemented in the VoIP module, though in a simplified manner. As an embedded system, the system features real timeness and task distributiveness. A number of additional techniques are used to improve the performance, including noise suppression, zero copy, and buffer structure optimization. When refined in interoperability, the system will also readily serve as a product.展开更多
In this paper, the capability of IEEE 802.11b Distributed Coordination Function （DCF） mode supporting of Constant Bit Rate （CBR） and Variable Bit Rate （VBR） Voice over IP （VoIP） traffic is investigated. Then, ...In this paper, the capability of IEEE 802.11b Distributed Coordination Function （DCF） mode supporting of Constant Bit Rate （CBR） and Variable Bit Rate （VBR） Voice over IP （VoIP） traffic is investigated. Then, the capacity of 802. lib Wireless Local Area Network （WLAN） system carrying zoice calls in a wide range of scenarios, including varying delay and packet loss rate constraints is analyzed and evaluated. Both G. 711 and G. 729 voice encoding schemes and a range of voice inter-arrival time are considered. The analyses and simulation results show that capacity is highly sensitive to the delay budget allocated to the sum of pucketization and wireless netzoork delays. For a given packet loss rate constrained, G. 729 is shown to have a capacity greater than that when G. 711 is used. The simulation results show that by supporting VBR under DCF mode the network has the approximately twice much capacity as supporting CBR has, regardless of the encoding schemes and the inter-arrival time.展开更多
VHF(Very High Frequency)通信系统是民航在空中交通指挥期间的主要手段,VoIP(Voice over Internet Protocol)技术则是基于IP网络进行语音传输,被引入到VHF通信系统中,可拓展通信范围,实现航空器与地面控制中心的实时语音和数据传输。...VHF(Very High Frequency)通信系统是民航在空中交通指挥期间的主要手段,VoIP(Voice over Internet Protocol)技术则是基于IP网络进行语音传输,被引入到VHF通信系统中,可拓展通信范围,实现航空器与地面控制中心的实时语音和数据传输。若发生间断性话音失真等问题,将会严重影响航空器的飞行安全。因此,重点研究面向VoIP的民航VHF通信系统话音质量增强技术,主要从丢包恢复、丢包补偿、综合编码、优化硬件链路四方面展开分析。展开更多
文摘Because of the best-effort service in Internet, direct routing path of Internet may not always meet the VoIP quality requirements. Thus, many researches proposed Peer-to-Peer VoIP systems such as SIP+P2P system, which uses relay node to relay RTP stream from the source node to the destination node and uses application-layer routing scheme to lookup the best relay nodes. The key of those systems is how to lookup the appropriate relay nodes, which we call relay lookup problem. This paper presents a novel peer relay lookup scheme based on SIP+P2P system. The main ideas are to organize the P2P network using a Cluster overlay and to use topology-aware to optimize relay selection. We introduce the mechanism in detail, and then evaluate this mechanism in NS2 network simulation environment. The results show that our scheme is scalable and can get high relay hit ratio, which confirm the feasibility of a real system. We also make comparison with traditional schemes and the results show that our scheme has good path quality.
文摘VoIP (Voice over IP) is a rapidly growing area with great market potential. To promote it for both commercial and research purposes, a prototype VoIP system based on state-of-the-art Motorola communication techniques has been developed. It is a gateway system integrating a PBX and a VoIP module. All components that H.323 defines to support VoIP are implemented in the VoIP module, though in a simplified manner. As an embedded system, the system features real timeness and task distributiveness. A number of additional techniques are used to improve the performance, including noise suppression, zero copy, and buffer structure optimization. When refined in interoperability, the system will also readily serve as a product.
文摘In this paper, the capability of IEEE 802.11b Distributed Coordination Function （DCF） mode supporting of Constant Bit Rate （CBR） and Variable Bit Rate （VBR） Voice over IP （VoIP） traffic is investigated. Then, the capacity of 802. lib Wireless Local Area Network （WLAN） system carrying zoice calls in a wide range of scenarios, including varying delay and packet loss rate constraints is analyzed and evaluated. Both G. 711 and G. 729 voice encoding schemes and a range of voice inter-arrival time are considered. The analyses and simulation results show that capacity is highly sensitive to the delay budget allocated to the sum of pucketization and wireless netzoork delays. For a given packet loss rate constrained, G. 729 is shown to have a capacity greater than that when G. 711 is used. The simulation results show that by supporting VBR under DCF mode the network has the approximately twice much capacity as supporting CBR has, regardless of the encoding schemes and the inter-arrival time.
文摘VHF(Very High Frequency)通信系统是民航在空中交通指挥期间的主要手段,VoIP(Voice over Internet Protocol)技术则是基于IP网络进行语音传输,被引入到VHF通信系统中,可拓展通信范围,实现航空器与地面控制中心的实时语音和数据传输。若发生间断性话音失真等问题,将会严重影响航空器的飞行安全。因此,重点研究面向VoIP的民航VHF通信系统话音质量增强技术,主要从丢包恢复、丢包补偿、综合编码、优化硬件链路四方面展开分析。
