For a video surveillance system connected by a computer network, one problem to be solved is the network communication problem of video monitoring of multiple sites, requiring the transmission delay to be as small as possible, occupying the existing network bandwidth as little as possible, and having Better site size and scale features. Based on the analysis and comparison of unicast, broadcast and multicast working methods, this paper points out that the use of IP multicast technology is of special significance for multi-point real-time transmission of network video and real-time monitoring of network multi-point, and introduces the group. Broadcast closely related IP address format.
I. Comparison of unicast, multicast and broadcast technologiesMulticast technology is considered to be one of the most exciting network technologies that emerged after the promotion of WWW technology. Multicast is a network technology that allows one or more senders to send a single packet to multiple recipients. The multicast source sends the packet to a specific multicast group, and only the address belonging to the multicast group can receive the packet. Multicast can greatly save network bandwidth because only a single packet is transmitted on any link of the entire network regardless of the number of destination addresses. The network structure based on three communication methods and the working mode of the data transfer 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. But it is difficult to achieve if a large number of hosts want to get the same copy of the packet. This will result in a heavy burden on the sender, long delays, and network congestion; hardware and bandwidth are required to ensure a certain quality of service.
Multicast (MulTIcast) transmission: It improves data transfer efficiency. Reduce the possibility of congestion on the backbone network. Hosts in a multicast group can be on the same physical network or from different physical networks (if supported by a multicast router).
Broadcast transmission: Broadcasts packets within an IP subnet, and all hosts inside the subnet will receive these packets.
Broadcasting means that the network delivers a packet to the subnet host, whether or not the host is happy to receive the packet. However, the range of use of the broadcast is very small and is only valid within the local subnet because the router blocks broadcast traffic. Broadcast transmission increases the overhead of non-recipients.
Currently, the most widely used multicast technology is IP MulTIcast. IP multicast technology is a technology generated to optimize the use of network resources. It is usually used in applications under multi-point mode. It is an extension of standard IP network layer protocol technology.
From the definition of IETF's RFC1112 "Host Extension for IP MulTIcast" proposed by Steve Deering in 19*, 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 the group to receive the information. All the receivers receive the same data stream. The members in the group are dynamic and can join at any time according to their own wishes. Or quit. Each host can be added to multiple groups at the same time. Each multicast address can have multiple data streams on different ports or different sockets, and many actual applications can share a group address. IP multicast technology can effectively avoid broadcast storms that may be caused by repeated transmissions, and can break the limitations of routers and transmit data packets to other network segments.
Second, remote video network technology program analysisIn the conventional point-to-point communication mode, the video transmission of N video sites must repeat the transmission of the same data packet at least N-1 times, and the transmission delay is large, and as the number of monitoring stations increases, the delay will rapidly increase. Therefore, the peer-to-peer communication method is not suitable for network transmission requiring multi-point video surveillance with large-scale and short delay.
On the other hand, broadcast communication consumes resources on all machines on the local area network. On the wide area network, the entire network broadcast consumes a large amount of network bandwidth, and the communication range of directed broadcast is limited to a certain local area network, so the broadcast technology is only applicable to Video surveillance network transmission that does not span physical network segments.
Some physical networks and their software implementation technologies support a limited range of multi-drop delivery methods, namely multicast. LAN multicast uses the address reserved for multicast by the physical network for multipoint communication. When a group of machines is to communicate, they choose a special multicast address for communication. Once the network interface hardware is configured, the multicast address is recognized and all machines in the group receive a copy of each packet sent to the multicast address. Since multicast can distinguish multiple different groups of communication in the network hardware (such as Ethernet network card), it does not need software to distinguish, so it can save host resources more than the broadcast mode.
At present, IP is almost the lowest consistent protocol for data communication and network interconnection. IP multicast (IP MuTIcasting) in IP protocol abstracts different hardware multicast addresses in the interconnection network, forming a consistent across the LAN. The multicast interface can fully utilize the multicast features of the hardware on the physical network. In WAN communication, it can use DVMRP, MOSPF, CBT or PIM routing protocols to ensure that there is always only one copy of data on a physical link, which greatly saves bandwidth.
Therefore, regardless of the communication mode of the local area network and the wide area network, or from the development consistency of the communication application, the use of IP multicast to achieve large-scale video surveillance has great advantages.
Third, the advantages of IP multicast technology in multi-point video data transmissionSince digital video has a large data throughput when transmitting over the network, if end-to-end IP unicast technology is used for multicasting digital video, first of all, the video server must always be in the listening state to understand each dynamic join. The client's service request, and the socket's interception consumes the CPU resources of the system. The too frequent listening is easy to cause the system to be unstable, and it also affects the real-time performance of the video transmission, causing the video to appear when transmitted over the network. Frequent jitter, which ultimately affects the quality of service (QoS) of video transmission. Secondly, the video server faces the same video service request from different clients and needs to be repeatedly sent. N clients need to occupy N times of network bandwidth resources, which is greatly wasted. The network bandwidth resources, if not controlled, will also cause broadcast storms, causing the system to collapse.
Therefore, under the premise that the network bandwidth environment can meet the video transmission requirements infinitely, there is no essential difference in performance between peer-to-peer transmission and multicast. However, this ideal state basically does not appear. Otherwise, in addition to studying network bandwidth, other networks Technology loses the foundation and meaning of research. We envisage that in the 10BASE-T LAN environment, when there are only 2 or a single client requesting a video service, there is no significant performance difference between the two; when there are 3 to 5 clients requesting a video service, the two The difference between the two is relatively significant. The video server adopting the point-to-point transmission method is obviously unable to do so. The network packet loss and delay are serious, and the video at the receiving end is obviously lagging and discontinuous. When there are more than 5 clients requesting video services, A broadcast storm was caused and the system was on the verge of collapse.
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 in the network, IP multicast technology can reduce unnecessary overlap. Sending, compared with multiple peer-to-peer unicast (Unicast), reduces the burden on the system and the network, improves the utilization of CPU resources and network bandwidth, and greatly improves the real-time performance of video data transmission. Each host participating in the communication uses the same program regardless of whether it is the source site or the destination site, and there is no client or server division, which is equivalent.
Fourth, IP multicast address formatThe IP address scheme specifically allocates an address range for multicast, and is a class D address in IPv4, ranging from 224.0.0.0 to 239.255.255.255, and classifying the class D address into a local link multicast address, a reserved multicast address, The administrative rights multicast address is as follows:
Local link address: 224.0.0.0 ~ 224.0.0.255, for LAN, the router does not forward IP packets belonging to this range.
Reserved multicast address: 224.0.1.0~238.255.255.255 for global scope or network protocols.
Administrative authority address: 239.0.0.0~239.255.255.255, used internally by the organization to limit the multicast range.
In summary, IP multicast technology is more in line with multi-point, multi-network platform and scalable video surveillance requirements, and can achieve remote video surveillance across buildings. It can be seen that the use of IP multicast technology is particularly important for multi-point real-time transmission of network video and real-time monitoring of network multi-point.
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