What are LANs and WANs? A Local Area Network links all the PCs in a building or group of buildings. Primarily it is a local system and data runs around it at high speed.
A Wide Area Network is made up of two or more LANs linked together. The link is usually a leased line and, of necessity, limited in bandwidth. In simple terms, the data that travelled around the LAN at high speed travels over the WAN at a much reduced speed.
To make a speed comparison: most LANs use 10 or 100 base T systems. They are capable of moving local data around at 10 million or 100 million bits per second. A leased line connecting two LANs may only be able to transmit data at 64,000 bits per second. In perspective, that is the difference between walking speed and travelling at two miles per second.
CCTV images and network bandwidth
As mentioned in previous articles, there is nothing that worries a Network Administrator more than the prospect that all those CCTV images flying around the network will eventually cause "congestion collapse" or "aliasing instabilities"or perhaps even the debilitating "address exhaustion'.
In reality the administrator's main concern is bandwidth, which is the amount of traffic that can be on a network at any one time. Video can take up a lot of bandwidth and your job is to ensure that the bandwidth being used by the CCTV images does not degrade the performance of the network.
Packets: When we use a network to bring an image from a video server to a PC screen, you may imagine that the whole picture arrives at once, in a big block of data. Not so, it gets sliced up into small pieces that are sent one a time across the network. These small pieces are called packets. An image may consist of several packets.
As all the packets that are flying around a network are similar in size there is no real discrimination against any particular packets. A packet could contain video, data, text, e-mail etc. So video packets are not the culprits that clog networks, just lots of packets or in network parlance, heavy traffic, can be the culprit.
Collisions: Collisions are what happens when the network traffic starts to become congested. One packet of data smashes into another, usually destroying both packets. The busier the network gets, the more collisions happen. Eventually the network becomes overloaded, and as data travels more and more slowly the network becomes unusable, like the M25!
WANs and bandwidth restrictions: While you will rarely have any trouble with congestion on Local Area Networks the bandwidth available for a WAN link is often very restricted.
As an example take two busy LANs, one in Manchester and the other in London. They are linked by a leased line with a bandwidth of 128K. Data transfer is busy so most of the time the 128K is fully utilised. If you need to send video images across this link then the usual data transfer will slow down.
To assuage the by now apoplectic Network Administrator, the video server should be restricted to transfer images at a suitable rate for the link. If the link is 128K a good compromise would be to set the video server to run at 64K. Remember that the performance restriction across a WAN link is one relating to the network, not to the video server.
Installation: Once you have all your CCTV gear set up and ready to go, it is best to try and find the network and plug right in.
What do networks actually look like?
Imagine you have just sold your first network CCTV installation. The cameras are up, the cables installed, the video server is screwed to the wall and all there is left to do is connect up to the network. But where is it? What does it consist of? Is it OK to plug in here?
The network will consist of a number of wires connecting to other wires running all over the building, and there will be lots of wires. Somewhere in the building you should find a patchbay for the network. Most of the network wires running through the building will terminate here in a socket on the patchbay.
It is a bit like being infested with ants: until you find the nest, there is not a lot you can do about them. If you look at the example nest in the picture on the previous page, you could be forgiven for thinking network engineers are an untidy lot. Yet as far as things go, this example is relatively neat.
One word of warning, don't mess around with any of the connections on a patchbay. They are all planned and they all do something. The Network Manager will be able to advise you where you can plug in.
More likely than not you will have already requested a network point be installed close to the CCTV equipment and can connect straight away.
Hubs: Alternatively you may plug straight into a hub. You can think of a hub as a distribution point. Most small networks consist of several hubs connecting each unit on the network to every other unit.
The nice thing about connecting to a hub is that there is always a reassuring LED that lights up to say you are correctly connected. There is also a LED that blinks to show when data packets are travelling through your cable link.
On most network hubs there is even a collision LED, lighting up when things are colliding. This is pretty useful, if it is flickering away you have big problems. The occasional blip can be ignored.
Hubs come in many varieties, from the 32-way 19-inch rack types found in the patchbay to smaller units with four to eight inputs to feed a small office.
Switched hubs: A switched hub is used to reduce overall traffic on a network. Generally any packets sent from a point on a local network will actually traverse the whole network, even if the destination is close by. A switched hub will specifically direct packets to the required destination, and this can dramatically reduce network traffic.
Conclusion
Hopefully everything will slot neatly into place and your system will run perfectly. If not you will have to start troubleshooting, but we can cover that in another edition.
Source
Security Installer
Postscript
Jeff Berg is currently Technical Director of AD Network Video and can be contacted on jberg@ad-networkvideo.com
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