Due to many developments and technological breakthroughs in the relevant hardware, software, protocols and standards, the widespread use of VoIP will soon become a reality. Technological advances and developments in these areas have contributed to the creation of a more efficient, functional, and interoperable VoIP network. The technical factors that promote the rapid development and even the wide application of VoIP can be summarized into the following aspects.
1、 Digital Signal Processor
Advanced Digital Signal processors (DSPS) perform the computationally intensive tasks required for voice and data integration.DSP processing of digital signals is mainly used to perform complex computations that might otherwise have to be performed by a general-purpose CPU. Their specialized processing power combined with low cost makes DSPS well suited to perform signal processing functions in VoIP systems
The computational overhead of G.729 speech compression on a single voice stream is usually large, which requires 20MIPS. If a central CPU is required to process multiple voice streams, perform routing and system management functions at the same time, it is unrealistic. Therefore, the use of one or more DSPS can offload the computational tasks of the complex speech compression algorithm within it from the central CPU.In addition, DSPS are also suitable for voice activity detection and echo cancellation functions, so they can process the voice data stream in real time and have fast access to the on-board memory .So,In this chapter, how to implement speech coding and echo cancellation on TMS320C6201DSP platform is introduced in detail.
Protocols and standard Software and hardware H.323 Weighted fair queuing method DSP MPLS label switching weighted random early detection Advanced ASIC RTP, RTCP Double Funnel Universal Cell rate algorithm DWDM RSVP rated access Rate SONET Diffserv, CAR Cisco Fast Forwarding CPU Processing power G.729, G.729a:CS-ACELP Extended Access Table ADSL, RADSL, SDSL FRF.11/FRF.12 token bucket algorithm Multilink PPP Frame Relay data rectification SIP Integration of priority-based CoS Packet over SONET IP and ATM QoS/CoS
2、Advanced dedicated integrated circuits
The development of Application-Specific Integrated Circait (ASIC) has produced a faster, more complex and more functional ASIC. Asics are specialized application chips that execute a single application or a small set of functions. By focusing on a narrow application target, they can be highly optimized for a specific function and are usually one or several orders of magnitude faster Just as reduced instruction Set computer (RSIC) chips focus on performing a limited number of operations quickly, ASICS are preprogrammed to perform a limited number of functions faster. Once developed, ASIC mass production is not expensive and is used for network devices including routers and switches, performing routing table checking, grouping forwarding, grouping sorting and checking, and queuing. The use of ASIC gives the device higher performance and lower cost. They provide increased broadband and better QoS support for the network, so they play a great role in promoting VoIP development.
3、 IP transmission technology
Most of the transmission telecom networks use the time division multiplexing mode, while the Internet must adopt the statistical reuse and long packet exchange mode. Compared with the two, the latter has a high utilization rate of network resources, simple and effective interconnection and communication, and is very suitable for data services, which is one of the important reasons for the rapid development of the Internet. However, broadband IP network communication puts forward severe requirements on QoS and delay characteristics, so the development of statistical multiplexed variable length packet switching technology has attracted people's attention. At present, in addition to the new generation of IP protocol-ipv6, the World Internet Engineering Task Force (IETF) has proposed Multi-protocol Label Switching technology (MPLS), which is a kind of label/label switching technology based on network layer routing, which can improve the flexibility of routing, extend the network layer routing ability, simplify the integration of routers and cell switching. Improving Network Performance. MPLS can not only work as an independent routing protocol, but also be compatible with the existing network routing protocol. It supports various operation, management and maintenance functions of IP network, and greatly improves the QoS, routing and signaling performance of IP network communication, reaching or approaching the level of statistical multiplexed Fixed Length packet switching (ATM). It is simpler, more efficient, cheaper and more applicable than ATM.
The IETF is also working on new packet management techniques in place to enable QoS routing. Tunnelling technology is being studied in order to achieve broadband transmission over unidirectional links. In addition, how to choose the IP network transmission platform is also an important field of research in recent years, and IP over ATM, IP over SDH, IP over DWDM and other technologies have appeared successively.
IP layer provides high-quality IP access services with certain service guarantees to IP users. The user layer provides access form (IP access and broadband access) and service content form.In the base layer, Ethernet is the physical layer of IP network, it is a matter of course, but IP overDWDM is the latest technology, and has great development potential.
Dense Wave Division MultipLexing (DWDM) has breathed new life into fiber networks and provided amazing bandwidth in telecom companies' new fiber backbone networks. DWDM technology utilizes the capabilities of optical fibers and advanced optical transmission equipment. The name of wave division multiplexing is derived from the transmission of multiple wavelengths of light (LASER) from a single strand of an optical fiber. Current systems are capable of sending and identifying 16 wavelengths, while future systems can support 40 to 96 full wavelengths. This is significant because each additional wavelength adds an additional flow of information. So the 2.6 Gbit/s (OC-48) network can be expanded 16 times without having to lay new fibers.
Most new fiber networks run OC-192 at (9.6 Gbit/s), generating capacity over 150 Gbit/s on a pair of fibers when combined with DWDM.In addition, DWDM provides the interface protocol and speed independent characteristics, in a fiber can support ATM, SDH and Gigabit Ethernet signal transmission at the same time, so it can be compatible with the various networks that have been built now, so DWDM can not only protect the existing infrastructure, but also can provide a more powerful backbone network for ISP and telecom companies with its huge bandwidth. And make broadband cheaper and more accessible, which provides strong support for the bandwidth requirements of VoIP solutions.
The increased transmission rate can not only provide a thicker pipeline with less chance of blocking, but also make the delay much lower, and therefore can reduce the QoS requirements on IP networks to a large extent.
4. Broadband access technology
User access of IP network has become a bottleneck restricting the development of the whole network. In the long run, the ultimate goal of user access is fiber-to-the-home (FTTH). Broadly speaking, optical access network includes optical digital loop carrier system and passive optical network. The former is mainly in the United States, combined with open mouth V5.1/V5.2, transmitting its integrated system on optical fiber, showing great vitality. The latter are mainly in Japan and Germany. Japan has persisted in research for more than a decade, and taken a series of measures to reduce the cost of passive optical networks to a similar level with copper cables and metal twisted-pair wires, and a large number of use. Especially in recent years, ITU has proposed ATM based Passive optical Network (APON), which combines the advantages of ATM and passive optical network. The access rate can reach 622M bit/s, which is very beneficial to the development of broadband IP multimedia services, and can reduce the failure rate and the number of nodes, and expand the coverage area. At present, ITU has completed the standardization work, and various manufacturers are actively developing it. Soon there will be products on the market, and it will become the main development direction of broadband access technology facing the 21st century.
At present, the main access technologies are :PSTN, IADN, ADSL, CM, DDN, X.25, Ethernet and broadband wireless access system. These access technologies have their own characteristics, among which the fastest developing are ADSL and CM; CM (Cable Modem) adopts coaxial cable with high transmission rate and strong anti-interference ability; but not two-way transmission, there is no unified standard.
ADSL(Asymmetrical Digital Loop) provides exclusive access to broadband, makes full use of the existing telephone network, and provides asymmetric transmission rate. The download rate at the user side can reach 8 Mbit/s, and the upload rate at the user side can reach 1M bit/s. ADSL provides the necessary broadband for businesses and individual users, and greatly reduces costs. Using lower-cost ADSL regional circuits, companies can now access the Internet and Internet service provider-based VPN at higher speeds, allowing for higher VoIP call capacity.
5. Central processing unit technology
Central processing units (cpus) continue to evolve in terms of functionality, power, and speed. This enables multimedia PCS to be widely used and improves the performance of system functions that are limited by CPU power. The ability of PCS to handle streaming audio and video data has long been expected from users, so delivering voice calls over data networks was a logical next step. This computational capability enables both advanced multimedia desktop applications and advanced features in network components to support voice applications.
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