The devil, as they say, is in the details.
One of the key tenets of engineering is to reduce complexity, but in doing so it is important to understand the implications. While we might try to view one technology as it relates to another to help us simplify the details, it is important that we recognise how and where they differ.
Case in point.
When it comes to wireless networks, I often talk about how there are two questions I dislike being asked more than any others:
- How many clients can connect to an access point?
- What is the maximum range of an access point?
The reason is that I believe they are the wrong questions. They are being asked from perspective of someone trying to relate to a wireless network as if it were a wired network. What they are really asking is “how many switch ports do I need to cover this area?”
But wireless networks are not switched networks. While each connected device in a wired network has its own physical cable, and thereby its own gigabit Ethernet link, in a wireless network, every device connected to a particular access point shares the same RF spectrum, the same total available bandwidth.
For a standard access point in today’s deployments, that means a maximum total bandwidth of 144Mbps on the 2.4GHz band with a 20MHz channel and 300Mbps on the 5GHz band with a 40MHz channel using channel bonding.
But that is an over simplification.
Those aggregate bandwidths assume each client is connected at the highest available data rate. As we increase range, however, the data rate decreases, thereby reducing the overall channel utilisation. Therefore, with fewer access points, we are not just sharing a limited amount of bandwidth with more clients, but we are actually reducing the total available bandwidth.
Interference, particularly as access points cover larger areas, becomes an even greater issue. An increase in the signal to noise ratio leads to a decrease in the maximum sustainable data rate. This again reduces the overall channel utilisation. The key here is that a wireless network’s ability to not only detect, but where possible mitigate interference is critical to its ability to sustain higher data rates and maximise the total available bandwidth in each cell.
All this assumes that the wireless clients connecting to the network are even capable of supporting those high data rates.
Most smartphones on the market today support only 802.11g in the 2.4GHz band, meaning that at most they can support 54Mbps.
Newer devices, such as the iPhone 4, support 802.11n, but only in 2.4GHz, and only with a single antenna, limiting them to a single “spatial stream”—in simple terms that means the maximum data rate they can support is 72Mbps.
This applies to tablet devices as well. While the new iPad2 supports 802.11n in both the 2.4GHz and 5GHz band, it too is limited to a single spatial stream. The Cius goes one step further with support for channel bonding in 5GHz, increasing the maximum data rate to 150Mbps.
Interestingly, we are now starting to see new access points enter the market using Atheros’ first-generation silicon supporting three spatial streams. While this increases the maximum data rate in the 5GHz band to 450Mbps, as we have just seen, this will have no impact on the multitude of mobile devices given their single spatial stream limitation.
Three spatial streams represents a key milestone for the 802.11 standard, and will become increasingly important over the next 2 to 3 years as battery technology improves and wireless chipsets incorporate better power saving designs. Of course, by that time we will be looking at access points supporting four spatial streams and 600Mbps—and again, be waiting for the mobile devices to catch up.
Adding to the complexity in all this are the applications these devices are running across the network. From FaceTime and Skype, to Business Video and Personal Telepresence, voice, and video in particular, are replacing data as the primary traffic type. However, the wireless networks that have been built over the last several years were not designed for voice and video, and certainly not at the device densities we are now seeing.
As we look to support these many different mobile devices entering the market today along with their high bandwidth applications, clearly the two key areas we must consider in our wireless network designs are access point density to control cell sizes, and interference detection and mitigation capabilities to ensure that we maximise the channel utilisation in each cell.
And so, I’d like to propose two different questions to consider at the start of a wireless deployment:
- How many different devices do you expect to connect to the wireless network?
- And what are the applications that will run across the network and what are their associated bandwidth requirements?
Wireless and wired networks fundamentally differ at the physical layer. While its not necessarily important to understand the details of RF communications, it is important to understand the implications.
Stay mobile. Stay secure.
Tags: Cius, cleanair, Consumerization, iPad, iphone, mobility, Smartphones, Tablets, wireless
From my home network, I can successfully ping or traceroute to some IPv6 hosts, but I cannot subsequently open a web page or use other applications with it. How can this be? Maximum Transmission Unit (MTU) gotchas…
There is a subtle difference between IPv4 and IPv6 fragmentation strategies. IPv4 routers fragment traffic in the network when needed and then the receiving host reassembles those fragments. This generally works well, but there are a number of potential issues. Because of these issues, the IETF developed means for higher layer protocols such as TCP to determine the smallest MTU on a path and send appropriately sized datagrams in order to avoid fragmentation. The IPv6 designers presumed the presence of this Path MTU Discovery so that in IPv6, fragmentation no longer happens in the network but only at the hosts – and then only in special cases in that absolutely require it.
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Tags: Borderless Networks, IPv6, MTU
The classic traceroute tool has become an essential tool for network engineers. Traceroute is able to discover layer-3 nodes (routers) along the path towards a destination. This information provides operators with visibility about the path towards a destination.
However, there are limitations to traceroute such as issues with traceroute following the right path (as it’s IP source address might be different), no layer-2 (switches and bridges) discovery and really only a single piece of information is returned (IP address of the router).
With mediatrace, which shares the IP header of the flow you would like to trace, you can have much better path congruency—and confidence in the discovery. The mediatrace will also not only discover the routers (as with traceroute), but also switches that are only doing layer 2 forwarding.
Mediatrace does not need to be enabled on every hop. If it is not enabled on node, the mediatrace packet will simply be forwarded through that part of the network. This is exactly what would happen in the case of your traditional MPLS-VPN network.
Figure 1. Mediatrace tracing a flow while the operator chillaxes
Now for the best part! Mediatrace can dynamically engage the performance monitor feature we talked about a few weeks ago. This allows a dynamic surgical monitoring policy to be applied for the flow we are tracing that results in hop by hop performance measurements such as loss and jitter. As is the case with all mediatrace runs, the information is brought back into a single report where it can be quickly analyzed.
Figure 2. Mediatrace integration with performance monitor
Despite the name, mediatrace is not only for voice/video flows. It is able to trace any IP flow, and is even able to engage performance monitor to gather hop by hop TCP stats.
Mediatrace is a new tool that cisco released in IOS 15.1(3)T for the ISR platforms as part of the medianet program. Over the course of 2011, this feature will proliferate across cisco’s enterprise line of routers and switches.
Mediatrace Configuration Guide
Tags: business video, enterprise network, medianet, mediatrace, performance monitor, rich media applications, video
“The philosophy of the school room in one generation will be the philosophy of government in the next.” – Abraham Lincoln
Given its technical complexities, it’s understandable that some people have been skeptical about business video adoption over the past few years. But video is now much more than just a technology. Like printing and voice were not so long ago, it’s an irresistible force that is fundamentally changing the way all generations create and experience culture, business, and much of our everyday existence. For example:
- Video and computer game time for kids 8-18 has doubled in the past 10 years, and only 4-6% of their time is spent on print media (source: Arstechnica).
- In a recent enterprise survey, 57% of respondents are planning or have already implemented some desktop video conferencing, and 44% are planning or have already implemented some IP video for training, demos, and other purposes (source: Forrester Research).
- By the end of 2010, almost half of all mobile data traffic was already video, and it’s expected to grow 26 fold from 2010 to 2015 (source: Cisco Visual Networking Index).
Forward-thinking organizations embracing these trends have already come up with some wonderfully innovative new business models built on delivering video everywhere. For instance, the Khan Academy delivers free education via YouTube to millions of people worldwide, and Marriott Marquis hotels are delivering unique new guest experiences for discriminating travelers via Cisco technology.
Here’s a dramatization of delivering video anywhere to enhance education:
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Tags: branch, medianet, routers, video
Most people already have IPv6 capability whether they know it or not. All Microsoft operating systems such as Windows Vista and all MacOS releases since 10.2 have IPv6 installed enabled by default. Mobile devices running Android 2.1, Apple iOS 4.0, and Symbian 7.0 are configured likewise as is nearly every *nix variant you can name. Even the venerable and ubiquitous Windows XP has a latent IPv6 stack which can be activated with a single command.
Typically, IPv6 enabled systems will prefer IPv6 connections over IPv4, so a misconfigured or malfunctioning IPv6 network will cause connectivity problems. Many popular troubleshooting regimens simply prescribe disabling IPv6 as the “solution,” which really does nothing more than to hide the underlying problem with the IPv6 network. When you have a network problem that is “solved” by disabling IPv6, you have masked the symptom of a bigger problem that warrants further investigation.
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Tags: firewall, IPv6