Published in October 2004

Network Video Multicasting
By Richard Mavrogeanes

Will MPEG video kill your network?



    The thought that more bandwidth will cure network ills is an illusion like the thought that more money will ensure human happiness. Certainly, more is better. But when “There is not enough of bandwidth” is stated as quickly as “We can’t afford it,” either or both statements may have been offered to dismiss a request because of misunderstanding. We’ll explain bandwidth perceptions, issues and solutions as they relate to sending MPEG video over modern networks.

Broadband?
     In recent years, the term “broadband” has been used so widely it’s beginning to lose its meaning. Cable companies and DSL providers would have you believe that “broadband” is anything better than a dial-up connection. But the industry has long recognized three bandwidth segments:
     • Narrowband: 0 to about 56Kbps
     • Wideband: 56 Kbps to 2Mbps
     • Broadband: more than 2Mbps
     Narrowband defines the speeds provided by analog modems, wideband is the T1 and E1 data range, and above T1/E1, we have broadband. Without a qualifier, such as “broadband-
access,” the terms imply a sustained and continuous throughput capability. For example, saying you have a T1 connection implies you have 1.536Mbps of connectivity. But if your T1 connects you to a Frame Relay network that delivers just 512 Kbps, saying you have a “T1” can be misleading1.
     Let’s say an internet service provider has a DS3 (45Mbps) connection to the internet. If that provider had 45 subscribers, it might fairly state that each user has 1Mbps of bandwidth. But if it has twice that number (2:1 over-subscription), can it still make the claim? How about at 100:1 over-subscription? 1000:1?
     The answer lies in subscriber usage patterns, and the expected nature of the data. Subscribers generally do not send 1Mbps of data all the time, and this fact makes room for statistical gain. In other words, it does not matter if there is a million:1 over-subscription as long only a few subscribers are actually using the network at any given instant. As networks gain more subscribers, and as those users become more dependent on the network, usage goes up, which drives performance down. For example, a DSL or cable modem provider may claim high-speed local connectivity, but at some point all of the users squeeze through the provider’s internet access pipe that typically is much smaller than the sum total of bandwidth available to all users.
     The same is true for our 10 or 100Mbps private Local Area Networks (LAN), too, but now we are talking about true broadband that delivers your 1s and 0s at blistering speeds.

Fire Hose Principle
     Modern networks deliver a 10 or 100Mbps connection to each and every computer, and often 1000Mbps to high-capacity shared devices such as file servers. Inside our buildings, the “garden hoses” that once interconnected our computers have given way to “fire hoses.”
     This has happened because the cost of high-speed local area networking has dropped from more than $1000 per connection just five years ago, to less than $100 per connection today. This 10X cost reduction also brought a dramatic increase in other network capability, including a move to fully switched Ethernet, data priority mechanisms, better management techniques and much more.
     But in most cases, Wide Area Network (WAN) bandwidth did not see the same dramatic cost reduction. Hence, the gap between WAN and LAN bandwidth has only widened. Although five years ago we may have had a 10Mbps Ethernet network connected to a 256Kbps private WAN via Frame Relay, today we often have a 1000Mbps network connected via T1 (1.536Mbps). And the post-internet bubble demise of promising new native LAN wide-area carriers has not helped matters. At the same time, our traffic patterns have changed. Not long ago, the primary destination for data traffic was “inside” our networks: file servers, printers, mail servers, etc. But today, the “outside” World Wide Web has become a dominant traffic destination, putting even more stress on the WAN.
     “WAN” once meant a point-to-point connection in a private network, but today it usually means “a connection to the internet.” This change in meaning is not trivial because the behavior and capabilities of the internet are quite different from the behavior and capabilities of a private network. Moreover, “off network” traffic (that is, data that originates in your LAN but is destined for the internet) easily can saturate expensive WAN bandwidth.
     So, today we are faced with the Fire Hose Principle: Connecting a LAN to a WAN is like drinking from a fire hose, and the user evaluates the performance of the LAN based on the performance of the WAN. It would be easy to conclude that you are bandwidth-challenged everywhere, when in fact you only have one bottleneck point.
     It is a common misperception that our local networks are saturated. In fact, this is far from true for most networks. Just as with a sports stadium with only one entrance, getting through the gate can be a problem; once you are inside, there is plenty of room. Live and stored DVD-quality MPEG video typically originates and terminates within our true broadband networks (i.e., our LANs), which are more than able to carry the traffic.

Do the Math
     Local area networks are built using Ethernet switches and a Category 5 wire connecting each port of a switch to each computer. If a switch has 16 ports, and each port is operating at 100Mbps, then the switch would have to support 1.6Gbps (100Mbps x 16 = 1600Mbps) for it to be “non-blocking”2. Happily, modern Ethernet switches are fully non-blocking, and a 1.6G switching capacity for a 16-port switch is today as common as 2.4G capacity is for a 24-port switch.
     But non-blocking switching really is meaningful only if there is a higher speed port that can accept all of the wireline speed data from all of the other ports, or with somewhat artificial traffic patterns: port 1 sends to port 2, port 3 sends to port 4 and so on. The reality is that normal traffic patterns typically require ports 1 to 15 to all send data to port 16—because port 16 may be the port that connects the workgroup to the corporate backbone and then on to the internet.
     One could easily conclude that sending 1.5Gbps (15 ports at 100Mbps each) to a single 100Mbps port would be a huge issue, but surprisingly it is not. This is because, although each computer can send data at the 100Mbps rate, they don’t send very much data! For example, consider what happens when you download a 1MB file over a 100Mbps network:

1MB file = 1,048,576 x 8 bits = 8,388,608 bit file
100Mbps = 1/100M = 0.00000001 seconds per bit
8388608 x 0.000000001 = 0.08388608

     Thus, it will take only 83.8 thousands of a second to download your file (it actually takes much longer because of computer disk operations and other factors). The point is that you are not using the network at all for most of the time, leaving time for others to use it. The sharing of an uplink from your workgroup switch, like the sharing of your WAN connection, is possible because of the bursty nature of most data sources and the statistical nature of the network usage. As long as there is not too much data, all is well.
     But, as traffic increases, the likelihood of multiple users contending for the same network port increases. At some point, typically about 80% of network capacity, there is so much contention that the network seriously slows down, leading to complaints. Therefore, higher speed uplinks such as Gigabit Ethernet (1000Mbps) are a superior solution.
     Considering this discussion, one might conclude it would be a bad idea to send an 8Mbps video stream from one port to every other port of an Ethernet switch. That would require the source port to provide 120Mbps (15 x 8Mbps), well in excess of a 100Mbps port’s capacity, right? Wouldn’t this more than saturate a 100M uplink? Wouldn’t all mission-critical applications slow to a crawl? Not when sending well-regulated video and when multicasting techniques are employed.

Multicasting to the Rescue
     While conventional packet data normally is sent from one source to one destination, multicast traffic is sent from one source to multiple destinations but without using more bandwidth.
With multicast, the source delivers only one packet stream to the switch (for example, at exactly 5Mbps), and the switch replicates the packets and delivers them to anyone connected to that switch who requests them. In this local Ethernet switch environment, it is rather pointless to worry about bandwidth when everything is happening at wireline speed.
     Modern Ethernet switches replicate multicast packets locally without using any additional uplink bandwidth. As a result, sending 5Mbps to every user will have the same network load as sending 5Mbps to one user.
     But if one Ethernet switch has 16 ports, and one port is connected to the router and 15 ports are connected to users who each wish to view the 5Mbps video, wouldn’t it require 75Mbps (15 x 5Mbps), dangerously close to maximum uplink capacity? The answer is no, because there is only one stream coming from the video source and it is delivered via multicast.

Bursts and Priority
     In our discussion, a 1MB file is transferred in about 83 milliseconds. It is important to understand that the intended nature of the Local Area Network is to allow everything to happen as quickly as possible. If you send a 1MB file, the network will attempt to use all of the bandwidth to complete the transfer. If you have 10Mbps Ethernet, the network will try to send your file at 10Mbps for as long as it takes; if you have 100Mbps Ethernet, the network will try to use 100Mbps for as long as it takes. In other words, you are “betting” that your file will be done before someone else needs the network3.
     With this in mind, you can see why giving one network user priority over another can become complex. If one user can send data at 100Mbps on a 100Mbps network, and if he has priority over everyone else, that priority user could lock out everyone else each time he uses the network!
     However, if a device such as an MPEG encoder were given top priority, it could never use more than the rate at which it was running. For example, if an encoder unit were sending video at 5Mbps, it would use exactly 5% of the 100Mbps Ethernet connection at all times. It could never use more because the video is a well-regulated continuous stream that does not burst, unlike conventional web, email, file transfers and other traffic.
     Exactly 95% of the Ethernet port would simply be unused. To the extent the video data were to leave the Ethernet switch via an uplink port (perhaps destined to a router), it will use exactly 5Mbps, never more. If that uplink were 100Mbps, 95% is available for other traffic; if that link were Gigabit Ethernet, exactly 99.5% remains available for other traffic. Using our previous example, if a 5Mbps MPEG video stream were present, a file transfer that might otherwise require 83 milliseconds would now require 88 milliseconds—not much of a difference!

Mix It Up
     For the most part, Ethernet and IP networks have grown in an unplanned way. It is a rare IT manager who actually has an up-to-date map of his network, and it is not uncommon for there to be pockets of old shared-media wiring hubs in some areas and modern switches in other areas.
Hubs do not support multicast and can be a problem for the deployment of network video. In fact, hubs do not really support unicast because all computers connected to a hub receive all traffic at all times.
     Video can still be deployed successfully with hubs, but the trick is not to have too much high bandwidth traffic. For example, if 15 computers were connected to a hub via 10Mbps and one 5Mbps video data stream were present in that hub, all computers would receive the stream (whether they like it or not…just as they receive all email, web and other traffic whether they like it or not).
     Because the video is a continuous stream, the effect on a 10Mbps Ethernet network is to reduce network capacity by 50% (5Mbps/10Mbps). However, this fact alone may not have any practical meaning! If a hub-based network is used primarily to access the internet via a T1, the real maximum capacity of the network is only 1.536Mbps, meaning 8.464Mbps (10Mbps-1.536Mbps) is not used. In this case, adding 5Mbps to the mix has no adverse effect. If two such 5Mbps streams were added, there would not be adequate bandwidth on a 10Mbps network, although there would be ample bandwidth on a 100Mbps network.
     In a mixed corporate network, it would be good practice to deploy multicast video in switched Ethernet segments, and to filter it out, or allow only a limited number of lower bandwidth streams to flow to hub-based segments. With hubs and other legacy devices in your network, the best practice is to go slow, and try it before committing to full-scale deployment in those areas of your network.

Bottom Line
     Very high quality video is deployed easily on modern networks, and even on networks that are not so modern. Multicasting makes it possible to practically eliminate bandwidth concerns, but for some organizations, multicast-ing is new4.
     Perceptions still linger that video requires more bandwidth than is available. While this easily can be true for wide-area networks, it is rarely true for local-area networks, particularly with good network knowledge and pre-deployment planning.
     With simple, straightforward and conventional network planning, an unlimited number of users connected to a broadband network can reap the benefit of DVD-quality video on desktops and TV monitors for better communications, training, and enhanced security and monitoring.

Bandwidth Rule #1:
Applications will grow to fill available bandwidth.

Bandwidth Rule #2:
Network bottleneck points limit the apparent bandwidth.

Bandwidth Rule #3:
Multicasting saves an enormous amount of bandwidth.

Bandwidth Rule #4
Quality of Service affects both real and apparent
bandwidth.

 

References
1 Another example would be fractional T1, which is sold in increments of 64Kbps. Similar to Frame Relay, you would have a full T1 connected between you and your provider, but less than the full T1 is actually available for use.
2 A non-blocking switch has enough switching capacity for all ports to transfer data at wireline speed to any other port. Most modern Ethernet switches are non-blocking, although older switches that are not non-blocking are still in use.
3 There are many complex mechanisms that prevent one user from consuming all bandwidth for too long and enable bandwidth sharing, but the general idea of rapidly bursting your data in the LAN is a fundamental principal of modern networks. Many router and switch vendors have implemented policy-based features to control priority and QoS.
4 The history of the Internet Protocol shows that multicasting has been with us longer than the world wide web!



Richard Mavrogeanes is founder and CTO of Vbrick Systems, Wallingford CT.

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