Significance of IP multicast in Multi-Point Network Video Transmission

Digital video transmission is a communication method that people use vision to obtain information. Compared with other information transmission methods, digital video transmission is accurate, intuitive, and efficient. Due to the large amount of information transmitted by digital videos and limited bandwidth, the compression and encoding of videos, selection of transmission channels and network protocols, and IP multicast technology (IP Multicast) software Implementation Based on the Windows operating platform has become a key technology in network-based digital video transmission applications.

For a video monitoring system connected to a computer network, a problem to be solved is the network communication problem of video monitoring on multiple sites. It is required that the transmission latency be as small as possible, use the existing network bandwidth as little as possible, and have a good number of sites to scale. Based on the analysis and comparison of unicast, broadcast, and multicast modes, it is pointed out that the use of IP multicast technology is of special significance for multi-point real-time transmission of Network Videos and network multi-point real-time monitoring. It also introduces IP address formats closely related to multicast.

I. Comparison of unicast, multicast, and broadcast Technologies

Multicast technology is considered to be one of the most exciting network technologies that emerged after the spread of WWW technology. Multicast is a network technology that allows one or more senders to send a single packet to multiple receivers. The Multicast Source sends data packets to a specific multicast group, and only the addresses belonging to the multicast group can receive data packets. Multicast can greatly save network bandwidth, because no matter how many destination addresses there are, only a single packet is transmitted on any link of the network. The network structure based on three communication methods and the working mode of the data transmission process can be seen:

Data Channel. If a host transmits data to a small number of recipients at the same time, there is generally no problem. However, it is difficult for a large number of hosts to obtain the same copy of data packets. This will lead to a heavy burden on the sender, long latency, and network congestion. To ensure a certain level of service quality, you need to increase hardware and bandwidth.

Multicast Transmission: It improves data transmission efficiency. Reduces the possibility of congestion on the backbone network. The host in the multicast group can be in the same physical network or from different physical networks (if supported by the multicast router ).

Broadcast Transmission: broadcasts packets in the IP subnet. All hosts in the subnet will receive these packets.

Broadcast means that the network delivers a packet to the subnet host, whether or not these hosts are willing to receive the packet. However, the broadcast range is very small and only valid in the local subnet, because the vro blocks broadcast communication. Broadcast Transmission increases the overhead of non-recipients.

Currently, IP multicast is the most widely used multicast technology. IP multicast is a technology generated to optimize the use of network resources. It is usually used in applications that work in multi-point mode. It is an extension of the standard IP network layer protocol technology.

From the definition of IETF rfc1112 "host extension for IP multicast" proposed by Steve Deering in 1989, we can know that: the core idea of IP multicast is to send data (UDP packets) to a group of hosts through an IP address ). The sender only sends information to a group address. The receiver only needs to join this group to receive information. All receivers receive the same data stream, and the group members are dynamic, you can join or quit at will. Each host can be added to multiple groups at the same time. Each multicast address can have multiple data streams on different ports or sockets, at the same time, many practical applications can share a group address. The IP multicast technology can effectively avoid the possible broadcast storm caused by repeated sending, and break through the limitations of the router to transmit data packets to other network segments.

Ii. Analysis of remote video network technical solutions

In the conventional point-to-point communication mode, the video transmission of N video sites must at least repeatedly send the same data packet of the N-1, the transmission delay is large, and as the number of monitoring sites increases, latency will increase rapidly. Therefore, point-to-point communication is not suitable for network transmission of multi-point video monitoring that requires large-scale and short latency.

On the other hand, broadcast communication consumes resources on all machines on the LAN, while on the WAN network, the whole network broadcast consumes a large amount of network bandwidth, and the communication scope of targeted broadcast is limited by a certain LAN, therefore, broadcast technology is only applicable to video monitoring network transmission that does not span physical network segments.

Some physical networks and their software implement technical support for a limited range of multi-point delivery methods, that is, multicast. Lan multicast uses the addresses reserved by the physical network for multicast for multi-point communication. When a group of machines want to communicate, they select a special multicast address for communication. After the network interface hardware is configured, the multicast address can be identified, and all machines in the group will receive a copy of each group sent to the multicast address. Because Multicast can differentiate communications between multiple different groups in a local network hardware (such as an Ethernet NIC) without software, multicast can save host resources more than broadcast.

Currently, IP is almost the lowest protocol for data communication and network interconnection. IP multicast (IP muticasting) in the IP protocol abstracts the multicast addresses of different hardware in the interconnection network, A consistent multicast interface that can span the LAN is formed. The hardware multicast feature can be fully utilized on the physical network. In Wan communication, it can use the dvmrp, mospf, CBT, or PIM routing protocols to ensure that there is always only one data copy on a physical link, thus greatly saving bandwidth.

Therefore, whether in terms of the communication methods between the local network and the wide area network or the development consistency of communication applications, IP multicast is a great advantage to achieve large-scale video monitoring.

Iii. Advantages of IP multicast technology in Multi-Point Video Data Transmission

Because digital videos have a high data throughput during network transmission, if you use the client-to-end IP Unicast Technology for multi-point transmission of digital videos, first, the video server must always be in the listening state to understand the service requests of every dynamically added client. The socket listening consumes a lot of CPU resources of the system, too frequent listening may cause system instability and affect the real-time video transmission, resulting in frequent jitters during video transmission over the network, this ultimately affects the quality of service (QoS) for video transmission. Second, the video server must repeatedly send the same video service request from different clients, N clients need to use n times of network bandwidth resources, which greatly wastes network bandwidth resources. If the control is ineffective, it will also cause a broadcast storm, resulting in a complete system crash.

Therefore, the performance of point-to-point transmission and multicast is basically the same if the network bandwidth environment can satisfy the video transmission needs infinitely, otherwise, in addition to studying network bandwidth, other network technologies will lose the foundation and significance of the study. We imagine that in a 10 BASE-T LAN environment, when only two or a single client initiates a video service request, there is no significant performance difference between the two; when three to five clients make video service requests, the difference between the two is significant. The video server using point-to-point transmission is obviously unable to cope with the problem, and the network packet loss and delay are serious, the receiver video lags behind and is not continuous. When more than five clients Initiate video service requests, a broadcast storm occurs and the system is on the verge of a crash.

It can be seen that IP multicast technology has great advantages in Multi-Point Video data transmission. When an IP site sends the same video data to multiple IP sites on the network, the IP multicast technology can reduce unnecessary overlapping sending, which reduces the burden on the system and network compared with multiple point-to-point Unicast (unicast, improves the utilization of CPU resources and network bandwidth, and greatly improves the real-time video data transmission. Each host involved in communication uses the same program, whether it is the source site or the target site, and there is no distinction between the client and the server, so as to have an equality.

Iv. IP multicast address format

The IP address scheme specifies an address range for multicast. In IPv4, It is a Class D address in the range of 224.0.0.0 to 239.255.255.255, class D addresses are divided into local link multicast addresses, reserved multicast addresses, and management permission multicast addresses as follows:

Local link address: 224.0.0.0 ~ 224.0.0.255 is used for LAN. The Router does not forward IP packets in this range.

Reserved multicast address: 224.0.1.0 ~ 238.00000000255, used for global or network protocols.

Management permission address: 239.0.0.0 ~ 239.00000000255, used within the organization to restrict the multicast range.

In conclusion, the IP multicast technology meets the requirements of multi-point, multi-network platform and scalable video monitoring, and supports remote video monitoring across buildings. It can be seen that the use of IP multicast technology is particularly important for the real-time transmission of Network Videos and network multi-point real-time monitoring.