When we talk about the Convergence Wave, especially in terms of technology, it is generally understood that those on either side have some amount of learning to do. A good deal of it is polished over time, but there are usually some foundational elements that need to be understood from the get-go.

In talking to traditional security integrators, there still seems to be some confusion regarding a couple of very fundamental network devices that need to be understood if you are going to enter this world. The question that usually comes up is, “What is the difference between a hub and a switch, and when would I use one or the other?” 

Both of these devices are crucial to a network’s ability to function. While not a perfect analogy, I would compare them to a matrix switcher. You can connect one camera to one monitor and get a picture, but if you have many cameras and many monitors, you need something in the middle to coordinate the video signals. Likewise, if you only have two computers, you can connect them with a single cable and chat all day long. But if you add any more, you will need a device to link them all together.

A Quick Collision Course
Before we get into the devices themselves I want to quickly discuss an extremely fundamental yet important concept in networking: the collision domain.

Like a video signal, or any other form of data, when an Ethernet packet leaves a device, it needs to make sure it has a physically clear pathway to any other device on the network. As on a freeway at rush hour, if multiple packets attempt to occupy the same space at the same time, collisions occur. Even though a packet isn’t necessarily a physical device, the electrical impulses on the wire interfere with each other, causing the collision. 

Collisions on a network are pretty much the No. 1 thing to avoid. In fact, most of the standards that define the different types of networking (Ethernet, token-ring, etc.) are written around different ways of avoiding collisions. 

A collision domain, then, is an area on the network where computers connected together are susceptible to collisions. In other words, they are sharing the same pathways. In our one-to-one example above, if both computers tried to send data at the same time, the packets would very likely collide, hindering communication. 

Keep this in mind as we talk about each device. 

Hubs Have Very Limited Use for Us
The central networking device that most people are familiar with is the hub, partly because the name is used to generically describe a central point on a network and also because up until a couple of years ago it was the most common network device for the consumer. 

The hub is basically nothing more than a repeater. It takes the data that comes in one port and blasts it out all other ports. Notice I said “it blasts it out all the other ports.” What about the data coming into those other ports? Therein lies the problem. 

A hub doesn’t pay any attention to what or when something is coming in on each port. All it does is take what it gets and sends it along. Make sure you understand; it forwards any data it receives out all the other ports at the same time. In other words, it creates a single collision domain. 

Hubs work totally at the physical layer, dealing only with the cabling and electrical impulse component of the data. A hub has no way of organizing or sorting the packets that travel through it. 

Hubs are basically fine if you want to share an Internet connection with a couple of computers, but how does it relate to what we do? Imagine this scenario: You have a DVR, and three or four people want to monitor that DVR simultaneously. Will it work? Maybe, but not very well. 

Since a hub blasts the data out of all ports, as soon as one person accesses the DVR and starts to stream video that video gets replicated out all the other ports on the hub. So when the second person connects, his or her data now has to compete with what’s coming from the first person, and it just gets worse as you add more video streams. 

The bottom line: a hub is not a good choice for a digital video application. Now, if you wanted to hook a single DVR to a single client, go for it. But you could probably do that with just a crossover cable and not worry about a box. 

So, if the hub isn’t really 21st century stuff, what should we use? 

Switches Are Like Hubs on Steroids
Today, most networking applications are totally reliant on switches. A switch can, at its most fundamental level, be considered a smart hub, but only in the sense that it is at the center of a group of connected devices. The similarities end there. 

Unlike the hubs that only see things at the physical level, a switch can understand a great deal more about not only the data it passes, but also the computers/devices connected to it. Switches can operate at higher layers, which basically mean they can actually read parts of the packets in order to understand what they are, where they are going and where they came from. 

Switches are also better than hubs because they isolate collision domains. Basically, each port is isolated from every other port and has its own path to the backplane of the switch, so data only goes to the port for which it is intended (with some exceptions). 

So how does a switch know where a packet is destined and, more importantly, the location of that destination? By paying attention to what’s in the packet. It looks at packets for two basic pieces of imprinted information — a source and a destination address. 

When a computer connects to a switch and sends its very first packet of data, the switch looks at the source address and records it to a database. It then looks at the destination address and determines if that packet is destined for another computer hooked to it by checking that same database. 

If the destination address is there, the switch forwards the packet out the proper port, and only that port. If the packet is destined for an unknown address, the switch can broadcast the packet out all other ports (except the one it came from) and see if it gets a response from an address that needs to be added to the database. If it is part of a larger network, the packet will be sent out an uplink port to another switch or a router, either of which will do the same thing and try to determine the correct destination. 

This basic function of a switch is performed at Layer 2, or the data link layer, which uses hard-coded MAC addresses to identify devices. Remember, MAC addresses are set at the factory and are not supposed to change like IP addresses. There are only a few instances where you might need to change the MAC address of a device, but it’s very rare. 

Devices Add Advanced Functions
There are some functions of newer switches that, when designing a digital video network, become very important. Switches today are gaining abilities at higher layers of the network. 

For instance, a Layer 3 switch can look at IP addresses the same way a router does. Among other things, this gives a switch the ability to create a virtual local area network (VLAN). This is a crucial ability when you get into large-scale video networks, and is used to further isolate traffic that if left alone, could create broadcast floods, such as streaming video, and take down that network. 

This capability necessitates another advanced function, the ability to manage, or program that switch. Most highto mid-end switches now are managed. They have some kind of user in
terface to make any necessary settings to the switch itself, like setting up VLANs.

Other cool features in more advanced switches can include port security, where only certain computers are allowed to plug in and access the network, and network-based management, where you can monitor and program a whole group of switches from one point.

Systems Getting More Demanding

Let’s review. If you are going to do a very small one-to-one video setup, a hub is all you need. If you are going to do any job of size, however, be very careful. As more and more fully distributed network systems are coming to market, more stringent switch requirements are becoming necessary.

As well as the basic functions discussed here, you also need to make sure the switch can handle the large amount of traffic video demands. We’ll talk more about that kind of throughput in a future column. Do your homework before selecting any piece of network gear.

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