Comparison of IP transmission technology of cable TV network
Abstract: The IP transmission technologies in cable TV networks include three forms: IP over ATM, IP over SDH, and IP over WDM. This article introduces these three IP transmission technologies in detail and compares them.
Keywords: IP technology, cable TV network, IP over ATM, IP over SDH, IP over WDM.
With the rapid development of the global Internet (Internet), the number of people online is growing rapidly in geometric ranges, and the proportion of data communication dominated by Internet technology in the total number of communication services has increased rapidly. Internet services have become the fastest-growing and most competitive field in the multimedia communication industry. The 21st century is a period of continuous development of the information industry. IP technology has made it possible to integrate information into the existing network, and IP over everything has become an indisputable fact.
At present, the Internet access speed through telecommunications is extremely slow. The modem of a general telephone can only provide transmission rates of tens of Kbit/S, and its speed and bandwidth cannot support broadband services such as multimedia information well.
With the development of multimedia communication, the demand for broadband Internet access is becoming increasingly urgent. Cable TV networks have rich bandwidth resources. At the same time, my country has currently reached 80 million cable TV users, and the mileage of cable TV network exceeds 2.4 million kilometers. China has become the world's largest cable TV user country. Cable TV network has huge industrial development value. Building an Internet broadband information network based on cable TV network is not only the hope of the majority of users, but also the key to the second takeoff of the cable TV network.
What kind of technology is used to transmit IP in cable TV networks depends on the transmission technology used in cable TV networks. IP transmission technologies in cable TV networks include three forms: IP over ATM, IP over SDH, and IP over WDM.
1. IP over ATM
ATM is a high-speed, low-latency multiplexing switching technology. It is developed based on the analysis and summary of the technical advantages and disadvantages of circuit switching and packet switching. It combines the advantages of both, namely, connection-oriented, service quality and statistical multiplexing to achieve high bandwidth. It uses short packets of fixed length to transmit various communication information in the network, which facilitates high-speed processing of hardware and realizes high-speed and large-capacity broadband switching. Moreover, it has quite complete flow control functions and congestion control functions to ensure bandwidth utilization and network security and reliability. In cable TV networks, using ATM traffic control can realize hierarchical services for video transmission, and ATM can also realize real-time asymmetric transmission of TV programs. At present, some cable TV transmission networks in provinces and cities still use ATM technology.
IP over ATM is a combination of IP and ATM. There are currently two technical methods: overlapping technology and integrated technology. The overlapping technology is to overlap the IP network layer protocol on the ATM, that is, the ATM network overlaps with the existing IP network, and uses the mapping function of ATM and IP addresses at the ATM endpoint at the same time. After the sending end obtains the ATM address at the receiving end, it can establish an ATM/SVC connection and transmit LAN packets. Integrated technology integrates the intelligence and management performance of IP routers into ATM switches to form an integrated platform. It only requires identification of IP addresses and does not require the address resolution protocol of ATMs. It simplifies the routing function of ATMs, improves IP forwarding efficiency, and retains routing flexibility.
The advantage of IP over ATM technology is that it can make full use of the fast switching of ATM and the complete QoS functions to ensure the service quality of the network; the network has good scalability and flexibility; it supports the convergence of multiple services, data, voice and videos on one network to provide different service quality QoS for different service types; it has good network traffic management and control performance, which is very fine in ATM traffic control. This is very important for wide-area networks with very valuable bandwidth and very high line costs. This is one of the reasons why ATMs can be widely used in wide-area networks.
Disadvantages of IP over ATM technology: Since IP packets must be mapped into ATM cells, the transmission overhead formed by this is called "cell tax", so the transmission efficiency is low; network management is relatively complex and equipment is expensive; it is not suitable for super-large IP backbone networks.
2. IP over SDH
ATM can support multiple services once was its unique feature, but with the development of IP technology and the continuous improvement of network hardware, today's IP has become the core of various services. Data voice and video services can be carried by IP. The advantages of ATM have been replaced by IP technology, especially when the data traffic volume exceeds voice and video, it seems that ATM is not necessary. Moreover, removing ATM can also improve transmission efficiency. Therefore, IP over SDH came into being, and this technology has greatly shaken the position of ATM in the wide area network.
The concept of SDH transmission network was originally proposed by the Bell Communications Institute in the United States in 1985 and is called Synchronous Optical NETwork (SONET). It consists of a complete set of standard transmission structures of grades and is suitable for transmission of various adapted net loads (i.e., the part of the network node interface bitstream that can be used for telecommunications services) on physical media such as optical fibers, microwaves, satellites, etc. This standard became the new standard for the American digital system in 1986. The International Telecommunication Union Standards Department (ITU-T), the predecessor of the International Telecommunication Union Standards Department (CCITT), accepted the SONET concept in 1988 and reached an agreement with the American Standards Association (ANSI) to modify and rename SONET as the Synchronous Digital Hierarchy (SDH), making it a general technical system that is adapted to optical fiber, microwave and satellite transmission at the same time.
The SDH transmission network is composed of some SDH network units, synchronous information transmission is carried out on optical fibers, microwaves or satellites, integrating multiplexing, transmission and exchange functions, and is a comprehensive information network that is unified in network management and operation. It can realize effective network management, dynamic network maintenance, and monitoring of service performance, effectively improve the utilization rate of network resources, meet the requirements of information transmission and exchange of radio and television trunk transmission networks, and make a qualitative leap in improving the quality of radio and television transmission. Therefore, SDH technology is becoming a hot topic in the development and application of transmission technology in the radio and television field.
IP over SDH uses the SDH network as the physical transmission network of the IP data network. It uses link and point-to-point protocol (PPP: Point To Point Protocol) to encapsulate data packets and simply insert IP packets into information segments in PPP frames according to RFC1662 specification. Then the service adapter of the SDH channel layer maps the encapsulated IP packets into the SDH synchronous payload, then passes through the SDH transmission layer and segment layer, plus the corresponding overhead, and loads the payload into an SDH frame, and finally reaches the optical network and transmits in the optical fiber. IP over SDH, also known as PACKET over SDH (PoS), retains the IP-oriented connection-free feature.
The advantages of IP over SDH are: strong support capabilities for IP routing and high IP transmission efficiency; they are in line with the characteristics of Internet services, such as being conducive to the implementation of multicast methods; they can use the loop and network self-healing capabilities of SDH technology itself to achieve the purpose of link error correction; at the same time, they use the OSPF protocol to prevent link failures and cause network pauses, improving network stability; establishing IP network technology on the SDH transmission platform can easily cross regions and national boundaries, and implement global networking in accordance with different technical standards; they can simplify the network structure and reduce operating costs. IP over SDH on cable TV network platforms is suitable for IP transmission on inter-provincial and intra-provincial networks.
The disadvantages of IP over SDH are: IP over SDH currently does not support virtual private network VPN and circuit simulation; among all packet switching technologies, ATM's QoS is the best, which can perform circuit simulation, while IP over SDH technology can only perform service grading and cannot provide better QoS; large-scale networks must handle huge and complex routing tables, and searching is difficult, and routing information occupies a relatively large bandwidth.
Judging from the development trend of optical communication technology, SDH/SONET will give way to wavelength division multiplexing technology in the future. Therefore, IP over SDH will eventually develop into IP over WDM (IP over OPTICAL)
3. IP over WDM
With the development of transmission technology, the further optimization design of network based on IP services will be IP over WDM.
Wavelength division multiplexing technology (WDM) is a technology that can transmit multiple wavelengths of optical signals at the same time in an optical fiber. Its principle is: combine optical signals of different wavelengths at the transmitting end, and send the combined optical signals to different terminals at the receiving end. This means that it turns out that only one wavelength can be used as a single channel for carrier waves, and becomes several optical channels of different wavelengths at the same time to transmit in the optical fiber, thereby increasing the capacity of optical communication exponentially. The implementation of WDM technology is mainly done by a wavelength division multiplexer. The wavelength division multiplexer is a passive optical device with a simple structure, small size and easy to couple with optical fibers. The WDM system has three basic structures, namely optical multiplexed unidirectional single fiber transmission, optical multiplexed bidirectional single fiber transmission and optical split insertion transmission. Flexible networking is of great significance to developing new bandwidth services, fully tapping and utilizing optical fiber bandwidth capabilities, and realizing high-speed communications.
IP over WDM is to allow IP packets to run directly on the optical path, reducing the redundancy between network layers. Because the intermediate ATM and SDH layers are eliminated, its transmission efficiency is the highest, network operation costs are saved, and user costs are also reduced. It is the most direct and economical IP network structure system, which is very suitable for super-large backbone networks.
IP over WDM has the following advantages: making full use of the bandwidth resources of optical fibers, greatly improving bandwidth and relative transmission efficiency; transparently conserving transmission code rate, data format and modulation method, it can transmit ATM, SDH/SONET and Gigabit Ethernet format services of different code rates; it can not only be compatible with existing communication networks, but also supports future broadband service network and network upgrades, and has the characteristics of promotion and high survival.
The disadvantage of IP over WDM is that it has not yet achieved wavelength standardization, and the network management of the WDM system should be separated from the signals it transmits and network management; the network management of the WDM system is not yet mature; the current network topology of the WDM system is only based on a point-to-point method, and the "optical network" has not yet been formed.
4. Comparison of IP over ATM, IP over SDH, IP over WDM
The three transmission solutions of IP have their own advantages and disadvantages, and they need to be treated separately in actual applications according to specific circumstances. If the backbone network has already adopted ATM equipment, the IP over ATM solution can be adopted. Because the ATM port speed is high, the complete QoS (quality of service) guarantee, and the product is mature, the IP network switching rate can be improved and the service quality of the IP network can be ensured. If the backbone has not yet involved ATM, the IP over SDH solution is adopted. Since the ATM equipment is removed, the investment is small, the effect is fast and the line utilization is high. Therefore, for now, IP over SDH is a better choice. In the metropolitan backbone network, IP over SDH technology is relatively high, and using IP over WDM technology will be more practical. The advantage of IP over WDM is to reduce the intermediate redundancy between the various layers of the network, reduce the functional overlap between the layers such as SDH, ATM, IP, etc., and reduce equipment operation, maintenance and management costs. In addition, IP over WDM technology can greatly expand the existing network bandwidth and maximize line utilization. With the peripheral network Gigabit Ethernet becoming the mainstream, this technology can truly achieve seamless access, which indicates that IP over WDM represents the future of broadband IP backbone network.
The development of broadband network communication has always been people's purpose and ideal, and it is also a direction for the development of broadband comprehensive service networks. Since its inception, ATM technology, as its representative, has been considered to have undertaken the mission of integrating multiple services (telephone, television, data, dedicated lines), but due to its complex technology and high price, its development has been limited. Today's popular IP technology has the characteristics of simplicity, flexibility, wide application and low price, which makes IP not only widely used in the Internet, local area network, etc., but is also considered to be a choice for broadband network technology. Using cable TV networks to build an IP broadband access network to achieve Internet data transmission will bring great development opportunities to cable TV networks.
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