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Networking tutorial, part 2

Figure 1. Depending upon the switch bandwidth, a server may or may not deliver full performance to workstations. Click here to see an enlarged diagram.

This month we will look at several things that I wish someone had told me before I became heavily involved in computers and networks. The first topic is bandwidth and throughput.

Backplane bandwidth

A 10Base-T system does not deliver 10Mb/s, and a gigabit Ethernet system does not deliver 1Gb/s. Because of overhead, collision resolution algorithms and other factors, the actual data rate across a network will always be less than the data rate specified by the nomenclature (10Mb/s for 10Base-T, etc.). A good rule of thumb is to load the network to about 70 percent of its total capacity. Some would argue that even this is too high, and that 60 percent or 50 percent is more realistic.

While we are on the topic of throughput, let's talk about routers and switches. It is common to find Ether-net switches that are labeled as 10/100 compatible or even 10/100/Gig-E. What the manufacturer is telling you is that the switch will automatically adjust its transmission rate to match whatever you plug into that port. If you plug two 10Base-T equipped computers and two Gig-E computers into the same switch, they will all be able to communicate. Furthermore, if the two Gig-E computers exchange data, they will do it at Gig-E speeds. In other words, having mixed rate computers on the switch will not lower the overall speed of the switch. The link between any two ports will operate at the lowest rate between the two ports.

If you have a station full of older computers with 10Base-T cards, the switch will automatically adjust its rate to 10Mb/s so that it is able to link to the Network Interface Cards (NICs) in these computers. When you buy a new server with a 100Base-T card, you can plug this server into the switch, and the link between the server and the switch will operate at 100Mb/s. In Figure 1, the server is connected to two workstations through a multi-speed switch. Each of the workstations is configured with 10Base-T NICs.

Given the configuration shown in Figure 1, what is the actual throughput between the server and each workstation? Assuming that the server has lots of internal bandwidth and keeping in mind our 70 percent figure above, the server should be able to source somewhere around 70Mb/s of data. (I am going to use “source” and “sink” terminology here, even though the conversation is actually two-way.) Presuming that each workstation contains a 10Base-T NIC, and that there are no internal bottlenecks inside the workstations (this may be a faulty supposition), and continuing to keep in mind our 70 percent figure, each workstation is capable of sinking about 7Mb/s of data. The aggregate sink bandwidth is then approximately 14Mb/s — so it appears that the server should be able to deal with the load on the network without any problem.

What if we slightly change the scenario? Let's say we move to a 10/100/Gig-E switch, the majority of workstations are equipped with 100Base-T cards, and there are a few graphics station clients with Gig-E cards. Will the system still work at full bandwidth? You might look at the front of the switch and assume that the number of ports times the maximum bandwidth of each port would give you the total bandwidth available on the backplane of the switch. Let's say you are using a 10 port 10/100/Gig-E switch, and that this switch cost a few hundred dollars. You calculate that the switch should have a total backplane bandwidth of 10Gb/s (10Gb/s × 1Gb/s per port). At this price, it is unlikely that this is the case. The switch may have a backplane bandwidth on the order of 2Gb/s to 3Gb/s at best. Of course, if you have spent several thousand dollars on your switch, then it is probably non-blocking.

When designing networks for professional teleproduction, there is one important question to ask: What is the non-blocking bandwidth of the switch? In other words, what is the total bandwidth of the switch backplane? A completely non-blocking 10 port 10/100 Ethernet switch would have to be able to switch a maximum of 10 streams of data, with all of those streams running at their theoretical maximum of 100Mb/s. The switch must have a total bandwidth capacity of 10Mb/s × 100Mb/s, or 1Gb/s. If someone argued that in this example a switch might still be considered non-blocking if it has 700Mb/s of backplane capacity, he or she might be right. However, if the switch only has 200Mb/s of backplane capacity, network speeds will certainly suffer.

If the switch is a blocking switch, what will happen as the switch runs out of bandwidth? Depending upon the protocols and application software, things may still function normally, although the network will be slower than if you had a non-blocking switch.

On cabling

The moral of the story is: If you really need high throughput, you may have to do more than upgrade your NIC cards to 100Base-T or Gig-E. You may have to replace your network switch, even if it is labeled 10/100/Gig-E.

Readers continue to write me regarding the August 2004 column on network cabling. The article had an error, so I would like to cover this again just to be sure we set the record straight. To build your own Ethernet cable, first things first — get some cable — and not just any cable. Be sure to get CAT-5 cable. Use CAT-5E if you want to use the cable for gigabit Ethernet now or in the future. Cable that is not specifically made for Ethernet Unshielded Twisted Pair applications will not work. You will also need two RJ-45 connectors and an RJ-45 crimp tool.

The pin out for a normal Ethernet cable that you would use between a computer and a switch is as follows:

Pin 1 — White/Orange — Pin 1 Pin 2 — Orange — Pin 2 Pin 3 — White/Green — Pin 3 Pin 4 — Blue — Pin 4 Pin 5 — White/Blue — Pin 5 Pin 6 — Green — Pin 6 Pin 7 — White/Brown — Pin 7 Pin 8 — Brown — Pin 8

The pin out for a crossover Ethernet cable that you would use between two computers without a switch is as follows:

Pin 1 — White/Orange — Pin 3 Pin 2 — Orange — Pin 6 Pin 3 — White/Green — Pin 1 Pin 4 — Blue — Pin 4 Pin 5 — White/Blue — Pin 5 Pin 6 — Green — Pin 2 Pin 7 — White/Brown — Pin 7 Pin 8 — Brown — Pin 8

Out with the old

To determine which pin is Pin 1, hold the connector in your hand with the opening of the cable facing you and with the metal contacts facing up. Pin 1 is the far left pin.

Many of us have drawers full of old networking hardware. When a new project comes up, it is tempting to dig into the drawer to see if you have the hardware you need. This is fine if you are dealing with equipment that will be used to feed non-critical systems.

However, network technology and protocols are being constantly improved, especially in areas of network security and quality of service. Therfore, if you are dealing with equipment that is being used for mission-critical applications at the core of a network, or you are working with firewall systems that will protect your network from intrusions, you will be much better off buying new equipment rather than recycling old hardware.

| Part 3 >>

Brad Gilmer is president of Gilmer & Associates, executive director of the AAF Association and executive director of the Video Services Forum.

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