High-speed networks

Just what constitutes a high-speed network depends upon your point of view. If you are running a 10Base-T network using a simple hub, then a 100Base-T network using switches will be a major step up. If you currently have a 100Base-T network but feel the need for speed, a fiber-based Gigabit Ethernet network may be in your future.

The networking industry has been trying hard to make these upgrades as painless as possible. Let's say you currently have a 10Base-T network in your facility that's been around for a while, and you would like to upgrade it without breaking the bank.

The current network consists of three interconnected hubs with two servers and 12 workstations. One of the servers is used to store graphics that have been generated by the art department until they can be moved to the on-air systems in news. Your users have been complaining that transfer times have been getting very long lately, and that overall network performance is suffering. Upon further investigation, you find that people are now really using the network in everyday production. When you put the network in five years ago, no one used it. Even a couple of years ago, people were still exchanging files using CD-ROM. But now, the network has slowed down enough that they're using CD-ROM because they have to. What to do?

Figure 1. A typical older 10Base-T installation. All workstations “hear” all other workstations on the network. Click here to see an enlarged diagram.

The first thing I would do is replace the hubs with switches. For a small amount of money, you can replace all three hubs with a single Ethernet switch. Switches are fundamentally different from hubs. Hubs connect all computers together. When one computer talks, all the computers on the network “hear” that computer, whether it is talking to them or not. (See Figure 1.) If two workstations want to exchange some files while another workstation is moving a big file onto the server, the workstations will have to compete for available network bandwidth.

On the other hand, switches provide a dedicated 10Base-T connection from each computer back to the switch. Traffic is switched from one port to another on a packet-by-packet basis. This means that in the example above, the two computers involved in a peer-to-peer transfer can do so at full bandwidth without “hearing” the traffic generated by the workstation engaged in a file transfer to the server. The switch does this by reading the Ethernet packet headers to determine the destination for the packet, and then switching the packet to its destination port without having to read through the entire packet.

When you start shopping for Ethernet switches, you will find that you cannot purchase a 10Mb switch anymore. You have to buy a 10/100Mb switch. 10/100 means that the switch will automatically sense the speed capabilities of the network interface card (NIC) connected to each port. Users may connect a mixture of 10Base-T and 100Base-T NICs to the switch. (See Figure 2.)

To upgrade your network, all you have to do is install the 10/100 switch in place of the three older hubs. But, of course, nothing in life is easy. You will need to check to see if any of the cards in your workstations or servers are 10/100 compatible. Second, you should check your cables. If they are not CAT-5, then you may not be able to get reliable performance at 100Mb/s, even though the card in the workstation, and the switch, are both 10/100. You may have to upgrade your cabling to get the higher network speeds. Remember that using telephone cable or other non-network cabling can cause you headaches. The switch will not detect that you are using the wrong cable. If you have a 10/100 NIC, the switch will try to operate in 100Base-T mode, but will have a very high error rate resulting in slow network connections and many time-out messages (if it works at all).

Figure 2. A multimode Gigabit Ethernet switch allows the user to mix 10Base-T and 100Base-T with Gigabit Ethernet on the same switch. Click here to see an enlarged diagram.

One of the nice things about having 10/100-compatible equipment is that you can upgrade your network a piece at a time. If you determine that your servers are a choke point, then you can buy 10/100 cards for them too, but you have the flexibility to leave the workstations with their old 10Base-T cards if you like.

If you move everything from 10Base-T to 100Base-T, it is likely that the new configuration will be more than 10 times faster than the old one. Why would that be? The old network operated at 10Mb/s; the upgraded network operates at 100Mb/s, 10 times faster. The answer lies in the difference between a hub and a switch. Because the switch gives each computer a dedicated full-bandwidth connection, and workstations and servers never “hear” traffic from another port, they will never encounter a collision.

If your 10Base-T network is really overloaded, then throughput across the network will be severely limited. Talk time is seldom available and NICs must frequently wait for an opportunity to transmit their packets. Also, in a hub-based network, when an Ethernet collision occurs, the NIC card is forced to wait a random amount of time before attempting to resend. This algorithm works very well on lightly loaded networks, but things slow to a crawl when network utilization becomes more than about 75 percent. So, if your old network was running at 80 percent utilization, your effective throughput might have been somewhere around 4Mb/s — a long way from 10Mb/s. Moving to a 100Base-T switched network likely will get you throughput in the range of 70- to 80Mb/s. But remember, you will only see this performance improvement if your original network was heavily saturated, and if the new cards on your network are 10/100 capable. Also, it is still possible to hit major bandwidth limitations either because of limits in switch bandwidth or because of limits on connectivity to a high-demand destination on your network, such as a server or Internet firewall.

So what is the difference between a switch that costs $10 per port and one that costs $150 per port? Why do large organizations continue to purchase more expensive switches? There are two main answers. First, more expensive switches come with network management, usually operating over the network using Simple Network Management Protocol (SNMP). SNMP allows a network engineer to use tools to look at the performance of network components from his desktop. You may or may not need these tools, depending on the size of your network. Many network products now come with a management interface you can access with a Web browser, making management unnecessary for most small- to medium-sized networks. The second reason a switch may be much more expensive is because of the device's backbone bandwidth. If a 10-port 10/100 switch has a backbone bandwidth of 100Mb/s, guess what — assuming equal loading on all ports, each port will get a real throughput of 10Mb/s, in spite of the fact that the connection will support a throughput of 100Mb/s. For a 10-port 100Mb/s switch to provide 100Mb/s to each port (less actually, due to Ethernet overhead), the switch must have a backplane bandwidth of 1Gb/s. Switch designers take a number of approaches to keep bandwidth requirements from spiraling out of sight in large commercial switches, but you can see that there can be some major performance differences between different switches with the same 10/100 label on the box.

There are still a couple of other things we can do to increase network performance. If you are charged with increasing the speed of a post-production network, you may already have a 100Base-T network in place. The good news is that much of what is written above applies just as well to moving up from 100Base-T to Gigabit Ethernet or Gig-E. Gig-E still uses RJ-45 connectors, but may require you to upgrade to CAT-6 cable. If you need longer distance, or electrical isolation, Gig-E supports fiber as well as unshielded twisted pair (UTP) cable. If you are moving from 100Base-T to Gig-E switches, you may not experience as big a jump in speed, relatively speaking, as you would in moving from a 10Base-T hub to a 100Base-T switch because you are already in a switched environment and do not have problems with collisions on your existing 100Base-T network.

Let's assume you replace your 100Base-T switch with a new Gig-E switch and CAT-6 cable. Everyone is happy for a few minutes until they notice that even with a Gig-E NIC card in the server, things are still too slow. After a little checking you discover the problem. Your workstations with new Gig-E cards are running the Gig-E card in your server out of bandwidth. (Yes, this is possible, even though it might seem that 1Gb/s is a lot of bandwidth.) As soon as people start moving high-res moving video in and out of the server, new bottlenecks will appear. We have one last trick available in this month's column. If your server NIC is the culprit, check to see if your server operating system supports aggregating bandwidth across multiple NIC cards. Most do. Drop another Gig-E NIC card in your server, and the bottleneck will go away. Of course, now you probably have bandwidth limitations caused somewhere else in the network (like the server backbone), but that is an article for another month.

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

Send questions and comments to:brad_gilmer@primediabusiness.com

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