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Transcript
Three Key Design Considerations of IP
Video Surveillance Systems
© 2012 Moxa Inc. All rights reserved.
Three Key Design Considerations of IP
Video Surveillance Systems
Copyright Notice
© 2012 Moxa Inc. All rights reserved.
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The MOXA logo is a registered trademark of Moxa Inc.
All other trademarks or registered marks in this manual belong to their respective manufacturers.
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Moxa.
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Table of Contents
CCTV Becoming a Deprecated Solution .................................................................................................. 1-4
Advantages of IP-based Video Surveillance ............................................................................................ 1-4
IP Video Surveillance Design Considerations........................................................................................... 1-5
Network Architecture ................................................................................................................... 1-5
Bandwidth Requirements .............................................................................................................. 1-6
QoS ........................................................................................................................................... 1-7
Begin Every New Network with the Right Plan ...................................................................................... 1-11
7
CCTV Becoming a Deprecated Solution
In the past, video surveillance systems were built using closed circuit television (CCTV) technology, which has
been used for many years. In the CCTV system, the video image is transmitted from camera to monitor in a
purely analog signal, and uses coaxial cable to transmit the analog data. A multiplexer is used to interconnect
the video camera, CRT monitor, VCR recorder, and control joystick.
This technology was in use for many years. However, CCTV is a closed system, so it is difficult to be integrate
with other systems. Also, CCTV requires high installation and cabling costs, especially for system maintenance
or system expansions. For this reason CCTV technology is increasingly becoming displaced by more capable
IP-based video surveillance technology.
Advantages of IP-based Video Surveillance
In an IP-based system, video streams are converted to digital data and shared through the IP network. The
benefits of IP surveillance are:
Remote Accessibility: IP-based video encoders and IP cameras use an IP network to deliver the video stream,
so as long as there is an access to the network, the video image can be easily obtained and restored. Also, most
video encoders/IP cameras are web based, which provides a user-friendly way to access and configure the
video devices.
Scalability and Flexibility: IP networking is a widely used and understood standard, and IP-based video
devices can freely exchange data on this network as long as they have network access. This provides a
convenient way to combine many different type of devices on one IP network. An IP video surveillance can
easily be integrated with an existing IP network-based system by using the same network resources, and the
system is highly scalable because it uses standard networking architecture.
Cost-Effective: Thanks to the integration of IP and video technology, the video surveillance system can save
a substantial amount of time and money by using existing IP network infrastructure to build the video
surveillance system. No separate cabling or infrastructure is needed. In addition, Video streams encoded in IP
data can even be transmitted wirelessly, through WLAN devices, which is a substantial cost and time-saver in
environments that are difficult to cable.
IP Video Surveillance Design Considerations
There are many ways to implement an IP network, and there is no single best way design a perfect IP network.
As mentioned above, any digital data can be exchanged within an IP network. For IP networks that need to
support IP video surveillance applications, keep the following key design considerations in mind
Network Architecture
Your network should optimize the network architecture for efficient bandwidth utilization. Some network
architectures are less efficient, and consume more bandwidth. For example, figure 1 illustrates a network with
bus topology. The PC is placed at the edge of the network to monitor the video images from all cameras. All
video data will flow to the PC eventually. Each section of the bus will increase the bandwidth load on the
network, until in the rightmost network segment the network much support data coming from all four cameras.
For this reason, bus architectures are usually inefficient network topologies and not recommended for IP video.
In the Figure 2, the PC is placed in the middle of the network. Compared to the figure 1, the total bandwidth
load of four cameras is split between the two halves of the network. There is never a network segment that
needs to support data from more than two cameras at once. This architecture distributes the bandwidth more
evenly and decreases the possibility of network issues.
Bandwidth Requirements
In IP video surveillance, the communications model is typically server-client or host-client communication. The
server is the IP camera, and the client is the PC or storage hard disk. The IP camera will generate a video
stream as long as there is a client request. The number of clients in the system depends on the type of
connection. Depending on the specific network and system requirements, sometimes a unicast communication
model is sufficient, but in other circumstances a multicast communication model is required.
Unicast: Unicast is one-to-one communications. In unicast transmissions, the packets are sent from one node
to another individually. The network uses the unicast address to identify the source and destination addresses.
When many clients request unicast transmissions from one host, the host will generate multiple duplicate
packets of that transmission. This will increase the load on the host, and the bandwidth requirements on the
network.
Multicast: Multicast transmission is one-to-many communications. In multicast transmissions, the host sends
the packets to a special address that represents a group clients or destination receivers. When a client wants
to request the multicast transmission, it must join the multicast group. The host sends just one copy of the
multicast data to the multicast address, and all members of the multicast group will receive the packets at the
same time. Even if many clients request a packet from the host, the host needs to send just one copy of the
packet. This minimizes the impact to the performance of the host, and the load on the network.
The L3 multicast IP addresses are between 224.0.0.0 and 239.255.255.255, and the L2 multicast MAC
addresses are from 01.00.5e.00.00.00 to 01.00.5e.7f.ff.ff.
In order to send multicast packets, the network devices must support multicast and a multicast protocol. For
IP cameras, which are the host in the network, look for multicast support that enables them to send traffic to
a multicast group. For L2 switches and L3 routers, look for IGMP snooping and IGMP, so these network devices
can understand the multicast process and deliver multicast traffic to group members.
In addition, multicast traffic has an additional restriction in that it usually can be transmitted within one subnet
only. If you need to send multicast traffic from one subnet to another subnet, the L3 router must support a
multicast routing protocol such as DVMRP to route multicast traffic between subnets.
In a system that only has one client that needs the video stream, as illustrated in the figure below where there
is only one control room monitoring all the cameras, then unicast communication is sufficient. On the other
hand, if a system possesses multiple clients, such as illustrated in the figure below where the control room and
two other remote sites are monitoring the cameras, then multicast communications is a superior solution.
Determine the right traffic type for your network based on the number of video streams required in order to
minimize device load and bandwidth requirements.
QoS
QoS, or Quality of Service, is a mechanism that ensures higher quality network performance for critical
applications. Traditionally, without QoS, all traffic will be processed equally by the network, and transmissions
will be based on the network’s best-effort. The QoS assigns different network traffic different priority. The
traffic with higher priority will be processed before lower priority traffic. This ensures the network performance
is reserved for the most critical applications, and can guarantee that these critical applications will experience
more reliable communication. Therefore, the transmission of critical applications will be optimized by QoS to
reduce frame loss, stabilize the jitter and minimize the latency. Layer 2 devices use 802.1p and Layer 3 devices
mostly use DSCP to prioritize traffic.
In order to ensure that the traffic is prioritized from sender all the way to receiver, all network devices must
support QoS. For video surveillance systems, this means the IP camera should have the ability to generate
packets with a priority tag.The priority tag identifies the packet to the L2 switch or L3 router, so that it can be
assigned the appropriate level of priority. Of course, the L2 switches and L3 routers themselves should also
support QoS, in order to process the priority tags. If any devices do not support QoS, then the traffic wil simply
by processed in First In First Out (FIFO) order.
The following figure is an example of QoS configuration of an IP camera and L2 switch(es). The IP camera
should have a field where the user can configure the DSCP value, which is the priority tag in the IP header.
When the video stream is generated by the camera, the DSCP value will be attached to the IP header of the
outgoing packets. When the L2 switch receives the packets on its incoming port, the switch will check the value
of the DSCP field in the IP header, and map to its DSCP mapping table. The switch will then process the packet
according to the priority queue.
QoS can be helpful to optimize the performance of video application. There are many network performance
issues that can be reduced or minimized by QoS, such as:
Frame Loss: The L2 switch can handle multiple types of traffic at the same time. The figure below illustrates
a scenario where there are multiple incoming streams to one port. In normal operations, the L2 switch would
have no problem handling this setup. With 100 Mbps of bandwidth available, the switch can easily support the
typical bandwidth usage rate of the three attached devices. The problem only arises when all three devices
experience a bandwidth spike simultaneously. This sudden burst in outgoing traffic can create up to 300 Mb of
traffic, overwhelming the port. Packets will be dropped, creating frame loss.
With the QoS, the traffic is prioritized, and the switch will process the traffic in priority order. Each stream will
be processed one by one based on priority, which reduces the risk of packet loss, as illustrated in the figure
below.
Latency: Network latency is the transmission time of a packet from a sender all the way to receiver. In IP video
systems, there is usually some latency between the actual event being captured, and when the image appears
on a monitor. This video latency comes from a combination of three sources:
Time spent processing the image on the camera, or “video processing delay”
Time spent transmitting data through the network, or “propagation delay”
Time spent processing the image on the computer or monitor, or “decoding/buffering delay”
With QoS, users can configure the video traffic as higher priority traffic. When the video stream is delivered
through the network, the switches and routers will process the video stream prior to other traffic. This reduces
the latency in the switch and router, minimizing the “propagation delay” component of video latency.
Jitter: When the video packets are generated by the camera, the packets are delivered one by one with gaps
between each packet. During the transmission through the network, the packets might be processed differently
in a switch or router, e.g. a switch might handle multiple streams in different order. This causes the gaps
between packet to vary when they finally reach the network destination. This phenomenon is called jitter, and
creates intermittent video streams.
the different gaps between packets when arrives to the destination, this is called jitter, and it will result the
intermittent video image display.
To prevent jitter, the video application attempts to buffer the video by collecting and consolidating many
incoming packets. After a certain number of packets ar stored in the buffer, the application will process the
packets so the video image will display smoothly.
QoS can help to reduce the jitter. When the video stream is set to high priority, the switch or router will process
the video stream prior to other data streams. This will maintain consistent gaps between packets.
Begin Every New Network with the Right Plan
Every network is different, and each network must be designed to meet the specific requirements and variables
of each specific application. However, for any network, it’s useful to review the three key design considerations
outlined in this guide. Identifying the right network architecture, bandwidth requirements, and quality of
service settings for an IP video network are the important first step to building a data network that will
successfully become a consistent, dependable communications platform that empowers IP video surveillance
systems.